Grassland productivity and diversity changes in responses to N and P addition depend primarily on tall clonal and annual species in semiarid Loess Plateau

Abstract

Ecological benefits and production of grassland on the Loess Plateau are limited by low soil N and P availability. Extraneous N and P fertilization is an efficient management measure to enhance grassland productivity and accelerate grassland restoration. However, biodiversity decline and species loss induced by N and P addition must be noticed. Two grassland communities dominated by tall clonal grass (TCG) and tall clonal forb (TCF) on Loess Plateau were selected. A two-year split-plot experiment (main-plot: 0, 25, 50, and 100 kgN ha−1 yr−1; subplot: 0, 20, 40 and 80 kg P2O5 ha−1 yr−1) was conducted to evaluate the effect of N and P addition on aboveground net primary production (ANPP), light availability, diversity and functional group composition. N and P addition slightly increased ANPP mainly benefiting from clonal species and had few effects on species composition and diversity due to the deficient precipitation during growing season in 2017. However, N or P addition alone resulted in significant ANPP increase, and adding N and P together had larger effects in 2018. The changes in ANPP and diversity after P alone addition were driven by legumes. The ANPP responses to 50 and 100 kg N ha−1 yr−1 combined with P addition in 2018 were mainly driven by pronounced increases in tall clonal or annual species. This large shift of species composition caused diversity decline only in TCF community. The extent of diversity decline was significantly correlated with the degree of light availability reduction. Diversity decline in TCF was a result of strong and uniform light availability reduction induced by increased tall annual forb. Tall clonal and annual species are the key functional groups that drive changes in productivity and diversity in the two communities. The optimum N and P amount to tradeoff productivity improvement and diversity decline were 50 kg N ha−1 yr−1 and 20 kg P2O5 ha−1 yr−1 combination for TCG and 25 kg N ha−1 yr−1 and 20 kg P2O5 ha−1 yr−1 combination for TCF.