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

TitleCross-Ecosystem Comparisons of In Situ Plant Uptake of Amino Acid-N and NH4 +
Publication TypeJournal Article
Year of Publication2010
AuthorsMcFarland, JW, Ruess, RW, Kielland, K, Pregitzer, K, Hendrick, R, Allen, M

Plant and microbial use of nitrogen (N) can besimultaneously mutualistic and competitive, particularlyin ecosystems dominated by mycorrhizalfungi. Our goal was to quantify plant uptake oforganic and inorganic N across a broad latitudinalgradient of forest ecosystems that varied with respectto overstory taxon, edaphic characteristics,and dominant mycorrhizal association. Using 13Cand 15N, we observed in situ the cycling dynamicsof NH4+ and glycine through various soil pools andfine roots over 14 days. Recovery of 15N as soil Nvaried with respect to N form, forest type, andsampling period; however, there were similaritiesin the cycling dynamics of glycine and NH4+ amongall forest types. Microbial immobilization of 15Nwas immediately apparent for both treatments andrepresented the largest sink (25%) for 15N amongextractable soil N pools during the first 24 h. Incontrast, fine roots were a relatively small sink(<10%) for both N forms, but fine root 13Cenrichment indicated that plants in all forest typesabsorbed glycine intact, suggesting that plants andmicrobes effectively target the same labile soil Npools. Relative uptake of amino acid-N versus NH4+varied significantly among sites and approximatelyhalf of this variation was explained by mycorrhizalassociation. Estimates of plant uptake of aminoacid-N relative to NH4+ were 39 higher in ectomycorrhizal-dominated stands (1.6 ± 0.2) thanarbuscular mycorrhizae-dominated stands (0.5 ±0.1). We conclude that free amino acids are animportant component of the N economy in allstands studied; however, in these natural environmentsplant uptake of organic N relative toinorganic N is explained as much by mycorrhizalassociation as by the availability of N forms per se.