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The effects of grazing on natural grasslands' plant composition, diversity, and productivity depend on the intensity of grazing. Besides grazing intensity, animal composition is also important. However, whether and how sheep grazing intensity affects the temporal biomass stability of plant communities is unclear. Here, we conducted a 5‐year grazing experiment to evaluate the effects of four grazing intensities on community biomass stability and the underlying mechanisms. Our results showed that the higher grazing intensity significantly decreased community biomass stability, community‐wide asynchrony, functional groups asynchrony, dominant species stability, and species dominance, but did not affect species richness. The results of structural equation modeling revealed that grazing decreased community biomass stability by decreasing dominant species stability and community‐wide asynchrony, which was attributable to the reduction in plant functional group asynchrony. Our results highlight the importance of functional group composition and dynamics in predicting the changes in community function in sheep grazing grassland ecosystems. Under continuous seasonal grazing conditions, the sustainable function and human services of grasslands in the agropastoral ecotone might decrease in the future. [ABSTRACT FROM AUTHOR]

The fraction of net primary productivity (NPP) allocated to belowground organs (fBNPP) in grasslands is a critical parameter in global carbon cycle models; moreover, understanding the effect of precipitation changes on this parameter is vital to accurately estimating carbon sequestration in grassland ecosystems. However, how fBNPP responds to temporal precipitation changes along a gradient from extreme drought to extreme wetness, remains unclear, mainly due to the lack of long‐term data of belowground net primary productivity (BNPP) and the fact that most precipitation experiments did not have a gradient from extreme drought to extreme wetness. Here, by conducting both a precipitation gradient experiment (100–500 mm) and a long‐term observational study (34 years) in the Inner Mongolia grassland, we showed that fBNPP decreased linearly along the precipitation gradient from extreme drought to extreme wetness due to stronger responses in aboveground NPP to drought and wet conditions than those of BNPP. Our further meta‐analysis in grasslands worldwide also indicated that fBNPP increased when precipitation decreased, and the vice versa. Such a consistent pattern of fBNPP response suggests that plants increase the belowground allocation with decreasing precipitation, while increase the aboveground allocation with increasing precipitation. Thus, the linearly decreasing response pattern in fBNPP should be incorporated into models that forecast carbon sequestration in grassland ecosystems; failure to do so will lead to underestimation of the carbon stock in drought years and overestimation of the carbon stock in wet years in grasslands. [ABSTRACT FROM AUTHOR]

Reabsorbing nutrients from senescent tissues before leaf falling has been recognized as a strategy to adapt to nutrient deficiency. However, how nutrient resorption modulates the nitrogen (N)–phosphorus (P) balance inside plants remains unclear, especially under increased soil N availability. We examined the impacts of N addition at varying rates (0–32 g N m−2 yr−1) on nutrient resorption and the performance of nutrient resorption on controlling the internal N–P balance in the leaf and stem of a dominant grass species, Leymus secalinus , in a saline–alkaline grassland in northern China. After 6 years of N addition, N concentration and N:P ratio in green and senesced tissues (leaf and stem) rose with increasing N addition. The P concentration in green tissues decreased, but did not significantly change in senesced tissues with increasing N addition. The N resorption efficiency (NRE), P resorption efficiency (PRE), and NRE:PRE ratio significantly decreased along the N addition gradient. Moreover, we found more sensitive responses of N:P ratio in senesced tissues than in green tissues; such exacerbation of plant internal N–P imbalances mainly resulted from a disproportionate reduction in nutrient resorption, especially NRE. Overall, our study suggested that differences in NRE and PRE further exacerbated the internal N–P imbalances in plant litters. [ABSTRACT FROM AUTHOR]

