15N Natural Abundance of Soil Microbial Biomass as a Tool for Assessing Controls on N-cycling Processes in Ecosystems

氮限制了世界上许多土壤中的植物生长。 植物与土壤中的微生物竞争氮。 当碳以有机物或死亡生物的形式丰富，而氮相对缺乏时，微生物将吸收大部分可用的氮。 相反，当微生物的生长受到碳的限制时，土壤微生物会通过分解含有氮的有机物来增加植物可利用的氮量。 目前，很难测量土壤微生物是否与植物竞争氮，或者它们是否通过提供氮来帮助植物生长。 氮原子在环境中至少以两种形式存在。大多数原子有7个质子和7个中子（14N），但少数原子将由7个质子和8个中子（15N）组成。 Schwartz、Hungate、哈特和Dijkstra发现，相对于土壤溶液中的氮，土壤生物富集15 N。 他们提出，微生物生物量中15 N与14 N的比例表明微生物生长是否受到氮或碳的限制。如果正确，将有可能确定植物生长是否受到土壤生物的促进或限制。 该项目将首次测量土壤微生物生物量的稳定氮同位素组成，并测试如何控制微生物15 N富集的概念模型。本研究为分析土壤微生物对植物生产力的影响提供了新的手段。此外，该项目将促进本科和研究生教育。主要研究人员教授环境微生物学、普通生态学、微生物生态学、生态系统生态学和林业等课程。拟议的研究将纳入这些课程。

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.