Enrichment of lignin-derived carbon in mineral-associated soil organic matter.

Abstract

A modern paradigm of soil organic matter proposes that persistent carbon (C) derives primarily from microbial residues interacting with minerals, challenging older ideas that lignin moieties contribute to soil C because of inherent recalcitrance. We proposed that aspects of these old and new paradigms can be partially reconciled by considering interactions between lignin decomposition products and redox-sensitive iron (Fe) minerals. An Fe-rich tropical soil (with C4 litter and either 13C-labeled or unlabeled lignin) was pre-treated with different durations of anaerobiosis (0-12 days) and incubated aerobically for 317 days. Only 5.7±0.2% of lignin 13C was mineralized to CO2 versus 51.2±0.4% of litter C. More added lignin-derived C (48.2±0.9%) than bulk litter-derived C (30.6±0.7%) was retained in mineral-associated organic matter (MAOM; density > 1.8 g cm-3), and 12.2±0.3% of lignin-derived C vs. 6.4± 0.1% of litter C accrued in clay-sized (< 2 \uf06dm) MAOM. Longer anaerobic pre-treatments increased added lignin-derived C associated with Fe, according to extractions and nanoscale secondary ion mass spectrometry (NanoSIMS). Microbial residues are important, but lignin-derived C may also contribute disproportionately to MAOM relative to bulk litter-derived C-especially following redox-sensitive biogeochemical interactions.