Temporal coupling of plant and microbial nitrogen uptake and plant nitrogen reallocation improve grassland nitrogen retention under nitrogen addition

Efficient nitrogen (N) acquisition by plants and microbes and internal reallocation of plant N are crucial for improving ecosystem N retention. However, little is known about shifts in N use strategy between plants and microbes under N addition. We used a¹⁵N tracer to investigate effect of N addition (0–25 g N m⁻² yr⁻¹) on the N use and retention of plant–microbe–soil system in a temperate Chinese steppe. Adding 2–5 g N m⁻² yr⁻¹ increased the ecosystem's ability to retain ¹⁵N (65–76%) after 1 year. Plant N demand during the peak growth period depended strongly on rapid N immobilization by microbes during 7 days (immobilized 45% of ¹⁵N within 24 h). When root biomass exceeded 0.36 kg m⁻², plants competed more strongly for N than the microbes. N addition increased N reallocation from roots to support new shoot growth (79–88% of N), because increased shoot N demand triggered root N transfer when external N was supplied. Thus, the temporal coupling between plant and microbial N use and in N reallocation within plants significantly altered the semi-arid grassland N cycle, and better predicting impacts of future N deposition scenarios.