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Effects of seasonal microbial biomass and community dynamics on organic N availability and plant-microbe competition.
Lipson, David*,1, 1 San Diego State University, San Diego, CA
ABSTRACT- The importance of plant uptake of organic nitrogen (ON) sources in natural ecosystems has become increasingly recognized, even for non-mycorrhizal plants. The importance of ON in plant nutrition poses several challenges to our understanding of the terrestrial N cycle: How can plants compete with microbes for C- and N- rich substrates such as amino acids? How can ON availability to plants be measured? What are the relative contributions of mycorrhizae and non-mycorrhizal roots to ON uptake? Here I present examples from the Colorado alpine of approaches to these problems. Declines in microbial biomass after snow melts in spring create pulses of ON availability to plants. The hydrolysis of proteins and peptides in soil create a continuous source of amino acids throughout the growing season, but this source is subject to control by microclimate and microbial competition. In the alpine because of the relatively low plant N sink and the high soil organic matter content, this source of amino acids can account for 50-100% of plant N requirements, despite plants competing for only about 2-4% of available amino acids. Niches exist within the spectrum of amino acids, with plants preferentially utilizing the C-poor amino acid, glycine, while microbes preferentially utilize the higher energy molecule, glutamate. In dual chamber experiments, mycorrhizal associations provided the host plants with about 1.3% of total glycine added to a chamber that excluded plant roots. Analysis of plant-microbe competition suggests that the best predictor of competitive success is absorptive surface area. The field would benefit from more measurements of ON availability, plant-microbe competition for ON, and root and mycorrhizal surface area in soils.
KEY WORDS: microbial community, plant-microbe interactions, alpine, Kobresia myosuroides