Soil GHG fluxes are altered by N deposition: New data indicate lower N stimulation of the N2 O flux and greater stimulation of the calculated C pools

Nonbiodegradable microplastic types determine the diversity and structure of soil microbial communities: A meta-analysis

As an emerging contaminant, microplastics (MPs) have received considerable attention for their potential threat to the soil environment. However, the response of soil bacterial and fungal communities to MPs exposure remains unclear. In this study, we conducted a global meta-analysis of 95 publications and 2317 observations to assess the effects of nonbiodegradable MP properties and exposure conditions on soil microbial biomass, alpha and beta diversity, and community structure. Our results indicate that MPs increased (p < 0.05) soil active microbial biomass by 42%, with the effect varying with MPs type, exposure concentration, exposure time and soil pH. MPs concentration was identified as the most important factor controlling the response of soil microbial biomass to MPs. MPs addition decreased (p < 0.05) the soil bacterial Shannon and Chao1 indices by 2% and 3%, respectively, but had limited effects (p > 0.05) on soil fungal Shannon and Chao1 indices. The type of MPs and exposure time determined the effects of MPs on bacterial Shannon and Chao1 indices, while the type of MPs and soil pH controlled the response ratios of fungal Shannon and Chao1 indices to MPs. Specifically, soil organic carbon (SOC) was the major factor regulating the response ratio of bacterial alpha diversity index to MPs. The presence of MPs did not affect soil bacterial community structure and beta diversity. Our results highlight that MPs reduced bacterial diversity and richness but increased the soil active microbial biomass, suggesting that MPs could disrupt biogeochemical cycles by promoting the growth of specific microorganisms.

Contrasting impacts of fertilization on topsoil and subsoil greenhouse gas fluxes in a thinned Chinese fir plantation

Globally, forest soils are considered as important sources and sinks of greenhouse gases (GHGs). However, most studies on forest soil GHG fluxes are confined to the topsoils (above 20 cm soil depths), with only very limited information being available regarding these fluxes in the subsoils (below 20 cm soil depths), especially in managed forests. This limits deeper understanding of the relative contributions of different soil depths to GHG fluxes and global warming potential (GWP). Here, we used a concentration gradient-based method to comprehensively investigate the effects of thinning intensity (15% vs. 35%) and nutrient addition (no fertilizer vs. NPK fertilizers) on soil GHG fluxes from the 0-40 cm soil layers at 10 cm depth intervals in a Chinese fir (Cunninghamia lanceolata) plantation. Results showed that forest soils were the sources of CO2 and N2O, but the sinks of CH4. Soil GHG fluxes decreased with increasing soil depth, with the 0-20 cm soil layers identified as the dominant producers of CO2 and N2O and consumers of CH4. Thinning intensity did not significantly affect soil GHG fluxes. However, fertilization significantly increased CO2 and N2O emissions and CH4 uptake at 0-20 cm soil layers, but decreased them at 20-40 cm soil layers. This is because fertilization alleviated microbial N limitation and decreased water filled pore space (WFPS) in topsoils, while it increased WFPS in subsoils, ultimately suggesting that soil WFPS and N availability (especially NH4+-N) were the predominant regulators of GHG fluxes along soil profiles. Generally, there were positive interactive effects of thinning and fertilization on soil GHG fluxes. Moreover, the 35% thinning intensity without fertilization had the lowest GWP among all treatments. Overall, our results suggest that fertilization may not only cause depth-dependent effects on GHG fluxes within soil profiles, but also impede efforts to mitigate climate change by promoting GHG emissions in managed forest plantations.

Depth-dependent responses of soil organic carbon under nitrogen deposition

Emerging evidence points out that the responses of soil organic carbon (SOC) to nitrogen (N) addition differ along the soil profile, highlighting the importance of synthesizing results from different soil layers. Here, using a global meta-analysis, we found that N addition significantly enhanced topsoil (0-30 cm) SOC by 3.7% (±1.4%) in forests and grasslands. In contrast, SOC in the subsoil (30-100 cm) initially increased with N addition but decreased over time. The model selection analysis revealed that experimental duration and vegetation type are among the most important predictors across a wide range of climatic, environmental, and edaphic variables. The contrasting responses of SOC to N addition indicate the importance of considering deep soil layers, particularly for long-term continuous N deposition. Finally, the lack of depth-dependent SOC responses to N addition in experimental and modeling frameworks has likely resulted in the overestimation of changes in SOC storage under enhanced N deposition.

Soil properties and fungal community jointly explain N2O emissions following N and P enrichment in an alpine meadow

Nutrient enrichment, such as nitrogen (N) and phosphorus (P), typically affects nitrous oxide (N2O) emissions in terrestrial ecosystems, predominantly via microbial nitrification and denitrification processes in the soil. However, the specific impact of soil property and microbial community alterations under N and P enrichment on grassland N2O emissions remains unclear. To address this, a field experiment was conducted in an alpine meadow of the northeastern Qinghai-Tibetan Plateau. This study aimed to unravel the mechanisms underlying N and P enrichment effects on N2O emissions by monitoring N2O fluxes, along with analyzing associated microbial communities and soil physicochemical properties. We observed that N enrichment individually or in combination with P enrichment, escalated N2O emissions. P enrichment dampened the stimulatory effect of N enrichment on N2O emissions, indicative of an antagonistic effect. Structural equation modeling (SEM) revealed that N enrichment enhanced N2O emissions through alterations in fungal community composition and key soil physicochemical properties such as pH, ammonium nitrogen (NH4+-N), available phosphorus (AP), microbial biomass carbon (MBC), and microbial biomass nitrogen (MBN)). Notably, our findings demonstrated that N2O emissions were significantly more influenced by fungal activities, particularly genera like Fusarium, rather than bacterial processes in response to N enrichment. Overall, the study highlights that N enrichment intensifies the role of fungal attributes and soil properties in driving N2O emissions. In contrast, P enrichment exhibited a non-significant effect on N2O emissions, which highlights the critical role of the fungal community in N2O emissions responses to nutrient enrichments in alpine grassland ecosystems.

Legacy effects of slag and biochar application on greenhouse gas emissions mitigation in paddy field: A three-year study

The utilization of slag and biochar in croplands has been proposed as a management approach to mitigate greenhouse gas (GHG) emissions, specifically methane (CH4) and nitrous oxide (N2O), from agricultural fields. However, there is limited understanding of the long-term effects of single and combined applications of slag and biochar on GHG emissions in rice paddy fields. We investigated the legacy effects of one-year applications of slag, biochar, and slag+biochar on CH4 and N2O emissions, physicochemical properties, and rice yields during a three-year period (2016-2018) in southeast China. Over the study period, the application of slag reduced CH4 emissions by 24 %, biochar by 45 %, and the combined application of slag+biochar by 44 %. Across the study period, slag, biochar, and slag+biochar applications resulted in respective N2O emissions increases of 78 %, 63 %, and 80 %. Methane emissions contributed to approximately 70 % of the global warming potential (GWP) in the paddy field, which was reduced by 20 % with biochar application and by 15 % with the combined application of slag+biochar. Additionally, the total rice yield in the slag, biochar, and slag+biochar treatments increased by 7 %, 5 %, and 10 %, respectively, compared to the control group. Based on our findings, we recommend the combined application of slag+biochar as a sustainable rice management strategy to effectively reduce GHG emissions from paddy fields while enhancing yield production.

