Our objective was to explore changes in soil respiration of three temperate forests in Mt. Dongling, Beijing over the last 20 years from the year of 1994-1995 to 2012-2015.

We re-investigated the permanent plots of three temperate forests (Betula platyphylla forest, Quercus wutaishanica forest and Pinus tabuliformis forest) which were established in 1992. We measured soil respiration for 3 years (2012-2015) using a LI-8100 Soil Respiration System. Continuous soil temperatures at 5 cm depth were measured at the same time. Annual soil respiration was accumulated using a relationship between soil respiration and soil temperature .

We found that soil respiration rates were significantly correlated with soil temperature at 5 cm depth and that these correlations differed remarkably among the three forests. Annual soil respiration in B. platyphylla forest was highest, with a 3-year average of (574 ± 21) g C·m^-2, followed by Q. wutaishanica forest ((455 ± 31) g C·m^-2) and P. tabuliformis forest ((414 ± 35) g C·m^-2). In the past 20 years, annual soil respiration in all these forests increased significantly: compared to 1994-1995, the average in 2012-2015 increased by 85%, 17% and 73% for B. platyphylla, Q. wutaishanica, and P. tabuliformis forests, respectively.

Our objective was to explore the effects of different levels of nitrogen (N) fertilization on soil respiration and its temperature sensitivity during growing season in winter wheat (Triticum aestivum) in East China.

Three levels of N fertilization, N1 (15 g·m^-2·a^-1), N2 (30 g·m^-2·a^-1), and N3 (45 g·m^-2·a^-1), and the control group (CK) were set up in winter wheat fields. The LI-8100 Automated Soil CO₂ Flux System was used to measure soil respiration rate during the growing season (December 2013 to May 2014) of winter wheat.

During the growing season of winter wheat, mean soil respiration rates of N1, N2 and N3 treatments were 5.29, 6.17 and 6.75 μmol·m^-2·s^-1, respectively, which were 7.8%, 23.6% and 37.8% greater than that of the CK (4.90 μmol·m^-2·s^-1). Compared to CK, the N1, N2, and N3 treatments increased the aboveground biomass by 39.9%, 104.4%, and 200.2%, respectively, and the increases were significantly correlated with total soil respiration during the growing season. Soil respiration increased exponentially with soil temperature at the depth of 5 cm, which explained 65%-75% of the variation (p < 0.05). The temperature sensitivity of soil respiration (Q₁₀) calculated based an exponential equation was between 2.09 and 2.32. These results suggested that nitrogen fertilization promoted plant growth, significantly increased biomass of winter wheat, and stimulated the soil respiration.

Land use change affects ecosystem carbon dynamics by changing the plant community structure and soil micro-environment in grassland ecosystems. The aims of this study were to determine the effects of land use on soil respiration and litter decomposition in the temperate grasslands of Nei Mongol and to identify the effects of litter quantity, quality and decomposition on soil respiration during growing season.

We measured soil respiration during growing season in 2011 and 2012 under three land use types, i.e. grazing, mowing, and grazing exclusion, by using an automatic infrared gas analyzer (LI-8100) that was connected to a multiplexer system (LI-8150). Quadrat surveys and litter bags were utilized to measure litter production and decomposition. Several chemical indicators of litter quality were measured to calculate the litter decay rates. All data were analyzed with ANOVA and Pearson correlation procedures of SPSS.

Soil respiration and litter decomposition differed greatly among the three land-use types. In the drought year, the total soil respiration at the grazing site was 1.5 times greater than at the mowing site and 1.29 times greater than at the grazing-exclusion site. However, in the wet year, the total soil respiration at the mowing site reached 309 g C∙m^-2∙a^-1 and was greater than at both the grazing site and the grazing-exclusion site. Precipitation increased soil respiration and litter decomposition, indicating that soil water availability was a primary constraint on plant growth and ecosystem C processes. Also, the responses of soil respiration and litter composition to rainfall differed among the land-use types. Further analysis showed that the litter C:N decreased and the litter N content and lignin:N increased after 2-years of decomposition. In addition, soil respiration was significantly correlated to litter production (r = 0.78, p < 0.01), decay rates, C:N (r = -0.84, p < 0.01), and lignin:N (r = 0.62, p < 0.05).

Aims Soil respiration is a major way that CO₂ is emitted into the atmosphere, and it is important in global change research. Our objective was to examine the effects of degradation on carbon flux in alpine grassland.
Methods We measured soil respiration rates in alpine grassland under four degrees of degradation (no, light, moderate, and heavy degradation) using a LI-8100A open-circuit soil carbon flux measuring system. We analyzed the relationship between soil respiration and soil temperature, as well as between soil respiration and soil moisture.
Important findings Soil respiration under each level of degradation showed a monthly dynamic, but it varied by degree of degradation. With an increase of degradation, average soil respiration of the growing season first increased and then decreased. The highest soil respiration occurred under the moderate level ((2.46 ± 0.27) μmol·m^-2·s^-1), which was significantly higher than under no degradation ((1.92 ± 0.11) μmol·m^-2·s^-1) and heavy degradation ((1.30 ± 0.16) μmol·m^-2·s^-1) (p < 0.01). There was no significant difference between the moderate degradation and the light degradation (p > 0.05). The respiration under heavy degradation was significantly lower than under the other degradation levels (p < 0.01). There was a significant positive linear correlation between aboveground biomass and soil respiration (p = 0.004), but not between soil respiration and underground biomass (p = 0.056). There was a significant positive correlation between soil respiration and soil temperature at each level except heavy degradation. There were correlations between soil respiration and soil moisture (binomial fitting) with no degradation as well as moderate and heavy degradation (p < 0.05), and it was significantly correlated with light degradation (p < 0.01).

Aims Partitioning the soil respiration is an important step in understanding ecosystem-level carbon cycling. In addition, the heterotrophic and autotrophic components of soil respiration may respond differently to climate change. Our objectives were to evaluate the impact of soil temperature and water content on soil respiration and its components in Castanopsis carlesii and Cunninghamia lanceolata plantations, to determine the relative contributions of autotrophic and heterotrophic respiration to soil respiration, and to explore how different forest types would affect soil respiration and its components.
Methods The study site is located in the Nature Reserve of Castanopsis kawakamii, Fujian Province, eastern China. By using a field setup through trenching method and LI-8100 open soil carbon flux system, the dynamics of soil respiration were measured from August 2012 through July 2013. Soil temperature at 5 cm depth and water content of the 0-12 cm soil layer were measured concurrently with the measurements of soil respiration. Relationships of soil respiration with soil temperature and water content were determined by fitting both an exponential model and a two-factor model.
Important findings Soil respiration and its components showed significant correlations with soil temperature. There were significant monthly changes, in the form of a single-peaked curve, in soil respiration and its components in the two forest types. Soil temperature explained 70.3%, 73.4%, and 58.2% of the monthly variations in soil respiration, autotrophic respiration, and heterotrophic respiration, respectively, in the Castanopsis carlesii plantation; whilst it explained 77.9%, 65.7%, and 79.2% of the monthly variations in the three variables in the Cunninghamia lanceolata plantation. There was no significant relationship between soil respiration and soil water content in both forest types. The annual estimates of CO₂ efflux through autotrophic respiration in the two types forests were 4.00 and 2.18 t C·hm^-2·a^-1, respectively, accounting for 32.5% and 24.1% of soil respiration. The annual estimates of CO₂ efflux through heterotrophic respiration were 8.32 and 6.88 t C·hm^-2·a^-1, respectively, accounting for 67.5% and 75.9% of soil respiration. The annual estimates of CO₂ efflux through soil respiration and partitioning of the components were all higher in the Castanopsis carlesii plantation than in theCunninghamia lanceolata plantation.

