Efficient utilization of winter nitrogen sources by soil microorganisms and plants in a temperate grassland

In seasonal snow-covered regions, the soil microbial community remains highly active, enabling nitrogen (N) mineralization processes to occur even during the winter. Therefore, winter N accumulation is likely to be a primary N source for plant growth in the subsequent spring. However, the availability of winter N resources (i.e., ammonium and nitrate N) to soil microorganisms and plants in temperate grasslands remains unclear. Here, we applied 15NH4Cl and K15NO3 tracers before autumn-winter freezing, and then investigated the retention of both 15N tracers in soil microorganisms and different plants during the winter and spring period in a typical steppe. The results showed that approximately 33 % and 27 % of 15NH4Cl and K15NO3 tracers were immobilized by soil microorganisms, while little had been uptake by plants during winter. In the subsequent spring, soil retained 32 % and 34 %, microorganisms immobilized 5.1 % and 5.2 %, plant acquired 40 % and 28 % of the 15NH4Cl and K15NO3 tracers, respectively. At a temporal scale, perennial bunch grasses first acquired 15N in the winter-spring transition, followed by perennial rhizome grasses and forbs in early spring, while legumes utilized little 15N. In contrast, the 15N absorption capacity of perennial rhizome grasses and perennial forbs was similar to that of bunch grasses. Additionally, only dominant bunch grasses showed a preference for ammonium N over nitrate N, while other plant species did not exhibit a clear preference for different forms of N. Our results suggest that the temporal differentiation of N utilization between plants and microorganisms enhances the availability of winter N sources. Importantly, the differences among plant species in the timing and quantity of winter N resource utilization contribute to species coexistence in temperate grasslands. This study highlights the crucial role of winter N accumulation as a primary N source for plant growth in temperate grasslands. Understanding the availability of winter N sources for plant and microbial growth is essential for predicting plant community dynamics and species coexistence in these ecosystems.