Attention on ecosystem cycling of amino acids is increasing due to the potentially high flux rates in nitrogen limited ecosystems. Amino acids serve as substrate for fungi and bacteria in the soil, and plants are able to take up small amino acids as intact compounds. Amino acids in soil water origin as root exudates, lyzed microbes and decomposing plants. We investigated uptake of amino acids and ammonium in plants and soil microorganisms by injecting 15N labelled ammonium or fully 15N and 13C labelled amino acids into the soil. As in previous studies, the obtained enrichment in plant shoots showed species specific preference for the different nitrogen sources (Andresen & Michelsen 2005). There was a significant plant uptake of carbon from free amino acids in the soil solution. Though measurements of amino acids (the 20 essential; HPLC) yielded low concentrations in the soil pore water (~ 1% of dissolved organic nitrogen) and showed seasonal variations, the 15N13C tracer experiment revealed high plant and microbial enrichment, suggesting a significant cycling of amino acids. Furthermore, the influence of colonisation and type of mycorrhizal association (ericoid and arbuscular) of the roots on amino acid uptake was investigated. Mineralization (decarboxylation) of the 13C labelled amino acids at uptake (through mycorrhizae or directly through root) and after xylem transport decreased the 13C enrichment of plant shoots, which therefore may not necessarily reflect root acquisition of intact amino acids. Furthermore, tannin addition tended to reduce plant uptake of label. By combining data on 15N recovery after 1 day in shoots and roots (fine and coarse) of the dominant heathland plants: the evergreen dwarf shrub Calluna vulgaris and the graminoid Deschampsia flexuosa, in soil microorganisms (chloroform fumigation extraction) and in soil water, we discuss the relative importance of free amino acids and ammonium as plant nutrients and microbial substrates in natural N-limited ecosystems with a high proportion of soil N held in tannin-N complexes.
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The 5th International Conference on Applications of Stable Isotope Techniques to Ecological Studies, 2006