Clay-Associated Organic Matter: A Hidden Source of Nitrogen in Agricultural Soils?.

Nitrogen (N) transformations and bioavailability limit productivity as well as regulate N losses in agricultural soils. However, it is challenging to predict bioavailable N due to uncertainty regarding the specific origins and eventual fate of N mobilized from soil organic matter (SOM). The breakdown of N-containing polymers to monomers (proteolysis) is now recognized as the rate-limiting step in N mineralization and the production of bioavailable N. Here we present a new conceptual model arguing that while depolymerization is a critical first step, clay minerals may be an important and overlooked mediator of bioavailable N, and especially in the soil rhizosphere where they are both a large source and sink for N.

Mineral-associated organic matter (MAOM) is a rich reservoir for N in agricultural soils and often holds 5-7x more N than particulate or labile fractions. However, MAOM is considered largely unavailable to crops as a source of N due to stabilizing physicochemical interactions with clays. We argue that several biochemical strategies enable plants and microbes to disrupt mineral-organic interactions and effectively access MAOM. In particular, root-deposited low molecular weight exudates enhance the direct and indirect destabilization, solubilization, and subsequent bioavailability of MAOM. We show that the balance between the potential fates of N monomers—bound to mineral surfaces vs. dissolved and available for assimilation—depends on the specific interaction between clay mineral properties, soil solution, mineral-bound organic matter, and the microbial community. Finally, we discuss several opportunities wherein agricultural management can exploit MAOM’s N-supplying capacity.