Subsoil tillage and straw mulching are synergistic for long-term improvement of soil carbon and structural characteristics

Subsoil tillage (S) improves the stability and quality of soil organic carbon (SOC) and soil structure. Combining straw mulching with subsoil tillage (SS) may further improve soil physical and biogeochemical properties, whilst enabling abundant straw resources to be productively and sustainably recycled. To address knowledge gaps regarding these treatments’ effects under long-term application and at deeper soil layers, we conducted a 14-year field experiment and analyzed changes to SOC, soil aggregate characteristics, SOC associated with various soil aggregate sizes, and soil structural stability indicators at high spatial resolution down to 1 m depth. Results indicate that SS increased the proportion of 0.5–2.0 mm soil aggregates throughout much of the soil profile, but decreased the proportion of smaller <0.25 mm aggregates at 0–20 cm depth. SS increased the total organic carbon (TOC) at 0–20 cm, TOC and labile organic carbon (LOC) content of various aggregate sizes at various depths, the relative contribution of 0.5–2.0 mm aggregates to TOC at 0–40 cm, and multiple soil structure stability indices at 0–20 cm. Although both S and SS improved soil properties, the spatial and quantitative extents of the improvements are greater under SS. Correlation analyses indicate that improvements in SOC, soil stability, and aggregate properties are positively correlated, implying minimal trade-offs in prioritizing SS over S. These findings highlight long-term synergistic interactions: subsoiling mixes decomposed straw mulch applied in previous years into deeper soil, reinforcing the interdependent processes of aggregate formation and stabilization, along with SOC generation and protection, across more extensive soil depths.