Effects Of Grazing On Plant Community And Soil Gross Nitrification And Denitrification Rate In Desert Steppe

Grazing is the most important ways in the utilization of grassland.Overgrazing is one of the main causes of grassland degradation.Nitrogen is one of the main limiting factors affecting plant growth in Inner Mongolia grassland,second only to water.Soil nitrogen nitrification-denitrification is an important process for soil nitrogen transformation and a key step in studying the soil nitrogen cycle.In recent years,grasslands have been affected by global climate change and human activities to varying degrees.Therefore,this study conducted a corresponding study on plant community characteristics and soil nitrogen nitrification-denitrification under grazing conditions,and tried to reveal that impact of grazing affected grassland soil nitrification-denitrification and its limiting factors.In the study,the Xilamuren grassland of Inner Mongolia was used as the research object.Two treatments,enclosure(CK)and heavy grazing(HG),were set up.From 2017 to 2018,the plant community characteristics,nitrogen nitrification and denitrification rate and soil physical and chemical properties(soil pH,bulk density,soil moisture content,soil temperature,soil carbon and nitrogen content)were measured.The purpose of this study was to investigate the change of soil nitrification-denitrification rate and its relationship with vegetation and soil physical and chemical properties,and to provide a certain reference for the analysis of rational use of grassland and grassland restoration.The results show as follows:1.Effects of grazing on plant community characteristics showed that reduction rate of the community height in the grazing plot was 26.62%,reduction rate of the dominant species was 22.06%,and reduction rate of the total grass yield was 16.35%.In general,grazing land should be slightly degraded.In the CK area plot,Leymus chinensis,Stipa krylovii,Allium tenuissimum,Agropyron mongolicum,and Cleistogenes songorica are relatively important and dominant species;the dominant species in the HG plot are S.krylovii,A.tenuissimum,Halophyllum daurica,and Allium polyrhizui and Artemisia fi-igida.The number of species and Shannon-wiener index in HG plot were significantly higher than CK in July(p<0.05),and the Simpson index in HG plot was significantly higher than CK in August(p<0.05).Except for Gramineae and Leguminosae,the heights of other families in the HG plot were significantly lower than CK in August and September(p<0.05).In the HG plot,the height of the Gramineae and C.songorica,and the aboveground biomass of Leguminosae were significantly lower than CK in September(p<0.05).The height of L.chinensis,the density of C.songorica,the abovegrand biomass of C.songorica,the density of the Gramineae,and the aboveground biomass of the Gramineae in HG plot were significantly lower than CK from July to September(p<0.05).The density of L.chinensis and the Leguminosae,and abovegrand biomass of L.chinensis in HG plot were significantly lower than CK in July and September(p<0.05).2.From March to November,after 5 years of grazing,compared with CK,the total nitrogen,inorganic nitrogen,nitrate nitrogen,soil pH,bulk density,water content,organic carbon,and carbon-nitrogen ratio in topsoil in HG plot increased.The soil gross nitrification rate ranged from 23.98 to 173.17μg kg-1 h-1.Both GNCK and GNHG showed a trend of rising first and then decreasing,and both reached their maximum values in July,173.17μg kg-1 h-1 and 140.04μg kg-1 h-1,respectively.The overall GNCK was higher than that of GNHG,indicating that severe grazing could inhibit soil gross nitrification rate,but not significant.3.The rates of gross nitrification and denitrification rates,soil physical and chemical indicators through linear regression analysis showed that soil gross nitrification rate had a significant linear correlation with soil moisture content,inorganic nitrogen,ammonium nitrogen,and nitrate nitrogen in the CK plot(p<0.05),Soil gross nitrification rate had a significant linear correlation with soil moisture content and ammonium nitrogen in the HG plot(p<0.05).Denitrification rates had no significant linear relationship with the soil physical and chemical indexes in the CK and HG plot.4.The stepwise regression equation analysis of the gross nitrification-denitrification rate under different treatments showed that the soil moisture content and soil temperature are the two major promoting factors of gross nitrification in the CK plot,and the soil moisture content promoted the most.The soil ammonium nitrogen content is the main contributing factor of denitrification.The soil moisture content in HG plot is the most promoting factor for gross nitrification,and the soil ammonium nitrogen content and soil nitrate nitrogen content are the two major inhibitors.The soil moisture content is the biggest promoting factor for denitrification,and the three,soil ammonium nitrogen content,soil pH and soil nitrate nitrogen content,are the inhibitory factor.