Effects Of Nitrogen Addition On The Early Decomposition Process Of Urban Forest Litter

As an important component of terrestrial ecosystem,forest is the direct recipient of large-scale nitrogen deposition.The significant increase of atmospheric nitrogen deposition has seriously threatened the stability of forest ecosystem.Litter is an important component of forest ecosystem nutrient pool,and litter decomposition plays an important role in material cycle and energy flow of forest ecosystem.So,it is urgent to study the effect of exogenous nitrogen on the litter decomposition under the background of globalization of nitrogen deposition,which is of great significance for understanding the response mechanism of forest ecosystem to nitrogen deposition.Water conservation forests in the upper reaches of Huangpu River has been selected as the study object to research the effects of simulated nitrogen deposition on the dynamics of litter decomposition,litter nitrogen turnover and soil microorganism by addition of a small amount of nitrogen,with the method of outdoor orientation observation combining with laboratory analysis.The purpose of this study was to reveal the effects of nitrogen addition on the early decomposition process of urban forest litter.The mainly conclusions were as follows.?1?The content of NH₄⁺-N and NO₃^--N in surface soil increased significantly with nitrogen addition,but soil enzyme activity decreased.Nitrogen addition had no significant effect on the microbial biomass,and the impact on microbial community structure varied with site conditions.?2?The differences of the initial properties of litters influenced the decomposition rate directly.What's more,the decomposition rate of some litters decreased with nitrogen addition.During decomposition process,the concentrations of N,Ca increased,but the K concentrations and C/N decreased,and the concentrations of C,P and Mg in different litters were different.Except the N and P in leaves of Cinnamomum camphora,the Mg in fine root and the Ca in fine branches of Quercus variabilis,the element content in other litters declined during decomposition process.And the net release rate of K was the highest,followed by C.Nitrogen addition had no significant effect on litter element concentration and nutrient releasing,and only the individual elements of individual litters responded to nitrogen addition.?3?The percentage composition of soil aggregates in the study area increased with the decrease of particle size,and<53?m soil aggregates were dominant in the distribution.The content of C and N in soil aggregates correlated positively with particle size,but negatively with soil depth.The nutrient contribution rate of soil aggregates had similar characteristics with aggregate composition,and the contribution rate of microaggregates was higher than 80%.Nitrogen addition had no significant effect on soil aggregate composition,C/N and nutrient contribution rate.?4?Allocation proportion of the N from litters increased with the decrease of particle size in soil aggregates,and most of the litter nitrogen transported into microaggregate?<250?m?.Nitrogen addition promoted the translocation of litter nitrogen to<53?m aggregates.