Turnover of proteins as a controller of soil nitrogen cycling

This project is supported by a seed grant from the Agricultural Research Foundation at Oregon State University.

Organic nitrogen (N) turnover has long been recognized as key controller of N availability in soil ecosystems, but the processes that break down macromolecular N compounds into assimilable N-monomers have received little attention so far. The proposed research will examine the turnover of organic N, particularly proteins, which make up the greatest fraction of soil organic N. A new conceptual approach towards the soil N cycle is proposed that recognizes “matrix” and “microbial” controls on the bioavailability of organic N: Interacting chemical and physical processes associated with the soil matrix control the accessibility of proteinaceous N to a diverse complement of proteases produced by a complex microbial community in response to environmental stimuli.

1. Determine the fate of two 15N-labeled 'substrate' proteins with distinct physicochemical characteristics in soils across a gradient of resource availability, mineralogy, and microbial ecology, with special emphasis on the relative importance of protein-mineral and protein-OM interactions as contributors to matrix regulation.
2. Determine the relative contributions of different microbial groups to protease activity in soils varying in N availability, and the control of protease activity by C versus N limitation as a means of exploring the microbial regulation.

Research questions associated with these objectives will be addressed using soils of well-characterized, long-standing experiments in temperate forests. 15N-labeled proteins will be used to determine their turnover rates and the fate of the C and N in the proteins. This will be coupled with characterization of proteases and measurements of their activities. A manipulative experiment will be done to determine the relative contribution to protease activity by bacteria and fungi. These data will provide insight into the control mechanisms functioning in our conceptual model of organic N turnover.

Graduate students: Trang Nguyen