Turnover of Proteins as a Controller of Soil Nitrogen Cycling

Oregon State University, Corvallis OR

Investigators

Abstract

Plant productivity in natural and managed ecosystems is often limited by nitrogen (N), which plants obtain from the soil mainly as ammonium or nitrate - both of which are inorganic forms of N. Yet, more than 95% of the N in soil exists in organic forms, primarily enzymes and other proteins that originated from decaying plant material or from the microorganisms that decompose it. This research will study protein breakdown in forest soil, which is the major bottleneck in converting organic N into forms available to plants. Proteins are broken down by enzymes (proteases) that are produced and released into the soil environment by bacteria and fungi. Interactions between proteins and soil-mineral and organic components can affect the activity of microbial proteases. This research will directly measure microbial and soil controls on protein degradation and thus provide a deeper understanding of the bottleneck in the supply of N to plants and other environmental fates of N. This research has the potential to be transformative in that it will reveal a core mechanism coupling carbon (C) and N cycling. Understanding how proteases de-polymerize and recycle organic N is needed to more accurately model how limited natural nitrogen availability affects interactions between C & N cycles. Research will be conveyed to groups underrepresented in science through four activities that extend beyond the laboratory: (1) the annual HJA Day at the Andrews Long-Term Ecological Research (LTER) site, which reaches large numbers of the general public; (2) assist in a short-course in soil science that is taught each summer for middle and high school teachers; (3) provide a K-12 teacher with research experience; and (4) design curriculum and teach a short-course in environmental proteomics for graduate students. Organic 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. 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. This model will be explored through two objectives: (1) determining 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-organic matter interactions as contributors to matrix regulation; (2) determining the relative contributions of different microbial groups to protease activity in soils that vary in N availability, and the control of protease activity by C versus N limitation as a means of exploring the microbial regulation of protein turnover. 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. Collectively, data generated in Objectives 1 and 2 will allow us to identify the mechanistic functions that the soil matrix and microbial community exert on the fate of protein N as it is degraded in soils. It will reveal how the activities of microbial proteases vary among soil ecosystems and characterize them by their catalytic types. Knowing the properties and behavior of proteases is essential to reconcile how they access the different forms of soil-associated proteins, and thus constitutes a fundamental prerequisite for successful future studies. To this end, the proposed research will generate quantitative information of N transformation and retention in soil, which is also critical to the management of soil productivity and environmental health. The data and functional relationships generated by the proposed research are eventually intended to be coupled to N cycling models that separate the activity of microbial functional groups or enzymes. The outcomes of this research will include a more quantitative understanding of the role of microbial proteases in organic N cycling in terrestrial ecosystems and of the sources and diversity of proteases.