The driving effects of nitrogen deposition on nitrous oxide and associated gene abundances at two water table levels in an alpine peatland

Alpine peatlands are recognized as a weak or negligible source of nitrous oxide (N2O). Anthropogenic activities and climate change resulted in the altered water table (WT) levels and increased nitrogen (N) deposition, which could potentially transition this habitat into a N2O emission hotspot. However, the underlying mechanism related with the effects is still uncertain. Hence, we conducted a mesocosm experiment to address the response of growing-season N2O emissions and the gene abundances of nitrification (bacterial amoA) and denitrification (narG, nirS, norB and nosZ) to the increased N deposition (20 kg N ha-1 yr-1) at two WT levels (WT-30, 30 cm below soil surface; WT10, 10 cm above soil surface) in the Zoige alpine peatland, Qinghai-Tibetan Plateau. The results showed that the WT did not affect N2O emissions, and this was attributed with the limitation of soil NO3-. The higher WT level increased denitrification (narG and nirS gene abundance) resulting in the depletion of soil NO3-, but the consequent NO3- deficiency further limited denitrification, while the WT did not affect nitrification (bacterial amoA gene abundance). Meanwhile, the N deposition increased N2O emissions, regardless of WT levels. This was associated with the N-deposition induced increase in denitrification-related gene abundances of narG, nirS, norB and nosZ at WT-30 and narG at WT10. Additionally, the N2O emission factor assigned to N deposition was 1.3 % at WT-30 and 0.9 % at WT10, respectively. Our study provided comprehensive understanding of the mechanisms referring N2O emissions in response to the interactions between climate change and human disturbance from this high-altitude peatland.

Effects of nitrogen addition on the combined global warming potential of three major soil greenhouse gases: A global meta-analysis

Increased nitrogen (N) deposition has a great impact on soil greenhouse gas (GHG) emissions, and numerous studies have revealed the individual effects of N addition on three major GHGs (CO2, CH4, and N2O). Nevertheless, quantitative evaluation of the effects of N addition on the global warming potential (GWP) of GHGs based on simultaneous measurements is needed not only to better understand the comprehensive effect of N deposition on GHGs but also for precise estimation of ecosystem GHG fluxes in response to N deposition. Here, we conducted a meta-analysis using a dataset with 124 simultaneous measurements of the three major GHGs from 54 studies to assess the effects of N addition on the combined global warming potential (CGWP) of these soil GHGs. The results showed that the relative sensitivity of the CGWP to N addition was 0.43%/kg N ha-1 yr-1, indicating an increase in the CGWP. Among the ecosystems studied, wetlands are considerable GHG sources with the highest relative sensitivity to N addition. Overall, CO2 contributed the most to the N addition-induced CGWP change (72.61%), followed by N2O (27.02%) and CH4 (0.37%), but the contributions of the three GHGs varied across ecosystems. Moreover, the effect size of the CGWP had a positive relationship with N addition rate and mean annual temperature and a negative relationship with mean annual precipitation. Our findings suggest that N deposition may influence global warming from the perspective of the CGWP of CO2, CH4, and N2O. Our results also provide reference values that may reduce uncertainties in future projections of the effects of N deposition on GHGs.

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 .

Effects of nitrogen addition on greenhouse gas fluxes during continuous freeze-thaw cycles in a cold temperate forest

Both nitrogen (N) deposition and soil freeze-thaw cycles (FTCs) induce pulses of greenhouse gas (GHG) emissions in cold temperate zones due to changes in soil carbon (C) and nitrogen (N) turnover. However, the combined effects of N addition and FTCs on GHG fluxes have received little research attention, particularly in boreal forests. We conducted a laboratory incubation experiment using intact soil cores from Rhododendron dauricum-Larix dahurica plots to investigate the GHG flux response to these combined effects. We separated the soil samples into seven groups (no, low, medium, and high sodium nitrate addition and low, medium, and high ammonium chloride addition) and exposed each group to continuous FTC conditions. The N2O and CO2 emissions were eventually stimulated by the FTCs, while CH4 uptake was inhibited by FTCs but responded differently under different N addition treatments. All the treatments had substantially increased N2O emissions compared to the control. However, the soil respiration rate significantly increased only with medium sodium nitrate addition, and high levels of N addition (regardless of form) inhibited CH4 uptake. These findings demonstrate that FTCs and N addition (in various forms and levels) have considerable effects on GHG emissions in temperate forest ecosystems. Moreover, dissolved organic carbon (DOC), microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and inorganic nitrogen in soil are potential factors that drive GHG emissions and are necessary considerations in predicting future feedback effects of GHG emissions on climate change.

Mapping global nitrogen deposition impacts on soil respiration

Soil respiration (R_s) is a key indicator of belowground biological activities of terrestrial ecosystems. Despite ongoing atmospheric nitrogen (N) deposition due to anthropogenic activities, it remains uncertain how R_s responds to globally varied atmospheric N deposition. Based on a meta-analysis of 340 simulated experimental nitrogen addition studies, we aimed to identify the key factors altering the responses of R_s to N deposition and extrapolate these results to the global mapping of R_s changes under N deposition. We found the overall experimental N addition effect on R_s was insignificant, but the responses of R_s significantly shifted from positive to negative with increasing accumulated N addition amount and lower soil pH, and the negative responses to increasing N amounts were significantly intensified in acid soils. Also, the response of heterotrophic respiration to N addition significantly increased with a lower N amount, and both responses of heterotrophic and autotrophic respiration were significantly more negative in soils with lower pH. Our mapping efforts showed that global R_s overall increased by 2.8 % in response to the accumulated N deposition from 2000 to 2020. Regions with combined characteristics of high accumulated N deposition amounts and low soil pH, including Eastern U.S., Europe, and Eastern Asia, were hotspots of R_s declines under current and future atmospheric N deposition. Our findings challenge the long-held notion that N deposition has universal negative impacts on R_s, and suggest the spatial heterogeneity in the impacts of N deposition on belowground activities and carbon release across the globe.

Variation in methane uptake by grassland soils in the context of climate change - A review of effects and mechanisms

Grassland soils are climate-dependent ecosystems that have a significant greenhouse gas mitigating function through their ability to store large amounts of carbon (C). However, what is often not recognized is that they can also exhibit a high methane (CH₄) uptake capacity that could be influenced by future increases in atmospheric carbon dioxide (CO₂) concentration and variations in temperature and water availability. While there is a wealth of information on C sequestration in grasslands there is less consensus on how climate change impacts on CH₄ uptake or the underlying mechanisms involved. To address this, we assessed existing knowledge on the impact of climate change components on CH₄ uptake by grassland soils. Increases in precipitation associated with soils with a high background soil moisture content generally resulted in a reduction in CH₄ uptake or even net emissions, while the effect was opposite in soils with a relatively low background moisture content. Initially wet grasslands subject to the combined effects of warming and water deficits may absorb more CH₄, mainly due to increased gas diffusivity. However, in the longer-term heat and drought stress may reduce the activity of methanotrophs when the mean soil moisture content is below the optimum for their survival. Enhanced plant productivity and growth under elevated CO₂, increased soil moisture and changed nutrient concentrations, can differentially affect methanotrophic activity, which is often reduced by increasing N deposition. Our estimations showed that CH₄ uptake in grassland soils can change from -57.7 % to +6.1 % by increased precipitation, from -37.3 % to +85.3 % by elevated temperatures, from +0.87 % to +92.4 % by decreased precipitation, and from -66.7 % to +27.3 % by elevated CO₂. In conclusion, the analysis suggests that grasslands under the influence of warming and drought may absorb even more CH₄, mainly because of reduced soil water contents and increased gas diffusivity.

