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.

Aims The objectives of this study were to reveal the changing trends and regional differences of vegetation fractional coverage on the Loess Plateau 20 years after the implementation of the “Grain for Green (GFG)” policy, and to quantify the contribution of climate and human activities to the change of vegetation fractional coverage and its spatial distribution in the region.

Methods The spatial and temporal variation of photosynthetic vegetation (PV) fractional coverage on the Loess Plateau from 2001 to 2020 and its drivers and contributions were analyzed based on MODIS-PV and meteorological data, and using the methods of the Mann-Kendall method, the Sen estimator, and multivariate residual trend analysis.

Important findings Regional vegetation fractional coverage increased from 40% in 2001 to 60% in 2020. Vegetation fractional coverage of the Loess Plateau showed a significant increasing trend over 20 years, with an increasing rate of 0.8%·a^-1. The proportion of the area with an increasing trend of vegetation fractional coverage for the entire region was 90%, and the proportion of the area with a significant increase was 71%. The contribution to the increase of vegetation fractional coverage in the region was mainly in the loess hilly region (2/5), followed by the sandy hilly region (1/4) and the rocky mountain region (1/5). Within the different geomorphology divisions, vegetation fractional coverage in the loess hilly region increased rapidly in Yulin and Yanʼan in Shaanxi. Vegetation fractional coverage in Ordos, Nei Mongol, changed the fastest in the sandy hilly region. Human activities and climate change contributed 76% and 24%, respectively, to the increase of vegetation fractional coverage on the Loess Plateau during the study period. The areas where human activities contributed positively to vegetation fractional coverage were mainly located in the loess hilly and sandy hilly regions in the northern part of Yanʼan in Shaanxi, the southern part of Taiyuan in Shanxi, the southern part of Tongxin in Ningxia, and the hills and plateaus of Pingliang and Qingyang in Gansu where the ecological projects funded by the Chinese government have been well implemented.

Aims This study investigated the spatial and temporal variation of spring and autumn photosynthetic phenology of vegetation in the Dongting Lake basin and revealed its response to climate change, and provides a useful reference for the establishment of model of subtropical vegetation phenology and the evaluation of carbon budget.

Methods Using solar-induced chlorophyll fluorescence (SIF) data, we extracted spring photosynthetic phenology (the start date of photosynthesis) and autumn photosynthetic phenology (the end date of photosynthesis) of vegetation in Dongting Lake basin, and evaluated temporal and spatial patterns of vegetation spring and autumn photosynthetic phenology and its response to climate change.

Important findings (1) From 2000 to 2018, the vegetation spring photosynthetic phenology was significantly advanced at the rate of 0.75 d·a^-1, the autumn photosynthetic phenology was delayed at the rate of 0.17 d·a^-1, and the vegetation growing season length was significantly prolonged at the rate of 0.90 d·a^-1. (2) The preseason maximum air temperature and minimum air temperature were the main factors affecting the advance of spring photosynthetic phenology. The autumn photosynthetic phenology of vegetation was positively correlated with preseason precipitation, minimum air temperature and radiation intensity, but negatively correlated with preseason maximum air temperature. (3) In addition, we found that the spring photosynthetic phenology of vegetation in the study area was more sensitive to climate change, especially the increase of preseason minimum air temperature led to the significant advance of spring photosynthetic phenology of evergreen needleleaf forest, evergreen broadleaf forest, bush and grassland. In conclusion, the advance of vegetation spring photosynthetic phenology in Dongting Lake basin played a dominant role in prolonging the growth season, indicating that spring photosynthetic phenology plays a more important role in enhancing the carbon sink function than the autumn photosynthetic phenology in the context of global warming. The vegetation spring photosynthetic phenology was more sensitive to climate change and the air temperature was the main factor controlling the vegetation spring photosynthetic phenology, which provides a scientific basis for the simulation and prediction of evergreen vegetation phenology.