Aims Soil respiration plays a critical role in the process of carbon cycling in terrestrial ecosystems, and it often shows spatio-temporal variations in response to diverse abiotic and biotic factors. Our objective was to examine the seasonal and spatial variations of soil respiration under five typical plant communities in the meadow steppe of western Songnen Plain.
Methods Using a LI-6400 soil CO₂ flux system, we investigated soil respiration and environmental factors under five vegetation types (Suaeda glauca, Chloris virgata, Puccinellia distans, Phragmites australis and Leymus chinensis) in the meadow steppe of Songnen Plain during the growing seasons of 2011 and 2012.
Important findings Soil temperature was the dominant controlling factor of soil respiration, which explained approximately 64% of the changes in soil CO₂ effluxes. Soil water content was not the limiting factor of the seasonal variations in soil respiration. The sensitivities of soil respiration to temperature (Q₁₀) ranged from 2.0 to 6.7, showing significant differences among vegetation types. The cumulative CO₂ emission averaged 316.6 g C·m^-2 during the growing season. The magnitude of soil CO₂ emission during the growing season was positively correlated with aboveground plant biomass, soil organic carbon content, and mean soil water content, and negatively linked to mean soil temperature, pH, electrical conductivity, and percentage of exchangeable sodium. The spatial variations of soil CO₂ emission were mainly caused by changes in soil microclimate, plant biomass, and soil properties.

Aims Due to concurrent variations of multiple factors influencing soil heterotrophic respiration (Rh), it is difficult to determine the responses of Rh to changes in individual factors and their interactive effects under field conditions. In this study, we conducted a laboratory incubation experiment with controlled temperature and water levels to determine the responses of Rh to changes in soil temperature and moisture using soil samples collected from an Stipa krylovii (Stipa sareptana var. krylovii) grassland in Nei Mongol.
Methods The incubation experiment consisted of five temperature treatments (9, 14, 22, 30, and 40 °C) and five water treatments (20%, 40%, 60%, 80%, and 100% of water holding capacity (WHC)), with a full factorial arrangement. We measured Rh at an interval varying from every two days to once a week, and soil dissolved organic carbon (DOC) and microbial biomass carbon (MBC) at an 18-day interval during the 71 days of incubation period.
Important findings The results showed that Rh differed significantly among different temperature treatments (p < 0.001) and was positively related to temperature (p = 0.001); the temperature sensitivity of Rh (Q₁₀) also increased with increasing moisture level. The relationship between Rh and water was best fitted using quadratic equations, and the optimal moisture condition increased when temperature rose. There existed significant interactions between soil temperature and moisture (p < 0.001) and their interactions could be best fitted using the function lnRh = 0.914 + 0.098T + 0.046M + 0.001TM - 0.002T² - 0.001M², it suggested that the models in additive form could explain the Rh response better than those in multiple form. Our results also showed that the relationship between Rh and MBC varied during incubation, and DOC was not significantly related to Rh (except for the 20th incubation day), suggesting that microbial turnover and community transformation could lead to the changes of gross microbial activity.

Aims Soil respiration (R_s) is the largest fraction of carbon flux in forest ecosystems, but the effects of forest understory removal on R_s in Chinese fir (Cunninghamia lanceolate) plantations is poorly understood. In order to quantify the effects of forest understory removal on R_s and microbial community composition, a field experiment was conducted in a subtropical Chinese fir plantation. Methods Forest understory was removed manually in June 2012. R_s was measured monthly using a LI-COR 8100 infrared gas analyzer from July 2012 through July 2014. Soil temperature and moisture were also measured at 5 cm depth at the time of R_s measurements. Surface soil (0-10 cm) samples were collected in July 2013 and 2014, respectively, and the soil microbial community structures were determined by phospholipid fatty acids (PLFAs) analysis. Important findings R_s decreased by 32.8% over a two-year period following understory removal (UR), with a greater rate of decrease in the first year (42.9%) than in the second year (22.2%). The temperature sensitivity of R_s was affected by UR, and was 2.10 and 1.87 in the control and UR plots, respectively. UR significantly reduced the concentration of fungal PLFAs by 18.3%, but did not affect the concentration of bacterial PLFAs, resulting in an increase in the fungal:bacterial ratio; it significantly increased the concentration of gram-positive bacterial PLFAs by 24.5%, and the ratio of gram-positive to gram-negative bacterial PLFAs after one year of treatment, but decreased the concentration of gram-positive bacterial PLFAs by 9.4% and the ratio of gram-positive to gram-negative bacterial PLFAs after two years of treatment. The results suggested that R_s and microbial community composition were both affected by UR in Chinese fir plantation, and the effects were dependent of the duration following the UR treatment.

Aims Seasonal snow cover is one of the most important factors that control winter soil respiration in the cold biomes. The warming-induced decreases in snowpack could affect winter soil respiration of subalpine forests. The aim of this study was to explore the effects of snow removal on winter soil respiration in a Picea asperata forest.Methods A snow removal experiment was conducted in a P. asperata forest stand in western Sichuan during the winter of 2015/2016. The snow removal treatment was implemented using wooden roof method. Soil temperatures, snow depth and soil respiration rate were simultaneously measured in plots of snow removal and controls during the experimental period.Important findings Compared to the control, snow removal increased the fluctuations of soil temperatures. The average daily temperature of the soil surface and that at 5 cm depth were 1.12 °C and 0.34 °C lower, respectively, and the numbers of freeze-thaw cycles of the soil surface and that at 5 cm depth were increased by 39 and 12, respectively, in plots of snow removal than in the controls. The average rate of winter soil respiration and CO₂ efflux were 0.52 μmol·m^-2·s^-1 and 88.44 g·m^-2, respectively. On average, snow removal reduced soil respiration rate by 21.02% and CO₂ efflux by 25.99%, respectively. More importantly, the snow effect mainly occurred in the early winter. The winter soil respiration rate had a significant exponential relationship with soil temperature. However, snow removal significantly reduced temperature sensitivity of the winter soil respiration. Our results suggest that seasonal snow reduction associated with climate change could inhibit winter soil respiration in the subalpine forests of western Sichuan, with significant implications for the carbon dynamics of the subalpine forests.

Aims Our objective was to explore the effects of different land use types on soil respiration rates in the mountain meadows of Tianshan Mountain, Zhaosu Racecourse, Xinjiang, China from 2015 to 2016.

Methods Four impermanent plots with different land use types (legume-grass mixture, LG; reseeding grassland, RG; natural grassland, NG; cropland, CR), which were established in 2013, were selected. The soil respiration rates in the growing seasons of two consecutive years (from April to September in 2015 and 2016) were measured using LI-8100A Soil Respiration System. Soil temperatures at 5 cm depth and soil water content at 0-10 cm depth were measured simultaneously. We also investigated soil biological properties including soil microflora structures, soil microbial biomass carbon, and soil enzyme activity. The hydrothermal and soil biological impacts on soil respiration rates were analyzed using the relationship among soil hydrothermal factors, soil microflora factors, and soil enzyme activities.

