Organic Matter Accumulates in Soil Silt and Clay Fractions Following Decomposition of High Quality Substrates.

Substrate quality impacts the rate of microbial decomposition of soil organic matter (OM), with higher quality substrates leading to faster rates of decomposition. Since OM decomposition is the opposite of OM stabilization, one might presume higher quality substrates would lead to less OM stabilization. Yet, there is growing evidence that C stabilized in the soil silt and clay fractions is preferentially derived from microbial metabolites. We hypothesized that the decomposition of higher quality substrates would stimulate silt and clay-associated soil (S)OM despite higher initial mineralization rates. Soils low in initial organic carbon were incubated for 139 d with substrates spanning a range of quality/lability including: (a) bermudagrass forage cut after 14d, 21d, 28d, 35d, and 42d of re-growth, and (b) ruminal digesta produced from these forages. We then monitored the production of CO₂; the carbon abundance and isotopic composition in the bulk, silt, and clay fractions; and counts of bacterial and fungal colony forming units (CFU). Undigested forage was respired at higher initial rates than ruminal digesta and resulted in higher bacterial abundance and more carbon (C) and nitrogen (N) in the clay fraction. Overall, substrate quality—assessed as the ratio of neutral detergent fiber (NDF-cellulose, hemicellulose, and lignin) to crude protein (CP)—was directly related to decomposition kinetics and with higher substrate quality resulting in more clay C and N. Our results show that the addition of labile substrates to sandy soils will promote the accumulation of mineral-associated SOM. These findings suggest there is relationship between SOM stability and organic matter amendments that should be incorporated into conceptual and numerical models of SOM generation and turnover.