Role of proteins in soil carbon and nitrogen storage:: controls on persistence

Mechanisms of soil organic carbon (C) and nitrogen (N) stabilization are of great interest, due to the potential for increased CO2 release from soil organic matter (SOM) to the atmosphere as a result of global warming, and because of the critical role of soil organic N in controlling plant productivity. Soil proteins are recognized increasingly as playing major roles in stabilization and destabilization of soil organic C and N. Two categories of proteins are proposed: detrital proteins that are released upon cell death and functional proteins that are actively released into the soil to fulfill specific functions. The latter include microbial surface-active proteins (e.g., hydrophobins, chaplins, SC15, glomalin), many of which have structures that promote their persistence in the soil, and extracellular enzymes, responsible for many decomposition and nutrient cycling transformations. Here we review information on the nature of soil proteins, particularly those of microbial origin, and on the factors that control protein persistence and turnover in the soil. We discuss first the intrinsic properties of the protein molecule that affect its stability, next possible extrinsic stabilizing influences that arise as the proteins interact with other soil constituents, and lastly controls on accessibility of proteins at coarser spatial scales involving microbial cells, clay particles, and soil aggregates. We conclude that research at the interface between soil science and microbial physiology will yield rapid advances in our understanding of soil proteins. We suggest as research priorities determining the relative abundance and turnover time (age) of microbial versus plant proteins and of functional microbial proteins, including surface-active compounds.