Size, distribution, and vulnerability of the global soil inorganic carbon.

Global estimates of the size, distribution, and vulnerability of soil inorganic carbon (SIC) remain largely unquantified. By compiling 223,593 field-based measurements and developing machine-learning models, we report that global soils store 2305 ± 636 (±1 SD) billion tonnes of carbon as SIC over the top 2-meter depth. Under future scenarios, soil acidification associated with nitrogen additions to terrestrial ecosystems will reduce global SIC (0.3 meters) up to 23 billion tonnes of carbon over the next 30 years, with India and China being the most affected. Our synthesis of present-day land-water carbon inventories and inland-water carbonate chemistry reveals that at least 1.13 ± 0.33 billion tonnes of inorganic carbon is lost to inland-waters through soils annually, resulting in large but overlooked impacts on atmospheric and hydrospheric carbon dynamics.

Disentangling the variability of symbiotic nitrogen fixation rate and the controlling factors.

Symbiotic nitrogen (N) fixation (SNF), replenishing bioavailable N for terrestrial ecosystems, exerts decisive roles in N cycling and gross primary production. Nevertheless, it remains unclear what determines the variability of SNF rate, which retards the accurate prediction for global N fixation in earth system models. This study synthesized 1230 isotopic observations to elucidate the governing factors underlying the variability of SNF rate. The SNF rates varied significantly from 3.69 to 12.54 g N m-2 year-1 across host plant taxa. The traits of host plant (e.g. biomass characteristics and taxa) far outweighed soil properties and climatic factors in explaining the variations of SNF rate, accounting for 79.0% of total relative importance. Furthermore, annual SNF yield contributed to more than half of N uptake for host plants, which was consistent across different ecosystem types. This study highlights that the biotic factors, especially host plant traits (e.g. biomass characteristics and taxa), play overriding roles in determining SNF rate compared with soil properties. The suite of parameters for SNF lends support to improve N fixation module in earth system models that can provide more confidence in predicting bioavailable N changes in terrestrial ecosystems.

Relative importance between nitrification and denitrification to N2 O from a global perspective.

Nitrous oxide (N2 O) is a potent greenhouse gas, and its mitigation is a pressing task in the coming decade. However, it remains unclear which specific process between concurrent nitrification and denitrification dominates worldwide N2 O emission. We snagged an opportunity to ascertain whence the N2 O came and which were the controlling factors on the basis of 1315 soil N2 O observations from 74 peer-reviewed articles. The average N2 O emission derived from nitrification (N2 On ) was higher than that from denitrification (N2 Od ) worldwide. The ratios of nitrification-derived N2 O to denitrification-derived N2 O, hereof N2 On :N2 Od , exhibited large variations across terrestrial ecosystems. Although soil carbon and nitrogen content, pH, moisture, and clay content accounted for a part of the geographical variations in the N2 On :N2 Od ratio, ammonia-oxidizing microorganisms (AOM):denitrifier ratio was the pivotal driver for the N2 On :N2 Od ratios, since the AOM:denitrfier ratio accounted for 53.7% of geographical variations in N2 On :N2 Od ratios. Compared with natural ecosystems, soil pH exerted a more remarkable role to dictate the N2 On :N2 Od ratio in croplands. This study emphasizes the vital role of functional soil microorganisms in geographical variations of N2 On :N2 Od ratio and lays the foundation for the incorporation of soil AOM:denitrfier ratio into models to better predict N2 On :N2 Od ratio. Identifying soil N2 O derivation will provide a global potential benchmark for N2 O mitigation by manipulating the nitrification or denitrification.

Opposing effects of warming on the stability of above- and belowground productivity in facing an extreme drought event.

