Ensuring the maintenance and efficient cycling of soil organic matter (SOM) is key to the provision of several ecosystem services, including food production and climate regulation, but poses what Dr. Henry Janzen named the “Soil C dilemma”. Independent of climate, plant inputs, microbial, and soil mineral traits drive the formation of SOM, in particulate (POM) or mineral-associated (MAOM) form, as well as its mineralization. While MAOM is thought to form from the microbial transformation of POM, we recently proposed that the two are formed from independent pathways of residue decomposition. MAOM would form from the efficient microbial transformation of solubles and accessible polymers, and require soil mineral capacity for its accumulation, but could persist over centuries to millennia. By contrast, POM would form from the fragmentation and partial transformation of residue structural components, and mineralize co-metabolically on a yearly-to-decadal time frame, contributing to nutrient recycling. We have been conducting several laboratory incubations and field studies, using isotopically labeled above and below-ground plant residues, and differing soil mineral matrices to test this hypothesis. More recently we have formalized it into the MEMS model, a parsimonious model of soil C cycling which uses physico-chemically defined MAOM and POM pools.
Results/Conclusions
Using dual (13C and 15N) differentially labeled plant residue we demonstrated that soluble residue components contributed, in the short term, to the formation of MAOM. Conversely, C and N-derived from residue structural compounds were only found in light POM. In slurry incubations, more efficient MAOM formation was realized from above than below-ground plant residues. We suggest this was due to the relative high concentration of acid unhydrolizable structures, and higher C:N of solubles in below-ground residues. These results were largely confirmed in the field as well as in another laboratory experiment using a higher variety of litters and two contrasting soils. On-going work aims to capture the longer-term fate of POM, to verify whether it contributes to MAOM formation or only to nutrient recycling. Representation of the two pathway hypothesis in the MEMS model is proving to be robust, further supporting the concept that there are discrete pathways to MAOM and POM formation in soil.