Coastal salt marshes are an effective blue carbon sink to mitigate climate warming, but their ecosystem stability and carbon sink function are threatened by the large amount of nitrogen input caused by coastal eutrophication. Under the action of regular tides, the high nitrogen content in the coastal waters will have a profound effect on the key processes of carbon cycle such as plant photosynthetic carbon fixation, carbon allocation in plant-soil system, and soil carbon release in the salt marsh. This study reviewed the effects of nitrogen input on plant photosynthetic carbon fixation, carbon allocation in plant-soil system, decomposition of soil organic carbon, formation and release of soil dissolved organic carbon (DOC), and carbon sequestration in the salt marsh. Based on the shortcomings of current research, this review proposed the directions of future research, including the effects of nitrogen input on plant photosynthetic carbon fixation and carbon allocation in plant-soil system, the microbial mechanism of soil organic carbon decomposition, production and lateral exchange of soil DOC, and the potential impact of different forms of nitrogen input on soil carbon sequestration in the salt marsh. Overall, this study aims to improve the understanding of impacts of nitrogen input on the key carbon processes and the mechanisms of carbon sequestration in a salt marsh, and to provide new ideas for assessing the potential changes of carbon pools under the influence of eutrophication of coastal waters in the salt marsh wetlands.

Aims Overgrazing induced reduction in supply of soil nutrients is a major mechanism leading to extensive grassland degradation in China. The capacity of carbon (C) sequestration was demoted in the degraded grassland ecosystems. However, it remains unclear whether the capacity of carbon sequestration in a degraded grassland can be restored by the supplement of nutrients to the soil. Net ecosystem CO₂ exchange (NEE), ecosystem respiration (ER) and gross ecosystem productivity (GEP) are important parameters describing the processes of ecosystem carbon cycle. Nitrogen (N) and phosphorus (P) are two limiting nutrients in typical steppe in China. To date, how these two nutrients alone or in combination affect the three parameters of carbon cycle (i.e., NEE, ER and GEP) in a degraded steppe community, especially their interactive effect, is poorly understood.

Methods To address these scientific questions, we conducted a field experiment in a degraded typical steppe community. Four treatments of nutrient addition were implemented: no nutrient addition (CK, control), adding N alone (10.5 g·m^-2, NH₄NO₃), adding P alone (7 g·m^-2, KH₂PO₄), and adding two nutrients in combination. Two stages were selected for fertilization: early-spring (April 21) and middle-summer (July 15).

Important findings Neither N nor P alone had significant effect on NEE, ER and GEP when nutrients applied at early-spring (April 21) or middle-summer (July 15), while their combination significantly increased the values ofNEE and GEP. 2) N and P exhibited strong synergistic effect on NEE, GEPand ER when applied in combination at early-spring (April 21), while a consistent additive effect between the two nutrients on the three carbon exchange parameters was observed when applied at middle-summer (July 15). Our findings have implications for the restoration of degraded grasslands. To restore the capacity of carbon sequestration of the degraded typical steppe ecosystem, supplying N and P in combination is better than a single nutrient alone, and appling these nutrients at spring is better than at summer.

Aims Vegetation phenology is an important indicator to reflect the stages of vegetation growth, which is of great significance to the feedback to climate. Solar-induced chlorophyll fluorescence (SIF) is a by-product of photosynthesis, which provides the possibility to directly detect vegetation phenology at the global scale. In order to reveal the accuracy of phenology estimated by SIF of different forest types, we estimated phenology of three forest types in the Northern Hemisphere.

Methods Based on 35 eddy flux tower sites in the Northern Hemisphere during the period of 2007-2014, we estimated phenology of three typical forest types using SIF value and gross primary production (GPP) by double logistic growth model and dynamic threshold. Correlation analysis was used to evaluate the different potential of SIF in estimating phenology of different forest types.

