Low‐Quality Residues Drive Long‐Term Carbon Stabilisation in Humid Temperate Upland Soils

Understanding the formation of soil organic carbon (SOC) is key to assessing soil carbon (C) pools. This study examined how plant residue quality regulates SOC formation, with a focus on particulate organic matter (POM) and mineral‐associated organic matter (MAOM), by incubating C 4 residues (maize, sorghum, and miscanthus) separated into roots, stems, and leaves and spanning a wide range of C:N ratios (15.3–69.4) and lignin contents (6.0%–26.9%) in upland soil converted from a C 3 paddy field. Residue‐derived C was monitored in the bulk soil and organic matter fractions during a 1‐year in situ incubation. Measurements of total soil C and δ 13 C at 30, 90, 180, and 360 days after incubation (DAI) were used to trace the stabilisation of the added C 4 residues. These findings indicate that residual biochemical traits play a vital role in C stabilisation. Leaf residues with low C:N ratios and lignin content (high‐quality residues) decomposed rapidly and were incorporated into the MAOM within the first 30 DAI. In contrast, stem and root residues with higher C:N ratios and lignin contents (low‐quality residues) decomposed more slowly; however, by 360 DAI, root residues contributed 8.5% ± 3.9% more to MAOM formation than high‐quality residues. Additionally, MAOM formation was negatively associated with residue C:N ratio and lignin content during the early stage (≤ 30 DAI), but this trend reversed at later stages. Moreover, a comparison of the MAOM formation rate across incubation periods indicated that continuous microbial processing of the remaining POM during the later stages contributed to greater MAOM neoformation from low‐quality residues, despite their lower initial transformation rates. Therefore, recalcitrant residues represent an essential yet often underappreciated source of long‐term SOC sequestration in converted upland soils in humid temperate regions.