Plant residue quality regulates mineral-associated organic matter formation rate and pathway in converted upland soil

Driven by shifts in environmental conditions and changes in plant residue quality, converting lowland paddy fields to upland fields substantially alters soil organic matter (SOM) pools. However, the impact of residue quality on SOM dynamics in converted upland soils remains unclear. Here, we evaluated residue-derived carbon (C) accumulation patterns and stabilization pathways in converted upland soils using an in-situ incubation experiment with six different ¹³C/¹⁵N-labeled plant residues (leaf, stem, root) of soybean and maize (C:N ratios = 15.3-53.6; lignin = 5.9-9.2 %) applied to converted upland soils (pH = 6.15; SOM = 13.9 g C kg^-1; clay = 32.8 %). Total C, N, and the atom percentages of ¹³C and ¹⁵N were measured in bulk soil and SOM fractions at 30, 90, 180, and 360 days after installation (DAI). The results showed that residue-derived C in mineral-associated organic matter (MAOM) was similar, ranging 23.6 %-25.5 % of added C across residues, except for soybean root (28.3 %), while the accumulation rate and pathway varied by residue quality. In the initial stage (0-30 DAI), lower C:N ratios and lignin contents correlated with faster MAOM formation, indicating that high-quality residues rapidly generate MAOM. These rapidly neoformed residue-derived MAOM were dominant (74 % of total neoformed MAOM) and remained stable across 1-year incubation, as indicated by their consistent C:N ratio. Later (30-90 DAI), MAOM formation rates rose with increasing C:N ratios and lignin, showing lower-quality residues generated MAOM more rapidly and consistently-likely through continued conversion of free/occluded particulate organic matter, supported by a decline in their C:N ratios. Our study demonstrates that residue quality governs the rate and pathway of residue-derived C stabilization: high-quality residues rapidly form stable MAOM, whereas low-quality residues drive long-term MAOM accumulation. These insights underscore the critical role of residue type and quality in regulating SOM dynamics in converted upland soils.