Important findings Results showed that: 1) SIF was more suitable to estimate the timing of the start of growing season (SOS) than the timing of the end of growing season (EOS). 2) SOS based on SIF had the highest correlation with SOS based on GPP in mixed forests (MF). However, the SOS of deciduous broadleaf forest (DBF) and evergreen needleleaf forest (ENF) could not be accurately tracked by SIF value. 3) The preseason shortwave radiation (SR) was the primarily environmental factor of SOS.

Aims Understanding the processes and drivers of vegetation change in giant panda habitats plays an important role in their conservation and management.

Methods Based on the long-term normalized difference vegetation index (NDVI) time series that was constructed by all available Landsat TM/ETM/OLI images from 1986 to 2018, we employed the BFAST (Breaks For Additive Seasonal and Trend) method and harmonic model to monitor the vegetation change during the period of 1986-2018. Three types of NDVI changes (i.e. vegetation accumulated abrupt change, accumulated gradual change, and total change) were built to reveal the spatial distribution characteristics of vegetation change. The effects of different factors (i.e. mean annual precipitation, mean annual air temperature, elevation, slope, aspect, distance to rivers, soil type, land cover type, distance to roads and distance to engineering disturbance area) on the spatial distribution of the three types of vegetation change were evaluated by Geodetector.

Important findings 1) A total of 9.13% of vegetation abrupt change in the study area was detected, which was mainly distributed around the eastern boundary of the habitats, and the largest abrupt change areas occurred in 2011 and 2013. 2) The proportion of vegetation accumulated abrupt change showing degradation accounted for 40.17% of the vegetation accumulated abrupt change area, and the accumulated gradual change and total change which presented increasing trends accounted for 94.58% and 97.02% of the study area, respectively. 3) The spatial distribution of vegetation changes was mainly affected by four factors: mean annual precipitation, mean annual air temperature, elevation, and soil type. The strongest explanatory factors of vegetation accumulated abrupt change, accumulated gradual change, and total change were mean annual precipitation, elevation, and soil type, respectively. The interactions between driving factors were mutually enhanced and nonlinearly enhanced.

Aims Understanding the relationship between transpiration and environmental factors is critical to the establishment of efficient irrigation strategies for Populus tomentosa. Therefore, we studied transpiration and environmental responses of P. tomentosa under varied irrigation treatments.

Methods This study was carried out at the triploidP. tomentosaplantation in Gaotang County, Shandong Province. The irrigation treatments were set as full irrigation (DF, irrigated when the soil water potential at 20 cm directly below the dripper reached -18 kPa), water-controlled irrigation (DC, irrigated when the soil water potential at 20 cm directly below the dripper reached -45 kPa) and no irrigation (CK, blank control). Thermal dissipation probes (TDP) were used to observe the continuous sap flow ofP. tomentosa during the growing season of 2019. The environmental factors were simultaneously monitored, including solar radiation (SR), air temperature (T_a), soil moisture content (SWC) and wind speed (WS).

Important findings 1) The trend of the diurnal sap flow variation of P. tomentosa was similar among the contrasting treatments, showing a single-peak curve, and there was significant sap flow at night. Irrigation treatments did not influence the start of stem sap flow but caused differences in the timing of the stem peak flow. The peak flow arrived earlier in the DF treatment than in other treatments. Irrigation increased the transpiration ofP. tomentosa. However, the transpiration would increase with the decrease of soil moisture deficit threshold of irrigation, namely transpiration of DC followed by DF and CK in sequence. 2) There was significant time-lag between the sap flow and SR and VPD. The sap flow was ahead of the VPD and lagged behind the SR. Irrigation treatments have no significant effect on the time-lag effect between the sap flow and environmental factors. 3) Under the three treatments, transpiration has a significantly positive correlation with VPD, SR and SWC, and negatively correlated with wind speed. The transpiration responses to the environmental factors varied among irrigation treatments. In summary, irrigation could effectively mediate the hydraulic adaptation of plantation trees to the environment factors. However, increased irrigation does not necessarily lead to transpiration enhancement. Based on the differences of stand transpiration and natural rainfall, irrigation during the beginning of the growing season (April to July in this study) would benefit the hydraulic physiological activity of theP. tomentosa plantation.