Important findings We found that: 1) in 2015, the temporal variation of soil respiration showed double peaks in NG and RC plots, but showed a single peak in RG and LG plots, and it reached the maximum in August in all plots. This temporal pattern was different in 2016. Soil respiration reached the maximum at the end of June in RG and LG, and at the end of July in NG and CR. 2) For the whole study period, the average soil respiration rate was in the order of: NG > RG > CR > LG. 3) Soil respiration rate was positively correlated with soil temperature, and negatively correlated with soil volumetric water content. The temperature sensitivity of soil respiration (Q₁₀) was in the order of: NG > CR > RG > LG. 4) Bacteria were dominant among soil microbes in all type of plots, followed by actinomycetes and fungi were the least abundant. The total soil microbial biomass was in the order of: NG > RG > CR > LG, which was consistent with the average soil respiration rate. The fitting analysis showed that soil respiration was positively correlated with the abundance of actinomycetes in RG (p < 0.05), and was positively correlated with the abundances of bacteria and actinomycetes in LG (p < 0.05). 5) The average microbial biomass carbon was in the order of: CR > NG > LG > RG. Fidelity analysis showed that soil respiration rate was significantly positively correlated with microbial biomass carbon in GR and CR (p < 0.05). 6) Among the examined enzymes, only protease and sucrase had a correlation with soil respiration, with sucrase having a greater effect. Changing the degraded mountain meadow to legume-grass mixture and reseeding grassland could decrease soil respiration rates, potentially benefiting carbon sequestration.

Aims Soil respiration from terrestrial ecosystems is an important component of terrestrial carbon budgets. Compared to forests, natural or semi-natural shrublands are mostly distributed in nutrient-poor sites, and usually considered to be relatively vulnerable to environmental changes. Increased nitrogen (N) input to ecosystems may remarkably influence soil respiration in shrublands. So far the effects of N deposition on shrubland soil respiration are poorly understood. The aim of this study is to investigate the soil respiration of Vitex negundo var. heterophylla and Spiraea salicifolia shrublands and their response to N deposition.
Methods We carried out a N enrichment experiment in V. negundo var. heterophylla and S. salicifolia shrublands in Mt. Dongling, Beijing, with four N addition levels (N₀, control, 0; N₁, low N, 20 kg N·hm^-2·a^-1; N₂, medium N, 50 kg N·hm^-2·a^-1 and N₃, high N, 100 kg N·hm^-2·a^-1). Respiration was measured from 2012-2013 within all treatments.
Important findings Under natural conditions, annual total and heterotrophic respiration were 5.91 and 4.23, 5.76 and 3.53 t C·hm^-2·a^-1 for the V. negundo var. heterophylla and S. salicifolia shrublands, respectively and both were not affected by short-term N addition. In both shrubland types, soil respiration rate exhibited significant exponential relationships with soil temperature. Temperature sensitivity (Q₁₀) of total soil respiration in V. negundo var. heterophylla and S. salicifolia shrublands ranged from 1.44 to 1.58 and 1.43 to 1.98, and Q₁₀ of heterotrophic soil respiration ranged from 1.38 to 2.11 and 1.49 to 1.88, respectively. Short-term N addition decreased only autotrophic respiration rate during the growing season, but had no significant effects on total and heterotrophic soil respiration in V. negundo var. heterophylla shrubland. In contrast, N addition enhanced the heterotrophic soil respiration rate and did not influence autotrophic and total soil respiration in S. salicifolia shrubland.

Under elevated atmospheric CO₂ concentration, an increase in the net primary production is likely to enhance the amount of litter inputs to forest soil. This study aims to determine the dynamics of soil respiration and soil carbon pool as affected by increased litterfall production.

A litterfall manipulation experiment was conducted in Cunninghamia lanceolata plantations and secondary Castanopsis carlesii forest stands in Chenda township of Sanming in Fujian Province, China, from January 2013 to December 2014, with treatments of litterfall exclusion, litter addition, and control (normal litterfall condition).

(1) The value of temperature sensitivity index (Q₁₀) shows a positive relationship with soil water content in the range 10%-25%, and drops below 1 at water content < 10%. Drought stress altered Q₁₀ value and interrupted the coupling between temperature and soil respiration, as it reduced the diffusion of soluble carbon substrate and the extracellular enzymes, consequently, limited the microbial activity. (2) Linear regression analysis shows that soil respiration is significantly correlated with monthly litter mass (p < 0.05). In the treatments of the control and litter addition in the Cunninghamia lanceolata stands and that of the control in the Castanopsis carlesii stands, soil respiration was best correlated with litter mass two months ago; in the treatment of the litter addition in the Castanopsis carlesii stands, soil respiration was best correlated with litter mass of the current month. (3) On average, the annual CO₂ efflux was significantly reduced by litterfall exclusion, by about (362.0 ± 64.9) g C·m^-2·a^-1 in the Castanopsis carlesii stands and (96.2 ± 37.3) g C·m^-2·a^-1 in the Cunninghamia lanceolata stands compared with the control. Litter respiration contributed to 34.4% of soil respiration in the Castanopsis carlesii stands and 15.1% in the Cunninghamia lanceolata stands. Litter addition increased the soil respiration rate in both Castanopsis carlesii and Cunninghamia lanceolata stands, but the magnitude of the increase did not match up with the doubling of litter inputs, implying that under elevated atmospheric CO₂ concentration, an increased litterfall inputs due to increases in the net primary production would be advantageous to the forest soil carbon sequestration.

Aims As the primary pathway for CO₂ emission from terrestrial ecosystems to the atmosphere, soil respiration is estimated to be 80 Pg C·a^-1 to 100 Pg C·a^-1, equivalent to 10 fold of fossil fuel emissions. As an important management practice in plantation forests, fertilization does not only increase primary production but also affects soil respiration. To investigate how nitrogen (N) fertilization affects total soil, root and microbial respiration, a N fertilization experiment was conducted in a five-year-old Cunninghamia lanceolata plantation in Huitong, Hunan Province, located in the subtropical region. MethodsOne year after fertilization, soil respiration was monitored monthly by LI-8100 from July 2013 to June 2014. Soil temperature and water content (0-5 cm soil depth) were also measured simultaneously. Available soil nutrients, fine root biomass and microbial communities were analyzed in June 2013. Important findings Total soil, root and microbial respiration rates were 22.7%, 19.6%, and 23.5% lower in the fertilized plots than in the unfertilized plots, respectively. The temperature sensitivity (Q₁₀) of soil respiration ranged from 1.81 to 2.04, and the Q₁₀ value of microbial respiration decreased from 2.04 in the unfertilized plots to 1.84 in the fertilized plots. However, neither the Q₁₀ value nor the patterns of total soil respiration were affected by N fertilization. In the two-factor model, soil temperature and moisture accounted for 69.9%-79.7% of the seasonal variations in soil respiration. These results suggest that N fertilization reduces the response of soil organic carbon decomposition to temperature change and may contribute to the increase of soil carbon storage under global warming in subtropical plantations.

