Effects of intensified freeze-thaw frequency on dynamics of winter nitrogen resources in temperate grasslands

In seasonal snow-covered temperate regions, winter serves as a crucial phase for nitrogen (N) accumulation, yet how intensified freeze-thaw cycles (FTC) influence the fate of winter-derived N remains poorly understood. We simulated intensified FTC regimes (increased 0, 6, and 12 cycles) in situ across two contrasting temperate grasslands, employing dual-labeled isotopes ( 15 NH 4 15 NO 3 ) to trace the dynamics of winter N sources. Our results showed that soil microbes exhibited a strategic adaptation to FTC stress characterized by C-N decoupling: despite a decline in microbial biomass C, they maintained or even increased biomass N. Intensified FTC did not cause ecosystem-level losses of winter N sources, primarily because the soil and microbes functioned as a crucial N reservoir during the vulnerable early spring period. The convergence in ecosystem-level 15 N retention emerged through distinct compensatory pathways: while the meadow steppe exhibited higher N mineralization potential, the sandy steppe achieved functionally equivalent retention through more efficient plant 15 N uptake, comparable microbial 15 N immobilization, and similarly constrained 15 N leaching. While HFTC reduced community-level plant 15 N acquisition, it amplified competitive asymmetry among plant functional types: dominant cold-adapted species (early spring phenology and deeper roots) increased 15 N uptake, while subordinate species (later-active, shallow-rooted species) exhibited reduced 15 N acquisition. These findings reveal that winter climate change restructures grassland N cycling primarily through biological mechanisms, microbial resilience and trait-mediated plant competition, rather than promoting N losses. Future climate models must incorporate these plant-microbe-soil interactions to accurately predict ecosystem trajectories under changing winter conditions.