Aims Root turnover is a primary driver of belowground ecological processes, and root functional traits can indicate species ecological strategies, hence root lifespan and morphological traits are essential for understanding ecosystem carbon and nitrogen cycling as well as community diversity. Yet, data on root ecological processes in subtropical evergreen forest is very rare.

Methods We observed root dynamics of six tree species across root orders for two years in an experimental forest farm in Zhangshu, Jiangxi Province. Based on 28 000 minirhizotron photos, we analyzed interannual and seasonal changes of absorptive fine roots in relation to both lifespan and morphology.

Important findings 1) The variation of root lifespan among the six species in subtropical forest was as high as 4.6-fold, the variation of coefficient was 73%, with median lifespan in the sequence of: Taxus wallichiana(426 d) > Koelreuteria bipinnata (155 d) > Nageia nagi(145 d) > Cinnamomum camphora (126 d) > Cerasus yedoensis (93 d) > Michelia maudiae (92 d); 2) Absorptive fine root lifespan appeared remarkable in both seasonal and interannual variations, a pattern seemingly related to the monsoon climate which is characterized by summer-to- autumn drought and the supplies of soil water resources; 3) The lifespan of absorptive roots was positively associated with diameter, but negatively correlated with specific root length, suggesting that the root construction cost is a key predictor of lifespan. These results provide parameters for modeling belowground carbon and nitrogen cycling processes in subtropical evergreen broadleaf forest, and pave the way for exploring species coexistence mechanisms from belowground.

Aims Vessel, fibers, and parenchyma are the main components of tree xylem. They are responsible for water transport, mechanical support, and water and nutrients storage. Given the limited xylem space, consistent investment in one type of tissue would constrain the space available for other types of tissue, thus resulting in a possible trade-off among different tissues in their fractions. Analysis of the fractions of tissue types in xylem and the trade-off would contribute to better understanding of the eco-physiological adaptation of plants.

Methods We selected 21 characteristic tree species (10 deciduous and 11 evergreen) from a mixed evergreen and deciduous broad-leaved forest located in the mid-subtropical karst region, and measured their xylem tissue fractions. In addition, we calculated the hydraulic-related structural traits in xylems and examined the correlations among various traits.

Important findings Compared to the global average values of xylem tissue fractions, the karst tree species tended to have a higher proportion of parenchyma. The fraction of vessel lumen was not correlated with fiber and parenchyma fractions across the tree species investigated. Instead, a significant trade-off was observed between fractions of fiber and parenchyma. A trade-off between the hydraulic efficiency (i.e. theoretical hydraulic conductivity) and safety (vessel wall reinforcement) was observed across both the deciduous and the evergreen tree species. The two contrasting group of karst trees differenced significantly in the intercepts of the lines for trade-offs. For given conductivity, the deciduous tree species exhibited stronger vessel well reinforcement (safety) than the evergreen tree species, which might be due to the fact that evergreen trees species had more axial parenchyma. Hence, this study revealed the specificity of xylem anatomy in karst tree species. Water and resource storage in xylem parenchyma are vital to karst trees (evergreens in particular) for their adaptation to the water-limiting environment.

Aims Photodamage to a shade-tolerant species is common due to a sudden increase in growth light intensity. However, it is unknown about the underlying mechanism of the sensitivity of the shade-tolerant species to high light. The objective of the present study was to elucidate the mechanisms involved in the inability of the typically shade-tolerant species Panax notoginseng to survive under natural full-light condition.

Methods The relative chlorophyll content (SPAD), photosynthetic parameters and chlorophyll fluorescence parameters were continuously examined in P. notoginseng when transferred from shade (10% of full sunlight) to full sunlight for three days.

