The Effects Of Different N Deposition Simulation Methods On Productivity And Biodiversity Of Grassland

With the increasing of human activities,nitrogen?N?compounds are more and more introduced into the ecosystem through atmospheric wet and dry deposition.Nitrogen deposition caused great impacts on the ecosystems,such as the increase of productivity and the decrease of biodiversity.There are two important features of atmosphere N deposition.First,natural N deposition is composed of inorganic N?IN?and organic N?ON?,and these sources of N may have different impacts on ecosystems.Second,N deposition occurs every day in every moment and is closely related to rainfall.However,our understandings of the impacts of N deposition are largely based on experimental gradients of IN or more rarely ON.And in most of the existing simulation experiments of N deposition,N was added once or several times a year,which is inconsistent with the natural N deposition patterns.Thus,we designed two experiments in a typical temperate grassland to explore whether existing studies about N deposition could follow the natural N deposition.First,we explored the impacts of different ratios of IN to ON?0:10,3:7,5:5,7:3 and 10:0?on annual net primary productivity?ANPP?and plant species richness.Second,we designed a resin-coated urea,whose N release pattern is very similar to natural atmospheric N deposition,and thus can well simulate the natural nitrogen deposition.We compared the impacts of two methods of N deposition?normal urea and coated urea addition?in addition level of 2.5,5,7.5,10,15,20 and 30 g N·m^-2·yr^-1 on ANPP and species richness.There are 4main results follows:?1?N addition significantly increased ANPP,but there are little significant effects of different IN:ON on ANPP.?2?N addition significantly reduced species richness,and more species loss occurred with increasing IN:ON ratios.?3?N addition significantly increased ANPP,and urea increased more ANPP compared with coated urea in lower N addition rate;coated urea increased more ANPP in higher N addition rate,which suggests that N deposition existing maybe overestimate the effects of N deposition on the grassland productivity in lower N addition rate and underestimate the effects of N deposition on the grassland productivity in higher N rate.?4?N addition significantly decreased species richness,and coated urea decreased less species than urea,which suggest that N deposition existing maybe overestimated the effects of N deposition on the grassland productivity.Our findings indicated that significant different effects exists in the N deposition on the productivity and biodiversity of grassland.And the previous evaluation of N deposition on ecosystems might be overestimated or underestimated without considering the natural N deposition patterns.Our study is meaningful to accurate evaluations for the effects of N deposition on the globel ecosystems.