Soil N availability influences primary productivity, competition within and among plant and microbial communities, and soil organic matter formation. Despite decades of research progress, ecologists are still debating in different ecosystems what pools and fluxes provide N to plants and microbes. The origin of most N in soil is ultimately plant litter decomposition, with the breakdown of protein N to amino acids (proteolysis) now recognized as the rate-limiting step in N mineralization and the production of bioavailable N. Here we present emerging evidence and new conceptual models from both the ecological and biogeoscience communities to argue 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 soils and can hold up to 20x more N than particulate fractions. MAOM fractions also preferentially accumulate N compounds such as proteins, amino acids, and nucleic acids. While some MAOM is protected from degradation, other MAOM can be mobilized by plants, microbes, and their interactions, leading to substantial amounts of MAOM-derived N in the soil solution.
While soil amino acids and inorganic N originate from litter and soil organic matter mineralization, we argue that in rhizospheres MAOM is the largest proximal source of N for plants and microbes. Several biochemical strategies enable plants and microbes to compete with mineral-organic interactions and effectively access MAOM. In particular, root-deposited low molecular weight exudates enhance the direct and indirect (via microbial communities) destabilization, solubilization, and subsequent bioavailability of MAOM. We show that the competitive balance between the potential fates of N monomers—bound to mineral surfaces or dissolved and available for assimilation—depends on the specific interaction between clay mineral properties, soil solution, mineral-bound organic matter, and microbial community. Thus, MAOM contributes to the characteristically low C/N ratio of soil organic matter compared to litter and is a potentially significant source of bioavailable N that supplies primarily small, soluble peptides that plants and microbes access through different dynamics than they do other N pools. These factors together suggest that the strategies to mobilize and acquire MAOM N create unique plant and microbial community dynamics that promote niche differentiation and species coexistence in plant and microbial communities.