Effects Of Nitrogen Addition On Litter Decomposition For Dominant Species In Semi-arid Grassland In Northern Agricultural And Pastoral Ecotone

Nitrogen deposition is an important process of nitrogen cycle in nature.Nitrogen element is the main limiting factor of grassland ecosystem productivity.In order to explore the response of main nutrient cycle process in semi-arid grassland ecosystem to nitrogen deposition,we carried on an experiment in grassland ecosystem field observation and research station in the agriculture-pasture ecotone of northern China.In the experiment,we set 8 different levels(respectively 0,1,2,4,8,16,24 and 32 g N.m-2.y-1)of nitrogen addition accepting randomized block design with 6 repetitions.The decomposition bags method was used to test the litters decomposition of advantage plant including Leymus secalinus,Puccinellia distans and Artemisia anethifolia in this ecosystem.We put the decomposition bags on the field in May,2018 and sampled in July,September in 2018 and May,July,September in 2019 to study:(1)how mass loss dynamics and decomposition rate of litter response to nitrogen addition,(2)how decomposition dynamics of main nutrient in litter response to nitrogen addition,(3)the effect mechanism of nitrogen addition on litter decomposition in terms of decomposition environment.The results showed that:1.The mass loss of the three litters showed a consistent dynamic trend,the mass loss was faster at the early stage and was slower at the middle and late stage in the decomposition process.The changes of litter mass loss followed the Olson negative exponential decay model,litter decomposition coefficient with nitrogen addition was higher than that of the control,suggesting that the addition of nitrogen tends to accelerate the decomposition of litters.2.In the period of decomposition,the total carbon(TC),cellulose and lignin were gradually released,and nitrogen addition promoted the degradation of TC,but had no significant effect on cellulose and lignin(P>0.05).Nitrogen addition significantly inhibited TN release of litters(P<0.05),and the higher the nitrogen addition levels was,the lower the decomposition ratio of TN was.The changes of total phosphorus(TP)of the three litters were obviously in stages including two stages of release followed by increase.Nitrogen addition could promote P element to release in the first stage,and a significant inhibitory effect on the accumulation P element.3.The litter decomposition under the action of nitrogen addition was mainly affected by the change of C and P element.C/N,C/P,N/P and cellulose /N showed a decreasing trend with the decomposition time going by.The nitrogen addition will reduce C/N and cellulose /N,increasing C/P and N/P,while the effect of nitrogen addition on lignin /N will change from decreasing to increasing with the decomposition time going by.4.Nitrogen addition reduced soil p H and temperature,with an increasing in soil NO3--N content,while had no significant effect on soil moisture,NH4+-N and microbial biomass carbon and nitrogen.The increase of soil inorganic nitrogen content is the main factor that accelerates the decomposition of litters and the foundation affecting other factors.At the same time,the addition of nitrogen alleviates the original high soil p H,which also becomes a potential factor to promote decomposition to some extent.However,the addition of nitrogen can promote the rapid growth of vegetation,especially the increase of vegetation layers under the high nitrogen addition.Besides blocking the sunlight,increasing of vegetation layers also hinders the leaching,which greatly reduces the photo-degradation and the application of nitrogen fertilizer,which thus reduced the effect of nitrogen addition.In conclusion,nitrogen addition alleviated the nitrogen limitation of semi-arid grassland in the northern agricultural and pastoral ecotone to some extent,improving the vegetation productivity and changed the surface microclimate.On this basis,all the related factors affect the decomposition of litter synthetically.At the current nitrogen deposition level in northern China,the decomposition of plant litter in semi-arid grassland will be accelerated in a long period of time in the future,thus accelerating the material cycle of grassland ecosystem.The decomposition process of litters is the release and enrichment process of each constituent element.