Amonette, James E (Pacific Northwest National Laboratory, PO Box 999, K8-96 Richland, WA, 99354; Phone: 509-376-5565; Fax: 509-376-3650; Email: jim.amonette@pnl.gov)

Enhancing Soil Humification: Insights from a Model System

J.E. Amonette*, J.B. Kim, C.T. Garten, C.C. Trettin, R.S. Arvidson, A. Luttge

Our research has focused on understanding the fundamental process by which humus is created (i.e., humification) and extending this knowledge to enhance the rate of humification. The rate-limiting step in the humification process appears to be the oxidation of polyphenols to quinones. These quinones then react with peptides and amino acids to form large melanin-like polymers that resist further degradation by microorganisms.

Soil fungi produce enzymes such as polyphenol oxidases and laccases that catalyze the oxidation step. Soil minerals, such as iron and manganese oxides, can also perform this function. We have observed a significant synergetic effect when a polyphenol oxidase (tyrosinase) and a mineral phase (e.g., mesoporous silica, manganese oxide, alkaline fly ash) are both present. As soil enzyme activity depends on structural conformation, and longevity depends on protection from microbial predation, we are examining the nature of enzyme attachment to soil particles and the impact of physical properties such as pore size on activity and longevity.

This presentation summarizes our laboratory results obtained using a model tyrosinase-enzyme-based reaction system. We perturbed the system by adding various co-catalysts and by changing the availability of oxygen and moisture. Our results yield insights into reaction mechanisms and suggest possible management strategies for enhancing soil-C sequestration.

We conclude that co-catalysis of humification occurs by three mechanisms involving physical stabilization of tyrosinase, direct oxidation of the monomers, and promotion of the oxidation and condensation steps by alkaline pH. Although tyrosinase activity is greatest at neutral pHs, the large pH dependence of the condensation step drives the overall reaction to maximum rates under alkaline conditions. Liming of soils to slightly alkaline pH should enhance net carbon sequestration.

Alkaline fly ash is a potential liming agent for soils, provided that the carbon costs associated with transportation from the source are less than the organic carbon that is humified. Soils that contain carbonate require flyash with relatively high-unburned C content (charcoal residual) to sustain a net positive sequestration. The porous charcoal residue provides sorption sites for the enzyme and for the organic monomers involved, which otherwise would attack and dissolve soil carbonates.