Effects of experimental nitrogen deposition on soil organic carbon storage in Southern California drylands

Atmospheric nitrogen (N) deposition is enriching soils with N across biomes. Soil N enrichment can increase plant productivity and affect microbial activity, thereby increasing soil organic carbon (SOC), but such responses vary across biomes. Drylands cover ~45% of Earth's land area and store ~33% of global SOC contained in the top 1 m of soil. Nitrogen fertilization could, therefore, disproportionately impact carbon (C) cycling, yet whether dryland SOC storage increases with N remains unclear. To understand how N enrichment may change SOC storage, we separated SOC into plant-derived, particulate organic C (POC), and largely microbially derived, mineral-associated organic C (MAOC) at four N deposition experimental sites in Southern California. Theory suggests that N enrichment increases the efficiency by which microbes build MAOC (C stabilization efficiency) if soil pH stays constant. But if soils acidify, a common response to N enrichment, then microbial biomass and enzymatic organic matter decay may decrease, increasing POC but not MAOC. We found that N enrichment had no effect on C fractions except for a decrease in MAOC at one site. Specifically, despite reported increases in plant biomass in three sites and decreases in microbial biomass and extracellular enzyme activities in two sites that acidified, POC did not increase. Furthermore, microbial C use and stabilization efficiency increased in a non-acidified site, but without increasing MAOC. Instead, MAOC decreased by 16% at one of the sites that acidified, likely because it lost 47% of the exchangeable calcium (Ca) relative to controls. Indeed, MAOC was strongly and positively affected by Ca, which directly and, through its positive effect on microbial biomass, explained 58% of variation in MAOC. Long-term effects of N fertilization on dryland SOC storage appear abiotic in nature, such that drylands where Ca-stabilization of SOC is prevalent and soils acidify, are most at risk for significant C loss.

Suppression of methane uptake by precipitation pulses and long-term nitrogen addition in a semi-arid meadow steppe in northeast China

In the context of global change, the frequency of precipitation pulses is expected to decrease while nitrogen (N) addition is expected to increase, which will have a crucial effect on soil C cycling processes as well as methane (CH₄) fluxes. The interactive effects of precipitation pulses and N addition on ecosystem CH₄ fluxes, however, remain largely unknown in grassland. In this study, a series of precipitation pulses (0, 5, 10, 20, and 50 mm) and long-term N addition (0 and 10 g N m^-2 yr^-1, 10 years) was simulated to investigate their effects on CH₄ fluxes in a semi-arid grassland. The results showed that large precipitation pulses (10 mm, 20 mm, and 50 mm) had a negative pulsing effect on CH₄ fluxes and relatively decreased the peak CH₄ fluxes by 203-362% compared with 0 mm precipitation pulse. The large precipitation pulses significantly inhibited CH₄ absorption and decreased the cumulative CH₄ fluxes by 68-88%, but small precipitation pulses (5 mm) did not significantly alter it. For the first time, we found that precipitation pulse size increased cumulative CH₄ fluxes quadratically in both control and N addition treatments. The increased soil moisture caused by precipitation pulses inhibited CH₄ absorption by suppressing CH₄ uptake and promoting CH₄ release. Nitrogen addition significantly decreased the absorption of CH₄ by increasing NH₄ ⁺-N content and NO₃ ^--N content and increased the production of CH₄ by increasing aboveground biomass, ultimately suppressing CH₄ uptake. Surprisingly, precipitation pulses and N addition did not interact to affect CH₄ uptake because precipitation pulses and N addition had an offset effect on pH and affected CH₄ fluxes through different pathways. In summary, precipitation pulses and N addition were able to suppress the absorption of CH₄ from the atmosphere by soil, reducing the CH₄ sink capacity of grassland ecosystems.

A simulated ecological restoration of bare cut slope reveals the dosage and temporal effects of cement on ecosystem multifunctionality in a mountain ecosystem

Cement is a critical building material used in the restorations of bare cut slopes. Yet, how cement affects ecosystem's functions and their undertakers remains elusive. Here, we revealed the dosage and temporal effects of cement on plant and soil traits, extracellular enzymes, greenhouse gas fluxes and microbiome using simulation experiments. The results showed that soil pH increased with the cement content at 1st day but relatively constant values around 7 to 7.5 were detected in the flowing days. The β-1,4-glucosidase, phenol oxidase, leucine aminopeptidase and acid phosphatase showed high activities under high cement content, and they generally increased with the cultivations except for acid phosphatase. CH₄ fluxes at 16th day were less than zero, and they increased to peak around at 37th to 44th days followed by decreasing until reaching to relatively stable fluctuations around 0. Despite of decrease patterns, N₂O fluxes stayed around zero across the temporal gradient except for the maximum around at 30th day in 2%, 5% and 8% cement treatment. Microbial diversity decreased with the cement content, in which there were a recovery trend for bacteria. By integrating above- and belowground ecosystem traits into a multifunctionality index, we identified a potential optimum cement content (11%). PLSPM showed that multifunctionality was affected by the shifts in soil bacterial community, enzyme activity and greenhouse gases while these components were effected by other environmental changes resulted from cement. Our results demonstrate that cement determines multifunctionality through mediating microbial community and activity, providing new insights for designing in situ experiments and ecological restoration strategies for bare cut slopes.

Elevational shifts in foliar-soil δ15 N in the Hengduan Mountains and different potential mechanisms

The natural abundance of stable nitrogen isotopes (δ¹⁵ N) provides insights into the N dynamics of terrestrial ecosystems, the determination of which is considered an effective approach for gaining a better understanding ecosystem N cycling. However, there is currently little information available regarding the patterns and mechanisms underlying the variation in foliar-soil δ¹⁵ N among mountain ecosystems. In this study, we examined the determinants of foliar-soil δ¹⁵ N in association with N transportation rates along an elevational gradient in the Hengduan Mountains. Despite the relatively high levels of available N produced from high N fixation and mineralization, we detected the lowest levels of foliar δ¹⁵ N at 3500 m a.s.l., reflecting the stronger vegetation N limitation at medium high elevations. The enhanced vegetation N limitation was driven by the combined effects of higher microbial immobilization and inherent plant dynamic (the shifts of δ¹⁵ N in vegetation preference, including vegetation community) with changing climate along the elevational gradient. Unexpectedly, we established that soil δ¹⁵ N was characterized by an undulating rise and uncoupled correlation with foliar δ¹⁵ N with increasing elevation, thereby indicating that litter input might not be a prominent driver of soil δ¹⁵ N. Conversely, soil nitrification and denitrification were found to make a more pronounced contribution to the pattern of soil δ¹⁵ N along the elevational gradient. Collectively, our results serve to highlight the importance of microbial immobilization in soil N dynamics and provide novel insights that will contribute to enhancing our understanding of N cycling as indicated by foliar-soil δ¹⁵ N along elevational gradients.

