Plant-microbe interactions and organic nitrogen availability in an alpine dry meadow
PhD: University of Colorado Boulder, 1998.
In alpine and arctic ecosystems, the standard model of the N cycle (net N mineralization) has failed to explain N uptake by plants. The recently recognized importance of organic N forms in alpine and arctic plant nutrition could explain why net mineralization cannot account for plant N uptake in these systems. However, this explanation leads to further questions:
- How are plants able to compete with microbes for amino acids, given that amino acids are rich substrates for which microbes should compete very strongly?
- What is the seasonality of organic N availability in soil, and what underlies its pattern?
This work addresses how interactions among alpine plants, soil microbes and their environment cause organic N to be available for absorption by plant roots. The study was performed in a Colorado alpine dry meadow community, dominated by the sedge Kobresia myosuroides. One set of studies tested hypotheses about plant-microbe competition for amino acids. I studied the effects of freeze-thaw and dry-rewet events on competion for glycine, compared the competitive abilities of plants and microbes for different amino acids, and tested the ability of mycorrhizal fungi to transfer amino acid N to their host plants. Another set described the links between seasonal patterns in microbial biomass and organic N availability, and analyzed the mechanisms underlying the seasonal patterns. Freeze-thaw and dry-rewet events do not favor plant uptake of amino acids, but K. myosuroides competes well for glycine when soils are warm and moist. There is a flush of amino acid availability after snowmelt, caused by the turnover of the large microbial biomass pool. The activity of soil proteases can account for the observed soil amino acids. The post-snowmelt decline of microbial biomass is caused by the sensitivity of the wintertime microbial community to prolonged temperatures above 0°C, and by the depletion of soil C. Competition between plants and the summer microbial community for N is lessened by the preference of soil microbes for more C-rich amino acids, thus increasing the availability of the light amino acid, glycine. The uptake of amino acid N by plants is also aided by mycorrhizal fungi.