As precipitation (PPT) regimes are becoming more extreme worldwide, the inter‐annual variation in PPT amount at a given location is becoming greater than ever before. Yet, our understanding of grassland responses to extreme drought and wet years, as well as the pattern of the relationship between PPT and above‐ground net primary productivity (ANPP) along a PPT gradient from extreme drought to extreme wetness scenarios, remains unclear, hampering accurate forecasting of the grassland feedbacks to future PPT changes.We addressed this research gap by a 4‐year, multi‐level PPT gradient experiment that included PPT amounts simulating extremely dry and wet years in combination with a long‐term (40 years) observational study in a grassland ecosystem.We found that the ANPP reductions due to extreme droughts were greater than the magnitude of positive responses to extreme wet years. The PPT‐ANPP relationship along the gradient of PPT followed a negative asymmetric (concave‐down) pattern. We identified the major mechanisms for this pattern were a shift in primary limiting resource for ANPP from water to nitrogen with increasing PPT and different responses of the plant functional groups presented in the community. With the increase of PPT from extremely low to extremely high levels, the relative abundance of forbs increased, while that of grasses decreased and annuals was not significantly affected.This study illustrates how a grassland responded to PPT extremes and highlights the role of shifts in resource limitation in affecting the PPT‐ANPP relationship. The negative asymmetric pattern suggests that the reduction of ANPP in extremely drought years cannot be mitigated by its positive response to extremely wet years. Thus, to accurately forecast future ecosystem feedback to climate change, we should take into account the negative asymmetric feature of the ecosystem response to stronger climate variation, as well as recognize that the resource limitations will be fundamentally altered by increasing frequency and intensity of climate extremes in the future. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

Anthropogenic disturbance, such as nitrogen (N) fertilization and mowing, is constantly changing the function and structure of grassland ecosystems during past years and will continue to affect the sustainability of arid and semiarid grassland in the future. However, how and whether the different N addition levels and the frequency of N addition, as well as the occurrence of mowing, affect the key processes of N cycling is still unclear. We designed a field experiment with five levels of N addition (0, 2, 10, 20, and 50 g N m−2 yr−1), two types of N addition frequencies (twice a year added in June/November and monthly addition), and mowing treatment in a typical grassland of northern China. The results showed that higher N addition and mowing interactively improved net primary productivity (NPP), including aboveground and belowground biomass, while different N addition frequency had no significant effects on NPP. Different N addition levels significantly improved gross ammonification (GA) and nitrification (GN) rates, which positively correlated to aboveground net primary productivity (ANPP). However, the effect of N addition frequency was differentiated with N addition levels, the highest N addition level (50 g N m−2 yr−1) with lower frequency (twice a year) significantly increased GA and GN rates. Mowing significantly increased the GA rate but decreased the GN rate both under the highest N addition level (50 g N m−2 yr−1) and lower N addition frequency (twice a year), which could improve N turnover by stimulating plant and microbial activity. However, a long-term study of the effects of N enrichment and mowing on N turnover will be needed for understanding the mechanisms by which nutrient cycling occurs in typical grassland ecosystems under global change scenarios. [ABSTRACT FROM AUTHOR]

Soil net nitrogen (N) mineralization is a key biogeochemical process influencing plant available N and net primary productivity in terrestrial ecosystems. However, the spatial variations and controlling factors of soil net N mineralization (RPNM) in arid and semi‐arid grasslands are less studied and unclear.In this study, we investigated the soil RPNM by performing a laboratory incubation experiment. Soil samples were collected from 30 sites in three east–west transects on the Inner Mongolia Plateau (MP), Loess Plateau (LP) and Tibetan Plateau (TP) along a 3200 km arid and semi‐arid grassland gradient, with each transect containing three different grassland types (meadow steppe [MS], typical steppe [TS] and desert steppe [DS], respectively).Results showed that the average RPNM values ranged from −0.37 to 1.29 mg N kg−1 day−1, with a significantly lower RPNM found in the DS (0.08 ± 0.01 mg N kg−1 day−1) compared with those in the MS (0.30 ± 0.03 mg N kg−1 day−1) and in the TS (0.33 ± 0.03 mg N kg−1 day−1) in the MP and LP transects (p < 0.05). This difference could be explained by variations in climatic and soil factors, such as hydrothermal index (HT), the soil pH, soil organic matter (SOM) and precipitation. However, no significant differences in RPNM were found among different grassland types in the TP transect, possibly due to the similarly low microbial activity, as indicated by the microbial biomass carbon values. Across all three grassland transects, HT, SOM and microbial variables were the major factors controlling RPNM, which together explained 20.7% of the variation in RPNM. Further structural equation model analysis indicated HT was an integral predictor of RPNM, directly or indirectly via SOM, under different conditions of precipitation and temperature.Our findings provide field evidence and parameters for biogeochemical cycling to better predict future N transformation processes under changing precipitation and temperature regimes across a wide range of arid and semi‐arid grassland ecosystems. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

