Mechanisms controlling the stabilization of soil organic matter in agricultural soils as amended with contrasting organic amendments: Insights based on physical fractionation coupled with 13C NMR spectroscopy

Soil organic matter (SOM) has vital roles in the global carbon (C) cycle and the use of organic amendments (OAs) to maintain or improve SOM levels is a promising practice. However, the mechanisms underlying SOM stabilization in OA-amended soils remain insufficiently resolved. For more effective understanding on such issues, we examined soil samples from a long-term experimental plot (26–31 years) that included six treatments, namely, a chemical fertilizer (CF) alone (CF), a bark compost plus CF (BC + CF), a coffee residue compost plus CF (CRC + CF), a cattle manure compost plus CF (CMC + CF), and a cattle manure (CMC) or sewage sludge compost (SSC) alone at a higher application rate, using physical fractionation. In the fractionation, free particulate SOM (fSOM), free SOM occluded in aggregates (oSOM), and SOM weakly bound to minerals (wSOM; s.g. 1.6–2.0 g cm−3) and strongly (sSOM; s.g. >2.0 g cm−3) were separated and characterized using 13C nuclear magnetic resonance (NMR) along with OAs and bulk soil samples. The long-term OA applications enhanced the total C accumulation and the amount of C accumulated as oSOM, wSOM, and sSOM correlated positively with the total C content. The application of BC or CRC resulted in a greater accumulation of fSOM. Conversely, the continuous application of SSC or CMC, which has a high N content and a low C/N ratio, led to a greater accumulation of C, mainly as wSOM. Our findings suggest that both the quality and quantity of OAs control the forms of C that accumulate and this involves different mechanistic pathways. We suggest that the abundant alkyl C in the wSOM and sSOM fractions was SSC-derived SOM in the SSC soil while in the case of the CMC soil, this was due to the increased contribution of microbial-derived SOM.