Impact of Long‐Term Organic No‐Tillage on Soil Aggregation, Aggregate‐Associated Carbon, and Soil Water Retention

In organic farming, intensive tillage for weed control can degrade soil structure and reduce organic carbon and water retention. Organic no‐tillage with surface mulch offers a potential solution, but its long‐term effectiveness in Andosol soil remains poorly understood. This study evaluated the long‐term effects of organic no‐tillage practices combined with organic mulch on soil organic carbon (SOC) content and storage, water‐stable aggregate distribution (WSAD%), aggregate‐associated carbon, carbon stabilisation within aggregates, bulk density (BD), mean weight diameter (MWD), and soil water retention. The field experiment was conducted using a randomised complete block design with four replications, comparing two treatments: no‐tillage (NT) and conventional tillage (CT), both supplemented with organic mulch. Compared with CT, NT treatment yielded a 65.4% increase in SOC content and a 16.9% decrease in bulk density in the 0–5 cm surface layer, triggering a 16.3% improvement in SOC accumulation in the 0–20 cm soil depth. Furthermore, WSAD and MWD increased at the 0–5 cm, 5–10 cm, and 10–15 cm soil depths. Compared with CT, at all depths, aggregates exceeding 4 mm showed high SOC content and increased SOC stock within macroaggregates in NT, with field capacity and total plant available water (PAW) content also increasing. This study emphasises the critical role of SOC in enhancing PAW directly and the mean weight diameter in indirectly improving it by regulating field capacity. Overall, in organic farming, NT with residue retention markedly facilitates the overall improvement of SOC, soil structure stability, SOC stabilisation within macroaggregates, and soil water retention.