The increase in atmospheric nitrogen (N) deposition has profound effects on soil respiration (SR). However, the responses of SR to the addition of different N compounds, particularly in saline–alkaline grasslands remain unclear. A 3-year controlled field experiment was conducted to investigate the responses of SR to different N compounds (NH4NO3, (NH4)2SO4 and NH4HCO3) during the growing seasons in a saline–alkaline grassland located in the agro-pastoral ecotone of northern China. Our results demonstrated that SR showed a bimodal pattern and a significant interannual difference that was regulated by air or soil temperature and precipitation. Nitrogen addition had a significant effect on SR, and the effect of N addition on SR varied yearly, which was related to seasonal precipitation. The mean SR across 3 years (2017–2019) was increased by 19.9%, 13.0% and 16.6% with the addition of NH4NO3, (NH4)2SO4 and NH4HCO3, respectively. The highest effect of NH4NO3 addition on SR across 3 years was ascribed to the highest aboveground net primary production, belowground net primary production (BNPP) and soil NO3− concentrations. SR (C loss) was significantly increased while plant productivity (C input) did not significantly change under NH4HCO3 addition, indicating a decrease in C sequestration. In addition, BNPP was the main direct factor influencing SR in this saline–alkaline grassland, and soil salinization (e.g. soil base cations and pH) indirectly affected SR through soil microorganisms. Notably, NH4NO3 addition overestimated the response of SR to N addition, and different N compounds should be considered, especially in saline–alkaline grassland. [ABSTRACT FROM AUTHOR]

Aims: Litter decomposition is a crucial component of nutrient recycling. Short-term nitrogen (N) deposition has been shown to influence litter decomposition in temperate steppe with significant variability due to differences in atmospheric N deposition, species identity, and experimental duration. Therefore, the effect of N addition, especially long-term, on litter decomposition in alpine grassland still needs further investigation. Methods: To address these knowledge gaps, we examined the influence of long-term N addition on litter decomposition, taking advantage of a field experiment with five N addition levels (0, 10, 30, 90, and 150 kg N ha−1 yr−1) with a meta-analysis, which has been running for 11 years in an alpine grassland, Northwest China. Results: Long-term N addition consistently decreased litter decomposition rates, and N negative effect became stronger with the increasing N addition rates. Reduced litter decomposition rates were related to lower soil enzymes activities. Litter decomposition rates were strongly correlated with litter quality, but weakly correlated with soil quality, but which suggested that litter quality and soil quality played important role in regulating litter decomposition. Furthermore, a regional meta-analysis revealed that N addition accelerated litter decomposition when all data were averaged. Although N addition indirectly increased litter decomposition, it had no direct effect on decomposition. However, the direction and degree of the direct effect of N on litter decomposition were regulated by N addition rate, experimental duration and form of N fertilizer. Conclusions: Overall, these results demonstrated that long-term N addition decreased litter decomposition and N negative effect increased over time. [ABSTRACT FROM AUTHOR]