Characteristics of nitrogen deposition research within grassland ecosystems globally and its insight from grassland microbial community changes in China

As global change continues to intensify, the mode and rate of nitrogen input from the atmosphere to grassland ecosystems had changed dramatically. Firstly, we conducted a systematic analysis of the literature on the topic of nitrogen deposition impacts over the past 30 years using a bibliometric analysis. A systematic review of the global research status, publication patterns, research hotspots and important literature. We found a large number of publications in the Chinese region, and mainly focuses on the field of microorganisms. Secondly, we used a meta-analysis to focus on microbial changes using the Chinese grassland ecosystem as an example. The results show that the research on nitrogen deposition in grassland ecosystems shows an exponential development trend, and the authors and research institutions of the publications are mainly concentrated in China, North America, and Western Europe. The keyword clustering results showed 11 important themes labeled climate change, elevated CO₂, species richness and diversity, etc. in these studies. The burst keyword analysis indicated that temperature sensitivity, microbial communities, etc. are the key research directions. The results of the meta-analysis found that nitrogen addition decreased soil microbial diversity, and different ecosystems may respond differently. Treatment time, nitrogen addition rate, external environmental conditions, and pH had major effects on microbial alpha diversity and biomass. The loss of microbial diversity and the reduction of biomass with nitrogen fertilizer addition will alter ecosystem functioning, with dramatic impacts on global climate change. The results of the study will help researchers to further understand the subject and have a deep understanding of research hotspots, which are of great value to future scientific research.

Responses of Soil N2O Emission and CH4 Uptake to N Input in Chinese Forests across Climatic Zones: A Meta-Study

Enhanced nitrogen (N) deposition has shown significant impacts on forest greenhouse gas emissions. Previous studies have suggested that Chinese forests may exhibit stronger N2O sources and dampened CH4 sinks under aggravated N saturation. To gain a common understanding of the N effects on forest N2O and CH4 fluxes, many have conducted global-scale meta-analyses. However, such effects have not been quantified particularly for China. Here, we present a meta-study of the N input effects on soil N2O emission and CH4 uptake in Chinese forests across climatic zones. The results suggest that enhanced N inputs significantly increase soil N2O emission (+115.8%) and decrease CH4 uptake (−13.4%). The mean effects were stronger for N2O emission and weaker for CH4 uptake in China compared with other global sites, despite being statistically insignificant. Subtropical forest soils have the highest emission factor (2.5%) and may respond rapidly to N inputs; in relatively N-limited temperate forests, N2O and CH4 fluxes are less sensitive to N inputs. Factors including forest type, N form and rate, as well as soil pH, may also govern the responses of N2O and CH4 fluxes. Our findings pinpoint the important role of Southern Chinese forests in the regional N2O and CH4 budgets.

Bait input altered microbial community structure and increased greenhouse gases production in coastal wetland sediment

Coastal wetland reclamation contributed to development of aquaculture industry, and the residual bait accumulation in aquaculture processes could influence biogeochemical elements cycling, which threaten ecological functions and services in aquaculture and adjacent ecosystems. However, systematic studies for changes in sediment microbial community structure and greenhouse gasses (GHGs) production, as well as environmental parameters following bait input at time scale are still rare. A 90-day incubation experiment was conducted using sediment collected from coastal wetland in Qi'ao Island in southern China, followed by the observations of temporal variations of physicochemical properties, sediment microbial community, and GHGs production in response to different amounts of bait input (0, 20, and 40 mg bait g^-1 wet sediment). The results showed that dissolved oxygen of overlying water was profoundly decreased owing to bait input, while dissolved organic carbon of overlying water and several sediment properties (e.g., organic matter, sulfide, and ammonium) varied in reverse patterns. Meanwhile, bait input led to significant loss of microbial community richness and diversity, and strongly altered microbial compositions from aerobic, slow-growing, and oligotrophic (Actinobacteriota, Chloroflexi, and Acidobacteriota) to anaerobic, fast-growing, and copiotrophic (Firmicutes and Bacteroidota). Moreover, both GHGs production and global warming potential were significantly enhanced by bait input, implying that aquaculture ecosystem is an important hotspot for global GHGs emission. Overall, bait input triggered quick responses of physicochemical properties, sediment microbial community, and GHGs production, followed by long-term resilience of the ecosystem. This study could provide new insight into temporal interactive effects of bait input on physicochemical properties, microbial community, and GHGs production, which can enhance the understanding of the temporal dynamics and ecological impacts of coastal aquaculture activities and emphasize the necessity of sustainable assessment and management in aquaculture ecosystems.

Effects of Nitrogen and Phosphorus Additions on Soil N2O Emissions and CH4 Uptake in a Phosphorus-Limited Subtropical Chinese Fir Plantation

Increased nitrogen (N) inputs in subtropical forest ecosystems were widely reported. Extra N additions were reported to cause nutrient imbalance and phosphorus (P) limitation in many tropical and subtropical forests, and further result in changes in soil nitrous oxide (N2O) and methane (CH4) fluxes. Here, we conducted experiments with N (high N addition: 15 g N/m2, HN), P (low: 5 g P/m2, LP; high: 15 g P/m2, HP) and their interactive (HNLP and HNHP) treatments to investigate how N and P additions affected CH4 and N2O exchanges in an N-rich Chinese fir plantation (Cunninghamia lanceolata), and further explored the underlying mechanisms through the structural equation model (SEM) analysis. The results indicated that N addition alone (HN) significantly (p < 0.05) increased the soil N2O emissions by 30.15% and 80.47% over annual and 4-month periods, mainly owing to the elevated NH4+-N content. P addition alone (LP and HP) did not significantly affect the soil N2O emissions as compared with the control. The SEM analysis indicated that increased N2O emissions under N addition were primarily explained by the increase in available N and contributed more to the stimulated NH4+-N contents. N and P interactive additions slightly (not significant) stimulated the N2O emissions as compared with that under the N addition alone treatment. High-dose P addition significantly increased the soil CH4 uptake by 15.80% and 16.23% under the HP and HNHP treatments, respectively, while N addition alone and low P addition (LP and HNLP) did not significantly affect CH4 uptake as compared with the control. The increased water-soluble organic carbon and microbial biomass carbon explained the increased CH4 uptake under high P addition. The fertilization effects on N2O emissions and CH4 uptake mainly occurred in the first 4 months and diminished after that. Our results suggested that the direction, magnitude and timing of the N and P addition effects on N2O emissions and CH4 uptake would depend on the soil nutrient status and plant–microbial competition for N and P in subtropical forests.

Effects of nitrogen and phosphorus enrichment on soil N2O emission from natural ecosystems: A global meta-analysis

Nitrogen (N) and phosphorous (P) enrichment play an important role in regulating soil N₂O emission, but their interactive effect remains elusive (i.e. whether the effect of P or N enrichment on soil N₂O emission varies between ambient and elevated soil N or P conditions). Here, we conducted a Bayesian meta-analysis across the global natural ecosystems to determine this effect. Our results showed that P enrichment significantly decreased soil N₂O emission by 13.9% at ambient soil N condition. This N₂O mitigation is likely due to the decreased soil NO₃^--N content (-17.6%) derived by the enhanced plant uptake when the P limitation was alleviated by P enrichment. However, this P-induced N₂O (and NO₃^--N) mitigation was not found at elevated soil N condition. Additionally, N enrichment significantly increased soil N₂O emission by 101.4%, which was associated with the increased soil NH₄⁺-N (+41.0%) and NO₃^--N (+82.3%). However, the effect of N enrichment on soil N₂O emission did not differ between ambient and elevated soil P subgroups, indicating that the P-derived N₂O mitigation could be masked by N enrichment. Further analysis showed that manipulated N rate, soil texture, soil dissolved organic nitrogen, soil total nitrogen, soil organic carbon, soil pH, aboveground plant biomass, belowground plant biomass, and plant biomass nitrogen were the main factors affecting soil N₂O emission under N enrichment. Taken together, our study provides evidence that P enrichment has the potential to reduce soil N₂O emission from natural ecosystems, but this mitigation effect could be masked by N enrichment.

