Cross-Ecosystem Comparisons of In Situ Plant Uptake of Amino Acid-N and NH4
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Authors: | Jack W McFarland Roger W Ruess Knut Kielland Kurt Pregitzer Ronald Hendrick Michael Allen |
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Institution: | (1) Department of Biology and Wildlife, University of Alaska, PO Box 757000, Fairbanks, Alaska 99775-7000, USA;(2) Institute of Arctic Biology, University of Alaska, Fairbanks, Alaska 99775, USA;(3) Department of Natural Resources and Environmental Science, University of Nevada, Reno, Nevada 89557, USA;(4) Warnell School of Forest Resources, University of Georgia, Athens, 30602, Georgia, USA;(5) College of Natural and Agricultural Sciences, University of California, Riverside, California 92521, USA |
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Abstract: | 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 13C and 15N, we observed in situ the cycling dynamics of NH4
+ and glycine through various soil pools and fine roots over 14 days. Recovery of 15N 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 15N was immediately apparent for both treatments and represented the largest sink (~25%) for 15N 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 13C 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 3× higher in ectomycorrhizal-dominated stands (1.6 ± 0.2) than arbuscular mycorrhizae-dominated stands (0.5 ± 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. |
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