The pools of plant community carbon (C) and nitrogen (N) are important sources of soil organic matter in terrestrial ecosystems and directly affect soil C and N cycling. A large amount of studies were manipulated with multiple N levels on soil C and N pools. However, how and whether the frequency of N addition can affect the plant C and N pools is still unclear. In order to comprehensively understand the N addition effects (including frequencies and levels) on C and N pools of the plant community, we executed a randomized complete block experiment with the addition of five levels of N, including 0, 2, 10, 20 and 50 g N m−2 yr−1 (designated as N-0, N-2, N-10, N-20 and N-50) and two N addition frequencies (twice a year vs. monthly, F2, F12) in August of 2008. After 5 years of treatment, the physical-chemical properties of the plants and soil were measured in 2013. The results indicated that with increasing N addition levels, the C and N pools of the plant community significantly increased, while N addition frequency had no significant effects. Moreover, significant interactions between N addition levels and the frequencies on the C and N pools of the plant community were also found in this typical grassland. Under different frequencies of N addition treatment, the plant community C and N pools showed different response patterns along with N addition levels in plants aboveground and belowground, respectively. Under different frequencies of N addition, the changes in the C and N pools of the plant community caused by N addition were regulated by different environmental factors. We highlight that long-term N deposition could affect the plant community C and N pools and would influence C and N cycling of terrestrial ecosystems based on global climate change in the future. [ABSTRACT FROM AUTHOR]

Aims Competition among plants in a community usually depends on their nitrogen (N)-use efficiency (NUE) and water-use efficiency (WUE) in arid and semi-arid regions. Artemisia frigida is an indicator species in heavily degraded grassland, however, how its NUE and WUE respond to N addition in different successional stages is still unclear, especially with mowing, a common management practice in semi-arid grasslands. Methods Based on a long-term controlled experiment with N addition and mowing in an abandoned cropland from 2006 to 2013, we investigated the NUE and WUE of A. frigida in two patches (i.e. grass and herb patches) in 2013 which represented two potential successional stages from herb to grass communities. The coverage of A. frigida was higher (about 50%) in the herb patch than in the grass patch (about 10%). Stable isotopic C (δ 13C) and N (δ 15N) as well as C and N pools were measured in plants and soils. NUE was calculated as leaf C/N, and leaf δ 13C values were used as a proxy for WUE. Important Findings N addition did not affect WUE of A. frigida , but significantly decreased NUE by 42.9% and 26.6% in grass and herb patches, respectively. The response of NUE to N addition was related to altering utilization of different N sources (NH4+ vs. NO3−) by A. frigida according to the changed relationship between leaf δ 15N/soil δ 15N and NUE. Mowing had no effect on NUE regardless of N addition, but significantly increased WUE by 2.3% for A. frigida without N addition in the grass patch. The addition of N reduced the positive effect of mowing on its WUE in grass patch. Our results suggested that decreased NUE and/or WUE of A. frigida under mowing and N addition could reduce its competition, and further accelerate restoration succession from the abandoned cropland to natural grassland in the semi-arid region. [ABSTRACT FROM AUTHOR]

While emissions of nitric oxide (NO), ammonia (NH3) and nitrous oxide (N2O) from grassland soils have been increasingly well constrained, soil dinitrogen (N2) emissions are poorly understood. However, N2 losses might dominate total gaseous nitrogen (N) losses. Knowledge on N losses is key for the development of climate-adapted management that balances agronomic and environmental needs. Hence, we quantified all gaseous N losses from a montane grassland in Southern Germany both for ambient climatic conditions and for a climate change treatment (+ 2 °C MAT, − 300 mm MAP). Monthly measurements of soil N2 emissions of intact soil cores revealed that those exceeded by far soil N2O emissions and averaged at 350 ± 101 (ambient climate) and 738 ± 197 µg N m−2 h−1 (climate change). Because these measurements did not allow to quantify emission peaks after fertilization, an additional laboratory experiment was deployed to quantify the response of NH3, NO, N2O, and N2 emissions in sub daily temporal resolution to a typical slurry fertilization event (51 kg N ha−1). Our results revealed that total N gas losses amounted to roughly half of applied slurry-N. Surprisingly, N2 but not NH3 dominated fertilizer N losses, with N2 emissions accounting for 16-21 kg or 31-42% of the applied slurry-N, while NH3 volatilization (3.5 kg), N2O (0.2-0.5 kg) and NO losses (0-0.2 kg) were of minor importance. Though constraining annual N2 loss remained uncertain due to high spatiotemporal variability of fluxes, we show that N2 losses are a so far overlooked key component of the N balance in montane grasslands, which needs to be considered for developing improved grassland management strategies targeted at increasing N use efficiency. [ABSTRACT FROM AUTHOR]

