No‐Tillage History and Residue Application Methods Regulate Net Carbon Balance via Contrasting Priming Responses in Andosols

ABSTRACT

No‐tillage (NT) is promoted as a climate‐change mitigation practice for its potential to enhance soil organic carbon (SOC) sequestration. Yet the effects of long‐term NT legacies and residue application methods on new C stabilization, priming effects (PE), and net C balance in humid Andosols remain unclear. Here, we conducted a 1‐year field incubation experiment using ¹³ C‐labelled plant residue (C/N = 25.8) in an Andosol collected from long‐term NT and conventional tillage (CT) sites in Japan. Residues were applied under three treatments (n = 4): surface application on intact NT soil (NTSA), surface application on CT soil with disrupted structure due to both tillage and sampling effects (CTSA), and incorporation into CT soil with similarly disrupted structure (CTMIX), and controls without residue additions (NTCK and CTCK). Soil‐ and residue‐derived CO ₂ emissions were measured over 375 days. Soils were collected at days 28, 91, 183, and 375 to quantify residue‐derived C remaining and microbial biomass C. PE and net C balance were also assessed. Surface residue application (NTSA and CTSA) resulted in greater residue mineralization than CTMIX, indicating that surface‐application‐driven CO ₂ emissions were stimulated regardless of tillage history. PEs were greater in CTSA (+2690 mg C kg ⁻¹ soil) than in NTSA or CTMIX (+154 and −518 mg C kg ⁻¹ soil, respectively). In the surface layer (0–2.5 cm), the ratio of microbially immobilized residue‐derived C to the total MBC was higher in CTSA than in NTSA, indicating that long‐term NT may reduce microbial responses and consequently suppress PE. Accordingly, NTSA and CTMIX showed a more favourable net C balance than CTSA. These results indicate that long‐term NT promotes SOC retention through soil structural preservation and microbial adaptation to surface C inputs, whereas residue mixing may compensate tillage‐induced SOC losses. Overall, long‐term NT legacies and residue application methods regulated SOC dynamics through distinct mechanisms in humid Andosols.