Root tissue chemistry influences the formation and composition of new mineral-associated organic matter

Interest in managing soil organic matter through plant inputs is increasing, but the role of plant litter chemistry in organic matter cycling is still debated. While roots are an important carbon input, there are conflicting findings on how root litter chemistry affects the formation and composition of organic matter across soil types. Roots of seven plant species with diverse chemical composition were incubated for six months in two soil types differing in texture and pH. Soil respiration was measured regularly and the movement of root carbon into soil organic matter fractions was tracked using carbon-13 natural abundance. In both soils, litters with high guaiacyl and syringyl lignin units had less respiration and less transformation of litter C into heavy particulate organic matter (POM) and mineral-associated organic matter (MAOM). High suberin content decreased respiration and increased the recovery of litter C in light POM, but had no effects on its transfer to heavy POM or MAOM. On the other hand, p -hydroxyphenyl lignin units had positive effects on the transformation of litter C into MAOM but limited effects on respiration or recovery of litter C in POM. The litter treatments had similar effects on litter-derived MAOM across both soils despite overall less litter C in that fraction for the coarse, low pH soil. We also found evidence of chemical changes to the MAOM, with the ratios of lignin subunits shifting towards the ratios found in the litters. Our results highlight that lignin composition, in addition to total amount, seems to shape decomposition dynamics. Our results also support the idea that microbial processing of high-quality litters facilitates stabilization of C in MAOM. However, we show that regardless of degradability roots leave a chemical imprint on MAOM, particularly through their lignin composition, suggesting that direct contributions of plant C to MAOM cannot be overlooked.