Aims Nitrogen (N) enrichment caused by human activities threatens biodiversity and alters plant community composition and structure. It has been found that heavy and infrequent N inputs may over-estimate species extinction, but it remains unclear whether plant community structure will equally respond to frequent reactive N enriched conditions. Methods We independently manipulated the rates and the frequencies of N addition in a temperate steppe, northern China, between 2008 and 2013. Important Findings We found that plant community structure changes, measured by 'Euclidean distance' involving species richness, composition and productivity, were significantly positively related to increasing N enrichment rates rather than frequencies. Changes in aboveground net primary productivity (ANPP), plant species richness and shifts in dominant species were observed. Community ANPP increased with N enrichment, whereas species richness reduced. The frequency of N enrichment increased species richness but had no impacts on community ANPP and the relative ANPP of the two dominant species, C3 perennial bunchgrass Stipa grandis and C3 perennial rhizome grass Leymus chinensis. The ANPP and relative ANPP of the two dominant species were significantly negatively correlated with each other. Moreover, changes in the relative ANPP of S. grandis was negatively associated with the changes in community structure. After 5 years' treatment, direct influence of the frequency of N enrichment on plant community structure was not observed, but the effects of the rate of N enrichment were apparent. Our results suggested that further study in various ecosystems and with long-term and well-controlled comparisons the frequency vs. the rate of N enrichment may still be needed. [ABSTRACT FROM AUTHOR]

Astragalus mongholicus Bunge (A.M.B), Potentilla anserina (A.P), Festuca ovina (F.O) The original article has been corrected. The original article can be found online at https://doi.org/10.1007/s11104-022-05537-8. [Extracted from the article]

Aims Vegetation type is important in determining variations in soil carbon (C) efflux under grassland managements. This study was conducted to examine the effects of mowing and nitrogen (N) addition on soil respiration and their dependences upon vegetation types in an oldfield grassland of northern China. Methods Soil respiration, temperature, moisture and aboveground net primary productivity (ANPP) and belowground net primary productivity (BNPP) were examined in response to mowing and N addition among the three patches dominated by different species (named as grass, forb and mixed patches, respectively) in the growing seasons (May–October) from 2006 to 2008. Important Findings Across the 3 years, soil respiration in the grass patch was greater than those in the forb and mixed patches, which could have been ascribed to the higher soil moisture (SM) in the grass patch. Mowing had no impact on soil respiration due to unaltered SM and plant growth. Soil respiration was stimulated by 6.53% under N addition, and the enhancement was statistically significant in 2006 but not in 2007 or 2008 because of the limited water availability in the later 2 years. There were no interactive effects between mowing and N addition on soil respiration. Soil respiration showed positive dependence upon SM, ANPP and BNPP across plots. The results suggest that soil water availability and plant growth could be the primary factors in controlling the temporal and spatial variations in soil respiration and its response to different treatments. Our observations indicate that grassland managements (i.e. mowing for hay once a year) may have little influence on soil respiration of the oldfield grassland in northern China. [ABSTRACT FROM PUBLISHER]