Climate warming, often accompanied by extreme drought events, could have profound effects on both plant community structure and ecosystem functioning. However, how warming interacts with extreme drought to affect community- and ecosystem-level stability remains a largely open question. Using data from a manipulative experiment with three warming treatments in an alpine meadow that experienced one extreme drought event, we investigated how warming modulates resistance and recovery of community structural and ecosystem functional stability in facing with extreme drought. We found warming decreased resistance and recovery of aboveground net primary productivity (ANPP) and structural resistance but increased resistance and recovery of belowground net primary productivity (BNPP), overall net primary productivity (NPP), and structural recovery. The findings highlight the importance of jointly considering above- and belowground processes when evaluating ecosystem stability under global warming and extreme climate events. The stability of dominant species, rather than species richness and species asynchrony, was identified as a key predictor of ecosystem functional resistance and recovery, except for BNPP recovery. In addition, structural resistance of common species contributed strongly to the resistance changes in BNPP and NPP. Importantly, community structural resistance and recovery dominated the resistance and recovery of BNPP and NPP, but not for ANPP, suggesting the different mechanisms underlie the maintenance of stability of above- versus belowground productivity. This study is among the first to explain that warming modulates ecosystem stability in the face of extreme drought and lay stress on the need to investigate ecological stability at the community level for a more mechanistic understanding of ecosystem stability in response to climate extremes.

Extreme precipitation causes divergent responses of root respiration to nitrogen enrichment in an alpine meadow.

Grassland roots are fundamental to obtain the most limiting soil water and nitrogen (N) resources. However, this natural pattern could be significantly changed by recent co-occurrence of N deposition and extreme precipitations, likely with complex interactions on grassland root production and respiration. Despite this nonlinearity, we still know little about how extreme precipitation change nonlinearly regulates the responses of root respiration to N enrichment. Here, we conducted a 6-year experiment of N addition in an alpine meadow, coincidently experiencing extreme precipitations among experimental years. Our results demonstrated that root respiration showed divergent responses to N addition along with extreme precipitation changes among years. Under normal rainfall year, root respiration was significantly stimulated by N addition, whereas it was depressed under high or low water. Moreover, we revealed that both root biomass and traits (i.e. specific root length) were critical mechanisms in affecting root respiration response, but their relative importance changed with water condition. For example, specific root length and specific root respiration were more dominant than root biomass in determining root respiration response under low water, or vice versa. Overall, this study comprehensively reveals the nonlinearity of root respiration responses to the interactions of N enrichment and extreme water change. These new findings help to reconcile previously conflicting results that obtain in a specific episode of water gradient, with important implications for understanding grassland belowground carbon dynamics in facing combined N deposition and extreme precipitation events.

Nitrogen enrichment delays the drought threshold responses of leaf photosynthesis in alpine grassland plants.

Extreme drought is found to cause a threshold response in photosynthesis in ecosystem level. However, the mechanisms behind this phenomenon are not well understood, highlighting the importance of revealing the drought thresholds for multiple leaf-level photosynthetic processes. Thus, we conducted a long-term experiment involving precipitation reduction and nitrogen (N) addition. Moreover, an extreme drought event occurred within the experimental period. We found the presence of drought thresholds for multiple leaf-level photosynthetic processes, with the leaf light-saturated carbon assimilation rate (Asat) displaying the highest threshold (10.76 v/v%) and the maximum rate of carboxylation by Rubisco (Vcmax) showing the lowest threshold (5.38 v/v%). Beyond the drought thresholds, the sensitivities of leaf-level photosynthetic processes to soil water content could be greater. Moreover, N addition lowered the drought thresholds of Asat and stomatal conductance (gs), but had no effect on that of Vcmax. Among species, plants with higher leaf K concentration traits had a lower drought threshold of Asat. Overall, this study highlights that leaf photosynthesis may be suppressed abruptly as soil water content surpasses the drought threshold. However, N enrichment helps to improve the resistance via delaying drought threshold response. These new findings have important implications for understanding the nonlinearity of ecosystem productivity response and early warning management in the scenario of combined extreme drought events and continuous N deposition.

A meta-analysis highlights globally widespread potassium limitation in terrestrial ecosystems.

