The retention dynamics of N input within the soil–microbe–plant system in a temperate grassland

In N-limited temperate regions, atmospheric N deposition remains high over the non-growing season. However, the retention dynamics of non-growing season N input within the ecosystem remain unclear. Using an isotopic approach, we investigated the initial retention and subsequent dynamics of 15N (1.5 g 15N m−2) in the soils, microbes, plants, and litter over three years in grazing-prohibited (PG) and heavily grazed treatments (HG) in northern China. For initial retention (21 days after 15N addition), most 15N was immobilized in soils and microbes, while less was taken up by plants. Soil and microbial 15N immobilization were significantly higher when grazing was prohibited, although plant 15N acquisition was not affected by grazing. After initial retention, rapid 15N loss was observed in microbes and soils, while 15N levels were sustained longer in plants and litter. The 15N residence times were longer when grazing was prohibited. The 15N acquisition capacity varies among plant taxa: perennial grasses and forbs accumulated 15N rapidly, while sagebrush and legumes acted slowly. Although the added 15N had significant contribution to early spring N demands of soil microbes and plants, it did not increase microbial or plant biomass N. Our results showed that non-growing season exogenous N was primarily retained by soil biota in temperate grasslands in the early stage, but N retention is finally sustained in soil and plants. The findings highlighted the importance of soil microbes in sustaining N upon N input, inferring the needs for considering the microbial role for better understanding N cycling in the temperate grasslands.