Aims Winter soil respiration plays a crucial role in terrestrial carbon cycle, which could lose carbon gained in the growing season. With global warming, the average near-surface air temperatures will rise by 0.3 to 4.8 °C. Winter is expected to be warmer obviously than other seasons. Thus, the elevated temperature can significantly affect soil respiration. The coastal wetland has shallow underground water level and is affected by the fresh water and salt water. Elevated temperature can cause the increase of soil salinity, and as a result high salinity can limit soil respiration. Our objectives were to determine the diurnal and seasonal dynamics of soil respiration in a coastal wetland during the non-growing season, and to explore the responses of soil respiration to environmental factors, especially soil temperature and salinity.
Methods A manipulative warming experiment was conducted in a costal wetland in the Yellow River Delta using the infrared heaters. A complete random block design with two treatments, including control and warming, and each treatment was replicated each treatment four times. Soil respiration was measured twice a month during the non-growing season by a LI-8100 soil CO₂ efflux system. The measurements were taken every 2 h for 24 h at clear days. During each soil respiration measurement, soil environmental parameters were determined simultaneously, including soil temperature, moisture and salinity.
Important findings The diurnal variation of soil respiration in the warming plots was closely coupled with that in the control plots, and both exhibited single-peak curves. The daily soil respiration in the warming was higher than that in the control from November 2014 to January 2015. Contrarily, from March to April 2015. During the non-growing seasons, there were no significant differences in the daily mean soil respiration between the two treatments. However, soil temperature and soil salt content in the warming plots were significantly higher than those in the control plots. The non-growing season was divided into the no salt restriction period (November 2014 to middle February 2015) and salt restriction period (middle February 2015 to April 2015). During non-growing season, soil respiration in the warming had no significant difference compared with that in control. During the no salt restriction period, soil respiration in the warming was 22.9% (p < 0.01) greater than the control when soil temperature at 10 cm depth in warming was elevated by 4.0 °C compared with that in control. However, experimental warming decreased temperature sensitivity of soil respiration (Q₁₀). During salt restriction period, soil warming decreased soil respiration by 20.7% compared with the control although with higher temperature (3.3 °C), which may be attributed to the increased soil salt content (Soil electric conductivity increased from 4.4 ds·m^-1 to 5.3 ds·m^-1). The high water content can limit soil respiration in some extent. In addition, the Q₁₀ value in the warming had no significant difference compared with that in control during this period. Therefore, soil warming can not only increase soil respiration by elevating soil temperature, but also decrease soil respiration by increasing soil salt content due to evaporation, which consequently regulating the soil carbon balance of coastal wetlands.

Aims As the second largest C flux between the atmosphere and terrestrial ecosystems, soil respiration plays a vital role in regulating atmosphere CO₂ concentration. Therefore, understanding the response of soil respiration to the increasing nitrogen deposition is urgently needed for prediction of future climate change. However, it is still unclear how nitrogen deposition influences soil respiration of shrubland in subtropical China. Our objectives were to explore the effects of different levels of nitrogen fertilization on soil respiration, root biomass increment, and litter biomass, and to analyze the relationships between soil respiration and soil temperature and moisture.
Methods From January 2013 to September 2014, we conducted a short-term simulated nitrogen deposition experiment in the Rhododendron simsii shrubland of Dawei Mountain, located in Hunan Province, southern China. Four levels of nitrogen addition treatments (each level with three replicates) were established: control (CK, no nitrogen addition), low nitrogen addition (LN, 2 g·m^-2·a^-1), medium nitrogen addition (MN, 5 g·m^-2·a^-1) and high nitrogen addition (HN, 10 g·m^-2·a^-1). Soil respiration was measured by LI-8100 soil CO₂ efflux system. At the same time, we measured root biomass increment and litter biomass in each plot.
Important findings Soil respiration exhibited a strong seasonal pattern, with the highest rates found in summer and the lowest rates in winter. Annual accumulative soil respiration rate in the CK, LN, MN and HN was (2.37 ± 0.39), (2.79 ± 0.42), (2.26 ± 0.38) and (2.30 ± 0.36) kg CO₂·m^-2, respectively. Annual mean soil respiration rate in the CK, LN, MN and HN was (1.71 ± 0.28), (2.01 ± 0.30), (1.63 ± 0.27) and (1.66 ± 0.26) μmol CO₂·m^-2·s^-1, respectively, and it was 17.25% higher in the LN treatment compared with CK (p = 0.06). The root biomass increment was increased by LN, MN, and HN treatments by 18.36%, 36.49% and 61.63%, respectively, compared to CK. The litter biomass was increased by LN, MN, and HN treatments by 35.87%, 22.17% and 15.35%, respectively, compared with CK. Soil respiration exhibited a significant exponential relationship with soil temperature (p < 0.01, R² is 0.77 to 0.82) and a significant linear relationship with soil moisture at the depth of 5 cm (p < 0.05, R² is 0.10 to 0.15). The temperature sensitivity (Q₁₀) value of CK, LN, MN and HN plots was 3.96, 3.60, 3.71 and 3.51, respectively. These results suggested that nitrogen addition promoted plant growth and decreased the temperature sensitivity of soil respiration. The increase of root biomass under N addition may be an important reason for the change of soil respiration in the study area.

Aims Soil respiration of the lands covered by biocrusts is an important component in the carbon cycle of arid, semi-arid and dry-subhumid ecosystems (drylands hereafter), and one of the key processes in the carbon cycle of drylands. However, the responses of the rate of soil respiration with biocrusts to water and temperature are uncertain in the investigations of the effects of experimental warming and precipitation patterns on CO₂ fluxes in biocrust dominated ecosystems. The objectives of this study were to investigate the relationships of carbon release from the biocrust-soil systems with water and temperature in drylands. Methods Intact soil columns with two types of biocrusts, including moss and algae-lichen crusts, were collected in a natural vegetation area in the southeastern fringe of the Tengger Desert. Open top chambers were used to simulate climate warming, and the soil respiration rate was measured under warming and non-warming treatments using an automated soil respiration system (LI-8150). Important findings Over the whole observational period (from April 2016 to July 2016), soil respiration rates varied from -0.16 to 4.69 μmol·m^-2·s^-1 for the moss crust-covered soils and from -0.21 to 5.72 μmol·m^-2·s^-1 for the algae-lichen crust-covered soils, respectively, under different rainfall events (the precipitations between 0.3-30.0 mm). The mean soil respiration rate of the moss crust-covered soils is 1.09 μmol·m^-2·s^-1, which is higher than that of the algae-lichen crust-covered soils of 0.94 μmol·m^-2·s^-1. The soil respiration rate of the two types of biocrust-covered soils showed different dynamics and spatial heterogeneities with rainfall events, and were positively correlated with precipitation. The mean soil respiration rate of the biocrust-covered soils without warming was 1.24 μmol·m^-2·s^-1, significantly higher than that with warming treatments of 0.79 μmol·m^-2·s^-1 (p < 0.05). By increasing the evaporation of soil moisture, the simulated warming impeded soil respiration. In most cases, soil temperature and soil respiration rate displayed a similar single-peak curve during the diel cycle. Our results show an approximately two hours’ lag between soil temperature at 5 cm depth and the soil respiration rate of the biocrust-covered soils during the diel cycle.

