Plant uptake of soil organic N in addition to inorganic N could play an important role in ecosystem N cycling as well as plant nutrition. We measured in situ plant uptake of organic and inorganic N by the dominant canopy species in two contrasting temperate forest ecosystems (bottomland floodplain forest, subxeric sandhills long-leaf pine forest). Seedlings of Acer rubrum L. and Pinus palustris Mill. in floodplain and sandhills forests, respectively, were treated with isotopically-enriched organic N (15N--13C-glycine) or inorganic N (15NH4 ) to examine in situ uptake. We also measured soil organic and inorganic N concentrations to assess the availability of N for plant uptake. Neither species took up organic N as intact 15N--13C-glycine, but significant root 15N enrichment in both species indicated that N mineralized from labeled glycine was taken up. Free amino-N dominated the total 2 M KCl-extractable N in floodplain (57 ± 3%) and sandhills soils (75 ± 3%), followed by NH4 then NO3− in both soils. Up to 13% of glycine label was mineralized to NH4 at both sites, suggesting that the majority of label was immobilized or adsorbed in the soil. Recovery of NH4 label also indicated strong soil immobilization, particularly in sandhills soils after 24 hours. Although uptake of intact organic N did not occur in either species, N mineralized from glycine was taken up by plants in these two contrasting temperate forested ecosystems.
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