Cross-Ecosystem Comparisons of In Situ Plant Uptake of Amino Acid-N and NH4 +

Plant and microbial use of nitrogen (N) can be simultaneously mutualistic and competitive, particularly in ecosystems dominated by mycorrhizal fungi. Our goal was to quantify plant uptake of organic and inorganic N across a broad latitudinal gradient of forest ecosystems that varied with respect to overstory taxon, edaphic characteristics, and dominant mycorrhizal association. Using C-13 and N-15, we observed in situ the cycling dynamics of NH4 (+) and glycine through various soil pools and fine roots over 14 days. Recovery of N-15 as soil N varied with respect to N form, forest type, and sampling period; however, there were similarities in the cycling dynamics of glycine and NH4 (+) among all forest types. Microbial immobilization of N-15 was immediately apparent for both treatments and represented the largest sink (similar to 25%) for N-15 among extractable soil N pools during the first 24 h. In contrast, fine roots were a relatively small sink (< 10%) for both N forms, but fine root C-13 enrichment indicated that plants in all forest types absorbed glycine intact, suggesting that plants and microbes effectively target the same labile soil N pools. Relative uptake of amino acid-N versus NH4 (+) varied significantly among sites and approximately half of this variation was explained by mycorrhizal association. Estimates of plant uptake of amino acid-N relative to NH4 (+) were 3x higher in ectomycorrhizal-dominated stands (1.6 +/- A 0.2) than arbuscular mycorrhizae-dominated stands (0.5 +/- A 0.1). We conclude that free amino acids are an important component of the N economy in all stands studied; however, in these natural environments plant uptake of organic N relative to inorganic N is explained as much by mycorrhizal association as by the availability of N forms per se.