Aims Recent studies have shown that artificial addition of biochar is an effective way to mitigate atmospheric carbon dioxide concentrations. However, it is still unclear how biochar addition influences soil respiration in Phyllostachys edulis forests of subtropical China. Our objectives were to examine the effects of biochar addition on the dynamics of soil respiration, soil temperature, soil moisture, and the cumulative soil carbon emission, and to determine the relationships of soil respiration with soil temperature and moisture.
Methods We conducted a two-year biochar addition experiment in a subtropical P. edulis forest from 2014.05 to 2016.04. The study site is located in the Miaoshanwu Nature Reserve in Fuyang district of Hangzhou, Zhejiang Province, in southern China. The biochar addition treatments included: control (CK, no biochar addition), low rate of biochar addition (LB, 5 t·hm^-2), medium rate of biochar addition (MB, 10 t·hm^-2), and high rate of biochar addition (HB, 20 t·hm^-2). Soil respiration was measured by using a LI-8100 soil CO₂ efflux system.
Important findings Soil respiration was significantly reduced by biochar addition, and exhibited an apparent seasonal pattern, with the maximum occurring in June or July (except LB in one of the replicated stand) and the minimum in January or February. There were significant differences in soil respiration between the CK and the treatments. Annual mean soil respiration rate in the CK, LB, MB and HB were 3.32, 2.66, 3.04 and 3.24 μmol·m^-2·s^-1, respectively. Compared with CK, soil respiration rate was 2.33%-54.72% lower in the LB, 1.28%-44.21% lower in the MB, and 0.09%-39.22% lower in the HB. The soil moisture content was increased by 0.97%-75.58% in LB, 0.87%-48.18% in MB, and 0.68%-74.73% in HB, respectively, compared with CK. Soil respiration exhibited a significant exponential relationship with soil temperature and a significant linear relationship with combination of soil temperature and moisture at the depth of 5 cm; no significant relationship was found between soil respiration and soil moisture alone. The temperature sensitivity (Q₁₀) value was reduced in LB and HB. Annual accumulative soil carbon emission in the LB, MB and HB was reduced by 7.98%-35.09%, 1.48%-20.63%, and -4.71%-7.68%, respectively. Biochar addition significantly reduced soil carbon emission and soil temperature sensitivity, highlighting its role in mitigating climate change.

Aims Our objective was to determine the effects of changes in global pattern of precipitation on soil respiration and the controlling factors. Methods Data were collected from literature on precipitation manipulation experiments globally and a meta-analysis was conducted to synthesize the responses of soil respiration to changes in precipitation regimes. Important findings We found that an increased precipitation stimulated soil respiration while a decreased precipitation suppressed it. When changes in rainfall were normalized to the average treatment level (41% of the current annual precipitation), the level of increases in soil respiration with increased precipitation (49%) were higher than that of decreases with decreased precipitation (21%), showing an asymmetric responses of soil respiration to increases and decreases in precipitation. Soil moisture occurred as the most predominant factor driving the changes in soil respiration under altered precipitation. Changes in soil moisture affected soil respiration directly and indiscreetly by changing aboveground/belowground net primary productivity and microbial biomass carbon, which collectively contributed 98% of variations in soil respiration. In addition, the responses of soil respiration to altered precipitation varied with background temperature and precipitation. The sensitivity of soil respiration increased with local mean annual temperature when precipitation was reduced, while remaining unchanged when precipitation was increased. Meanwhile, the sensitivity of soil respiration to either increases or decreases in precipitation decreased with increasing local mean annual precipitation. Under future altered precipitation regimes, the sensitivity of soil respiration to changes in precipitation is likely dependent of local environment conditions.

Aims Soil respiration component partitioning is pivotal to understand the belowground carbon (C) cycle. Mycorrhizal fungi have been proven to play an important role in the soil C turnover, but only a few studies have been conducted to quantify the contribution of mycorrhizal respiration to total soil respiration in grassland ecosystems.
Methods The mini-trenching mesh method was applied to partition soil respiration components of a semi-arid grassland in Inner Mongolia. A shallow collar (measuring soil total respiration), a deep collar (excluding roots and mycorrhizal hypahe) and a deep collar with 40 μm pore mesh window (excluding roots but not mycorrhizal hyphae) were installed in each plot. Soil respiration rate of each collar was measured every two weeks during the growing season from 2014 to 2016. The differences in the rate of soil respiration among different type of collars were used to partition the components of soil respiration.
Important findings The results showed that the contribution of heterotrophic, root and mycorrhizal respiration to total soil respiration was 49%, 28%, and 23%, respectively. Across the three years, the proportion of mycorrhizal respiration varied from 21%-26%, which is comparable with the results reported by other studies recently. Our results demonstrated that the mini-trenching mesh method is a suitable method for separating mycorrhizal respiration component in grassland ecosystems. Evaluating the contribution of mycorrhizal respiration to total soil respiration is very important for predicting the responses of soil carbon release to future climate change.

Soil respiration plays an important role in carbon cycling in grassland ecosystems. However, the effects of collar size and buried depth during field measurement on soil respiration are rarely assessed.

We conducted a two-factor experiment to examine how soil collar depth (2 cm and 5 cm) and size (15 cm × 15 cm and 30 cm × 30 cm) affected the soil respiration (SR), post aboveground net primary productivity (post-ANPP), soil temperature (ST), and soil water content (SWC) in a semi-arid steppe.

The results showed that the deep-inserted soil collar (5 cm soil depth) decreased the soil respiration by 8.0%-9.7% compared with the shallow-inserted soil collar (2 cm soil depth). The large-sized soil collar (30 cm × 30 cm) decreased the soil respiration by 9.1%-10.8% compared with the small-sized soil collar (15 cm × 15 cm). We also found that the deep-inserted and large-sized soil collars had higher ST but lower SWC compared with the shallow-depth and small-sized soil collars. Structural equation model indicated that the lower respiration in the deep-inserted and large-sized soil collars was due to the lower post-ANPP, ST, and SWC. Overall, we found that the soil collar size and buried depth can substantially alter the magnitude of soil respiration by changing plant biomass, ST, and SWC. These findings suggest that the influences of collar size and buried depth on soil respiration should be considered for better estimation and modeling of soil CO₂ fluxes in terrestrial ecosystems.

Aims Soil respiration is an important indicator for evaluation of ecosystem health in the grazing grasslands of arid regions, and thus can be used to assess dynamics of ecosystem functioning during the restoration of degraded grasslands from enduring intensive grazing.
Methods This study was carried out in a Nei Mongol desert grassland with four grazing intensity treatments, i.e., control, light, moderate, and heavy grazing intensity designated as CK, LG, MG, and HG, respectively. Our objectives of this study were to explore the responses of soil respiration in these treatments with additional nitrogen (N) and water (W) addition. The plant community was dominated by a grass species, Stipa breviflora.
Important findings Our results showed that: (1) previous grazing intensity had significant impacts on soil respiration in 2016 and 2017, but not in 2015. Grazing increased soil respiration. Moreover, both nitrogen and water addition significantly enhanced soil respiration in MG plots, while only combined addition of nitrogen and water significantly increased soil respiration in HG plots. (2) Neither grazing intensity nor addition of nitrogen and water changed the seasonal dynamics of growing season soil respiration in this desert grassland. Soil respiration showed a single-peak curve model, and the peak occurred in July with both rain and heat. (3) The effects of nitrogen and water addition varied in different growing seasons. Nitrogen addition had no significant effects in the first two years (2015 and 2016), while showed significant effects in the third year (2017). Water addition had significant effects in years with normal precipitation (2015 and 2017), while had insignificant effect in the year with high precipitation (2016). Combined addition of nitrogen and water showed stronger effects than only addition of water in CK, LG, and HG plots, indicating that the synergistic effects of nitrogen and water addition on soil respiration. (4) The sensitivity of soil respiration to soil temperature at 10 cm depth (i.e., the Q₁₀ value) ranged between 1.13 and 2.41, with an average value of 1.71. Without addition of nitrogen and water, Q₁₀ values in grazing plots were all lower than in CK plots, with the lowest value occurring in HG plots. With the addition of water and combined addition of water and nitrogen, the Q₁₀ value increased significantly by 100%. Taken together, our results indicated that soil moisture was the leading environmental factor affecting soil respiration in this desert grassland, while nitrogen played an effective role only after the minimum requirement of water availability was met. Results from this study will provide important helpful information for the restoration and rational utilization of the degraded desert steppe.