Grazing exclusion (GE) is an efficient management practice that prevents grassland degradation. The temperature sensitivity (Q 10) of soil net nitrogen (N) mineralization (N min) is an important parameter of N cycles, and it directly influences the feedback between the primary productivity of grassland ecosystems and climate warming. However, information remains limited about how GE affects the Q 10 of N min in N-limited grasslands. In this study, we conducted a laboratory incubation experiment at six temperatures (−10, 0, 5, 15, 25, and 35 °C) and three soil moisture contents (15%, 25%, and 35%) for 35 days by using intact soil cores from three different GE periods grassland (0, 4 and 24 years). The results showed that long-term GE decreased the Q 10 of N min , which was highest in GE0 (2.62), followed by GE4 (2.36) and GE24 (2.42). In addition, Q 10 declined in 15% soil moisture content under all three GE treatments. Furthermore, activation energy (E a) was negatively correlated with the substrate quality index (Q) in all three GE grasslands. These results support the carbon quality-temperature (CQT) hypothesis, showing that is can be applied to soil N mineralization in grasslands after long-term GE. This study demonstrated that GE potentially reduces the sensitivity of N min to increasing temperature under global warming scenarios, which might help prevent N losses in these N-limited grasslands. • Long-term grazing exclusion decrease the soil nitrogen mineralization rate. • Temperature sensitivity decrease with the duration of grazing exclusion. • Carbon quality-temperature hypothesis is applicable to soil N mineralization. [ABSTRACT FROM AUTHOR]

As the largest portion of the terrestrial ecosystems, the arid and semi-arid grassland ecosystem is relatively sensitive and vulnerable to nitrogen (N) deposition. Mowing, the main management in Inner Mongolia grassland also has deep direct and indirect effect on N transformation by removing the nutrient from soils. However, the interaction effect of N addition and mowing on N transformation is still unclear, especially in semi-arid grassland. Here, we conducted a field-manipulated experiment to assess N addition (10 g N m−2 y−1) and mowing (in the middle of August) effects on soil net N mineralization rate across 4 growing seasons (2006–2009) in a semi-arid grassland in Inner Mongolia of northern China. We found that N addition with or without mowing led to significant effect on soil ammonification rate and net N mineralization rate, but had no significant effect on nitrification rates. Furthermore, mowing had no significant effect on soil net N mineralization, ammonification and nitrification rates. N addition and Mowing decreased microbial respiration and metabolic quotient, whereas the interaction of N addition and mowing had no significant effect on microbial respiration and metabolic quotient. Our results indicated that the effects of mowing and N addition did not interactively weaken soil net N mineralization rates in a semi-arid grassland of Northern China. Therefore, the anthropic management (i.e. mowing for hay once a year) with N addition may be a sustainable approach for restoration and reconstruction of vegetation in the abandoned grassland of Northern China. [ABSTRACT FROM AUTHOR]

[Display omitted] • The bacterial MRC/SOC was regulated by the climate, soil, plant, and microbial factors. • The bacterial MRC/SOC was negatively correlated with the phosphorus stoichiometry. • The fungal MRC/SOC was notably correlated with mean annual temperature. • The fungal MRC/SOC benefitted from the high-quality substrate. Soil microbial residue carbon (MRC) plays an important role in the accumulation and stabilization of soil organic carbon (SOC) in grassland ecosystems. Numerous studies to date show that soil bacterial MRC and fungal MRC have different effects on SOC stability, however, there is a lack of understanding about what regulates the contributions of the bacterial MRC and fungal MRC to SOC in large-scale grassland ecosystems. In this study, we selected nine alpine and temperate grassland ecosystems to quantify the effects of biotic (plant and microbial) and abiotic (climate and soil) factors on the bacterial and fungal MRC/SOC. The results showed that 1) Both biotic and abiotic factors explained 97% and 89% of the variations in the bacterial MRC/SOC and fungal MRC/SOC, respectively. Compared with the fungal MRC/SOC, the bacterial MRC/SOC could be explained more by the biotic and abiotic factors. 2) The bacterial MRC/SOC was clearly correlated with climate and increased as the mean annual temperature increased and decreased as the mean annual precipitation increased. The bacterial MRC/SOC was negatively correlated with the soil phosphorus stoichiometry. 3) The fungal MRC/SOC was significantly correlated with mean annual temperature and was negatively correlated with the ratio of carbon to nitrogen. Our study demonstrates that the bacterial and fungal MRC/SOC respond differently to environmental changes, which can contribute to the regulation of soil carbon sequestration in grassland ecosystems by modulating different soil microbial communities under climate change. [ABSTRACT FROM AUTHOR]