Aims Nitrogen (N) is essential for the photosynthesis of plants. Shade plants are mostly exposed to dynamic light under natural growth conditions. However, little is known about the role of N levels in the photosynthetic regulation of shade plants under dynamic light. The objective of present study was to elucidate the mechanisms involved in the effect of N on dynamic photosynthesis in the typically shade-tolerant species Panax notoginseng.

Methods The gas exchange parameters and the activity and amount of Calvin cycle enzyme/proteins were examined under dynamic and steady light conditions in P. notoginseng grown under low N (LN, 112.5 kg·hm^-2) and high N (HN, 450.0 kg·hm^-2), respectively.

Important findings N content per unit of leaf area (N_area) was negatively correlated with the induction state at 60s of light (IS₆₀) and positively correlated with the time required to reach 90% of photosynthetic steady state (t_P90) and 100% of photosynthetic steady state (t_P-steady), suggesting that N_area does not regulate the photosynthetic induction during dynamic light by the total activity of ribulose-1,5-bisphosphate carboxylase (Rubisco). Moreover, a short low light interval only slightly decreased the activity of Rubisco, but significantly reduced the activity of fructose-1,6-bisphosphatase (FBPase) and sedoheptulose-1,7-bisphosphatase (SBPase); when the sunfleck of high intensity appeared suddenly, Rubisco was still highly active, but SBPase and FBPase need to be reactivated to match Rubisco activity, so that photosynthetic induction after low light intervals is largely limited by reactivation of SBPase and FBPase. Furthermore, the content of Rubisco was higher than that of FBPase and SBPase. During the high light period of dynamic light, HN leaves need to activate a higher proportion of FBPase and SBPase and a longer period of time to resume photosynthesis. The results of present study reveal that under dynamic light condition, LN could alleviate the decline of photosynthetic induction rate, while HN exacerbated the decline of photosynthetic induction rate. The limitation in enzymes related to the ribulose-1,5-disphosphate (RuBP) may be one of the reasons why HN exacerbates the decline of photosynthetic induction rate under dynamic light condition.

Aims Poplars (Populusspp.) are main timber and greening trees in the Northern Plains of China, and remains unclear whether nitrogen form defines the effects of nitrogen on poplar growth and physiology. Hence, this study aimed at investigating the effects of different nitrogen forms (ammonium nitrogen, nitrate nitrogen, ammonia-nitrogen mixed state and amido nitrogen) on photosynthesis and growth of poplar seedlings.

Methods Two poplar clones (Populus deltoides cv. ‘55/56’ × P. deltoides cv. ‘Imperial’, clone ‘546’ and P. euramericana cv. ‘74/76’, clone ‘107’) were used as test materials. Poplar ‘546’ has lower plant height and larger single leaf area and is more sensitive to low temperature than poplar ‘107’. An experiment was conducted to determine gas exchange, chlorophyll fluorescence, plant height and branch diameter, specific leaf area, biomass and root-shoot ratio in seedlings treated with different forms of nitrogen.

Important findings The results showed that leaf photosynthesis characteristics of clones ‘546’ and ‘107’ were similar, but their growth parameters differed. Nitrogen application significantly increased leaf net photosynthetic rate, stomatal conductance, electron transport rate of photosystem II (PSII), PSII actual photochemical quantum yield, photosynthetic electron transport rate, branch diameter, plant height, specific leaf area and plant biomass, but reduced root-shoot ratio. Except for leaf net photosynthetic rate, there were no significant differences in other photosynthesis characteristics and growth parameters among different nitrogen forms. The response significantly differed between the two clones. Relative to other nitrogen treatments, specifically, leaf net photosynthetic rate was significantly increased by ammonia-nitrogen mixed state treatment in clone ‘107’, while most of the indexes of ‘546’ poplar were not significantly different among different nitrogen treatments. These results suggested that the application of ammonia-nitrogen mixed state and amido nitrogen can promote the photosynthesis, growth and biomass accumulation of poplar seedlings. However, clone ‘107’ has a higher nitrogen use efficiency under amide nitrogen treatment, whereas the best suitable nitrogen application form for clone ‘546’ was nitrate nitrogen and ammonium nitrogen.

