Responses Of Soil Microbial Activities And Functions To Nitrogen And Phosphorus Addition In Alpine Grassland

In the context of global climate change,the increasing rate of nitrogen and phosphorus deposition in the atmosphere has become a major scientific issue.The structure and function of the grassland ecosystem will continue to be influenced by exogenous nitrogen and phosphorus.In recent years,due to the influence of human activities such as agricultural fertilization and industrial fuel combustion,the input rate of nitrogen and phosphorus in Qinghai-Tibet Plateau is increasing rapidly.Nitrogen and phosphorus addition can change plant biomass,community composition and soil physical and chemical characteristics,as well as the activity and function of soil microorganisms,and ultimate regulate the transformation and accumulation of soil organic carbon(SOC).However,understanding of how nitrogen and phosphorus addition affects the activity and function of soil microbial by affecting the physiological metabolism of soil microorganisms is still limited.We took advantage of the Haibei national field research station of alpine grassland ecosystem in Qinghai province and carried out the nutrient addition experiment since 2011.Focusing on the topsoil(0-10cm)and subsoil(20-30 cm)of different nitrogen and phosphorus addition treatments(N 100 kg N ha^-1 yr^-1,P 50 kg P ha^-1 yr^-1,NP 100 kg N ha^-1 yr^-1+50 kg P ha^-1 yr^-1)with different nitrogen and phosphorus addition duriation(2 and 10 years).Soil microbial biomass carbon and extracellular enzyme activity was quantified to assess microbial biomass and activity.Combining with the incubation experiments with ¹³C-labeled glucose amendment and biomarker(amino sugar),the mineralization potential of soil organic carbon,microbial carbon use efficiency(CUE)and carbon accumulation efficiency(CAE)was measured to comprehensively analyze the effect of nitrogen and phosphorus addition on microbial activities and functions.The main results are as follows:(1)Short-(2nd year)and long-term(10th year)nutrient addition had varied effects on different extracellular enzyme activities.Short-term nitrogen and phosphorus addition had no obvious effect on hydrolases.However,it had a significant effect on the activity of polyphenol oxidase.The activity of polyphenol oxidase in the topsoil was significantly decreased with nitrogen alone addition,but significantly increased the subsoil with phosphorus alone and nitrogen and phosphorus addition.Long-term nitrogen and phosphorus addition had no significant effect on polyphenol oxidase activity.However,it significantly promoted the activity of hydrolase.(2)Short-term nitrogen and phosphorus addition had no significant effect on the mineralization potential of SOC and glucose in soils.For the long-term nutrient addition,nitrogen alone application significantly decreased the mineralization potential of SOC in the topsoil,while other treatments had no significant effect on the mineralization potential of SOC and glucose.The mineralization potential of SOC after short-term nitrogen and phosphorus addition is mainly controlled by microbial biomass carbon,total nitrogen,inorganic nitrogen content,and extracellular enzyme activity.However,the mineralization potential of SOC after long-term nitrogen and phosphorus addition is mainly affected by soil total nitrogen content.(3)Short-and long-term nitrogen and phosphorus addition had no significant effect on microbial CUE,but the microbial CUE at the 10^th year(0.55±0.04%)was significantly higher than that at the 2^ndyear(0.21±0.02%).Short-term nitrogen and phosphorus addition had no significant effect on microbial CAE,while long-term of nitrogen alone addition significantly increased microbial CAE.Moreover,the microbial CAE after long-term nitrogen and phosphorus addition(2.06±0.19%)was significantly higher than that of short-term(0.97±0.17%).For short-term nitrogen and phosphorus addition,the main factors affecting microbial CUE is clay content,the main factor affecting soil microbial CAE is the content of total phosphorus in soil.After long-term addition of nitrogen and phosphorus,microbial CUE is mainly related to?-1,4-glucosidase activity,and the microbial CAE is mainly affected by the content of inorganic nitrogen in soil.In conclusion,different nitrogen and phosphorus addition treatments and durations had different effects on soil microbial activity and function.Short-term nitrogen and phosphorus addition mainly affects the activities of enzymes related to degradation of refractory compounds such as phenols,which is related to the fact that the activation energy of oxidase is lower than that of hydrolase,so oxidase has lower environmental stability and is more sensitive to nitrogen and phosphorus addition.Short-term nitrogen and phosphorus addition had no significant effect on microbial related functions,which may be due to the short time of nitrogen and phosphorus nutrient addition,and the effect on microbial related functions has not been reflected for the time being.With the addition of nitrogen and phosphorus nutrients,the activities and functions of soil microbial change continuously,and the microbial responses to the addition of nitrogen and phosphorus are different.Continuous addition of nutrients increased the input of plant carbon into soil,and significantly promoted the activity of hydrolase.At the same time,long-term of nitrogen alone addition significantly decreased the mineralization potential of SOC in the topsoil.The reason may be that long-term of nitrogen addition greatly relieved the nitrogen limitation of microorganisms,and microorganisms did not need to degrade organic carbon to obtain energy for more nitrogen nutrients.Although short-and long-term nitrogen and phosphorus addition had no significant effect on microbial CUE.However,compared with short-term nitrogen and phosphorus addition,long-term nitrogen and phosphorus addition significantly increased microbial CUE.In addition,the main factors affecting soil microbial CUE were?-1,4-glucosidase activity(long-term)and soil clay content(short-term).Long-term application of nitrogen fertilizer alone significantly increased microbial CAE in the topsoil,and the main factor affecting microbial CAE was the content of soil nutrients(total phosphorus and inorganic nitrogen).