Potassium (K+ ) is the most abundant inorganic cation in plant cells, playing a critical role in various plant functions. However, the impacts of K on natural terrestrial ecosystems have been less studied compared with nitrogen (N) and phosphorus (P). Here, we present a global meta-analysis aimed at quantifying the response of aboveground production to K addition. This analysis is based on 144 field K fertilization experiments. We also investigate the influences of climate, soil properties, ecosystem types, and fertilizer regimes on the responses of aboveground production. We find that: K addition significantly increases aboveground production by 12.3% (95% CI: 7.4-17.5%), suggesting a widespread occurrence of K limitation across terrestrial ecosystems; K limitation is more prominent in regions with humid climates, acidic soils, or weathered soils; the effect size of K addition varies among climate zones/regions, and is influenced by multiple factors; and previous N : K and K : P thresholds utilized to detect K limitation in wetlands cannot be applied to other biomes. Our findings emphasize the role of K in limiting terrestrial productivity, which should be integrated into future terrestrial ecosystems models.

Enhanced causal effect of ecosystem photosynthesis on respiration during heatwaves.

Because of global warming, Earth's ecosystems have been experiencing more frequent and severe heatwaves. Heatwaves are expected to tip terrestrial carbon sequestration by elevating ecosystem respiration and suppressing gross primary productivity (GPP). Here, using the convergent cross-mapping technique, this study detected positive bidirectional causal effects between GPP and respiration in two unprecedented European heatwaves. Heatwaves enhanced the causal effect strength of GPP on respiration rather than respiration on GPP across 40 site-years of observations. Further analyses and global simulations revealed spatial heterogeneity in the heatwave response of the causal link strength between GPP and respiration, which was jointly driven by the local climate and vegetation properties. However, the causal effect strength of GPP on respiration showed considerable uncertainties in CMIP6 models. This study reveals an enhanced causal link strength between GPP and respiration during heatwaves, shedding light on improving projections for terrestrial carbon sink dynamics under future climate extremes.

Drivers of microbially and plant-derived carbon in topsoil and subsoil.

Plant- and microbially derived carbon (C) are the two major sources of soil organic matter (SOM), and their ratio impacts SOM composition, accumulation, stability, and turnover. The contributions of and the key factors defining the plant and microbial C in SOM along the soil profile are not well known. By leveraging nuclear magnetic resonance spectroscopy and biomarker analysis, we analyzed the plant and microbial C in three soil types using regional-scale sampling and combined these results with a meta-analysis. Topsoil (0-40 cm) was rich in carbohydrates and lignin (38%-50%), whereas subsoil (40-100 cm) contained more proteins and lipids (26%-60%). The proportion of plant C increases, while microbial C decreases with SOM content. The decrease rate of the ratio of the microbially derived C to plant-derived C (CM:P ) with SOM content was 23%-30% faster in the topsoil than in the subsoil in the regional study and meta-analysis. The topsoil had high potential to stabilize plant-derived C through intensive microbial transformations and microbial necromass formation. Plant C input and mean annual soil temperature were the main factors defining CM:P in topsoil, whereas the fungi-to-bacteria ratio and clay content were the main factors influencing subsoil CM:P . Combining a regional study and meta-analysis, we highlighted the contribution of plant litter to microbial necromass to organic matter up to 1-m soil depth and elucidated the main factors regulating their long-term preservation.

Data-driven modeling on the global annual soil nitrous oxide emissions: Spatial pattern and attributes.

Previous assessments generated divergent estimates of global terrestrial soil nitrous oxide (N2O) emission and its spatial distributions, which did not match the observed data well. The objectives of this study were to generate a global map of terrestrial soil N2O emissions based on field observations (n = 5549) and quantify the contribution of different variables for predicting the global variation of N2O emissions. We provided spatially explicit maps of annual soil N2O emission rates across forest, grassland and cropland using the random forest approach. The global mean soil N2O emission rate in our data-driven model was 0.059 ± 0.006 g N m-2 year-1, which was lower than the estimates from previous model ensembles. Soil N2O emissions were higher in the northern than southern hemisphere. The average annual soil N2O emission rate of cropland (0.094 ± 0.009 g N m-2 year-1) was higher than that of forest (0.039 ± 0.004 g N m-2 year-1) and grassland (0.045 ± 0.007 g N m-2 year-1). In addition, we found that soil nitrogen substrates dominated the changes in soil N2O emissions and the relative importance of nitrate, ammonium, and fertilizer in predicting soil N2O emissions was greater than that of mean annual temperature and precipitation. Our data-driven model results implied that previous process-based model may overestimate the global soil N2O emission rates due to limited validation data and incomplete assumptions on related-mechanisms. This study highlights the importance of global field observations in N2O emission estimation, which can provide an independent dataset to constrain previous process-based models for better prediction.

