The Effects Of Nitrogen Deposition Simulation Method On Grassland Ecosystem Productivity And Species Diversity

Increase in anthropogenic activities due to growing human population has caused a rapid increase in the emission of reactive nitrogen.Furthermore,climate models have predicted that the global annual atmospheric nitrogen deposition would increase in coming decades.These emitted reactive nitrogen are added into the terrestrial ecosystems via deposition,and significantly alter the ecosystem functions and processes.Grassland ecosystems cover 24%areas of the Earth’s surface and provide many ecosystem services including carbon storage,biodiversity preservation and the production of livestock forage.Many studies have shown that nitrogen addition increases ecosystem productivity and reduces species diversity.In order to study how the increasing atmospheric nitrogen deposition impacts grassland ecosystems,numerous nitrogen addition experiments have been conducted globally with nitrogen deposition been simulated in several different ways.Meanwhile,the nitrogen addition rate,frequency,duration and the type of fertilizer could affect the experimental results and conclusions.Yet,the effects of nitrogen deposition simulation method on grassland ecosystem productivity and species diversity are rarely reported.This study is the first meta-analysis on the effects of nitrogen deposition simulation method on productivity and diversity of grasslands.The data were obtained from peer-reviewed articles that studied the impact of N addition on aboveground net primary productivity（ANPP）,species richness（SR）or both variables at the community level in grasslands.Relevant studies were dentified through searches of journals included on the Web of Science and Chinese Science Citation Database（CSCD）.The experimental data from the Nutirent Network,in which slow-release fertilizer was added to many grasslands across the world,were included in this meta-analysis.In this meta-analysis,simulation method was divided into four factors:nitrogen addition rate,frequency,duration and nitrogen fertilizer type（fast-release fertilizer vs.slow-release fertilizer）.Overall,this study was focused on whether these four factors had significantly affect the nitrogen addition experiment results,and whether the existing nitrogen simulation methods affect the estimation of atmospheric nitrogen deposition.The key findings are as follows.（1）At the global scale,nitrogen addition increased the ANPP of the grassland ecosystems by22.9%on the average,while the species richness reduced by 14.3%on the average.（2）Nitrogen addition rate significantly correlated with ANPP and SR.ANPP increased with the greater nitrogen addition rate,and reached the saturation state at 10-20 g N m^-2 yr^-1.With the increase in nitrogen addition rate,SR presented a nonlinear trend of decreasing at first and then recovering somewhat.（3）The responses of ANPP and SR under different nitrogen addition frequencies were significantly different.Under the addition frequency of 1 per year,2 per year,3 per year,4 per year and>4 per year,ANPP increased by 29.4%,33.8%,47.3%,53.1%and 25.7%,respectively,while SR decreased by18.4%,30.1%,15.0%,12.7%and 1.9%,respectively.（4）Under the duration of 0-5 years,5-10 years,10-20 years and>20 years,ANPP increased by22.2%,24.1%,30.6%and 19.3%,respectively.This shows that short-term nitrogen addition has a positive effect on plant productivity,but the positive effect may gradually disappear with the extension of nitrogen addition duration.Under the duration of 0-5 years,5-10 years,10-20 years and>20 years,SR decreased by 10.2%,18.2%,26.5%and 49.4%,respectively.This shows that the loss of species may increase with the extension of nitrogen addition duration.（5）Both the responses of ANPP and SR under fast-release fertilizer are significantly greater than those under slow-release fertilizer.The ANPP and SR were 33.4%and-20.5%respectively under the addition of fast-release fertilizer,while 16.1%and-11.3%respectively under the addition of slow-release fertilizer.Moreover,these significant differences will gradually expand with the extension of nitrogen addition duration.When the duration reached 10 years,the response difference in ANPP and SR between the two fertilizer types reached 26%and 22%,respectively.In conclusion,the nitrogen simulation method has a significant effect on the productivity and diversity of grassland ecosystem.The results of the existing nitrogen deposition simulation experiments,which mainly use fast-release fertilizer and low addition frequency,may overestimate the real effects of atmospheric nitrogen deposition on the grassland ecosystem.This study provides a scientific reference for the improvement of the nitrogen addition experiment and a new vision for the selection of important parameters in the future model research.It has a great significance for the accurate evaluation of global changes in ecosystem functions and services caused by increased nitrogen deposition.