Differential responses of grasses and forbs led to marked reduction in below‐ground productivity in temperate steppe following chronic N deposition

Abstract

Summary Enhanced deposition of atmospheric nitrogen (N) has profound impacts on ecosystem processes such as above‐ground productivity and community structure in grasslands across the globe. But how N deposition affects below‐ground processes of grasslands is less well known. Here, we evaluated the effects of chronic N amendment at a relatively low rate (20 kg ha−1 year−1) on root traits (root productivity, root biomass, root/shoot ratio) in Inner Mongolia steppes by rhizotron and ingrowth core and soil monolith techniques at levels of individual species, functional groups and ecosystem. For 8 years, N amendment suppressed above‐ground net primary production (ANPP), photosynthetic rates and root biomass of forbs, but enhanced ANPP and root biomass of grasses. This led to an overall reduction in below‐ground productivity of the grassland by 24–33%, while ANPP remained unchanged. Nitrogen amendment acidified soil and subsequently increased extractable soil manganese (Mn) concentration. Nitrogen amendment increased foliar Mn concentrations in forb, but not grass species, leading to a significant inhibition of photosynthetic rates in forb species. Synthesis. These findings highlight the importance of the differentiating responses of plant functional groups to long‐term N deposition and the important consequences of these responses for below‐ground productivity and long‐term soil C sequestration.