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

Abstract

Mechanisms of soil organic carbon (C) and nitrogen (N) stabilization are of great interest, due to the potential for increased CO 2 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 surfaceactive compounds.

The C:N:P:S stoichiometry of soil organic matter

“…Protein is a likely major source of the nitrogen in NRSOM. It is well-known that a high proportion of the N in material isolated from soils by alkaline extraction is derived from proteins (Stevenson 1986;Schulten and Schnitzer 1998;Knicker 2004Knicker , 2011, and a significant role for proteinaceous material in the formation of stable SOM has been advanced by a number of authors (Amelung 2003;Rumpel et al 2004;Kleber et al 2007;Rillig et al 2007;Knicker 2011). Our illustrative three-component mixture of NRSOM (Fig.…”

“…The proximate source of protein-derived N in NRSOM cannot be deduced from its stoichiometry, so it could be plant or microbial protein or both. Rillig et al (2007) stated that microbial proteins are thought to be the more persistent in soil, but that this was not yet proven.…”

The decomposition of ectomycorrhizal fungal necromass

“…Glomalin-related soil proteins may be resistant to decomposition because of its hydrophobic nature (Rillig and Mummey, 2006). It is unclear if EM fungi synthesize similar glycoproteins that are similarly resistant to degradation, but hydrophobins (cysteine-rich hydrophobic proteins) produced by Basidiomycetes and Ascomycetes (Wessels, 1996;W€ osten and de Vocht, 2000;Rillig et al, 2007) may be similarly resistant to decomposition. These proteins are arranged as a film on the outside of the cell wall, making it unwettable (W€ osten and de Vocht, 2000).…”

“…These proteins are arranged as a film on the outside of the cell wall, making it unwettable (W€ osten and de Vocht, 2000). The unwettable nature of these tissues is likely to then impede their enzymatic decomposition (Rillig et al, 2007). Hydrophobins are also important in the formation of ectomycorrhizas and play a role in the retention and transportation of water in the extramatrical mycelium (Unestam and Sun, 1995).…”

How relevant is recalcitrance for the stabilization of organic matter in soils?

“…Even carbohydrates and certain peptides produced by soil microorganisms seem to be more resistant to microbial degradation, since they make up a substantial part of the stable subsoil DOC (Guggenberger et al, 1994) and can persist in soils for several decades (Gleixner et al, 1999). Rillig et al (2007) point out that protein misfolding resulting in amyloid aggregates and so-called fibrils can greatly reduce their biochemical degradation. They also identified a number of microbially produced proteins such as hydrophobins and glomalin that appear to be stable in soils, although data on degradation or turnover rates of these compounds are sparse.…”