Effects Of Long-term Nitrogen And Phosphorus Addition On Soil Microbial Respiration In An Alpine Meadow On The Tibetan Plateau

Atmospheric nitrogen(N)and phosphorus(P)deposition greatly increased due to anthropogenic activities,which altering biogeochemical cycling in terrestrial ecosystems.The decomposition of soil organic matter is an important process in the carbon cycling of terrestrial ecosystems.The carbon in the soil is transferred to the atmosphere through soil microbial respiration,which regulates terrestrial carbon-climate feedbacks.The decomposition process of soil organic carbon is affected by soil physical and chemical properties,species composition of plant community,soil microbial community composition and soil microbial extracellular enzyme activity.However,the mechanisms of long-term nitrogen and phosphorus addition on soil microbial respiration are still not well understood.Our research was conducted in the long-term nitrogen and phosphorus addition experiment platform(9 years)from the National Field Scientific Observation and Research Station of Gannan Grassland Ecosystem(Azi Station).In September 2019,we collected soil samples and investigated the plant community composition from different concentrations of nutrient addition platforms:N addition(0,5,10,15 g N m^-2 year^-1),P addition(0,2,4,8 g P m^-2 year^-1)and NP mixed addition(fixed with 10 g N m^-2 year^-1 combined with 0,2,4,8 g P m^-2 year^-1).By constructing a structural equation model(SEM)to study the direct effects of nitrogen and phosphorus addition on soil microbial respiration,and the indirect effects on soil microbial respiration through their direct effects on plant communities,soil microbial community and soil microbial extracellular enzymes through soil physical and chemical properties.The results are mainly as follows:1)N addition significantly decreased arbuscular mycorrhizal fungi(AMF),fungi and fungi to bacteria ratio(F:B),but had no effects on actinomycete,Gram positive bacteria(GP),Gram negative bacteria(GN),bacteria,total PLFA and Gram positive bacteria to Gram negative bacteria(GP:GN).P and NP addition had no effects on different functional groups of soil microbes,fungi to bacteria ratio(F:B)and Gram positive bacteria to Gram negative bacteria(GP:GN).2)Acid phosphatase(AP)had an increased trend with N concentration increase,had a decreased trend of along a NP addition gradient and did not change under P addition.N,P,and NP addition had no significant effect on C acquisition enzyme(?-1,4-glucosidase(BG),and?-1,4-xylosidase(BX)),oxidase(phenol oxidase,and peroxidase),and N acquisition enzyme(?-1,4-N-acetyl-glucosaminidase(NAG)).3)Both N and P addition decreased soil microbial respiration,but NP addition had no effects on soil microbial respiration.4)In the N addition experiment,N addition indirectly decreased soil microbial respiration through directly increased soil N availability and altered plant community composition.Meanwhile,N addition also indirectly decreased soil microbial respiration through directly decreased soil p H(resulting from increases of soil N availability)and altered soil microbial community.5)In the P addition experiment,P addition indirectly decreased soil microbial respiration through directly increased soil P availability and altered plant community composition.Our research indicated that species composition of plant community change due to long-term N and P addition play a crucial role in the inhibition of soil microbial respiration.Compared with P addition,N addition altered soil microbial community due to soil acidification and then decreased soil microbial respiration.NP addition had no effects on soil microbes,soil extracellular enzyme and soil microbial respiration,and it indicated that under the condition of constant N addition,P addition did not alter the response of soil microbes and its function to N addition.The results of this study revealed the mechanisms of long-term N and P addition on soil microbial respiration in alpine meadow ecosystem,and can provide a scientific basis for the soil carbon dynamics of alpine meadow ecosystems on the Tibetan Plateau under the background of global eutrophication.