Effects And Mechanism Of Nitrogen Addition On Productivity And Community Stability In The Desert Grassland

Productivity and stability are important components of ecosystem functions.However,it is not clear how and the mechanism of change of the community productivity and stability of desert grassland under nitrogen(N)addition;does N affect the productivity and stability of desert grassland by affecting biodiversity?A manipulative N addition experiment located in the desert grassland has been conducted since 2018.The experimental treatment included five levels of N addition rate(i.e.,control,5,10,20 and 40 g·m-2·a-1)with four replicates for each treatment.The effects of N addition on species diversity,functional diversity,productivity,community stability and their relationships in desert grassland were investigated.The results showed that:(1)The height and coverage of desert grassland communities and Gramineae were increased by N addition treatment(2018-2021),and the species abundance of desert grassland communities were decreased;plant species diversity and functional diversity were changed by N addition,but the interannual trends are different.In the fourth year of N addition treatment(2021),the functional diversity(Rao index),community weighted average(CWM-height),functional evenness and functional dispersion of the desert grassland increased significantly,while the species richness and Shannon-wiener index of the desert grassland decreased significantly.Compared with the control treatment,Shannon-Wiener index and species richness decreased by 80.5%,71.0%and 79.4%,61.29%at high N addition(N20 and N40)treatment.(2)N addition treatment(2018-2021)significantly increased the productivity of desert grassland community and decreased the plant root-shoot ratio.In the first year and second year(2018 and 2019)of N addition,the belowground productivity were increased by N addition,while belowground productivity were not affected by N addition in the third year and fourth year(2020 and 2021).There were differences response of productivity among different plant functional groups to N addition.N addition significantly increased the productivity of Gramineae from 2019 to 2021,while N addition decreased the productivity of Leguminosae and Forbs in the third year and fourth year(2020 and 2021).The Leguminosae in the high N treatment community almost disappeared in the fourth year of N addition treatment.(3)In the second year(2019)of N addition,the community stability(ICV)was increased,and showed a trend of first rise and then fall with the increase of N addition and it was the most stable in N10 treatment,which increased by 52.72%compared with the control treatment.In the third year and fourth year(2020 and 2021)of N addition,the community stability(ICV)were decreased by N addition,while temporal stability of productivity were increased by N addition.(4)N addition could indirectly promote desert grassland productivity by affecting species richness and functional diversity.The community weighted trait value-plant height had a positive effect on productivity,while species richness and functional dispersion had a negative impact on productivity.The variability of community productivity in desert grassland were better explained by the biomass ratio hypothesis.The study demonstrates that N addition modulated desert grassland productivity mainly by altering the functional traits and biomass of dominant species within the community.(5)There were direct and indirect effect of N addition on community stability(ICV).The community stability(ICV)was negatively and directly affected by N addition.The functional dispersion had a positive effect on community stability(ICV),while the Shannon-Wiener index had a negative effect on community stability(ICV).The community stability of desert grassland were better explained by selection effect.