Aims Kobresia pygmaea is a perennial cushion herb from the Cyperaceae family with a height of 1-3 cm and small linear leaves about 1 mm wide. It is mainly distributed on the low slopes of the high mountains ranging from 3 000 m to 5 960 m on the Qingzang Plateau. Its habitat is harsh, and extreme climate conditions such as low temperature, strong wind, and high sunlight intensity are the main abiotic stresses during plants growing season. The objectives of this study were to analyze the photochemical and non-photochemical energy distribution of the photosystem II (PSII) reaction center in K. pygmaea leaves, and their quenching protection mechanism after nocturnal low-temperature (NLT) treatment.

Methods Kobresia pygmaea meadow turfs (30 cm × 15 cm) were collected from the Alpine Grassland Ecosystem Research Station of the Resource of Three Rivers. The turf blocks were separated into two groups, one group was kept in a culture room with a temperature of 24/18 °C (day/night) as a control treatment, and another was kept in an artificial climate chamber with 0 °C in the evening as an NLT treatment. During the daytime, the NLT group was moved back to the culture room and irradiated together with the control group. On day 0, day 1, day 3, and day 5 after NLT treatment, the chlorophyll fluorescence of K. pygmaea leaves including, the light-response curve, PSII photochemical efficiency at 400 and 1 500 μmol·m⁻²·s⁻¹ steady-state light intensities, and dark relaxation were monitored using CF imager. Then, based on the “Lake Model”, the variation of the PSII actual photochemical efficiency (Φ_PSII), the quantum yield of non-regulated energy dissipation (Φ_NO) and regulated energy dissipation (Φ_NPQ) were explored. Additionally, the fast and slow relaxation components of PSII non-photochemical quenching were determined.

Important findings Nocturnal low temperature had limited effects on the rapid light-response curves of PSII relative electron transfer rate through PSII (rETR), the fraction of open PSII centers (q_L), and PSII non-photochemical quenching coefficient (q_NP). The comparison of chlorophyll fluorescence between 400 and 1 500 μmol·m⁻²·s⁻¹ steady-state light intensities confirmed that NLT treatment did not affect the activity of the PSII reaction center and the process of non-photochemical quenching of K. pygmaea. On the third day after NLT treatment, under high light intensity, the ratios of Φ_PSII:Φ_NO:Φ_NPQ were 36:19:45 and 38:19:43 in the control and NLT groups, respectively; while under lower light intensity, they were 66:22:12 and 66:23:11, respectively. The fast relaxation component (NPQ_f) was the main component in non-photochemical quenching (NPQ); the proportion of the slow relaxation component in non-photochemical quenching was 11% and 10% on day 1 and day 3 in control group, and 13% and 12% in NLT group, respectively. Our results indicated that the probability of photoinhibition of the PSII reaction center in K. pygmaeawas increased after NLT treatment; low light intensity and NLT led to the prolongation of photosynthetic induction time. Overall, the NLT treatment did not increase the tendency of excess excitation energy to be difficult to regulate and dissipate in K. pygmaea leaves, since PSII photochemical energy dissipation and protective regulation mechanism still effectively distributed the absorbed light energy.

Aims The purpose of this study was to investigate the regulatory effect of exogenous H₂S on the photosynthetic carbon metabolism of plants under saline-alkali stress.

Methods The Avena nude was selected and a potted soil culture experiment was conducted to study the effects of spraying 50 µmol·L⁻¹ H₂S donor sodium hydrosulfide (NaHS) solution on photosynthetic and fluorescence parameters, the contents of monosaccharides, oligosaccharides and starch, the activities of key enzymes in Calvin cycle and sugar metabolism in leaves, and yield components under 3.00 g·kg^-1 saline-alkali stress.