Important findings The net photosynthetic rate (P_n) of P. notoginseng exposed to full sunlight condition showed a “double-peaked” diurnal curve, and P_n decreased with the prolonged days of full light treatment. The SPAD value, water utilization efficiency and light use efficiency were significantly decreased under full sunlight condition. Furthermore, the maximum fluorescence signal of the P700 reaction center, electron transfer rate of photosystem II (PSII), the maximum quantum efficiency of PSII under dark adaptation and maximum quantum efficiency of PSII under light adaptation were significantly lower in full sunlight than those under shading condition, while the fraction of energy passively dissipated in the forms of heat and fluorescence, energy dissipation due to acceptor side limitation of PSI, and cyclic electron flow were significantly higher under the full light condition. Moreover, the sudden increase in growth light intensity caused a significant change in the fluorescence induction kinetic curve and significantly increased the fluorescence yield on the donor and acceptor side of PSII. The oxygen-evolving complex activity in the donor side of PSII was impaired under full sunlight. Furthermore, the electron transfer in the acceptor side of PSII was inhibited and the over-reduction of the acceptor side of PSI was caused by PSI photoinhibition. The study reveals that the full sunlight might induce the irreversible damage to PSII and the moderate photoinhibition to PSI in shade-tolerant species, and it may be an important underlying mechanisms why the shade-tolerant speciesP. notoginseng cannot survive under full sunlight.

Aims To quantify the responses of stomatal conductance (g_s) to CO₂ concentration (g_s-C_a) under current and future climate conditions, it is necessary to build a generally applicable model suitable for simulating this process at plant leaf levels.

Methods The response curves (A_n-C_a) of photosynthesis of soybean (Glycine max) and wheat (Triticum aestivum) to CO₂ were fitted using data collected from a portable photosynthetic apparatus (LI-6400). Based on the comparison between the traditional Michaelis-Menten model (M-M model) and the CO₂ response model developed by Ye, a new g_s-C_a response model was proposed. Then, the measured g_s-C_a curves of soybean and wheat were fitted with the new model. The model results were compared with those of the traditional model and the corresponding observation data to judge the rationality of the model.

Important findings The A_n-C_a model developed by Ye could fit well the A_n-C_a curve of soybean and wheat, and the coefficient of determination (R²) is as high as 0.999. Although the R ² values of M-M model fitting the A_n-C_a curves of soybean and wheat were also high, the fitting curves deviated from the observation at higher CO₂ concentrations. Meanwhile, M-M model greatly overestimated the maximum photosynthetic rate and could not estimate the saturation CO₂ concentrations. Therefore, it was more feasible to developg_s-C_a model based on the A_n-C_a model of Ye. The new model of g_s-C_a could fit well the g_s-C_a curves of soybean and wheat, and the R ² were 0.995 and 0.994, respectively. Moreover, the maximum stomatal conductance (g_s-max), the minimum stomatal conductance (g_s-min) and the CO₂ concentration corresponding to g_s-min (C_s-min) could also be generated directly. g_s-max of soybean and wheat fitted by the g_s-C_a model was 0.686 and 0.481 mol·m^-2·s^-1, respectively, and there was no significant difference between the fitted values and corresponding observation values (0.666 and 0.471 mol·m^-2·s^-1, respectively). The new model of g_s-C_a could also obtain the minimum g_s (g_s-min) of soybean and wheat (0.271 and 0.297 mol·m^-2·s^-1, respectively), and there was also no significant difference between the fitted values and corresponding observation values (0.279 and 0.293 mol·m^-2·s^-1, respectively). In addition, the new model of g_s-C_a generated the C_s-min values of 741.45 and 1 112.43 μmol·mol ^-1for soybean and wheat, respectively, and also showed no significant difference from the observed value (732.78 and 1 200.34 μmol·mol^-1, respectively). Consequently, the g_s-C_a model developed in this paper can be used as an effective mathematical tool to quantitatively study the effect of stomatal conductance on CO₂ concentration.