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The recoveries of mesophyll (gm) and stomatal conductance to CO2 (gsc) after soil rewatering have received considerable attention in recent years, but the recovery mechanisms involving leaf anatomy and physiological activities are poorly understood. Moreover, it is also unclear whether leaf gas-phase conductance (gias) or liquid-phase conductance (gliq) is the main factor promoting gm recovery. By simultaneously using gas exchange and chlorophyll fluorescence, we measured the recoveries of gm and gsc in saplings of Manchurian ash (Fraxinus mandshurica Rupr.) and Mongolian oak (Quercus mongolica Fish. ex Ledeb) exposed to two initial water stress (medium water stress, MW, and severe water stress, SW) and following rewatering. Furthermore, leaf anatomical characteristics and the activities of aquaporin (AQP) and carbonic anhydrase (CA) were measured to explain the mechanisms of gm and gsc recoveries. The results showed that (i) both gm and gsc were partly recovered after rewatering, and the recoveries decreased with initial water stress in both species. (ii) The gm recovery was much greater in Mongolian oak than in Manchurian ash, while the gsc recovery was much greater in Manchurian ash. Consequently, the photosynthesis recovery in Manchurian ash was mostly affected by gsc recovery, while that in Mongolian oak was mostly affected by gm recovery. (iii) The gm recovery mainly resulted from the great increase in leaf gliq after rewatering rather than that in gias, as gias had a negative effect on gm recovery. The stomatal opening status improved after rewatering, as the stomatal pore size (SS) increased, greatly promoting gsc recovery. In addition, the activities of both AQP and CA increased after rewatering, which improved CO2 transmembrane transports and greatly promoted gm and gsc recoveries.

Carbon isotopic composition of soil respired CO2 (soil δ13CR) has been regarded as a good indicator of the linkages between aboveground processes and soil respiration. However, whether δ13CR of autotrophic or heterotrophic component of soil respiration dominates the temporal variability of total soil δ13CR was rarely examined by previous studies. In this study, carbon isotopic composition of atmospheric CO2 (δ13Cair) and soil δ13CR in control (with roots) and trenched (without roots) plots were measured in a temperated mixed forest. A 13C isotopic profile system and an automated soil respiration system were used for δ 13Cair and soil δ13CR measurements, respectively. We found that soil δ13CR in the control plots changed substantially in the growing season and it was more negative (by ~0.6‰) than that in the trenched plots, while soil δ13CR in the trenched plots showed a minor temporal variability. This suggests that δ13CR from the autotrophic respiration is the key decider of the seasonal variation pattern of the soil δ13CR. Moreover, the seasonal variation of soil δ13CR in the control plots showed a similar pattern with the seasonal variation of δ13Cair. A significant time-lag was found between δ13Cair and soil δ13CR, showing that soil δ13CR generally lagged behind δ13Cair 15 days. This result supports the hypothesis that soil respiration is closely related to carbon assimilation at the leaf-level and also stressed the importance of δ13Cair in shaping soil δ13CR. These findings are highly valuable to develop the process-based models of the carbon cycle of forest ecosystems.

Peatland carbon cycling is critical for the land-atmosphere exchange of greenhouse gases, particularly under changing environments. Warming and elevated atmospheric carbon dioxide (eCO2) concentrations directly enhance peatland methane (CH4) emission, and indirectly affect CH4 processes by altering hydrological conditions. An ecosystem model ELM-SPRUCE, the land model of the E3SM model, was used to understand the hydrological feedback mechanisms on CH4 emission in a temperate peatland under a warming gradient and eCO2 treatments. We found that the water table level was a critical regulator of hydrological feedbacks that affect peatland CH4 dynamics; the simulated water table levels dropped as warming intensified but slightly increased under eCO2. Evaporation and vegetation transpiration determined the water table level in peatland ecosystems. Although warming significantly stimulated CH4 emission, the hydrological feedbacks leading to a reduced water table mitigated the stimulating effects of warming on CH4 emission. The hydrological feedback for eCO2 effects was weak. The comparison between modeled results with data from a field experiment and a global synthesis of observations supports the model simulation of hydrological feedbacks in projecting CH4 flux under warming and eCO2. The ELM-SPRUCE model showed relatively small parameter-induced uncertainties on hydrological variables and their impacts on CH4 fluxes. A sensitivity analysis confirmed a strong hydrological feedback in the first three years and the feedback diminished after four years of warming. Hydrology-moderated warming impacts on CH4 cycling suggest that the indirect effect of warming on hydrological feedbacks is fundamental for accurately projecting peatland CH4 flux under climate warming.