Enhanced CO2 uptake is marginally offset by altered fluxes of non-CO2 greenhouse gases in global forests and grasslands under N deposition.

Despite the increasing impact of atmospheric nitrogen (N) deposition on terrestrial greenhouse gas (GHG) budget, through driving both the net atmospheric CO2 exchange and the emission or uptake of non-CO2 GHGs (CH4 and N2 O), few studies have assessed the climatic impact of forests and grasslands under N deposition globally based on different bottom-up approaches. Here, we quantify the effects of N deposition on biomass C increment, soil organic C (SOC), CH4 and N2 O fluxes and, ultimately, the net ecosystem GHG balance of forests and grasslands using a global comprehensive dataset. We showed that N addition significantly increased plant C uptake (net primary production) in forests and grasslands, to a larger extent for the aboveground C (aboveground net primary production), whereas it only caused a small or insignificant enhancement of SOC pool in both upland systems. Nitrogen addition had no significant effect on soil heterotrophic respiration (RH ) in both forests and grasslands, while a significant N-induced increase in soil CO2 fluxes (RS , soil respiration) was observed in grasslands. Nitrogen addition significantly stimulated soil N2 O fluxes in forests (76%), to a larger extent in grasslands (87%), but showed a consistent trend to decrease soil uptake of CH4 , suggesting a declined sink capacity of forests and grasslands for atmospheric CH4 under N enrichment. Overall, the net GHG balance estimated by the net ecosystem production-based method (forest, 1.28 Pg CO2 -eq year-1 vs. grassland, 0.58 Pg CO2 -eq year-1 ) was greater than those estimated using the SOC-based method (forest, 0.32 Pg CO2 -eq year-1 vs. grassland, 0.18 Pg CO2 -eq year-1 ) caused by N addition. Our findings revealed that the enhanced soil C sequestration by N addition in global forests and grasslands could be only marginally offset (1.5%-4.8%) by the combined effects of its stimulation of N2 O emissions together with the reduced soil uptake of CH4 .

Evidence for widespread thermal optimality of ecosystem respiration.

Ecosystem respiration (ER) is among the largest carbon fluxes between the biosphere and the atmosphere. Understanding the temperature response of ER is crucial for predicting the climate change-carbon cycle feedback. However, whether there is an apparent optimum temperature of ER ([Formula: see text]) and how it changes with temperature remain poorly understood. Here we analyse the temperature response curves of ER at 212 sites from global FLUXNET. We find that ER at 183 sites shows parabolic temperature response curves and [Formula: see text] at which ER reaches the maximum exists widely across biomes around the globe. Among the 15 biotic and abiotic variables examined, [Formula: see text] is mostly related to the optimum temperature of gross primary production (GPP, [Formula: see text]) and annual maximum daily temperature (Tmax). In addition, [Formula: see text] linearly increases with Tmax across sites and over vegetation types, suggesting its thermal adaptation. The adaptation magnitude of [Formula: see text], which is measured by the change in [Formula: see text] per unit change in Tmax, is positively correlated with the adaptation magnitude of [Formula: see text]. This study provides evidence of the widespread existence of [Formula: see text] and its thermal adaptation with Tmax across different biomes around the globe. Our findings suggest that carbon cycle models that consider the existence of [Formula: see text] and its adaptation have the potential to more realistically predict terrestrial carbon sequestration in a world with changing climate.

Dryness limits vegetation pace to cope with temperature change in warm regions.

