The ability of temperate forest trees to take up carbon dioxide from the atmosphere and incorporate it in their living tissues is limited by the availability of nitrogen (N) in soils. There remain significant uncertainties in the processes that control N availability in soils. Soil microorganisms release N from organic material in the soil through the release of enzymes. This study will focus on one class of enzymes, proteolytic enzymes, which break down proteins into amino acids that can be used by plants and microbes as a source of N. The goal of this research is to test a new conceptual model for proteolytic enzyme production and activity. This model applies economic theory to proteolytic enzyme activity in that it tests the hypothesis that (1) tree roots provide energy in the form of carbon to subsidize the production of proteolytic enzymes by soil microbes, and (2) microbial investment in the production of proteolytic enzymes depends on the availability of N in the soil. These hypotheses will be tested through the growing season and therefore in response to seasonal variations in temperature and protein-substrate availability. These analyses will be conducted in a series of integrated field and laboratory experiments.
This research will advance fundamental understanding of the processes that regulate N availability in the soil and hence the productivity of forests as well as their ability to sequester rising concentrations of atmospheric carbon dioxide. This is potentially transformative research through its detailed analysis of the processes controlling the production of proteolytic enzymes and the response of this large class of enzymes to global changes, including rising temperatures and changes in belowground plant carbon allocation. The doctoral candidate and his advisor will continue their substantial educational and outreach activities targeting K-12 schools in the greater Boston MA area, many of which serve underrepresented groups.
The productivity of forests is integral to sustaining human welfare and economic activity. By absorbing carbon dioxide (CO2) from the atmosphere through photosynthesis, trees help slow the rate at which fossil fuel emissions of CO2 accumulate and therefore help slow climate change. In the United States, the availability of nitrogen (N) in soil controls the rate at which forests grow and absorb CO2. In this project, we studied the factors that control the production and availability of a particular form of N in the soil called organic N that plants can use for growth. Organic N has carbon atoms attached to the N and the most common forms in soils are proteins which need to be broken down into amino acids for plants to use. This contrasts inorganic forms of N like ammonium and nitrate, which are typically used as fertilizer on lawns and agricultural fields. We made many important discoveries. First, we found that tree roots can increase the availability of organic N by releasing C into soils. This C provides microbes in the soil with an energy subsidy that they use to make enzymes that break down large organic N forms like proteins into amino acids that tree roots can use. This ability of roots to increase organic N availability differed between tree species depending upon the type of symbiotic fungi that colonized their roots. Tree roots trade C with these fungi in exchange for nutrients. Hemlock and beech roots, which associate with ectomycorrhizal (ECM) fungi were able to increase N cycling more than sugar maple and white ash roots, which associate with arbuscular mycorrhizal (AM) fungi. This difference between tree species is important because almost all trees associate with either ECM or AM fungi and suggests that changes in the amount of C going to soil microbes will have a larger impact on N availability in forests that have more ECM trees. Second, we found that higher soil temperatures could accelerate the production of organic N in the soil, but that this acceleration was quite small compared to the response other biological processes in the soil to temperature including respiration. Thus, there appeared to be another factor that limited the production of organic N. We found that the enzymes that breakdown large complex organic N into amino acids that plants can use were more strongly limited by their access to complex organic N substrates than to temperature. This result is important because it challenges the assumption that the availability of N will increase substantially with rising soil temperatures. This means that positive effects of climate warming on tree growth could be limited by the amount of soil N. Lastly, we found a common control on the response of organic N production to climate change across the country. We collected soil samples from climate-change studies that experimentally manipulated temperature and rainfall from across the US. The objective of this research was to determine whether climate change could in fact increase N availability and plant growth in many different ecosystems (e.g., arctic tundra, confer forests, hardwood forests, grasslands, deserts). We found increases in organic N production when changes in climate simultaneously increased the temperature and moisture content of soils, whereas changes in climate that warmed and dried soils caused moderate-to-large decreases in the production of organic N. This means that the future ability of plants and soils to store carbon is highly dependent upon how much water is in the soil. Thus, we cannot assume that plants will grow more rapidly with warmer temperatures and rising levels of CO2. Changes in soil moisture and N availability also control this response.
