The Effects And Driving Mechanisms Of Nitrogen Addition And Grazing On Soil Respiration And Its Components In Songnen Grassland

Soil respiration（SR）is one of the main sources of atmospheric CO₂.The changes in the magnitude of SR have a significant impact on carbon balance of terrestrial ecosystems.Autotrophic respiration（AR）and heterotrophic respiration（HR）,as components of SR,which are jointly affected by soil microclimate,soil physicochemical properties,soil microbial characteristics and plant biomass.There is substantial temporal and spatial variability in SR due to the diversity of driving factors,as well as periodic changes in environmental and climatic factors.Landuse（e.g.grazing）and global change（e.g.increasing nitrogen deposition）affect these environmental factors and add more levels of complexity to our understanding of variability in SR and its components.In the context of human disturbance and global change,in order to improve our understanding of the control mechanism of soil carbon flux,we carried out an experiment with four experimental treatments in the Songnen Leymus chinensis meadow steppe,including control（CK）,grazing（G）,nitrogen addition（N）and grazing plus nitrogen addition（NG）.In this study,Li-6400 Portable CO₂Infrared Gas Analyzer was used to measure SR,HR and AR in situ during the growing season（May to October）from 2019 to2020.We examined how soil respiration and its components respond to N addition(10 g N m^-2y^-1),grazing（moderate cattle grazing）and their interaction under different soil moisture conditions and explored the underlying ecological mechanisms.We discovered hot spots of soil respiration and generating mechanisms under the nitrogen addition and grazing treatments.The following results were obtained:（1）Soil temperature and soil water content could explain 59～68%of the seasonal variation of SR across four treatments.Nitrogen addition and grazing had no significant effects on the seasonal variation of AR,but nitrogen addition and grazing had significant effects on AR intensity.Nitrogen addition increased AR,while moderate grazing decreased AR,and nitrogen addition and grazing had an additive effect on AR.We found that plant aboveground biomass was the predominant controlling factor and key predictor of AR.Moderate grazing offset nitrogen addition-induced increases in aboveground biomass and AR in L.chinensis meadow steppe.Nitrogen addition increased the seasonal variation of HR,and grazing decreased the seasonal variation of HR.Although N addition or grazing caused changes in a range of biotic and abiotic factors（total phospholipid fatty acids,fungal:bacterial biomass,aboveground biomass,soil temperature and soil water content）that controlled soil microbial metabolic processes,these positive and negative effects on HR can be offset by each other,eventually resulting in that HR of G and N was not significantly different from CK treatment.（2）In Songnen grassland,soil water content was the key driving factor of soil respiration and its components,the effects of grazing and combination of N addition and grazing on SR were also regulated by soil water content.Under drought condition,G and NG significantly reduced SR due to the negative effects of livestock feeding and trampling on AR,these negative effects were far outweighed by the positive effects of excreting soil nutrients on HR.However,N significantly increased SR and AR,but had no significant effect on HR.Under wet condition,G and NG increased SR,because moderate grazing stimulated the compensatory growth of plants and promoted AR.In addition,suitable soil water content improved soil enzyme activity and accelerated the diffusion of soil soluble substrates,which significantly promoted HR.Compared with CK treatment,N had no significant effect on SR and its components.（3）During the main growing season（June to August）in the Songnen grassland,inputs of livestock feces can potentially generate hot spots of SR with extremely high respiration rates.The mechanism could be that livestock excretion reduced soil temperature,increased soil water content and soil nutrients,which in turn enhanced soil enzyme activities and stimulated plant growth,thereby promoting SR in dung subplots.However,inputs of urine did not significantly increase SR,which may be attributed to the rapid utilization of soil dissolved organic carbon,thus,the short-term change trend of SR in urine subplots was the same as that of control subplots.We explored the temporal and spatial variation characteristics and control mechanism of SR under nitrogen addition and grazing in Songnen grassland.We also illustrated the changes in soil water content could regulate the effect of nitrogen deposition and human disturbance on HR and AR.Our research provides necessary basic data to accurately estimate CO₂fluxes of regional ecosystems,and enables us to design a reasonable management and utilization strategy of carbon sequestration in L.chinensis meadow steppe under global change scenarios.