Important findings (1) Spraying NaHS significantly decreased chlorophyll content, intercellular CO₂ concentration, photosystem II (PSII) initial fluorescence, maximum fluorescence, regulatory energy dissipation, photochemical quenching, non-photochemical quenching, and significantly increased Hill reaction activity, net photosynthetic rate, transpiration rate, stomatal conductance, apparent CO₂ utility efficiency, PSII maximum photochemical efficiency, and the activities of ribulose-1,5-bisphophate carboxylase (Rubisco), Rubisco activase, glyceraldehyde-3-phosphate dehydrogenase and transketolase in A. nude leaves under salt-alkali stress. These indicated that exogenous H₂S could alleviate saline-alkali stress-induced photoinhibition and photosynthetic rate decrease by reducing light energy absorption and the portion of light energy absorbed by the PSII antenna pigment which was used for photochemical electron transfer, improving primary photochemical efficiency and promoting water photolysis of PSII, regulating key enzyme activities in the Calvin cycle, and enhancing CO₂ utility efficiency. (2) Under saline-alkali stress, the activities of α-amylase and sucrose phosphate synthase were significantly increased and the contents of starch and reducing sugar and neutral invertase activities were significantly decreased in A. nude leaves on the 7th day after spraying NaHS. On the 14th day, the contents of total soluble sugar and reducing sugar, and the activities of sucrose synthase and sucrose phosphate synthase decreased significantly, and the activity of neutral invertase increased significantly. The contents of glucose, fructose and galactose on the 7th and 14th days increased to varying degrees, and the content of raffinose decreased significantly, while the activities of total amylase, β-amylase, starch phosphorylase, adenosine diphosphate glucose pyrophosphorylase and acid invertase and the contents of fucose, trehalose and sucrose did not change significantly. These changes suggested that exogenous H₂S was involved in the regulation of starch and sucrose metabolism and the conversion between polysaccharides and oligosaccharides in A. nude under saline-alkali stress. (3) Spraying NaHS had no significant effect on plant height, spike numbers per plant, boll numbers per spike, thousand-grain weight and biological yield per plant of A. nude under salt-alkali stress, while grain numbers per spike and grain yield per plant were increased significantly. In summary, exogenous H₂S participates in the regulation of photosynthetic carbon metabolism in A. nude under salt-alkali stress, and it can enhance the tolerance of A. nude to saline-alkali stress.

Aims The research on dynamic niche partitioning of soil resource uptake is crucial for the understanding of plant coexistence mechanisms. However, there are still knowledge gaps in the interpretation of this field.

Methods In the growing season of 2019, a mature mixed forest of Populus tomentosa and Robinia pseudoacacia on the North China Plain was repeatedly sampled for water isotopes and soil water content, and fine root sampling was performed at the end of the growing season. Seasonal water uptake patterns of trees were determined by the hydrogen-oxygen stable isotope method and Bayesian mixture model (MixSIAR). The degree of niche overlap between P. tomentosaand R. pseudoacaciawas assessed by Pianka’s normalized overlap value.

Important findings Both tree species have deep root systems. Nevertheless, P. tomentosa tended to develop horizontal lateral roots and a higher proportion of fine roots were distributed in the shallow soil layers (0-30 cm). In contrast, R. pseudoacacia inclined to develop vertical taproots and a high proportion of fine roots were distributed in the deep soil layers (100-600 cm). In terms of the mean values of the whole growing season, the primary water sources for P. tomentosa and R. pseudoacacia were water from the middle (30-100 cm) and deep soil horizons. However, the contribution of water from shallow and middle soil horizons to the water uptake of P. tomentosa was higher than that of R. pseudoacacia, while the opposite was true for deep soil water and groundwater for the two species. Populus tomentosa and R. pseudoacacia showed completely opposite water uptake strategies in response to drought and heavy summer rainfall. During the dry season, P. tomentosa enhanced the water uptake contribution of the middle soil layer, while R. pseudoacacia promoted the relative water uptake from the groundwater. When heavy rainfall events occurred, P. tomentosa increased the water uptake contribution of the shallow soil layer, while R. pseudoacacia increased the water uptake contribution of the deep soil layer. In conclusion, there was niche complementarity in the fine root and water uptake between P. tomentosa and R. pseudoacacia. Furthermore, the degree of niche complementation in water uptake varied with seasons, and the water-uptake niche complementation degree in the dry season was relatively higher than that in other seasons. In addition, the study also showed that the root niche partitioning was not representative of the water uptake partitioning of trees. This study provides support for further understanding of plant coexistence mechanisms, and will provide an important reference for the formulation of future mixed forest management strategies to cope with climate change.