Climate change leads to increasing temperature and more extreme hot and drought events. Ecosystem capability to cope with climate warming depends on vegetation's adjusting pace with temperature change. How environmental stresses impair such a vegetation pace has not been carefully investigated. Here we show that dryness substantially dampens vegetation pace in warm regions to adjust the optimal temperature of gross primary production (GPP) ( T opt GPP ) in response to change in temperature over space and time. T opt GPP spatially converges to an increase of 1.01°C (95% CI: 0.97, 1.05) per 1°C increase in the yearly maximum temperature (Tmax ) across humid or cold sites worldwide (37o S-79o N) but only 0.59°C (95% CI: 0.46, 0.74) per 1°C increase in Tmax across dry and warm sites. T opt GPP temporally changes by 0.81°C (95% CI: 0.75, 0.87) per 1°C interannual variation in Tmax at humid or cold sites and 0.42°C (95% CI: 0.17, 0.66) at dry and warm sites. Regardless of the water limitation, the maximum GPP (GPPmax ) similarly increases by 0.23 g C m-2 day-1 per 1°C increase in T opt GPP in either humid or dry areas. Our results indicate that the future climate warming likely stimulates vegetation productivity more substantially in humid than water-limited regions.

Contextualized response of carbon-use efficiency to warming at the plant and ecosystem levels.

Carbon-use efficiency (CUE) has been widely used as a constant value in many earth system models to simulate how assimilated C is partitioned in ecosystems, to estimate ecosystem C budgets, and investigate C feedbacks to climate warming. Although correlative relationships from previous studies indicated that CUE could vary with temperature, and relying on a fixed CUE value could cause large uncertainty in model projections, however, due to the lack of manipulative experiment, it remains unclear how CUE at the plant (CUEp) and ecosystem (CUEe) levels respond to warming. Based on a 7-year manipulative warming experiment in an alpine meadow ecosystem on the Qinghai-Tibet Plateau, we quantitatively distinguished various C flux components of CUE, including gross ecosystem productivity, net primary productivity, net ecosystem productivity, ecosystem respiration, plant autotrophic respiration, and microbial heterotrophic respiration and explored how CUE at different levels responded to climate warming. We found large variations in both CUEp (0.60 to 0.77) and CUEe (from 0.38 to 0.59). The warming effect on CUEp was positively correlated with ambient soil water content (SWC) and the warming effect on CUEe was negatively correlated with ambient soil temperature (ST), but was positively correlated with warming-induced changes in ST. We also found that the direction and magnitude of the warming effects on different CUE components scaled differently with changes in the background environment, which explained the variation in CUE's warming response under environmental changes. Our new insights have important implications for reducing modelling uncertainty of ecosystem C budgets and improving our ability to predict ecosystem C-climate feedbacks under climate warming.

Nitrogen enrichment differentially regulates the response of ecosystem stability to extreme dry versus wet events.

Extreme climate events, such as severe droughts and heavy rainfall, have profound impacts on the sustainable provision of ecosystem functions and services. However, how N enrichment interacts with discrete extreme climate events to affect ecosystem functions is largely unknown. Here, we investigated the responses of the temporal stability (i.e., resistance, recovery, and resilience) of aboveground net primary productivity (ANPP) in an alpine meadow to extreme dry and wet events under six N addition treatments (0, 2, 4, 8, 16, 32 g N m-2 year-1). We found that N addition had contrasting effects on the responses of ANPP to the extreme dry versus wet events, which resulted in no overall significant effects on ANPP stability across 2015-2019. Specifically, high N addition rates reduced the stability, resistance, and resilience of ANPP in response to extreme drought, whereas medium N addition rates increased ANPP stability and recovery in response to the extreme wet event. The main mechanisms underlying the response of ANPP to extreme drought and wet events were discrepant. Species richness, asynchrony, and dominant species resistance contributed most to the reduction of ANPP resistance to extreme drought, while species asynchrony and dominant and common species resilience contributed most to the decrease of ANPP resilience from extreme drought with N enrichment. The ANPP recovery from the extreme wet event was mainly explained by dominant and common species recovery. Our results provide strong evidence that N deposition mediates ecosystem stability in response to extreme dry and wet events in different ways and modulates the provisioning of grassland ecosystem functions under increasing extreme climate events.