Aims Our objective was to determine the spatial variation of the temperature sensitivity of soil respiration (Q₁₀) and it’s controlling factors in forest ecosystems across China.
Methods Based on published papers, the field measurement data of soil respiration were collected to build the dataset of annual Q₁₀ in forest ecosystems across China. Further, the spatial variation and the drivers of Q₁₀ in different forest types were analyzed.
Important findings The results showed that 1) Q₁₀ ranges from 1.09 to 6.24, with a mean value (± standard error) of 2.37 (± 0.04) and no significant difference among different forest types; 2) When all forest types were considered, Q₁₀ increased with increasing latitude, altitude, soil organic carbon content (SOC) and soil total nitrogen content (TN), but decreased with increasing longitude, mean annual temperature (MAT) and mean annual precipitation (MAP). Climate (MAT, MAP) and soil (SOC, TN) factors together explained 32.8% variations in Q₁₀. MAT and SOC were considered as the primary factors driving the spatial variation of Q₁₀. 3) Q₁₀ of different forest types responded differently to climate and soil factors. Q₁₀ decreased with the increase of MAP in the deciduous needleleaf forest (DNF), while Q₁₀ showed no significant correlation with MAP in other forest types. Q₁₀ increased with the increase of TN in evergreen broadleaved forest (EBF), deciduous broadleaved forest (DBF), evergreen needleleaf forest (ENF), and the sensitivity of Q₁₀ to TN was the highest in EBF and the lowest in ENF. Although Q₁₀ showed concentrated distribution trend, more attention should be paid to the large range of variation in future C budget studies. The primary driving factors and the response to environmental factors of Q₁₀ varied among forest types. Under the scenario of future climate change, Q₁₀ may vary divergently among different forest types. Therefore, the divergent responses of key parameters of carbon cycle in different forest types to climate change should also be considered in future carbon-climate models.

Aims The agro-pastoral ecotone is considered as fragile ecosystems which are strongly affected by agriculture and animal husbandry. The saline-alkali grassland is a unique grassland type in the agro-pastoral ecotone. A large amount of fertilizers are used to increase productivity in this area, which also promotes the emission of reactive nitrogen (N) gases and leads to the changes in soil carbon and N cycles. Mowing is a primary management practice in the agro-pastoral grassland in northern China. In order to explore the impact of N addition and mowing on carbon dynamic in this saline-alkali grassland located in the agro-pastoral ecotone, we determined the response of soil respiration to N addition and mowing.
Methods This study area is located in Youyu County, an agro-pastoral grassland ecosystem in northern China. The field experiment was set up in May, 2017. The treatments included: control (without mowing and mowing), addition of urea, addition of slow release urea, addition of urea + mowing, addition of slow release urea + mowing. Each treatment included 6 replicates. Therefore, there were totally 36 plots in this experiment. Soil respiration rate, soil temperature, soil moisture content, microbial biomass, inorganic N content, above-ground and below-ground biomass were measured under different treatments, and the cumulative carbon emissions and CO₂ fluxes were calculated.
Important findings Our results showed that: (1) Short-term (2017-2018) N addition significantly increased soil respiration rates and soil cumulative carbon emissions. Meanwhile, soil respiration rates and cumulative carbon emissions were significantly higher under urea treatment than those under slow release urea addition. (2) Mowing significantly reduced soil respiration rates and cumulative carbon emissions. (3) The interaction of short-term N addition and mowing had no significant effect on soil respiration rate. Therefore, short-term N addition can promote soil carbon release from the saline-alkali grassland in the agro-pastoral ecotone of northern China. Mowing can reduce soil respiration and decrease cumulative of carbon emissions. This may be because that mowing reduced the input of litter and further reduced soil substrate for microbes, which led to a decrease in soil microbial activity. However, long-term effect of N addition and mowing on soil carbon dynamics in saline-alkaline grasslands in the agro-pastoral ecotone still needs to be further explored.

Aims Biological soil crust is an important type of surface cover in alpine sandy lands. Understanding of the effect of warming on respiration from the biological soil crust-soil system in alpine regions can provide theoretical reference to the assessment of the response and feedback of biological soil crusts to climate changes.
Methods The moss and algae crusts in the artificial vegetation restoration areas were taken as the research objects. The open top chamber (OTC) was used as a passive warming device to simulate warming. The daily and growing season dynamics of respiration rates in two types of biological soil crust-soil systems were measured. The effects of warming on CO₂ emission and its temperature sensitivity were discussed.
Important findings Both the daily and the growing season dynamics of respiration rate of the moss and algae crust-soil system showed “single-peak” curves and were not affected by warming. The daily peaks appeared around 13:00, and the growing season peaks appeared around August. Warming changed the daily peak value of respiration rate of the biological soil crust-soil system. In the relatively dry year (2017), moderate warming increased cumulative CO₂ emission from the two types of biological soil crust-soil system during growing season, but the increase declined under excessive warming. In the relatively wet year (2018), as warming got greater, CO₂ emission from the two types of biological soil crust-soil system increased more. The relationship between respiration rate and temperature of two types of biological soil crust-soil system followed the exponential function. In the relatively dry year, more increase of temperature induced smaller temperature sensitivity of CO₂ emission, and the temperature sensitivity varied from 1.47 to 1.61 and 1.60 to 1.95 in the moss and algae crust soil system respectively. In the relatively wet year, with the increase of temperature, temperature sensitivity of system respiration increased, and the temperature sensitivity varied from 1.44 to 1.68 and 1.44 to 1.76 in the moss and algae crust soil system respectively. This study shows that global warming has greatly increased the respiration of biological soil crust-soil system in alpine ecosystems. Therefore, we should fully consider the impact of climate warming on the wide spread biological soil crusts in this area for better evaluation of carbon cycling processes in alpine ecosystems.

Aims Increasing global nitrogen (N) deposition has exerted significant influences on productivity and carbon cycle of terrestrial ecosystems. More than 90% of the carbon in grasslands is stored in the soil, therefore any changes in soil total respiration (R_s) might have a vital impact on the carbon balance and the stability of soil carbon pool of grassland ecosystems. Most of our understanding about the responses of R_s to N deposition was based on N deposition manipulative experiments with short-term (<5 years) and low frequency (1-2 times per year) N addition treatments. It is still unclear how the long term N addition and different N addition frequency will affect R_s and its components in semiarid grasslands.
Methods Our study is based on a long term N addition manipulative experiment platform conducted in a typical temperate semiarid steppe, Nei Mongol. The experimental treatment consisted of six N addition amounts and two N addition frequencies. N addition treatments began at 2008. Soil respiration and its components were measured every two weeks during the growing season in 2018 and 2019.
Important findings 1) R_s significantly decreased with increasing N addition amount. The negative impact of N addition on R_s was mainly resulted from the inhibition of heterotrophic respiration (R_h). 2) No significant differences were observed in responses of R_s and its components to low and high frequency N addition treatments. 3) Soil acidification caused by long term N addition inhibited soil microbial activity and changed soil microbial community composition, consequently decreased R_s and R_h. Our results suggested that the negative effect of N addition on soil carbon release still lasted after a decade of N addition treatment. In particular, the decrease of R_h would enhance the stability of soil carbon pool. No significant differences in the two N addition frequency treatments indicated that the potential impacts caused by simulated N addition with different frequencies would be diminished with prolonged treatment period. Therefore, the results of long-term (>10 years) simulated N addition experiments can provide reliable references for evaluating the responses of natural ecosystems to atmospheric N deposition.