• We addressed soil carbon fluxes in saline-alkaline grasslands. • Light grazing showed a negative effect on SR h. • Seasonal fluctuation of grazing effects resulted in unaltered seasonal mean SR. • Annual precipitation regulated the interannual variation of soil C emissions. • Soil environment explained SR more than plants and soil microorganisms. Grazing is a prevalent management strategy in grassland ecosystems that has profound effects on plants and soil microorganisms, consequently altering soil carbon (C) fluxes. However, the difference responses and regulatory mechanisms of soil autotrophic respiration (SR a) and heterotrophic respiration (SR h) to different grazing intensities remain unclear, especially in saline-alkaline grasslands. A six–year (2017–2022) field experiment involving four grazing intensities (no grazing, light grazing, moderate grazing, and heavy grazing) was conducted in a saline-alkaline grassland located in the agro-pastoral ecotone of northern China. Soil total respiration (SR tot) and its components (SR a and SR h), and the related biotic and abiotic factors (plants, soil microorganisms, and soil environment) were measured. The results showed that light grazing significantly decreased the mean SR h by 17.7% across six measuring years. The effects size of grazing on soil respiration fluctuated consistently with soil water content during the growing seasons (May–September), resulting in a neutral effect of grazing on the seasonal mean SR a. The cumulative soil C emissions of the current year varied with the seasonal precipitation in the previous year, whereas the changes in cumulative soil C emissions induced by grazing showed the same trends as annual precipitation. In addition, the key control factors for SR h and SR a differed and varied with grazing intensity and the soil environment played a critical role in regulating soil C fluxes. In conclusion, light grazing reduces soil microbial-driven C emissions and the soil environment and precipitation co-regulate the response of soil C fluxes to different grazing intensities in saline-alkaline grasslands. [ABSTRACT FROM AUTHOR]

• The N threshold of the unimodal response of CH 4 uptake was ∼10 g N m−2 yr−1. • The critical N addition rate to +/- abundance of pmoA gene was ∼7.0 g N m−2 yr−1. • N -induced changes in the plant community affect pmoA gene abundance through rhizospheric effects. Methane (CH 4) uptake in dryland ecosystems is a pathway in mitigating atmospheric CH 4 concentrations. A number of studies have demonstrated that nitrogen availability is a key regulator of the CH 4 oxidation process. However, how plants mediate soil CH 4 oxidation in response to N addition, particularly in saline-alkaline grasslands in the vast regions of the northern China, has received little attention. We conducted a 3-yr (2017–2019) in-situ experiment with eight N addition levels to examine the effect of N on soil CH 4 oxidation, and to explore the linkages between the N -induced changes of plant community and the seasonal dynamics of CH 4 in a saline-alkaline grassland. We found that the saline-alkaline grassland was a weak CH 4 sink displaying nonlinear CH 4 uptake levels in response to N additions, possibly due to the nonlinear changes in the abundance of key functional genes (pmoA) that were responsible for CH 4 oxidation. The changes in plant productivity and diversity that were induced by N additions explained 21 % of the variations in CH 4 uptake. The N -induced increase in productivity indirectly enhanced CH 4 uptake at N addition rates of <10 g m−2 yr−1, while the decrease in biodiversity indirectly inhibited CH 4 uptake when N addition exceeded 10 g m−2 yr−1. The N -induced changes in the plant community can affect CH 4 uptake, mainly through the rhizospheric effects of plants. In conclusion, our findings underscore the importance of rhizosphere when assessing the CH 4 uptake in saline-alkaline grassland ecosystems. [ABSTRACT FROM AUTHOR]