Warming-induced greenhouse gas fluxes from global croplands modified by agricultural practices: A meta-analysis

Climate warming increases the emissions of soil greenhouse gases (GHGs) by stimulating carbon (C) and nitrogen (N) processes in terrestrial ecosystems, contributing to climate change. However, the responses of soil GHG fluxes to warming from global agricultural ecosystems remain unknown. Here, we evaluate the effects of warming on soil GHG fluxes from global croplands under different agro-ecosystems, cropping systems, crop species, and N fertilizer levels, and determine the potential mechanisms through a meta-analysis of field observations. The results showed that warming (+2.0 °C on average) significantly enhanced soil carbon dioxide (CO₂) emissions (i.e., soil respiration) by 14.7% and nitrous oxide (N₂O) fluxes by 12.6% across croplands and increased soil methane (CH₄) uptake by 21.8% in uplands and CH₄ release by 23.4% in paddy fields. The responses of C gas fluxes to warming were regulated by initial C substrates, initial wetness, and changes in temperature in croplands. The responses of N₂O fluxes to warming were mainly associated with changed NH₄⁺-N and NO₃^--N as well as initial wetness and N fertilizer in croplands. The responses of soil GHG fluxes to warming were generally comparable among different crop species and N fertilizer levels, respectively. However, the responses of CO₂ emissions and CH₄ release to warming were significantly higher in upland-paddy fields than in uplands and paddy fields; the warming-induced changes in CH₄ release was significantly greater in rotation cropping systems than in single- and double-cropping systems. This synthesis highlights the important role of climate warming in increasing soil GHG fluxes from croplands, underscoring the critical need for agricultural practice adjustment to mitigate climate change in the future.

The Impact of Modifications in Forest Litter Inputs on Soil N2O Fluxes: A Meta-Analysis

Although litter can regulate the global climate by influencing soil N2O fluxes, there is no consensus on the major drivers or their relative importance and how these impact at the global scale. In this paper, we conducted a meta-analysis of 21 global studies to quantify the impact of litter removal and litter doubling on soil N2O fluxes from forests. Overall, our results showed that litter removal significantly reduced soil N2O fluxes (−19.0%), while a doubling of the amount of litter significantly increased soil N2O fluxes (30.3%), based on the results of a small number of studies. Litter removal decreased the N2O fluxes from tropical forest and temperate forest. The warmer the climate, the greater the soil acidity, and the larger the soil C:N ratio, the greater the impact on N2O emissions, which was particularly evident in tropical forest ecosystems. The decreases in soil N2O fluxes associated with litter removal were greater in acid soils (pH < 6.5) or soils with a C:N > 15. Litter removal decreased soil N2O fluxes from coniferous forests (−21.8%) and broad-leaved forests (−17.2%) but had no significant effect in mixed forests. Soil N2O fluxes were significantly reduced in experiments where the duration of litter removal was <1 year. These results showed that modifications in ecosystem N2O fluxes due to changes in the ground litter vary with forest type and need to be considered when evaluating current and future greenhouse gas budgets.

The Influence of Harbin Forest–River Ecological Corridor Construction on the Restoration of Mollisols in Cold Regions of China

Artificial ecological corridors (AECs) are internationally approved ecological restoration and climate mitigation strategies. The width and recovery time indices of AECs directly affect the restoration efficiency of degraded soil nutrients. However, there is a lack of comprehensive and quantitative evaluation research on the construction factors of AECs from the perspective of soil fertility improvement. This research aimed to examine the critical ecological corridor construction factors affecting Mollisols’ eco-chemometrics and give a scientific scope. We collected 55 Mollisol samples at different restoration years (0–35 years) and different distances (0–280 m) from the AEC of the Ashi River, a typical Mollisol restoration area in Harbin, and the cold regions of China. We measured the distances, restoration years, soil thickness, pH, electrical conductivity (EC), cation exchange capacity (CEC), soil total organic carbon (SOC), soil total organic matter (SOM), dry matter content (DMC), and the proportion of nitrogen (TN), phosphorus (TP) and potassium (TK). The results are as follows: (1) Within the AEC, there were significant differences in soil stoichiometric characteristics in different restoration years and locations; after restoration for 10–35 years, the soil stoichiometric characteristics reach or exceed the reference value of Mollisols. (2) It is feasible to restore large-scale degraded Mollisols through ecological corridors. In this recovery process, the soil nutrients first decreased, then increased, and finally reached and exceeded the reference value of normal Mollisols. (3) Soil nutrient accumulation was related to ecological corridor width and recovery time. The recommended unilateral width of the ecological corridor based on Mollisols’ CEC and SOC indices for restoration is 175–225 m, and the restoration period is 22.7–35 years based on Mollisols’ EC and SOC indices for restoration. This study demonstrated the change mechanism of Mollisols in AECs based on recovery time and location, and provided the basis for the Chinese government to formulate policies for Mollisol remediation.

Evidence, causes, and consequences of declining nitrogen availability in terrestrial ecosystems

The productivity of ecosystems and their capacity to support life depends on access to reactive nitrogen (N). Over the past century, humans have more than doubled the global supply of reactive N through industrial and agricultural activities. However, long-term records demonstrate that N availability is declining in many regions of the world. Reactive N inputs are not evenly distributed, and global changes-including elevated atmospheric carbon dioxide (CO₂) levels and rising temperatures-are affecting ecosystem N supply relative to demand. Declining N availability is constraining primary productivity, contributing to lower leaf N concentrations, and reducing the quality of herbivore diets in many ecosystems. We outline the current state of knowledge about declining N availability and propose actions aimed at characterizing and responding to this emerging challenge.

Stimulation of ammonia oxidizer and denitrifier abundances by nitrogen loading: Poor predictability for increased soil N2 O emission

Unprecedented nitrogen (N) inputs into terrestrial ecosystems have profoundly altered soil N cycling. Ammonia oxidizers and denitrifiers are the main producers of nitrous oxide (N₂ O), but it remains unclear how ammonia oxidizer and denitrifier abundances will respond to N loading and whether their responses can predict N-induced changes in soil N₂ O emission. By synthesizing 101 field studies worldwide, we showed that N loading significantly increased ammonia oxidizer abundance by 107% and denitrifier abundance by 45%. The increases in both ammonia oxidizer and denitrifier abundances were primarily explained by N loading form, and more specifically, organic N loading had stronger effects on their abundances than mineral N loading. Nitrogen loading increased soil N₂ O emission by 261%, whereas there was no clear relationship between changes in soil N₂ O emission and shifts in ammonia oxidizer and denitrifier abundances. Our field-based results challenge the laboratory-based hypothesis that increased ammonia oxidizer and denitrifier abundances by N loading would directly cause higher soil N₂ O emission. Instead, key abiotic factors (mean annual precipitation, soil pH, soil C:N ratio, and ecosystem type) explained N-induced changes in soil N₂ O emission. Altogether, these findings highlight the need for considering the roles of key abiotic factors in regulating soil N transformations under N loading to better understand the microbially mediated soil N₂ O emission.

