Nitrogen limits plant growth in many soils around the world. Plants compete with microorganisms in the soil for nitrogen. When carbon is abundant in the form of organic matter or dead organisms, and nitrogen is relatively scarce, the microorganisms will take up most of the available nitrogen. In contrast, when microbial growth is limited by carbon, soil microorganisms will increase the amount of nitrogen available to plants by breaking down organic matter that contains nitrogen. At present it is difficult to measure if soil microorganisms are competing with plants for nitrogen or if they are helping plants to grow by supplying nitrogen. Nitrogen atoms occur in a least two forms in the environment. Most of the atoms have 7 protons and 7 neutrons (14N) but a few atoms will consist of 7 protons and 8 neutrons (15N). Schwartz, Hungate, Hart and Dijkstra have found that soil organisms are enriched in 15N relative to nitrogen in the soil solution. They propose that the ratio of 15N to 14N in the microbial biomass indicates if microbial growth is limited by nitrogen or by carbon. If correct, it will be possible to ascertain if plant growth is promoted or restricted by soil organisms. This project will be the first to measure the stable nitrogen isotope composition of the soil microbial biomass, and to test a conceptual model for how microbial 15N enrichment is controlled.
This study may provide a new tool to assay the impact of soil microorganisms on plant productivity. In addition, the project will contribute to education at undergraduate and graduate levels. The principal investigators teach courses in environmental microbiology, general ecology, microbial ecology, ecosystem ecology and forestry. The proposed research will be incorporated into these courses.
