Snow depth shifts greenhouse gas balance during freeze–thaw periods in grasslands

Freeze–thaw periods represent transient yet climatically significant episodes of greenhouse gas (GHG) exchange in seasonally frozen ecosystems. Projected changes in winter snowfall are expected to alter the balance among carbon dioxide (CO ₂ ), nitrous oxide (N ₂ O) and methane (CH ₄ ) fluxes, yet these short‐lived non‐growing season pulses remain poorly constrained because winter GHG measurements are technically and logistically challenging, hindering accurate assessment of their aggregate impact on ecosystem climate forcing across environmental gradients.

We combined a 1500‐km transect spanning arid to meadow grasslands with a controlled snow manipulation incubation experiment using intact soil cores to disentangle snow depth effects on freeze–thaw‐induced GHG balance and assess their consequences for ecosystem global warming potential (GWP) across ecoregions.

Deepened snow increased freeze–thaw GWP primarily through increases in water‐filled pore space (WFPS), which strongly stimulated N ₂ O emissions (980%–1589%) while exerting weaker and site‐dependent effects on CO ₂ (32%–54%) and CH ₄ (25%–100%). As WFPS increased, the relative contribution of N ₂ O to total GWP surged from 1.4% to 82.8%, while that of CO ₂ decreased from 98.3% to 17.1%. Meanwhile, CH ₄ shifted from a weak sink (−0.6%) to a net source (1.7%). This pattern reflected a moisture‐driven trade‐off between CO ₂ and N ₂ O‐derived contributions to freeze–thaw GWP. Crucially, plant species richness exerted dual control over GHG emissions: it promoted CO ₂ emissions through fungal‐mediated increases in enzyme activity, particularly associated with Basidiomycota, while mitigating N ₂ O emissions by reducing nitrate (NO ₃ ⁻ ‐N) availability and arbuscular mycorrhizal fungi‐associated suppression of denitrification.

Synthesis : Our findings reveal that snow‐driven changes in soil moisture reactivated a cross‐seasonal plant–fungal legacy that shaped microbial GHG dynamics. Thus, integrating vegetation–microbe linkages alongside winter climate processes is critical to improving the prediction of non‐growing season carbon–nitrogen feedback under climate change.