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 through persistent mineralization processes. Climate warming has accelerated snowmelt and intensified freeze-thaw cycle frequency (FTC), potentially altering the availability of winter N sources for plants. We simulated intensified FTC regimes (increased 0, 6, and 12 cycles) in situ across two contrasting temperate grasslands, employing dual-labeled isotopes (¹⁵NH₄¹⁵NO₃) to quantify winter N dynamics. Our results showed that intensified FTC significantly enhanced soil net ammonification rates and inorganic N levels in early spring, while net nitrification rates remained stable. This suggests that frequent FTC may provide a substantial N source for soil microorganisms and plant growth. Notably, soil microbial biomass N increased despite microbial C limitation, indicating efficient microbial N competition that restricted plant access to winter N sources. Intensified low-frequency FTC did not affect plant ¹⁵N acquisition, whereas high-frequency FTC significantly reduced plant ¹⁵N acquisition. Importantly, the impacts of FTC on plant ¹⁵N acquisition varied among functional types. Dominant cold-tolerant species (perennial bunch grasses and semi-shrubs) increased ¹⁵N acquisition, likely due to earlier root activity, while subordinate species (perennial rhizome grasses and forbs) exhibited reduced acquisition. In conclusion, while intensified FTC did not lead to the loss of winter N sources, it restructures N availability by favoring microbial retention and creating competitive hierarchies among plants in temperate grasslands. The high-frequency FTC-induced shifts in partitioning of winter N resources could substantially influence grassland productivity and community structure, highlighting the critical need to integrate winter climate change effects into temperate grassland ecosystem models.