Effects Of Nitrogen Addition On Soil Enzyme Kinetic Characteristics And Temperature Sensitivity In Alpine Meadow

Soil enzyme is the key driving force in the material cycle,and soil enzyme activity is an important index to indicate soil quality.Soil enzyme kinetic parameters(V_max,K_m,K_a)are used to represent the soil enzyme kinetic characteristics,which can reflect soil enzyme-substrate binding reaction.Temperature sensitivity Q₁₀(V_max),Q₁₀（K_m）and Q₁₀（K_a）can reflect the sensitivity of soil enzyme kinetic parameters to temperature changes.The Qinghai-Tibet Plateau is an important ecological barrier in China.The local nitrogen deposition is increasing day by day and the average annual temperature is increasing gradually.There are few reports on the response of soil enzyme activity and kinetic characteristics to nitrogen deposition and temperature change in the alpine meadow of the Qinghai-Tibet Plateau.Therefore,we used a nitrogen addition experiment（0,5,10,15g N/m²/yr）,which had been set up in Gansu Gannan Grassland Ecosystem National Observation and Research Station since 2011,to study effects of nitrogen addition on soil enzyme activity,enzyme kinetic parameters and temperature sensitivity.We further analyzed the driving factors which caused the variation of the above parameters under nitrogen addition,and our results may provide theoretical reference for the material cycling of alpine meadow in the background of climate change.The main research results were as follows:（1）Nitrogen addition significantly changed soil p H,available phosphorus,total phosphorus,available nitrogen,above-ground plant biomass,microbial biomass carbon（MBC）,microbial biomass nitrogen（MBN）,and microbial biomass phosphorus（MBP）,bacterial and fungalβdiversity,but had no significant effects on soil moisture content,soil organic carbon,total nitrogen,bacterial and fungalαdiversity.（2）Nitrogen addition increased the enzyme activities of carbon and phosphorus cycling,while decreased the enzyme activities of nitrogen cycling and the activities of oxidase.Compared with the control,β-glucosidase（BG）,β-xylosidase（BX）,Cellobiohydrolase（CBH）and Acid phosphatase（AP）activities increased by 22.90%,31.79%,32.33%and 22.81%under N15 treatment,respectively,while N-acetyl glucosaminidase（NAG）,L-leucine aminopeptidase（LAP）and Polyphenol oxidase（PPO）activities decreased by 26.89%,22.36%and 50.52%,respectively.The results of RDA showed that MBC and MBN were the key factors driving the change of soil enzyme activities.（3）Nitrogen addition increased V_maxof carbon and phosphorus cycling and decreased V_maxof nitrogen cycling;Nitrogen addition significantly affected the soil enzyme K_m,but there were not consistent patterns;The effects of nitrogen addition on K_awere similar to those of nitrogen addition on V_max,which were more strongly dominated by V_max.RDA results showed that the changes of soil enzyme kinetics parameters(V_max,K_m,K_a)were closely related to MBC,MBN,MBP and NO₃^--N,among which MBC and MBN were the key driving factors.（4）The V_maxand K_avalues of different enzyme activities increased with the increase of temperature in the range of 0～40℃;The temperature sensitivity of all enzyme K_mwere lower than that of V_max,with Q₁₀(V_max)values ranging from 1.24 to1.28,Q₁₀（K_m）values ranging from 1.00 to 1.02 and Q₁₀（K_a）values ranging from 1.21to 1.23.The RDA results showed that the change of temperature sensitivity Q₁₀of soil enzyme kinetics parameters were closely related to MBC,MBN,MBP,NO₃^--N and bacterialβdiversity,among which MBC,MBP and NO₃^--N were the key driving factor.Our results indicated that nitrogen addition increased carbon,phosphorus cycle enzyme activities and decreased nitrogen cycle enzymes activities and polyphenol oxidase activities.Nitrogen addition increased V_maxof carbon and phosphorus cycle enzyme and reduced V_maxof nitrogen cycle enzyme.K_achanged in a similar way to V_max,significantly affecting different soil enzyme K_mand Q₁₀values,but there were no consistent patterns.MBC and MBN were the main driving forces of the changes in soil enzyme activity,enzyme kinetic parameters under nitrogen addition.MBC,MBN,MBP and NO₃^--N were the main driving forces of the changes in temperature sensitivity of soil enzyme kinetic parameters under nitrogen addition.