Amorphous aluminum hydroxide and rice-husk biochar enhance new organic carbon stabilization via different mechanisms

Increasing soil organic carbon (SOC) levels is essential for sustainable agricultural productivity and climate change mitigation, particularly in alkaline soils with inherently low SOC. While amorphous Al hydroxide (Am-Al) significantly influences SOC stabilization in volcanic and humid-region soils, and biochar enhances SOC in temperate and tropical regions, their effectiveness and stabilization mechanisms in alkaline soils require further exploration. We conducted a 1-year incubation of low SOC alkaline soils (5.2 g kg−1) amended with 13C-labeled maize residue (1 g kg−1), with or without Am-Al or rice-husk biochar (each 10 g kg−1); residue mineralization/retention was quantified and molecular composition profiled by solid-state 13C NMR and Py-GC/MS. Rapid decomposition of plant residue ceased around 12 weeks, while plant residue-derived C and native SOC decomposition continued throughout the incubation period. Am-Al significantly reduced maize mineralization within the initial two weeks and retained a higher proportion of residue-derived C than the control soil with maize addition after one year (Am-Al: 36% vs. Control: 28%). 13C NMR and pyrolysis–GC/MS showed smaller decreases in carbohydrate-C and saccharides and a higher carbon preference index and odd–even predominance of alkanes, indicating that Am-Al better preserved carbohydrate- and cuticular-wax-derived components, proxies for less-degraded residues. Respiration dynamics and molecular fingerprints indicate Am-Al rapidly stabilizes labile plant compounds, possibly through non-electrostatic sorption and ligand-exchange. Biochar also retained more residue-derived C (33%) than the control, but its effects on mineralization emerged later in the incubation (>6 months). We attribute this lag to surface degradation/activation of the biochar, which may stabilize residue-derived C more efficiently. Overall, adding Am-Al or biochar with plant residues significantly increased residue-derived C retention through immediate and delayed mechanisms, respectively. Treatments combining Am-Al or biochar with plant residue yielded a net positive C balance over the incubation, whereas residue alone was negative. Thus, the application of Am-Al and biochar with plant residues represents a promising strategy for sustained C stabilization, thereby improving SOC in degraded alkaline soils.