Global variations and controlling factors of anammox rates.

Soil anammox is an environmentally friendly way to eliminate reactive nitrogen (N) without generating nitrous oxide. Nevertheless, the current earth system models have not incorporated the anammox due to the lack of parameters in anammox rates on a global scale, limiting the accurate projection for N cycling. A global synthesis with 1212 observations from 89 peer-reviewed papers showed that the average anammox rate was 1.60 ± 0.17 nmol N g-1 h-1 in terrestrial ecosystems, with significant variations across different ecosystems. Wetlands exhibited the highest rate (2.17 ± 0.31 nmol N g-1 h-1 ), followed by croplands at 1.02 ± 0.09 nmol N g-1 h-1 . The lowest anammox rates were observed in forests and grasslands. The anammox rates were positively correlated with the mean annual temperature, mean annual precipitation, soil moisture, organic carbon (C), total N, as well as nitrite and ammonium concentrations, but negatively with the soil C:N ratio. Structural equation models revealed that the geographical variations in anammox rates were primarily influenced by the N contents (such as nitrite and ammonium) and abundance of anammox bacteria, which collectively accounted for 42% of the observed variance. Furthermore, the abundance of anammox bacteria was well simulated by the mean annual precipitation, soil moisture, and ammonium concentrations, and 51% variance of the anammox bacteria was accounted for. The key controlling factors for soil anammox rates differed from ecosystem type, for example, organic C, total N, and ammonium contents in croplands, versus soil C:N ratio and nitrite concentrations in wetlands. The controlling factors in soil anammox rate identified by this study are useful to construct an accurate anammox module for N cycling in earth system models.

Threshold responses of soil gross nitrogen transformation rates to aridity gradient.

The responses of soil nitrogen (N) transformations to climate change are crucial for biome productivity prediction under global change. However, little is known about the responses of soil gross N transformation rates to drought gradient. Along an aridity gradient across the 2700 km transect of drylands on the Qinghai-Tibetan Plateau, this study measured three main soil gross N transformation rates in both topsoil (0-10 cm) and subsoil (20-30 cm) using the laboratorial 15 N labeling. The relevant soil abiotic and biotic variables were also determined. The results showed that gross N mineralization and nitrification rates steeply decreased with increasing aridity when aridity was less than 0.5 but just slightly decreased with increasing aridity when aridity was larger than 0.5 at both soil layers. In topsoil, the decreases of the two gross rates were accompanied by the similar decreased patterns of soil total N content and microbial biomass carbon with increasing aridity (p < .05). In subsoil, although the decreased pattern of soil total N with increasing aridity was still similar to the decreases of the two gross rates (p < .05), microbial biomass carbon did not change (p > .05). Instead, bacteria and ammonia oxidizing archaea abundances decreased with increasing aridity when aridity was larger than 0.5 (p < .05). With an aridity threshold of 0.6, gross N immobilization rate increased with increasing aridity in wetter region (aridity < 0.6) accompanied with an increased bacteria/fungi ratio, but decreased with increasing aridity in drier region (aridity > 0.6) where mineral N and microbial biomass N also decreased at both soil layers (p < .05). This study provided new insight to understand the differential responses of soil N transformation to drought gradient. The threshold responses of the gross N transformation rates to aridity gradient should be noted in biogeochemical models to better predict N cycling and manage land in the context of global change.

Unexpected no significant soil carbon losses in the Tibetan grasslands due to rodent bioturbation.

