The Dual Filter Model: Labile Plant Inputs Form Stable Soil Organic Matter

The decomposition of above and below-ground plant detritus (litter) is the main process by which soil organic matter (SOM) is formed. Yet, research on litter decay and SOM formation has been uncoupled, failing to provide an effective nexus between these two fundamental processes for carbon (C) and nitrogen (N) cycling and storage. However, a unified framework for integrating litter decomposition and SOM formation is emerging, whereby microbial resource use efficiency and allocation of C and N, in interaction with the soil matrix, controls the proportion of plant derived-C and -N that is retained in long-term SOM pools. We synthesize this emerging understanding and call it the Dual Filter Model, with the two filters being microbial resource use efficiency and soil matrix interactions. This model suggests that, since labile plant constituents are degraded more efficiently by microbes, they are the dominant source of microbial products, relative to input rates. Via promoting aggregation and through strong chemical bonding to clay particles, these microbial products of decomposition become the main precursors of stable SOM.