Evaluation of carbon mineralization and its temperature sensitivity in different soil aggregates and moisture regimes: A 21-year tillage experiment

Characterizing soil organic carbon (SOC) mineralization and its temperature sensitivity (Q₁₀) under different soil moisture in tillage systems is crucial for determining global carbon balance under climate warming and increasing precipitation. Aggregate protection can potentially govern SOC mineralization and its Q₁₀. However, how tillage and aggregate sizes affect SOC mineralization and its Q₁₀, especially under varying soil moisture, remains unclear. Soil samples (0-10 cm and 10-20 cm) were collected from a 21-year field study with four tillage treatments: conventional tillage (CT), reduced tillage (RT), no-tillage (NT), and subsoiling (SS). Bulk soil and dry-sieved aggregates were incubated at 15°C and 25°C at low, medium, and high moistures (i.e., 40%, 70%, and 100% water-holding capacity, respectively). Macro-aggregates (> 0.25 mm) had lower SOC mineralization relative to micro-aggregates (< 0.25 mm) across all soil temperatures, moistures, and depths (P < 0.01), which was attributed to their lower SOC quality (i.e., higher ratio of SOC to total nitrogen and lower ratio of dissolved organic carbon to SOC). Moreover, NT and SS promoted macro-aggregation relative to CT and RT, and thereby decreased mineralization (P < 0.001). However, Q₁₀ was higher in macro-aggregates than in micro-aggregates at low and medium moistures. The Q₁₀ was negatively correlated with the SOC quality in macro-aggregates (P < 0.05). The macroaggregate-associated SOC quality was lower under NT and SS than under CT and RT, which resulted in a greater Q₁₀ under NT and SS at low and medium moistures, suggesting that NT and SS may accelerate SOC losses under global warming. Furthermore, increased soil moisture could lower Q₁₀, and no differences among tillage practices were observed at high moisture levels (P > 0.05). Overall, our findings indicated that NT and SS decreased SOC mineralization but increased Q₁₀ because of their large amounts of macro-aggregates with low SOC quality, and the improvement of Q₁₀ was constrained by increasing soil moisture.