Climate change-driven increases in drought and atmospheric nitrogen (N) deposition frequency and severity across the world, and these changes have profound impacts on forest dynamics by affecting tree carbon balance. However, important knowledge gaps persist concerning the interactions between drought and N enrichment on carbon supply and reserve dynamics. We investigated gas exchange and carbohydrate reserve shifts and associated with leaf chemical composition across two temperate tree saplings coping with different levels of drought stress, N nutrition addition and their interactions. Our results showed that drought stress decreased net photosynthetic rate (A) and stomatal conductance (gs), while A and gs were increased by N addition, combined with increased gs under the interactive effects of drought and N addition, indicating that N nutrient availability had suppressing effect on drought stress and improved the drought-induced negative conditions by altering carbon exchange traits. In addition, we found that N addition reduced the concentrations of nonstructural carbohydrate (NSC) and its components in relation to foliar carbon and nitrogen changes and the fast growth of saplings. Under the interaction of drought and N nutrition addition, no variation in carbohydrate concentrations demonstrate that carbon reserves play a critical role in regulating carbon exchange and growth. Our observations provide evidence of N nutrient availability mitigated drought stress by improving carbon exchange and reserves, which is expected to contribute to the predictions of future vegetation dynamics.

Wetlands store approximately one-third of soil carbon (C) in terrestrial ecosystems, and the loss of C in wetlands has been accelerated by reclamation. However, how wetland reclamation affects microbial properties along soil profiles remains unclear. In this study, we sampled 100 cm soil cores from a wetland and an adjacent cropland that has been cultivated for 23 years to evaluate the impacts of land conversion on soil microbial community structure and soil element concentrations in the Sanjiang Plain, Northeastern China. The C and nitrogen (N) concentrations in microbial biomass declined exponentially with depth in both the wetland and cropland with different magnitudes. Continuous cultivation for 23 years tended to homogenized C, N, phosphorus (P), and sulfur (S) contents in soils and microbial biomass along the soil profile. Compared with the wetland, C and N in cropland soils were lower, and microbial biomass C, N, P and S were lower in surface soils (0–30 cm), while higher in both middle soils (40–70 cm) and deep soils (70–100 cm). Cultivation narrowed the C:N:P:S stoichiometry in soils and microbial biomass, with large changes in surface soils and minor changes in deep soils. The Bray-Curtis dissimilarity test confirmed the large changes in surface soils while minor changes in deep soils. After 23-years of cultivation the abundances of fungi and bacteria were significantly reduced by approximately 90% and 78% in surface soils, but the bacterial abundance was enhanced by approximately 2–3 times in middle soils, leading to a decreasing fungi:bacteria ratio along soil profile in cropland. Soil TC and pH were the predominant factors controlling the microbial composition in wetland. The homogenizing impact of wetland reclamation on microbial properties along soil profiles suggests the different responses of microbial and soil elements to human activities, indicating a critical need of differentiating microbes from soils when examining soil elements under a changing environment.

Snowpack in winter is very sensitive to climate change, and may in turn induce complex effects on terrestrial biochemistry processes. Here we synthesized the results from 49 papers based on snow depth manipulation experiments and conducted a meta-analysis to evaluate the general response of 9 variables related to terrestrial carbon pools and dynamics to the altered snowpack depth. Our meta-analysis results indicated that increasing snowpack depth significantly increased litter biomass (+18.4%), MBC (+18.2%) and CO2 emission (+15.5%), and significantly decreased litter C: N ratio (−5.8%) and total soil carbon (−28.5%) under snow addition manipulation, but did not significantly affect fine root biomass, MBC: MBN ratio and soil dissolved organic carbon (DOC). However, increasing snowpack depth only significantly altered litter C: N ratio (+11.2%) under snow removal manipulation. The different responses between snow addition and removal were possibly related to the higher soil temperature and moisture under snow addition than under snow removal manipulation experiments. In addition, the effect of increasing snowpack depth on CH4 flux was not significant, presumably because all these studies were compiled from snow-removal experiments. These results are useful for a better understanding of the effects of altered snowpack on terrestrial carbon cycling under climate change scenarios.

