Seasonal Synchrony of Aggregate Turnover and Carbon Stabilization Pathways in Mollisol Under Contrasting Tillage

ABSTRACT Soil structural degradation, driven by the loss of stable aggregates and organic carbon (OC), undermines productivity in Northeast China's Mollisol region. While seasonal shifts in surface soil structure are known, the vertical synchronization of these dynamics across the deeper soil profile and the specific mathematical reciprocity of aggregate mass transfer remain poorly understood. This study quantified seasonal trajectories of water‐stable aggregates (WSA), particulate organic matter (POM), and soil organic carbon (SOC) under conventional tillage (CT), no‐till (NT), and abandoned land (Ab) across a 0–40 cm profile. Using periodicity‐constrained sinusoidal regressions (T = 365 days), we identified a consistent “structural breathing” pattern across the soil continuum. Model selection using Akaike and Bayesian information criteria confirmed that these non‐linear periodic fits were the most parsimonious compared to linear models, proving the cyclical nature of the data. Results revealed a fundamental anti‐phase geometric reciprocity (Δ ϕ ≈ π ) between size classes: the mid‐summer contraction of macroaggregates (5–10 mm) was quantitatively mirrored by the expansion of intermediate hierarchies (0.5–1 mm), confirming a closed‐loop mass transfer triggered by the East Asian Monsoon. Conservation regimes (Ab and NT) fostered a “structural memory” that dampened oscillation amplitudes and enhanced surface SOC occlusion. Conversely, CT promoted macroaggregate disintegration and the translocation of carbon into the 20–40 cm layer as mineral‐associated OC. The high coefficients of determination ( R 2 : 0.722–0.999) are physically consistent with a phase‐locked turnover synchronized to the annual hydrothermal cycle. Adopting conservation tillage is therefore essential to buffer the seasonal climatic pulse, mitigate physical degradation, and enhance carbon sequestration in monsoon‐affected Mollisol ecosystems.