| The way and rate of nitrogen input from the atmosphere to terrestrial ecosystems have been greatly affected by the widespread application of nitrogen fertilizers and the large-scale combustion of fossil fuels.The increase of nitrogen deposition has a series of effects on nutrient cycling and carbon sequestration function of grassland ecosystems.As one of the main utilization and management methods of grassland ecosystem,grazing plays a unique and important role in regulating the structure and function of grassland ecosystem.In addition,grazing could alter the response intensity and direction of soil organic carbon storage to nitrogen deposition in grassland ecosystem.Understanding the response of topsoil organic carbon storage to nitrogen deposition and grazing,as well as the regulatory mechanisms of microbial community composition and metabolism on soil organic carbon dynamics in grassland ecosystem under nitrogen deposition and grazing,are essential for predicting soil organic carbon storage in the context of global change and grassland management.Here,to elucidate the regulatory mechanisms of soil microbial community composition and metabolism on topsoil soil organic carbon storage under the effects of nitrogen addition and grazing,we conducted a manipulative experiment with control(CK),nitrogen addition(N),grazing(G)and nitrogen addition plus grazing(NG)treatments in a Leymus chinensis meadow steppe,in northeastern China.Firstly,we measured soil microbial community composition and respiration,plant biomass,topsoil organic carbon storage and a series of soil micro-environmental and physicochemical properties during the growing season(spring,summer and fall)for two consecutive years(2018-2019),to explore the response of topsoil organic carbon storage to the nitrogen addition and grazing treatments.Secondly,we quantified and mapped the spatial patterns of topsoil organic carbon storage,soil microclimate,plant biomass,soil nutrients,soil microbial community structure and enzyme activities at fine scale(15 m × 15 m)using geostatistical analysis.We examined the response of spatial heterogeneity in topsoil organic carbon storage to the nitrogen addition and grazing treatments and identified key factors that regulated spatial heterogeneity in topsoil organic carbon storage.Thirdly,after four years of experimental treatments,soil samples from the field plots were collected.Then,we added flesh C into soil samples and ran a 70-day incubation experiment.Using isotope labeling technique,we calculated priming effect and net C balance and quantified the carbon use efficiency and assimilation capacity of soil microbial community to reveal the response of soil organic carbon stability and soil carbon retention capacity to the nitrogen addition and grazing treatments,further clarifying the response of topsoil organic carbon pool to flesh C addition and microbial regulation mechanisms.The main results are as follows:(1)Nitrogen addition and grazing regulated the topsoil organic carbon storage via their effects on plant and microbial factors.Compared with the CK treatment,topsoil organic carbon storage decreased by 8.7% and 15.5% under the N treatment,increased by 5.4% and 4.5% under the G treatment and decreased by 16.2% and 16.1% under the NG treatment in2018 and 2019,respectively.In this study,the N addition increased the cumulative microbial respiration C emission(by 3.7% in 2018 and 2.7% in 2019),and further decreasing SOC storage in topsoil.The grazing treatment reduced cumulative microbial respiration carbon emissions(by 8.7% and 4.5% in 2018 and 2019,respectively),thereby increasing topsoil organic carbon storage.The response of topsoil organic carbon storage to the grazing plus nitrogen addition treatment largely depends on nitrogen addition.In addition,the grazing treatment induced the changes in root:shoot ratio,further increasing the ratio of bacterial biomass in soil,and these changes reduced the stability of the soil carbon pool.These results suggest that soil microbial metabolism is a key factor controlling topsoil organic carbon storage under the N addition and grazing treatments,and soil microbial community structure play a vital role in regulating the stability of soil organic carbon pool.(2)Nitrogen addition and grazing restructured the spatial pattern of topsoil organic carbon storage.The topsoil organic carbon storage(SOCD)showed strong spatial dependence[C/(C + C0)≥ 0.6](the value was closer to 1.0,the spatial variation was stronger)at 2-7 m.The spatial variation(spatial dependence)in SOCD was 0.62,0.73,0.59 and 0.82 in the CK,N,G and NG treatment,respectively.In the CK treatment,soil dissolved organic carbon content and NAG activity explained 30% of the spatial variation in SOCD.The N treatment increased the spatial heterogeneity of SOCD by enhancing the spatial variations of soil dissolved inorganic nitrogen content and plant aboveground biomass.The homogenization in microbial community structure induced by the G treatment was a determinant of the reduction in spatial heterogeneity of SOCD.The combination of soil microbial community structure,litter mass and soil water content explained 63% of the spatial variability of SOCD in the NG plots.Regardless of the grazing treatment,the N addition enhanced the spatial heterogeneity of SOCD.The required number of SOCD measurements ranged from a minimum of 13 to a maximum of 38 depending on different treatments.(3)The responses of soil organic carbon stability and soil carbon retention capacity to N addition were regulated by grazing.The grazing treatment weakened the positive effect of the nitrogen addition on the accumulation of fungal necromass carbon.The nitrogen addition increased the contribution of fungal necromass carbon in soil organic carbon content by 33.8% and 19.2% in the ungrazed and grazed plots,respectively.In addition,we found that the grazing treatment amplified the promoting effect of the N addition on microbial carbon use efficiency.In the incubation experiment,the grazing treatment increased priming effect,while the N addition treatment and the interaction of N addition and grazing had no significant effects on priming effect.At the end of the incubation experiment,about 1/4 of the added carbon was remained in soil.Nitrogen addition increased the remained carbon by 32.8% and 13.4%,and increased net carbon accumulation by 79.6% and 12.3% in the ungrazed and grazed plots,respectively.In addition,random forest analysis indicated that the assimilation capacity of bacteria to added carbon was a key factor in controlling the strength of priming effects and the net carbon balance.Our results showed that grazing could regulate the responses of soil organic carbon stability and soil carbon retention capacity to N addition by changing the assimilation capacity of microbial communities.Overall,the nitrogen addition and grazing treatments affected topsoil organic carbon storage and stability by changing plant biomass,soil microbial community structure and microbial respiration.This study reveals the regulation mechanism of soil microbial community structure and enzymatic activities on the spatial pattern of topsoil organic carbon storage,which are helpful for us to design the best sampling strategy,and accurately assess the response of soil organic carbon storage to global change and land use.In addition,this study find that grazing-induced changes in bacterial-dominated carbon assimilation processes can alter the magnitude of priming effects and the net C balance in response to nitrogen addition,highlighting the regulatory effect of grazing on soil organic carbon stability and soil carbon retention capacity under nitrogen addition conditions.These results provide reliable data evidence and theoretical basis for assessing the risk of soil carbon emission and the potential of carbon sequestration in grassland ecosystem under the scenarios of nitrogen deposition and grazing utilization. |
