Enrichment of Lignin-Derived Carbon in Mineral-Associated Soil Organic Matter

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 redoxsensitive iron (Fe) minerals. An Fe-rich tropical soil (with C-4 litter and either C-13-labeled or unlabeled lignin) was pretreated with different durations of anaerobiosis (0-12 days) and incubated aerobically for 317 days. Only 5.7 +/- 0.2% of lignin C-13 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 mu m) MAOM. Longer anaerobic pretreatments 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.