Active Al/Fe and native soil organic matter exert time-dependent controls on residue-derived carbon retention in particulate and mineral-associated organic matter

Soil carbon (C) sequestration depends on the balance between the mineralization of plant residue inputs and their stabilization as soil organic matter (SOM). Particulate OM (POM) and mineral-associated OM (MAOM) are the major SOM fractions; however, the factors controlling residue-derived C mineralization and incorporation into these fractions remain unclear, especially over time. Herein, a 343-day incubation experiment was conducted by adding 13 C-labeled maize residue to soils varying in acid oxalate extractable Al and Fe (active Al/Fe: 10–127 cmol kg −1 ) and soil organic C (SOC: 11–116 g C kg −1 ) contents to investigate controls on residue-derived C mineralization and retention in SOM fractions. Native SOM composition was characterized using pyrolysis-GC/MS. Over the 343-day incubation, 36 ± 9% of added plant-residue was mineralized. In the early stage (0–28 days), soils with greater native OM lability (indicated by Py-GC/MS-derived molecular composition) retained more residue-derived C in POM, possibly due to competition between native SOM- and added OM-derived C for microbial utilization and mineral binding sites. During mid-to-late incubation stages (28–343 days), active Al/Fe was positively correlated with residue-derived C in MAOM and POM after controlling for SOC, suggesting the stabilization of plant residue by active Al/Fe promoted sorption and aggregation. SOC was negatively correlated with residue-derived C in MAOM after controlling for active Al/Fe, consistent with reduced availability of active Al/Fe surfaces for association with residue-derived OM due to competition. In addition, residue-derived C in MAOM declined by 32 ± 15% from day 28 to day 343, indicating a relative instability of some newly formed MAOM components. This study highlights two major stages of residue-derived C incorporation into OM fractions during decomposition and shows that the retention of residue-derived C in POM and MAOM was governed not only by the contents of active Al/Fe and SOC but also by native SOM composition.