No-tillage promotes topsoil carbon storage in humid Andisols through bacteria-centered trophic reorganization and microbial necromass accumulation

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Long-term no-tillage (NT) alters soil habitat and biota organization, yet its consequences for cross-kingdom interactions and soil organic carbon (SOC) storage in humid Andisols remain unclear. We investigated a 22-year field experiment comparing NT and conventional tillage (CT), with and without cover cropping, across topsoil (0–5 cm) and subsurface soil (5–15 cm) over three seasons. Soil physicochemical properties, microbial biomass C (MBC), β-glucosidase activity (BG), bacterial and fungal gene copy numbers were analyzed. We characterized bacterial, fungal, protistan, and nematode communities, inferred SPIEC-EASI-based cross-kingdom networks, and quantified microbial necromass as an indicator of accumulated microbial-derived SOC. NT exerted stronger effects than cover cropping and created a topsoil-oriented system. In topsoil, NT increased SOC, moisture, available N & P, MBC, BG, and bacterial and fungal gene copies, whereas CT maintained relatively higher values in subsurface soil. Bacterial, but not fungal, copies strongly correlated with MBC ( r = 0.86***), indicating a bacteria-associated agroecosystem. Tillage significantly altered all four biotic groups, and NT topsoil bacteria were enriched in taxa associated with polymer decomposition. NT topsoil also developed a larger, denser, and more robust network than CT topsoil, with more bacterial nodes, within-bacteria links, and bacteria–protist/nematode links. Functional composition of protistan and nematode nodes indicated a more developed bacteria-based predator–prey web under NT. Procrustes analysis suggested shared edaphic filtering for bacteria and fungi, whereas bacteria–protist/nematode concordance reflected bacteria-based trophic coupling that was maintained under NT but weakened under CT. Despite these shifts, BG/MBC remained similar, indicating limited changes in microbial metabolic efficiency. In contrast, bacterial and fungal necromass increased under NT in topsoil but decreased in subsurface soil. These results indicate that, in humid Andisols, NT promotes topsoil carbon storage mainly by increasing microbial biomass, strengthening bacteria-centered trophic coupling, and enhancing microbial necromass accumulation, rather than by shifting microbial metabolic efficiency.

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