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

Abstract

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 (15NH415NO3) 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 15N 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 15N uptake, comparable microbial 15N immobilization, and similarly constrained 15N leaching. While HFTC reduced communitylevel plant 15N acquisition, it amplified competitive asymmetry among plant functional types: dominant cold-adapted species (early spring phenology and deeper roots) increased 15N uptake, while subordinate species (later-active, shallowrooted species) exhibited reduced 15N 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.