• There was an interaction between N addition and precipitation change on R s and R a. • R h was insensitive to N addition and precipitation change in saline-alkaline grassland. • R a was controlled by plants biomass, while R h was controlled by ST and soil pH. • Soil salinization regulated the effects of N addition and precipitation change on R s. The responses of soil total respiration (R s) and its components (soil autotrophic respiration (R a) and heterotrophic respiration (R h)) to nitrogen (N) deposition have been widely evaluated in non-saline-alkaline grasslands. However, their responses to N addition under drought and wet conditions, especially in saline-alkaline grasslands, remain unclear. A two-factorial experiment involving N addition and precipitation changes (decreased or increased 50% precipitation relative to ambient) was conducted in a saline-alkaline grassland of Northern China, while soil respiration was measured in different treatment plots during two growing seasons (2018, 2019). Results showed that the addition of N or changes in precipitation alone had no significant effect on R s , R a or R h in both years because root productivity and soil microbial biomass were not affected. However, N addition with increased precipitation synergistically augmented seasonal mean R s and R a by 25.7% and 46.8% in 2018, and by 42.4% and 89.3% in 2019, respectively, owing to the increase in plant productivity. The variation in R a primarily contributed to variation in R s , and the effect size of N addition on R s and R a were increased with precipitation. In addition, a structural equation model showed that the response of soil respiration to N addition and precipitation changes was regulated by soil salinization, in which the R a was regulated by soil based cations while R h was controlled by soil pH values. Our study highlights that N addition with increased precipitation preferentially affects plants rather than microorganisms, and that R h was not sensitive to N addition and precipitation changes in saline-alkaline grassland. [ABSTRACT FROM AUTHOR]

Gross ammonification (GA) and gross nitrification (GN) are key regulators of the bioavailability of nitrogen (N) in terrestrial ecosystems. In arid and semi-arid grassland ecosystems, the impacts of precipitation change on in situ GA and GN and their seasonal variations are understudied. A manipulative experiment with five precipitation levels (−60%, −30%, unchanged ambient precipitation as control, +30%, +60% of ambient precipitation) was set up in May 2012. After a 2-year equilibration period, the 15N pool dilution technique was applied to determine GA and GN rates in surface soil (0–5 cm) within 8 sampling dates ranging from the onset to the end of the growing season. Pronounced temporal variability of GA and GN rates were observed with the highest GA and GN rates in the mid-growing season (August) and the lowest in the post-growing season (October). GA rates during the growing season were primarily controlled by precipitation and soil water availability, while this effect diminished at the onset and end of the growing season when the temperature dropped. In contrast, GN was predominately controlled by ammonium (NH 4 +) production through GA. In the peak-growing season, the +60% precipitation treatment enhanced GA and GN rates by up to 4-fold, while the −30% precipitation treatment marginally suppressed GA and GN rates. The −60% precipitation treatment suppressed GA and GN rates to below detection limits across the entire study period, suggesting that extreme and persistent droughts greatly inhibited N turnover in the surface soil, thereby forcing plants to mobilize and access nutrients from deeper soil layers. Our findings provide a season-specific mechanistic insight into precipitation impacts on topsoil gross N turnover and suggest that the N cycling represented in ecosystem models needs to consider a mechanism that allows a strong suppression of microbial functions under extreme droughts. • Gross ammonification/nitrification (GA/GN) was strengthened by precipitation. • Temperature is limiting GA before and after the growing season. • GN is tightly coupled to substrate supply by GA. • Extreme drought with 60% precipitation reduction might terminate GA and GN. [ABSTRACT FROM AUTHOR]