Aims Thermal dissipation probes (TDP) have been extensively applied in studying forest transpiration. The calculation accuracy of TDP data directly affects the precise quantification of water consumption of trees and stands. Granier’s original equation (F_d = 0.0119K^1.231, F_d is the sap flux density (cm·s^-1), K is the temperature difference coefficient) is a standard method for calculating the data measured by TDP, but its accuracy is questioned. The objective of this study is to systematically understand the applicability of Granier’s original equation to Populus tomentosa and clarify the need for calibration.

Methods With P. tomentosa as the experimental material, this study used the stem-weighing and the whole-tree potometer methods to evaluate the accuracy of Granier’s original equation for different types of TDP probes, and compared the applicability of equations calibrated by various methods.

Important findings Compared with the values measured by the stem-weighing method, the sap flux density calculated by Granier’s original equation was underestimated by 52.3%-61.4% on average. The calibrated equations by the stem-weighing method and the whole-tree potometer method were F_d= 0.0362K^1.870 and F_d = 0.0105K^0.976, respectively. The calibrated equation by one method produced a large deviation when applied to calculate the sap flux density measured by other methods. Relative to the values estimated by Granier’s original equation, the average sap flux density of seven field-grown trees calculated using the whole-tree potometer calibrated equation did not change significantly, but that calculated using the equations calibrated by the stem-weighing method or in other studies became significantly larger. Compared with the sap flux density measured by the whole-tree potometer method, the calculation precision of Granier’s original equation is considerably higher than that of other calibrated equations, and its relative average absolute error and root mean square error were 10% and 0.000 5 cm·s^-1, respectively. In addition, the coefficients of the calibrated equation differed greatly across different trees, but their values were negatively correlated with the length ratio of the probe inserted into the water conducting sapwood. To sum up, it may be necessary to calibrate original Granier’s equation when applying TDP to measure sap flux density. However, the application effects of calibrated equations by different methods varied considerably, indicating that the calibrated equations derived in previous studies have great limitations. Meanwhile, this study did not find sufficient evidence to support the viewpoint that it is necessary to use a calibrated equation for accurately estimate the sap flux density of P. tomentosa, especially considering that no significant difference was observed when using the calibrated equation by the whole-tree potometer method and Granier’s original equation to estimate the sap flux density of field-grown P. tomentosa. Therefore, continued application of Granier’s original equation is recommended for this tree species.

Aims Heavy metal chromium (Cr) contamination has toxic effects on crops and will disrupt soil microbial community homeostasis in agricultural soils. Nevertheless, the mechanisms underlying the responses of different crops and their rhizosphere soil microbial communities to Cr stress were different. By using time series data, this study analyzed the effects of Cr stress on ‘Jingu 21’ growth, functional pathways of differentially expressed genes (DEGs) in cereals and soil microbial community structure and function. Our objective was to elucidate the response mechanism of millet (Setaria italica) and soil microbial community, and provide a theoretical basis for the growth of cereals under Cr stress and the ecological restoration of the contaminated soil.

Methods We collected millet seedlings and soil samples from the pot experiments with planted millet before (CK) and 6-hour and 6-day after Cr stress (Cr_6h, Cr_6d), and determined the physiological traits of seedlings and soil physicochemical properties. The gene expression and the functional pathways enriched in the seedlings were investigated by transcriptome analysis; the dynamics of microbial community composition, structure, diversity as well as function in time series and their correlations with soil physicochemical properties were studied by high-throughput sequencing analysis.