The Tibetan grasslands store 2.5% of the Earth's soil organic carbon. Unsound management practices and climate change have resulted in widespread grassland degradation, providing open habitats for rodent activities. Rodent bioturbation loosens topsoil, reduces productivity, changes soil nutrient conditions, and consequently influences the soil organic carbon stocks of the Tibetan grasslands. However, these effects have not been quantified. Here, using meta-analysis and upscaling approaches, we found that rodent bioturbation impacts on the Tibetan grassland soil organic carbon contents were depth-dependent, with significant (P < 0.001) decreasing of 24.4% in the topsoil (0 to 10 cm) but significant (P < 0.05) increasing of 35.9% in the deeper soil layer (40 to 50 cm), and nonsignificant changes in other soil layers. The depth-dependent responses in soil organic carbon content were closely associated with rodent tunnel burrowing, foraging, excrement deposition, and mixing of the upper and deeper soil layers. Rodent bioturbation had shown nonsignificant impacts on soil bulk density, independent of soil layer. Tibetan grasslands totally lose -35.2 Tg C yr-1 (95% CI: -48.5 to -21.1 Tg C yr-1) and -32.9 Tg C yr-1 (-54.2 to -8.6 Tg C yr-1) due to rodent bioturbation in the 0 to 10 or 0 to 30 cm soil layer, while no significant net loss was found over the 0 to 90 cm layer. Our findings highlight the importance of considering depth-dependent factors to robustly quantify the net changes in the terrestrial soil organic carbon stocks resulting from disturbances such as rodent bioturbation.

Dynamic carbon-nitrogen coupling under global change.

Carbon-nitrogen coupling is a fundamental principle in ecosystem ecology. However, how the coupling responds to global change has not yet been examined. Through a comprehensive and systematic literature review, we assessed how the dynamics of carbon processes change with increasing nitrogen input and how nitrogen processes change with increasing carbon input under global change. Our review shows that nitrogen input to the ecosystem mostly stimulates plant primary productivity but inconsistently decreases microbial activities or increases soil carbon sequestration, with nitrogen leaching and nitrogenous gas emission rapidly increasing. Nitrogen fixation increases and nitrogen leaching decreases to improve soil nitrogen availability and support plant growth and ecosystem carbon sequestration under elevated CO2 and temperature or along ecosystem succession. We conclude that soil nitrogen cycle processes continually adjust to change in response to either overload under nitrogen addition or deficiency under CO2 enrichment and ecosystem succession to couple with carbon cycling. Indeed, processes of both carbon and nitrogen cycles continually adjust under global change, leading to dynamic coupling in carbon and nitrogen cycles. The dynamic coupling framework reconciles previous debates on the "uncoupling" or "decoupling" of ecosystem carbon and nitrogen cycles under global change. Ecosystem models failing to simulate these dynamic adjustments cannot simulate carbon-nitrogen coupling nor predict ecosystem carbon sequestration well.

Depth-dependent drivers of soil aggregate carbon across Tibetan alpine grasslands.

Elucidating the effects underlying soil organic carbon (SOC) variation is imperative for ascertaining the potential drivers of mitigating climate change. However, the drivers of variations in various SOC fractions (e.g., macroaggregate C, microaggregate C, and silt and clay C) at different soil depths remain poorly understood. Here, we investigated the effects and relative contributions of climatic, plant, edaphic, and microbial factors on soil aggregate C between the topsoil (0-10 cm) and subsoil (20-30 cm) across alpine grasslands on the Tibetan Plateau. Results showed that the C content of macroaggregates, microaggregates, and silt and clay fractions in the topsoil was 128.6 %, 49.6 %, and 242.4 % higher than that in the subsoil, respectively. Overall, plant properties were the most determinants controlling soil macroaggregate, microaggregate, and silt + clay associated C for both two soil depths, accounting for 32.2 %, 37.4 %, and 38.8 % of the variation, respectively, followed by edaphic, microbial, and climatic factors. The aggregate C of both soil depths was significantly related with the climatic, plant, edaphic, and microbial factors, but the relative importance of these determinants was soil-depth dependent. Specifically, the effects of plant root biomass and microbial (e.g., microbial biomass carbon and fungal diversity index) factors on each aggregate C weakened with soil depth, but the importance of edaphic factors (e.g., clay content, pH, and bulk density) strengthened with soil depth, except for the weakened effect of bulk density on the microaggregate C. And the effects of climatic factor (e.g., mean annual precipitation) on macroaggregate and microaggregate C increased with soil depth. Our results highlight differential drivers and their impacts on soil aggregate C between the topsoil and subsoil, which benefits biogeochemical models for more accurately forecasting soil C dynamics and its feedbacks to environmental changes.