Stimulation of ammonia oxidizer and denitrifier abundances by nitrogen loading: Poor predictability for increased soil N2 O emission

Unprecedented nitrogen (N) inputs into terrestrial ecosystems have profoundly altered soil N cycling. Ammonia oxidizers and denitrifiers are the main producers of nitrous oxide (N₂ O), but it remains unclear how ammonia oxidizer and denitrifier abundances will respond to N loading and whether their responses can predict N-induced changes in soil N₂ O emission. By synthesizing 101 field studies worldwide, we showed that N loading significantly increased ammonia oxidizer abundance by 107% and denitrifier abundance by 45%. The increases in both ammonia oxidizer and denitrifier abundances were primarily explained by N loading form, and more specifically, organic N loading had stronger effects on their abundances than mineral N loading. Nitrogen loading increased soil N₂ O emission by 261%, whereas there was no clear relationship between changes in soil N₂ O emission and shifts in ammonia oxidizer and denitrifier abundances. Our field-based results challenge the laboratory-based hypothesis that increased ammonia oxidizer and denitrifier abundances by N loading would directly cause higher soil N₂ O emission. Instead, key abiotic factors (mean annual precipitation, soil pH, soil C:N ratio, and ecosystem type) explained N-induced changes in soil N₂ O emission. Altogether, these findings highlight the need for considering the roles of key abiotic factors in regulating soil N transformations under N loading to better understand the microbially mediated soil N₂ O emission.

Effects of vegetation restoration types on soil nutrients and soil erodibility regulated by slope positions on the Loess Plateau

Soil degradation is significantly increased driven by soil nutrient loss and soil erodibility, thus, hampering the sustainable development of the ecological environment and agricultural production. Vegetation restoration has been widely adopted to prevent soil degradation given its role in improving soil nutrients and soil erodibility. However, it is unclear which vegetation type has the best improving capacity from soil nutrient and soil erodibility perspectives. This study selected three vegetation restoration types of grasslands (GL), shrublands (SL), and forestlands (FL) along the five slope positions (i.e., top, upper, middle, lower, and foot slope), to investigate the effects of vegetation restoration types on soil nutrients and soil erodibility. All vegetation restoration types were restored for 20 years from croplands (CL). We used comprehensive soil nutrient index (CSNI) and comprehensive soil erodibility index (CSEI) formed by a weighted summation method to reflect the effect of vegetation restoration on the improving capacity of soil nutrient and erodibility. The results showed the vegetation types with the highest comprehensive soil quality index (CSQI) at the top, upper, middle, lower and foot slope were FL (1.92), FL (1.98), SL (2.15), FL (2.37) and GL (3.93), respectively. When only one vegetation type was considered on the entire slope, SL (0.59) and FL (0.59) had the highest CSNI, the SL had the lowest CSEI (0.34) and the highest CSQI (1.89). The CSNI was mainly influenced by soil structure stability index (SSSI), sand content, silt + clay particles, and CSEI was controlled by soil organic matter (SOM), macroaggregates and microaggregates. Moreover, the CSQI was influenced by pH, silt and clay content, and biome coverage (BC). The study suggested the SL were advised as the best vegetation restoration type on the whole slope from improving soil nutrients and soil erodibility.

A global synthesis reveals increases in soil greenhouse gas emissions under forest thinning

Forest thinning is a major forest management practice worldwide and may lead to profound alterations in the fluxes of soil greenhouse gases (GHGs). However, the global patterns and underlying mechanisms of soil GHG fluxes in response to forest thinning remain poorly understood. Here, we conducted a global meta-analysis of 106 studies to assess the effects of forest thinning on soil GHG fluxes and the underpinning mechanisms. The results showed that forest thinning significantly increased soil CO₂ emission (mean lnRR: 0.07, 95% CI: 0.03-0.11), N₂O emission (mean lnRR: 0.39, 95% CI: 0.16-0.61) and decreased CH₄ uptake (mean Hedges' d: 0.98, 95% CI: 0.32-1.64). Furthermore, the negative response of soil CH₄ uptake was amplified by thinning intensity, and the positive response of soil N₂O emission decreased with recovery time after thinning. The response of soil CO₂ emission was mainly correlated with changes in fine root biomass and soil nitrogen content, and the response of soil CH₄ uptake was related to the changes in soil moisture and litterfall. Moreover, the response of soil N₂O emission was associated with changes in soil temperature and soil nitrate nitrogen content. Thinning also increased the total balance of the three greenhouse gas fluxes in combination, which decreased with recovery time. Our findings highlight that thinning significantly increases soil GHG emissions, which is crucial to understanding and predicting ecosystem-climate feedbacks in managed forests.

Effect of Nitrogen Addition on Soil Microbial Functional Gene Abundance and Community Diversity in Permafrost Peatland

Nitrogen is the limiting nutrient for plant growth in peatland ecosystems. Nitrogen addition significantly affects the plant biomass, diversity and community structure in peatlands. However, the response of belowground microbe to nitrogen addition in peatland ecosystems remains largely unknown. In this study, we performed long-term nitrogen addition experiments in a permafrost peatland in the northwest slope of the Great Xing'an Mountains. The four nitrogen addition treatments applied in this study were 0 g N·m-2·year-1 (CK), 6 g N·m-2·year-1 (N1), 12 g N·m-2·year-1 (N2), and 24 g N·m-2·year-1 (N3). Effects of nitrogen addition over a period of nine growing seasons on the soil microbial abundance and community diversity in permafrost peatland were analyzed. The results showed that the abundances of soil bacteria, fungi, archaea, nitrogen-cycling genes (nifH and b-amoA), and mcrA increased in N1, N2, and N3 treatments compared to CK. This indicated that nitrogen addition promoted microbial decomposition of soil organic matter, nitrogen fixation, ammonia oxidation, nitrification, and methane production. Moreover, nitrogen addition altered the microbial community composition. At the phylum level, the relative abundance of Proteobacteria increased significantly in the N2 treatment. However, the relative abundances of Actinobacteria and Verrucifera in the N2 treatment and Patescibacteria in the N1 treatment decreased significantly. The heatmap showed that the dominant order composition of soil bacteria in N1, N2, and N3 treatments and the CK treatment were different, and the dominant order composition of soil fungi in CK and N3 treatments were different. The N1 treatment showed a significant increase in the Ace and Chao indices of bacteria and Simpson index of fungi. The outcomes of this study suggest that nitrogen addition altered the soil microbial abundance, community structure, and diversity, affecting the soil microbial carbon and nitrogen cycling in permafrost peatland. The results are helpful to understand the microbial mediation on ecological processes in response to N addition.

Responses of soil N2 O emissions and their abiotic and biotic drivers to altered rainfall regimes and co-occurring wet N deposition in a semi-arid grassland

Global change factors such as changed rainfall regimes and nitrogen (N) deposition contribute to increases in the emission of the greenhouse gas nitrous oxide (N₂ O) from the soil. In previous research, N deposition has often been simulated by using a single or a series of N addition events over the course of a year, but wet N deposition actually co-occurs with rainfall. How soil N₂ O emissions respond to altered rainfall amount and frequency, wet N deposition, and their interactions is still not fully understood. We designed a three-factor, fully factorial experiment with factors of rainfall amounts (ambient, -30%) rainfall frequency (ambient, ±50%) and wet N deposition (with/without) co-occurring with rainfall in semi-arid grassland mesocosms, and measured N₂ O emissions and their possible biotic and abiotic drivers. Across all treatments, reduced rainfall amount and N deposition increased soil N₂ O emissions by 35% and 28%, respectively. A significant interactive effect was observed between rainfall amount and N deposition, and to a lesser extent between rainfall frequency and N deposition. Without N deposition, reduced rainfall amount and altered rainfall frequency indirectly affected soil N₂ O emissions by changing the abundance of nirK and soil net N mineralization, and the changes in nirK abundance were indirectly driven by soil N availability rather than directly by soil moisture. With N deposition, both the abundance of nirK and the level of soil water-filled pore space contributed to changes in N₂ O emissions in response to altered rainfall regimes, and the changes in the abundance of nirK were indirectly driven by plant N uptake and nitrifier (ammonia-oxidizing bacteria) abundance. Our results imply that unlike wetter grassland ecosystems, reduced precipitation may increase N₂ O emissions, and N deposition may only slightly increase N₂ O emissions in arid and semi-arid N-limited ecosystems that are dominated by grasses with high soil N uptake capacity.

