The role of plant input physical-chemical properties, and microbial and soil chemical diversity on the formation of particulate and mineral-associated organic matter

Soil organic matter (SOM) is a fundamental resource to humanity for the many ecosystem services it provides. Increasing its stocks can significantly contribute to climate change mitigation and the sustainability of agricultural production. Elucidating the mechanisms and drivers of the formation of the main components of SOM, particulate (POM) and mineral associated (MAOM) organic matter, from the decomposition of plant inputs is therefore critical to inform management and policy designed to promote SOM regeneration. We designed a two-tiered laboratory incubation experiment using 13 C and 15 N labeled plant material to investigate the effects of the physical nature (i.e., structural versus soluble) of plant inputs as well as their chemical composition on (1) the pathways of SOM formation, (2) the soil microbial community and chemical diversity, and (3) their interaction on the stabilization efficiency of litter-derived C in POM and MAOM, in a topsoil and a subsoil. We found that: i) the physical nature of the plant input (structural vs soluble) drove both the pathways and efficiencies of SOM formation; ii) POM formation from the decomposition of structural residues increased in efficiency the more decomposed were the residues, and linearly with soil microbial and chemical diversity, the latter only for subsoil; ii) more input-derived C and N were retained in subsoil because of both higher stabilization in MAOM and POM, and slower residue decay. Our results also confirm the importance of direct sorption of soluble inputs to silt- and clay-sized minerals for the formation of MAOM in bulk soils. Taken together these finding suggest that the highest potential for SOM accrual is in subsoils characterized by higher C saturation deficit, from the separate addition of decomposed residues and soluble plant inputs. • Plant soluble vs structural inputs drive pathways and efficiencies of SOM formation. • MAOM forms most efficiently from soluble plant inputs, likely by direct sorption. • POM forms most efficiently from highly decomposed plant residues. • Soil microbial and chemical diversity promote POM but not MAOM formation. • POM and MAOM form most efficiently in subsoil due to lower decay and C saturation.