Aims The balance between soil organic carbon (SOC) input and output processes determines SOC content. However, it is not clear which of the two processes dominantly affect SOC content during the degradation of alpine meadows in Zoigê Wetland. In this study, the changes in SOC contents of alpine meadows and their causes at different degradation stages (alpine meadow (AM), slightly degraded alpine meadow (SD), and heavily degraded alpine meadow (HD)) in the Zoigê Wetland were investigated using the method of spatial sequence instead of temporal successional sequence.
Methods First, the changes in C input to soil and their causes along the degradation gradient were analyzed by investigating main soil physicochemical properties, microbial biomass, plant biomass and community composition of plant functional groups at different degradation stages. Secondly, the changes in the C output from soil were estimated based on lab incubation experiments of soil C mineralization and the temperature sensitivity of soil respiration (Q₁₀) and monthly average air temperature of the Zoigê Wetland. Finally, the main causes and processes leading to changes in SOC content along the degradation gradient were analyzed.
Important findings The results showed that soil water content (SWC), SOC content, total nitrogen (TN) content, microbial biomass C and N content decreased with the increase of degradation. Plant community composition gradually changed from sedges and grasses dominated community to forbs dominated community. Plant biomass and SOC mineralization rate decreased during the degradation of alpine meadows. The potential accumulation of organic C reduced during the degradation (compared with AM, the potential input, output and accumulation of organic C in SD and HD decreased by 16%, 18%, 15% and 59%, 63%, 41%, respectively). The decrease in SWC changed soil physical and chemical properties, including bulk density, SOC content, TN content, total phosphorus content, and C:N, which led to the shifts in the distribution pattern of plant functional groups and in soil microorganisms, consequently reducing the inputs and outputs of SOC. The decrease in potential plant-derived C input to soil caused by decreased SWC was the main reason for the decline in SOC content along the degradation gradient of alpine meadows in Zoigê Wetland.

Aims Arid and semi-arid regions are typical ecologically fragile areas, and they also have an important impact on global warming. Those regions are considered to be important CH₄ sinks since most soils are under aerobic conditions. Studies have found that along with the increase of CH₄ uptake velocity, the rate of CO₂ emissions also has increased. This study was carried out to examine whether there is an offset phenomenon and under what environmental conditions it occurs.

Methods Based on the integration of soil greenhouse gas fluxes and relevant environmental data in arid and semi-arid regions of China, correlations between soil CO₂ and soil CH₄ fluxes, on seasonal and daily scales, were analyzed.

Important findings The results showed that there were three levels of soil CO₂ and soil CH₄ flux, i.e., synergy (positively correlated), offset (negatively correlated), and random (not correlated). Among which, the proportion of random relationships was the highest, on seasonal and daily scales 83% and 54%, respectively. Compared to water content and vegetation conditions, air temperature correlated with the correlations between the two fluxes more strongly, showing a quadratic relationship (the absolute values of correlation coefficients between fluxes decreased with increasing temperature). On a seasonal scale, the mean air temperature during the sampling period determined the correlations between the fluxes with an accuracy of 92%, and the air temperature threshold of flux coupling-decoupling was 12.5 °C. On the daily scale, the diurnal air temperature difference determined fluxes relationships with an accuracy of 79% and the temperature threshold of flux coupling-decoupling was 15.2 °C. In addition, when the soil was in the state of absorbing CH₄ on a daily scale, the relationship between soil CH₄fluxand soil CO₂flux was positive in more cases. This phenomenon was difficult to explain by temperature alone. We speculate that a one-way coupling relationship between soil respiration and CH₄ oxidation formed through O₂ competition, that is, soil respiration would inhibit the CH₄ oxidation by consuming O₂, resulting in an increase in soil CO₂ emissions and a decrease in CH₄ absorption. The study suggests that coupling-decoupling of soil CO₂ and CH₄ fluxes might be driven by a mechanism of temperature regulation linked with oxygen competition regulation. Climate warming may cause decoupling of the two fluxes across space and time and increase the complexity of carbon cycles, thereby increasing the uncertainty of regional carbon flux estimations.

Aims We aimed to explore the response of net ecosystem productivity (NEP) and carbon use efficiency (CUE) to asymmetric daytime vs. nighttime warming in Artemisia ordosica shrublands, and to examine the sensitivity of carbon balance components to daytime vs. nighttime warming.

Methods The BIOME-BGC model was parameterized and validated against eddy covariance measurements of ecosystem carbon fluxes, and used for simulating the impacts of different warming scenarios on NEP and CUE and their components, including gross primary productivity (GPP), net primary productivity (NPP), ecosystem respiration (Re), autotrophic respiration (AR), heterotrophic respiration (HR), maintenance respiration (MR), and growth respiration (GR). Two warming scenarios were simulated: (1) asymmetric warming according to the historical trends from 1954 to 2020 (i.e. daytime warming 1.2 °C, nighttime warming 1.8 °C); (2) daytime or nighttime warming separately with different temperature increase treatments (2, 4, 6 °C).

Important findings (1) Modeled GPP on the daily and annual scales, Re on the daily timescale and NEP on the annual scale showed good agreement with the observed values (coefficient of determination (R²): 0.72-0.88; Nash-Sutcliffe efficiency coefficient (NS): 0.72-0.79). Modeled Re on the annual timescale and NEP on the daily timescale showed weak agreement with observed values (R²: 0.57 and 0.26; NS 0.46 and 0.12, respectively). (2) All warming scenarios promoted GPP, NPP, Re and all respiration components. GPP, Re, AR, and MR were more sensitive to daytime than to nighttime warming, while NPP, HR, GR were more sensitive to nighttime than daytime warming. (3) Greater increases in Re (about 13%) and AR (about 16%) than that in GPP (about 10%) under all warming scenarios, leading to the decreases in NEP and CUE. In addition, both NEP and CUE were more sensitive to daytime than nighttime warming. (4) NEP and CUE decreased by about 68% and 5% under the historical trend of asymmetric daytime vs. nighttime warming treatment. Greater response of NEP and CUE to the daytime warming than nighttime warming. Our results highlight the negative impacts of climatic warming on carbon sink of the semiarid shrublands, and justify the efforts to mitigate climate change are vital for dryland ecosystems.

Phosphorus (P) is an essential but limited nutrient for plant growth, and global climate changes may affect soil P cycling and further aggravate P limitations in the soil. In this review, we focused on the response of plant P acquisition strategies to climate changes and subsequent influences on ecosystem productivity. By searching and analyzing the existing literatures, we summarized the P acquisition mechanism of plants and their response to global climate changes from following aspects: 1) plant P starvation response mechanisms; 2) plant P acquisition pathways and strategies; 3) involvements of soil microorganisms in plant P utilization; and 4) responses of plant P acquisition strategies to global climate changes (e.g., warming, nitrogen deposition and precipitation changes) and the underlying mechanisms. The review is expected to deepen our understanding of plant adaptation to low-P stress under the future climate scenario, and can also provide a theoretical basis for nutrient management in agriculture.

