Seasonal freeze-thaw cycles drive soil aggregate turnover and nutrient redistribution in black soil, Northeast China

Seasonal freeze–thaw cycles (FTCs) significantly influence soil nutrient sequestration by altering aggregate turnover dynamics (i.e., breakdown, stabilization, and formation). However, the specific responses of nutrients to seasonal FTCs at the aggregate level remain poorly understood. To bridge this knowledge gap, we conducted an in-situ seasonal FTC experiment to elucidate how FTC-driven aggregate turnover regulates nutrient redistribution through microbial-enzymatic coupling. To quantify soil aggregate turnover processes, we employed rare earth oxides (REOs) as tracers. Our results demonstrated that, compared to the non-FTC soil, although seasonal FTCs did not significantly alter overall aggregate stability, they enhanced the breakdown of ≥ 0.25 mm aggregates and reduced the formation of > 1 mm aggregates. Notably, FTC-treated soil exhibited a 5.21 % reduction in soil organic carbon within > 1 mm aggregates (P < 0.05). FTCs triggered nutrient redistribution across aggregate fractions, i.e., available phosphorus in < 0.25 mm aggregates and total potassium in both bulk soil and different-sized aggregates increased. Additionally, FTCs significantly reduced soil enzyme activities and enhanced bacterial alpha diversity. The partial least squares model (PLS-PM) confirmed that soil aggregate turnover drives nutrient redistribution directly or indirectly, with enzymes and microorganisms exerting the most significant effects. Our findings indicated that knowledge about soil aggregate turnover could be useful for exploring soil nutrient loss mechanisms and optimal agricultural fertilization in freeze–thaw agricultural ecosystems.