A hierarchical, multivariate meta-analysis approach to synthesising global change experiments

Meta-analyses enable synthesis of results from globally distributed experiments to draw general conclusions about the impacts of global change factors on ecosystem function. Traditional meta-analyses, however, are challenged by the complexity and diversity of experimental results. We illustrate how several key issues can be addressed by a multivariate, hierarchical Bayesian meta-analysis (MHBM) approach applied to information extracted from published studies. We applied an MHBM to log-response ratios for aboveground biomass (AB, n = 300), belowground biomass (BB, n = 205) and soil CO₂ exchange (SCE, n = 544), representing 100 studies. The MHBM accounted for study duration, climate effects and covariation among the AB, BB and SCE responses to elevated CO₂ (eCO₂ ) and/or warming. The MHBM revealed significant among-study covariation in the AB and BB responses to experimental treatments. The MHBM imputed missing duration (4.2%) and climate (6%) data, and revealed that climate context governs how eCO₂ and warming impact ecosystem function. Predictions identified biomes that may be particularly sensitive to eCO₂ or warming, but that are under-represented in global change experiments. The MHBM approach offers a flexible and powerful tool for synthesising disparate experimental results reported across multiple studies, sites and response variables.

Nitrogen-fixing trees increase soil nitrous oxide emissions: a meta-analysis

Nitrogen-fixing trees are an important nitrogen source to terrestrial ecosystems. While they can fuel primary production and drive carbon dioxide sequestration, they can also potentially stimulate soil emissions of nitrous oxide, a potent greenhouse gas. However, studies on the influence of nitrogen-fixing trees on soil nitrous oxide emissions have not been quantitatively synthesized. Here, we show in a meta-analysis that nitrogen-fixing trees more than double soil nitrous oxide emissions relative to non-fixing trees and soils. If planted in reforestation projects at the global scale, nitrogen-fixing trees could increase global soil nitrous oxide emissions from natural terrestrial ecosystems by up to 4.1%, offsetting climate change mitigation via reforestation by up to 4.4%.

Nitrogen deposition accelerates greenhouse gas emissions at an alpine steppe site on the Tibetan Plateau

The increase in atmospheric N deposition can alter the exchange of greenhouse gas (GHG) between the biosphere and atmosphere. The effect of N deposition on GHG is poorly understood in alpine steppe. Therefore, we conducted a 4-year experiment with multilevel N addition of 0, 10, 20, 40, 80, 160 kg N ha^-1 using the static chamber and gas chromatography method to quantify responses of GHG fluxes and characterize the relationship between GHG fluxes and N addition rate in Namco located in the alpine steppe in the central Tibetan Plateau (TP). The mean CO₂ efflux, CH₄ uptake, N₂O emission during growing seasons across four years were 33.59 ± 16.41 mg C m^-2 h^-1, -56.42 ± 29.20 μg C m^-2 h^-1, 0.67 ± 4.40 μg N m^-2 h^-1 respectively. CO₂ efflux increased 1.55-22.6%, CH₄ uptake decreased 5.96-20.1%, N₂O emission increased 37.3-133.5% with N addition. We also observed a critical threshold of 40 kg N ha^-1 a^-1 of N addition above which the increase rate of N₂O emission would diminish. Our current results implied that under the background of future N deposition, N induced C sequestration might be offset by GHG balance causing a heating effect to the climate.

Long-term, amplified responses of soil organic carbon to nitrogen addition worldwide

Soil organic carbon (SOC) is the largest carbon sink in terrestrial ecosystems and plays a critical role in mitigating climate change. Increasing reactive nitrogen (N) in ecosystems caused by anthropogenic N input substantially affects SOC dynamics. However, uncertainties remain concerning the effects of N addition on SOC in both organic and mineral soil layers over time at the global scale. Here, we analysed a large empirical data set spanning 60 years across 369 sites worldwide to explore the temporal dynamics of SOC to N addition. We found that N addition significantly increased SOC across the globe by 4.2% (2.7%-5.8%). SOC increases were amplified from short- to long-term N addition durations in both organic and mineral soil layers. The positive effects of N addition on SOC were independent of ecosystem types, mean annual temperature and precipitation. Our findings suggest that SOC increases largely resulted from the enhanced plant C input to soils coupled with reduced C loss from decomposition and amplification was associated with reduced microbial biomass and respiration under long-term N addition. Our study suggests that N addition will enhance SOC sequestration over time and contribute to future climate change mitigation.

Effects of increasing organic nitrogen inputs on CO2, CH4, and N2O fluxes in a temperate grassland

Understanding future climate change requires accurate estimates of the impacts of atmospheric nitrogen (N) deposition, composed of both inorganic and organic compounds, on greenhouse gas (GHG) fluxes in grassland ecosystems. However, previous studies have focused on inorganic compounds and have not considered the potential effects of organic N sources. Here, we conducted a grassland experiment that included organic, inorganic N, and a mix of them at a ratio of 4:6, with two input rates, to study N inputs induced CO₂, CH_4, and N₂O fluxes, as well as the potential abiotic and biotic mechanisms driving the fluxes. We found that N compositions significantly affected fluxes each of the three GHGs. Greater organic N decreased the impacts of N addition on CO₂ and N₂O emissions, caused primarily by low rates of increase in substrates (soil available N) for production of CO₂ and N₂O resulting from high ammonia volatilization rather than changes in microbial activity. Also, greater organic N slightly stimulated CH₄ uptake. Nitrogen composition effects on CO₂ emissions and CH₄ uptake were independent of N input rates and measurement dates, but N₂O emissions showed stronger responses to inorganic N under high N addition and in June. These results suggest that future studies should consider the source of N to improve our prediction of future climate impact of N deposition, and that management of N fertilization can help mitigate GHG emissions.