Understanding the response patterns and potential mechanisms of structure and function in grassland ecosystems to nitrogen (N) enrichment is essential to evaluate ecological impacts of external N input. The muti-level N manipulative experiment offers the possibility to explore the nonlinear response patterns and associated mechanisms of structure and function in grassland ecosystems to additional N input. In this review, we summarized the impacts of additional N inputs on community diversity, carbon (C) and N cycling in grassland ecosystems around the world. Numerous studies illustrated that N enrichment induced the decline of plant species diversity, plant functional diversity and soil bacteria richness in grassland ecosystems, yet the change of fungal diversity was not significant. Above- and below-ground plant productivity showed different responses to N input: aboveground plant productivity exhibited initial increasing and subsequent saturation trends, but root productivity and root:shoot ratio showed downward patterns, and root turnover rate appeared a single-peak pattern of first increasing and then decreasing with the continuous increase of N addition rate. Meanwhile, different C decomposition processes responded variously to N enrichment. Specifically, litter decomposition rates exhibited multiple response of “exponential decrease, liner increase or insignificant change with N addition level”. However, the relationship of soil respiration and CH₄ consumption with N addition was dominated by a single peak trend of increasing at low to medium N levels but declining at high N levels. Likewise, different soil C fractions showed multiple response patterns to N input. N addition generally stimulated soil C storage and particulate organic C accumulation, while the mineral-associated organic C exhibited divergent responses of “increase, unaltered, and decrease” along the N addition gradient. In addition, plant N uptake exhibited initial increasing and subsequent situation trends along N addition gradients, while different soil N transformation processes showed differentiated responses along N addition gradients and the relationship between N₂O emission and N addition rate varied among various grassland ecosystems. An exponential increase of N₂O fluxes with N addition rate was observed in temperate grasslands, while the patterns of first increase and then saturation or linear increase of the N-induced changes in N₂O emissions had been discovered in alpine grasslands. Future studies should focus on the nonlinear responses of rhizosphere processes and phosphorus (P) cycle to external N input, and also explore potential mechanisms from the aspect of multi-dimensional biodiversity changes.

Aims Soil respiration is one of the most critical components of carbon cycle in terrestrial ecosystems. The study on temporal dynamics of soil respiration and its linkage with environmental factors in desert steppes under changing precipitation can provide data supports for a deep understanding of the regulatory mechanisms of key carbon cycling processes in fragile ecosystems.
Methods A field experiment involving five precipitation treatments (50% reduction, 30% reduction, natural, 30% increase, 50% increase) was set up in 2014 in a desert steppe in Ningxia. The temporal dynamics of soil respiration rate were explored during the growing season (from June to October) in 2019, and the relationships between soil respiration rate and soil properties and plant characteristics were analyzed.
Important findings Soil respiration rate showed a seasonal variation of an increasing and a decreasing trend across the growing season, with the maximum values (2.79-5.35 μmol·m^-2·s^-1) occurring in late July or early August. Compared with the natural condition, 30% reduction in precipitation did not result in a significant effect on soil respiration rate, reflecting the adaptability of soil respiration to moderate drought. Overall, 50% reduction in precipitation reduced soil respiration rate, whereas increased precipitation (especially the 30% increase) enhanced soil respiration rate, and this positive effect was especially obvious in the early growing season (June to July). Soil respiration rate had a significantly exponential relationship with soil temperature and a significantly linear relationship with soil water content. Soil physicochemical property had a highly independent explanatory power for soil respiration rate (R² = 0.36), and its effect was highly correlated with soil biological property and plant diversity (R² = 0.31). Precipitation could affect soil respiration rate either directly or indirectly through the influences on soil biological property and plant biomass. The results indicated that a moderate increase in precipitation could accelerate soil respiration by alleviating soil water limitation, stimulating soil enzyme activity, promoting microbial activity and plant growth in the desert steppe, and that an extreme increase in precipitation would lead to a decrease in soil permeability and a hindrance to microbial metabolic activity, thus inhibiting soil respiration.

Soil respiration is mainly composed of the CO₂ released from atmosphere-soil interface and change of CO₂ stored in the soil. Understanding the production and migration of CO₂ in the soil is essential for measuring the carbon cycle in terrestrial ecosystems. The flux gradient method calculates soil CO₂ flux by measuring the diffusion-driven CO₂ concentration gradient and diffusion coefficient. The flux of soil CO₂ and its stable carbon isotopes composition (δ¹³C) at different depths can be calculated based on Fickʼs law. The amount of CO₂ released from soil and the amount of CO₂ stored in different soil layers can thus be measured. The underground soil CO₂ (¹³CO₂ and ¹²CO₂) concentration is mainly controlled by pore tortuosity, the depth of root distribution, microbial activity and total soil CO₂ production. The underground CO₂ transmission process is mainly controlled by the CO₂ concentrations, porosity and water content at different depths of the soil. These physical, chemical and biological features of the soil are key factors affecting the application of the soil flux gradient method, and directly determine the precision and accuracy of soil CO₂ and its δ¹³C flux calculation. The gradient method is a useful complement to the chamber method, which can clarify the process of production and migration of soil CO₂ at different depths and thus the impacts on the release and storage of soil CO₂, elucidating the contribution of soils at different depths to CO₂ release and uncovering the underlying environmental and physical mechanisms.

Aims This study aimed to explore change in soil respiration and its components after mild fire, as well as its influence on forest environmental factors, which could provide a scientific basis for the estimation of the soil carbon emission of coastal sandy plantation under the condition of forest fire disturbance.

Methods We conducted an experiment in the Casuarina equisetifolia plantation burnt area and the control plot to measure the total soil respiration rate (R_S) and heterotrophic respiration rate (R_H) in the coastal areas of southern Fujian from September 2019 to August 2020, using the LI-8100 soil carbon flux automatic measurement system. Meanwhile, the soil temperature at the depth of 10 cm (T₁₀), soil volumetric water content at the depth of 10 cm (W₁₀), and soil physical and chemical properties at the depth of 0-10 cm were measured, in order to explore the effects of mild fire on soil R_S, R_H and abiotic factors.

Important findings There were significant differences in soil respiration rate and its components between the burned area and the control area. Our results showed that the annual average soil R_S and R_H in the burned area were (2.37 ± 0.65) and (2.05 ± 0.63) μmol·m^-2·s^-1, respectively. In contrast, the annual average soil R_S and R_H in the control plot were (2.86 ± 1.08) and (2.51 ± 1.08) μmol·m^-2·s^-1, respectively. Soil respiration rate and its components were significantly correlated with soil temperatures in the two plots, except soil R_H in the control plot, but their relationships with soil moisture did not reach a significant level. There was significant positive correlation of soil respiration rate with dissolved organic carbon content, microbial biomass nitrogen content and dissolved organic nitrogen content, but significant negative correlation with microbial biomass carbon content. Overall, we found that mild burning inhibited soil respiration and its components in C. equisetifolia plantation, indicating that fire disturbance had an important impact on soil respiration and carbon cycle in forest ecosystems.