Pathways of mineral-associated soil organic matter formation: integrating the role of plant carbon source, chemistry, and point-of-entry.

To predict the behavior of the terrestrial carbon cycle, it is critical to understand the source, formation pathway, and chemical composition of soil organic matter (SOM). There is emerging consensus that slow-cycling SOM generally consists of relatively low molecular weight organic carbon substrates that enter the mineral soil as dissolved organic matter and associate with mineral surfaces (referred to as 'mineral-associated OM', or MAOM). However, much debate and contradictory evidence persists around: (1) whether the organic C substrates within the MAOM pool primarily originate from aboveground versus belowground plant sources, and (2) if C substrates directly sorb to mineral surfaces or undergo microbial transformation prior to their incorporation into MAOM. Here, we attempt to reconcile disparate views on the formation of MAOM by proposing a spatially-explicit set of processes that link plant C source with MAOM formation pathway. Specifically, because belowground versus aboveground sources of plant C enter spatially distinct regions of the mineral soil, we propose that fine-scale differences in microbial abundance should determine the probability of substrate-microbe versus substrate-mineral interaction. Thus, formation of MAOM in areas of high microbial density (e.g. the rhizosphere and other microbial hotspots) should primarily occur through an in vivo microbial turnover pathway, and favor C substrates that are first biosynthesized with high microbial carbon-use efficiency prior to incorporation in the MAOM pool. In contrast, in areas of low microbial density (e.g. certain regions of the bulk soil), MAOM formation should primarily occur through the direct sorption of intact or partially oxidized plant compounds to un-colonized mineral surfaces, minimizing the importance of carbon use efficiency, and favoring C substrates with strong 'sorptive affinity'. Through this framework, we thus describe how the primacy of biotic versus abiotic controls on MAOM dynamics are not mutually exclusive, but rather spatially dictated. Such an understanding may be integral to more accurately modeling soil organic matter dynamics across different spatial scales. This article is protected by copyright. All rights reserved.