Bacteria and fungi, representing two major soil microorganism groups, play an important role in global nutrient biogeochemistry. Biogeographic patterns of bacterial and fungal biomass are of fundamental importance for mechanistically understanding nutrient cycling. We synthesized 1323 data points of phospholipid fatty acid-derived fungal biomass C (FBC), bacterial biomass C (BBC), and fungi:bacteria (F:B) ratio in topsoil, spanning 11 major biomes. The FBC, BBC, and F:B ratio display clear biogeographic patterns along latitude and environmental gradients including mean annual temperature, mean annual precipitation, net primary productivity, root C density, soil temperature, soil moisture, and edaphic factors. At the biome level, tundra has the highest FBC and BBC densities at 3684 (95% confidence interval: 1678–8084) mg kg−1 and 428 (237–774) mg kg−1, respectively; desert has the lowest FBC and BBC densities at 16.92 (14.4–19.89) mg kg−1 and 6.83 (6.1–7.65) mg kg−1, respectively. The F:B ratio varies dramatically, ranging from 1.8 (1.6–2.1) in savanna to 8.6 (6.7–11.0) in tundra. An empirical model was developed for the F:B ratio and it is combined with a global dataset of soil microbial biomass C to produce global maps for FBC and BBC in 0–30 cm topsoil. Across the globe, the highest FBC is found in boreal forest and tundra while the highest BBC is in boreal forest and tropical/subtropical forest, the lowest FBC and BBC are in shrub and desert. Global stocks of living microbial biomass C were estimated to be 12.6 (6.6–16.4) Pg C for FBC and 4.3 (0.5–10.3) Pg C for BBC in topsoil. These findings advance our understanding of the global distribution of fungal and bacterial biomass, which facilitates the incorporation of fungi and bacteria into Earth system models. The global maps of bacterial and fungal biomass serve as a benchmark for validating microbial models in simulating the global C cycle under a changing climate.

Permafrost play an important role in regulating global climate system. We analyzed the gross primary productivity (GPP), net primary productivity (NPP), and evapotranspiration (ET) derived from MODIS and three earth system models participated in the Coupled Model Inter-comparison Project Phase 6 (CMIP6) in the Asian permafrost region. The water use efficiency (WUE) was further computed. The simulated GPP, NPP, and ET show slightly increasing trends during historical period (1900-2014) and strong increasing trends in projection period (2015-2100), and projected impacts of climate change on all variables are greater under high-emission scenarios than low-emission scenarios. Further analysis revealed higher increases in GPP and NPP than that of ET, indicating that vegetation carbon sequestration governs the growing WUE under historical and projected periods in this region. The GPP, NPP and ET showed higher changing rates in western, central and southeast areas of this region, and WUE (WUEGPP, and WUENPP) shows the similar spatial pattern. Compared to MODIS-derived GPP, NPP, and ET during 2000-2014, Earth system models yield the best estimates for NPP, while slight underestimations for GPP and ET, and thus slight overestimations for WUEGPP and WUENPP. This study highlights the predominant role of vegetation activity in regulating regional WUE in Asian permafrost region under future climate change. Vegetation domination of the growing water use efficiency implies that the permafrost region may continue acting efficiently in sequestrating atmospheric carbon in terms of water consumption throughout the 21st century.

Global atmospheric nitrogen deposition has increased steadily since the 20th century, and has complex effects on terrestrial ecosystems. This work synthesized results from 54 papers and conducted a meta-analysis to evaluate the general response of 15 variables related to plant root traits to simulated nitrogen deposition. Simulated nitrogen deposition resulted in significantly decreasing fine root biomass (

Summary Hydrological processes in river systems have been changing under the impacts of both climate variability and human activities. The non-parametric Mann–Kendall statistic was used to identify change trends and points in the annual streamflow in the Hun–Tai River basin in northeast China. The identifications were based on streamflow records from six hydrological stations during 1961–2006, and the purpose was to analyze the change characteristics of the hydrological processes. The results indicated that all hydrological stations presented downward trends in annual streamflows. Abrupt changes in the annual streamflow occurred around 1978 in the Hun River basin, and around 1998 in the Taizi River basin. The impact of climate variability on the mean annual streamflow was also analyzed based on the relationships among streamflow, precipitation, and potential evapotranspiration. Precipitation and meteorological data from 22 rainfall stations and 10 weather stations within the basin were employed in the analysis. Daily potential evapotranspiration was calculated using the Penman–Monteith equation. Climate variability was estimated to account for 43% of the reduction in the annual streamflow, and human activities accounted for about 57%.

Summary Based on the eddy flux and meteorological measurements, we analyzed variation of the components of energy balance before, during and after the snow coverage in dormant season over temperate meadow. The results showed that the energy balance ratio EBR was 0.76, lowest in the fresh snow phase and positively correlated with friction velocity u∗. Furthermore, the energy balance closure error had a diurnal cycle. The radiation partition and energy balance changed in the presence of the snow cover. The surface albedo was high during snow coverage (maximum in the fresh snow phase) and low in the snow-free period (including pre-snow and snow-melted phases). The ratio of net radiation Rn to solar radiation Q was higher in the snow-melted phase, and lower in the fresh snow phase, so did the peaks in diurnal courses of the energy fluxes (Rn, latent heat flux LE, sensible heat flux H, and soil and storage heat flux G + S). The daily-integrated value of H increased followed by Rn in the snow-melting phase, LE and G + S increased quickly followed by Rn in the snow-melted phase. Daily average Bowen ratio β was large in the snow-melting phase and low in the snow-melted phase, indicating that more energy partitioning of Rn into H in the snow-melting phase but LE in the snow-melted phase.