Important findings 1) Transcriptome analysis showed that Cr stress induced up-regulation of gene expression (54% for up-regulated DEGs); GO enrichment analysis showed that DEGs significantly down-regulated the expression of photosynthesis-related genes in CK & Cr_6h, Cr_6h & Cr_6d samples, and also significantly up-regulated the expression of defense and damage regulation-related genes, and down-regulated the expression of cell wall and cell membrane and cell division-related genes in Cr_6h & Cr_6d samples. 2) High-throughput sequencing revealed a significant change in the composition of soil bacterial and fungal communities at the phylum and genus level during the time series of Cr stress. The α diversity of bacterial communities showed a phase change from stress to stability (Shannon-Wiener diversity index for CK, Cr_6h, Cr_6d were 6.09, 5.93, 6.05, respectively. Simpson diversity index for CK, Cr_6h, Cr_6d were 0.006 8, 0.007 8, 0.006 8, respectively; Chao diversity index for CK, Cr_6h, Cr_6d were 2 818.49, 2 630.73, 2 769.38, respectively), while the α diversity of fungal community decreased significantly (Shannon-Wiener diversity index for CK, Cr_6h, Cr_6d were 4.17, 3.81, 3.23, respectively). The distribution of β diversity of bacterial community in the Cr stress time series was significantly different from that of fungal community. 3) Correlation analysis between soil physicochemical properties and microbial communities showed that soil physicochemical factors were significantly correlated with a variety of fungal flora, but weakly correlated with bacterial flora. Cr stress significantly inhibited the photosynthesis of millet seedlings by reducing chlorophyll content, photosystem activity and affecting structural components such as thylakoid, and inhibited the proliferation and differentiation of leaf cells by down-regulating the expression of cell wall and microtubule-related components. At the same time, Cr stress also activated the plant defense system to overcome toxicity. Meanwhile, soil bacterial and fungal communities adapted to Cr stress through changing community composition and diversity, and their response levels and strategies differed in the stress time series.

Aims Globally, coastal salt marshes have been considered as major blue carbon sinks and contributors for climate change mitigation. Understanding the effects of soil moisture and salinity on soil CO₂and CH₄ emissions will advance better understand of long-term storage of soil carbon in coastal salt marshes.

Methods We conducted a simulation experiment with a gradient of water treatments (25%, 50%, 75% and 100% soil saturated water content) and salt treatments (9 g·kg^-1and 18 g·kg^-1). And we investigated soil carbon mineralization rates, soil properties, microbial biomass and community structure of typical salt marsh soils in the Yellow River Delta.

Important findings We found that: (1) There was no interaction between soil moisture and salinity content on soil CO₂, CH₄ emissions and CH₄:CO₂, and soil CO₂ emissions showed a unimodal curve along the soil moisture gradients and a significant decrease with increasing soil salinity content. The increased soil moisture significantly promoted soil CH₄ emissions, but the increased soil salinity content significantly inhibited soil CH₄emissions. (2) There was a weak significant interaction between moisture and salinity content on dissolved organic carbon (DOC). Under low water treatment, DOC content decreased with increasing soil salinity content, but increased under high water treatment. There was a significant positive relationship between soil CO₂ emissions and DOC content. (3) Soil microbial biomass exhibited a trend of first increasing and then decreasing with the increasing soil moisture, while soil salinity content significantly decreased microbial biomass. There was a significant positive correlation of microbial biomass with CO₂ and CH₄ emissions. (4) Both soil moisture and salinity treatments modified soil microbial community structure. Soil moisture and salinity treatments significantly increased and decreased the number of bacteria and α diversity index, respectively. Both soil CO₂ and CH₄ emissions were positively correlated with the number of bacteria and α diversity index. The climate is gradually drying and warming in this region due to climate change. Therefore, we speculated that changes in microbial biomass and community structure, soil moisture and salinity content may have potentially profound effects on the carbon-sink function at coastal salt marsh.