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 N-15 (1.5 g N-15 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( 15)N addition), most N-15 was immobilized in soils and microbes, while less was taken up by plants. Soil and microbial N-15 immobilization were significantly higher when grazing was prohibited, although plant 15 Nacquisition was not affected by grazing. After initial retention, rapid N-15 loss was observed in microbes and soils, while N-15 levels were sustained longer in plants and litter. The N-15 residence times were longer when grazing was prohibited. The 15 Nacquisition capacity varies among plant taxa: perennial grasses and forbs accumulated N-15 rapidly, while sagebrush and legumes acted slowly. Although the added N-15 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.