Effects Of Nitrogen Additions On Soil Microbial Biomass And Enzymes Activities In Red Soil

Atmosphere nitrogen?N?deposition increased in the past decade,and the ratio of nitrate to ammonium in the atmosphere N deposition had improved over years,which caused decrease of soil microbial carbon.Soil microorganisms adjusted the decomposition of soil organic matter by producting enzyme,thus,both played key role in the turnover of soil organic matter.However,it is still unclear how different N dosage and N forms[ammonium?NH₄⁺-N?and nitrate?NO₃^--N?]additions effect on soil microbial communities and enzyme activities.In order to better predict the effects of elevated N deposition on carbon?C?cycling,it is necessary to evaluate the effects of different N dosage and N forms on soil microbial biomass and enzyme activities.Our study locates in Qianyanzhou Experimental Station of Red Soil and Hilly Land,Chinese Academy of Sciences,Jiangxi provience,southeastern China.A four-years field N additions experiment is conducted to evaluate the effects of different N dosage and N forms on soil microbial communities and enzyme activities.And the effects of N dosage on soil enzyme activities are evaluated by comparing 40,120 kg N ha-1 yr-1 NH4Cl additions,and the effects of N forms on soil microbial biomass and enzyme activities are evaluated by comparing 40 kg N ha-1 yr-1 NH4Cl and NaNO3 additions.The results showed that?1?The inhibition effects of N additions on ?-1,4-glucosidase??G?,cellobiohydrolase?CBH?,?-1,4-N-acetylglucosaminidase?NAG?and peroxidase?PER?activities were enhanced with increase of N dosage,which decreased from 17%to 51%.However,although N additions decreased a-1,4-glucosidase?aG?,(3-1,4-xylosidase??X?,acid phosphatase?AP?and phenol oxidase?PPO?from 15%to 39%,there were little difference between different N dosages.?2?Based on data used to evaluate the effects of N dosage on soil enzyme activities,redundancy analysis?RDA?and Pearson analysis showed that measured enzyme activities were positivitely correlated with soil pH and ammonium?NH₄⁺-N?contents,but negatively correlated with nitrate contents.It suggested that soil enzyme activities were restricted by acidification and nitrification.?3?The inhibition effects of NH₄⁺-N additions were stronger that NO₃^--N additions.Total PLFA contents were decreased by 24%response to NH₄⁺-N additions,while decreased by 11%response to NO₃^--N additions.N additions?NH₄⁺-N and NO₃^--N additions?decreased gram positive bacteria?G+?,gram negative bacteria?G-?fungi?F?,bacteria?B?,actinomycetes?A?,arbuscular mycorrhizal fungi?AMF?,saprophytic fungi?SAP?from 14%to 40%.?4?Both NH₄⁺-N and NO₃^--N additions inhibited C,N,P-acquisition hydrolase and oxidase,and NO₃^--N additions had a stronger inhibition effect on AP activities.NH₄⁺-N and NO₃^--N additions had no influence to C-acquisition and oxidase specific enzyme activities that enzyme activities were normalized by total PLFA contents.NO₃^--N additions had a stronger inhibition effects on N-acquisition specific enzyme activities than NH₄⁺-N additions,which N-acquisition specific enzyme activities were decreased by 21%and 43%due to NH₄⁺-N and NO₃^--N additions,respectively.However,only NH₄⁺-N additions increased P-acquisition specific enzyme activities by 19%.?5?Based on data used to evaluate the effects of different N forms on soil microbial communities and enzyme activities,RDA showed that microbial PLFA biomarkers were positively correlated with soil pH,total organic carbon?SOC?,total N?TN?,but negatively correlated with NH₄⁺-N contents.Soil enzyme activities were positively correlated with soil pH and NH₄⁺-N contents,but negatively correlated with NO₃^--N contents.?6?Soil enzyme activities were in the order of March>June>October.However,microbial biomarkers PLFA contents were higher in October than March and June.In sum,N additions inhibit soil microbial biomarkers biomass and soil enzyme activities.The inhibition effects of NH₄⁺-N additions are stronger than NO₃^--N additions.However,soil enzyme activities are less sensitive to N forms.In order to better predict the effect of elevated N deposition on nutrient cycling in forest ecosystem,it is necessary to evaluate the effect of N dosage and N forms on soil microbial communities and enzyme activities.