Sources and trends of oxidized and reduced nitrogen wet deposition in a typical medium-sized city of eastern China during 2010-2016

Fluxes and composition dynamics of atmospheric nitrogen deposition play key roles in better balancing economic development and ecological environment. However, there are some knowledge gaps and difficulties in urban ecosystems, especially for small and medium-sized cities. In this study, both flux and composition (ratio of NH₄⁺-N to NO₃^--N, R_N) of wet-deposited dissolved inorganic nitrogen (DIN, sum of NO₃^--N and NH₄⁺-N) were estimated and sources were identified at a long-term urban observation station in Tongling, a typical medium-sized city in eastern China during 2010-2016, respectively. Results showed that wet-deposited DIN fluxes were 33.20 and 28.15 kgN ha^-1 yr^-1 in Tongling city during 2010-2011 and 2015-2016, respectively. Compared to these two periods, both DIN and NO₃^--N fluxes decreased by 15.2% and 31.8% for a series of NOx abatement measures applied effectively, respectively. At the same time, the NH₄⁺-N flux remained stable and ranged from 19.53 to 20.62 kgN ha^-1 yr^-1, and the R_N increased from 1.7 to 2.2. Seasonally, winds from the southwest and west-southwest with higher frequencies and speeds in spring and summer brought more NH₄⁺-N and DIN wet deposition from an ammonia plant, which could threaten the safety of regional hydrosphere ecosystems. On the whole, the wet-deposited NH₄⁺-N was threatening regional ecosystems of both the hydrosphere and forest. The wet-deposited DIN including NH₄⁺-N in Tongling city stemmed mainly from a combined source of coal combustion and dust from Cu extraction and smelting, ammonia production, and roads. Therefore, production lines should be updated for Cu extraction and smelting industries, thermal power generations and the ammonia plant, old vehicles should be eliminated, and the use of new energy vehicles should be promoted for regional sustainable development and human health in the medium-sized city.

Green Infrastructure Offset of the Negative Ecological Effects of Urbanization and Storing Water in the Three Gorges Reservoir Area, China

Land use planning usually increases the uncertainties of the ecosystem structures and functions because various human demands usually bring both positive and negative ecological effects. It is critical for estimating various land use changes and their ecological effects, but the previous studies have failed to decouple the respective and the combined effects of different land use changes on ecosystem services. Net primary productivity (NPP) could be used to indicate many ecosystem services such as carbon sequestration and storage. Here, we employed a light use efficiency model to estimate the spatial and temporal dynamics of NPP in the Three Gorges Reservoir (TGR) area from 2000 to 2015, and designed four scenarios to analyze the relative roles of afforestation, urbanization and storing water on NPP dynamics. Our results documented that terrestrial NPP of the TGR area increased from 547.40 gC•m^-2 to 629.96 gC•m^-2, and carbon sequestration capacities were 31.66 TgC (1Tg = 10¹²g) and 36.79 TgC in 2000 and 2015, respectively. Climate change and land use change both could contribute to carbon sequestration with 4.08 TgC and 1.05 TgC. Among these land use changes, only afforestation could sequester carbon with 2.04 TgC, while urbanization-induced and impoundment-induced emissions were 0.12 TgC and 0.32 TgC, respectively, and other land use changes also could release 0.55 TgC of carbon. This finding suggested that although positive and negative environmental effects happened simultaneously over the past decades, green infrastructure could effectively offset the carbon emissions from urbanization and storing water in the TGR area, which provides some fundamental supports for further ecological restoration and contributes to empowering land use policies towards carbon sequestration and storage at the regional scale.

Global nitrogen input on wetland ecosystem: The driving mechanism of soil labile carbon and nitrogen on greenhouse gas emissions

Greenhouse gas emissions from wetlands are significantly promoted by global nitrogen input for changing the rate of soil carbon and nitrogen cycling, and are substantially affected by soil labile carbon and nitrogen conversely. However, the driving mechanism by which soil labile carbon and nitrogen affect greenhouse gas emissions from wetland ecosystems under global nitrogen input is not well understood. Working out the driving factor of nitrogen input on greenhouse gas emissions from wetlands is critical to reducing global warming from nitrogen input. Thus, we synthesized 72 published studies (2144 paired observations) of greenhouse gas fluxes and soil labile compounds of carbon and nitrogen (ammonium, nitrate, dissolved organic carbon, soil microbial biomass nitrogen and carbon), to understand the effects of labile carbon and nitrogen on greenhouse gas emissions under global nitrogen input. Across the data set, nitrogen input significantly promoted carbon dioxide, methane and nitrous oxide emissions from wetlands. In particular, at lower nitrogen rates (<100 kg ha^-1·yr^-1) and with added ammonium compounds, freshwater wetland significantly promoted carbon dioxide and methane emissions. Peatland was the largest nitrous oxide source under these conditions. This meta-analysis also revealed that nitrogen input stimulated dissolved organic carbon, ammonium, nitrate, microbial biomass carbon and microbial biomass nitrogen accumulation in the wetland ecosystem. The variation-partitioning analysis and structural equation model were used to analyze the relationship between the greenhouse gas and labile carbon and nitrogen further. These results revealed that dissolved organic carbon (DOC) is the primary factor driving greenhouse gas emission from wetlands under global nitrogen input, whereas microbial biomass carbon (MBC) more directly affects greenhouse gas emission than other labile carbon and nitrogen.

Simulated nitrogen deposition influences soil greenhouse gas fluxes in a Mediterranean dryland

Soil nitrogen (N) availability is a key driver of soil-atmosphere greenhouse gas (GHG) exchange, yet we are far from understanding how increases in N deposition due to human activities will influence the net soil-atmosphere fluxes of the three most important GHGs: nitrous oxide (N₂O), methane (CH₄) and carbon dioxide (CO₂). We simulated four levels of N deposition (10, 20 and 50 kg N ha^-1 yr^-1, plus unfertilised control) to evaluate their effects on N₂O, CH₄ and CO₂ soil fluxes in a semiarid shrubland in central Spain. After 8 years of experimental fertilisation, increasing N availability led to a consistent increase in N₂O emissions, likely due to simultaneous increases in soil microbial nitrification and/or denitrification processes. However, only intermediate levels of N fertilisation reduced CH₄ uptake, while increasing N fertilisation had no effects on CO₂ fluxes, suggesting complex interactions between N deposition loads and GHG fluxes. Our study provides novel insight into the responses of GHGs to N deposition in drylands, forecasting increases in N₂O emissions, and decreases in CH₄ uptake rates, with likely consequences to the on-going climate change.

Increased soil release of greenhouse gases shrinks terrestrial carbon uptake enhancement under warming

Warming can accelerate the decomposition of soil organic matter and stimulate the release of soil greenhouse gases (GHGs), but to what extent soil release of methane (CH₄ ) and nitrous oxide (N₂ O) may contribute to soil C loss for driving climate change under warming remains unresolved. By synthesizing 1,845 measurements from 164 peer-reviewed publications, we show that around 1.5°C (1.16-2.01°C) of experimental warming significantly stimulates soil respiration by 12.9%, N₂ O emissions by 35.2%, CH₄ emissions by 23.4% from rice paddies, and by 37.5% from natural wetlands. Rising temperature increases CH₄ uptake of upland soils by 13.8%. Warming-enhanced emission of soil CH₄ and N₂ O corresponds to an overall source strength of 1.19, 1.84, and 3.12 Pg CO₂ -equivalent/year under 1°C, 1.5°C, and 2°C warming scenarios, respectively, interacting with soil C loss of 1.60 Pg CO₂ /year in terms of contribution to climate change. The warming-induced rise in soil CH₄ and N₂ O emissions (1.84 Pg CO₂ -equivalent/year) could reduce mitigation potential of terrestrial net ecosystem production by 8.3% (NEP, 22.25 Pg CO₂ /year) under warming. Soil respiration and CH₄ release are intensified following the mean warming threshold of 1.5°C scenario, as compared to soil CH₄ uptake and N₂ O release with a reduced and less positive response, respectively. Soil C loss increases to a larger extent under soil warming than under canopy air warming. Warming-raised emission of soil GHG increases with the intensity of temperature rise but decreases with the extension of experimental duration. This synthesis takes the lead to quantify the ecosystem C and N cycling in response to warming and advances our capacity to predict terrestrial feedback to climate change under projected warming scenarios.