Contrasting Freeze–Thaw Regimes and Straw Amendment Reshape Microbial Necromass and Soil Carbon Pools in Saline–Alkali Paddy and Maize Soils

Soil carbon dynamics in seasonal freeze–thaw zones reflect the interplay of two temporal regimes: high-frequency symmetric cycles driven by diurnal temperature fluctuations, and low-frequency asymmetric cycles with extended freezing and brief thawing. Their differential association with microbially derived carbon pools remains unclear. We incubated saline–alkali paddy and maize soils for 45 days under three regimes: a 5 °C control (CK), symmetric freeze–thaw (12 h/12 h at 5 °C/−10 °C; SFT) and asymmetric freeze–thaw (12 h/60 h at 5 °C/−10 °C; ASFT), each with and without straw. Dissolved organic carbon (DOC), microbial biomass carbon (MBC), microbial necromass carbon (MNC), fungal necromass carbon (FNC), bacterial necromass carbon (BNC) and the FNC/BNC ratio were determined at six sampling times. The asymmetric regime depleted the biomass and necromass pools most strongly and accumulated the most DOC; the symmetric regime produced smaller changes. Straw amendment reversed this depletion under the asymmetric regime and raised every necromass pool above its initial value, but it had the opposite effect at constant temperature and only a marginal effect under the symmetric regime. Substrate supply, rather than freezing stress alone, therefore appears to constrain necromass accumulation. The biomass and necromass pools changed in a coordinated way, whereas DOC was uncorrelated with them. Straw amendment may thus offset the suppression of necromass accumulation under the asymmetric freeze–thaw regime. Because the two regimes differ in several cycle components at once, the differences are attributed to the regimes as a whole, and the results describe a 45-day incubation rather than long-term carbon stabilization.