Soil nitrogen availability and in situ nitrogen uptake by Acer rubrum L. and Pinus palustris Mill. in the southeastern U.S. Coastal Plain

Virginia L. Jin¹, Christopher S. Romanek², Lisa A. Donovan³, and Rebecca R. Sharitz⁴

¹USDA-ARS, Lincoln, NE 68583
²Department of Earth and Environmental Sciences, University of Kentucky, Lexington, KY 40506
³Department of Plant Biology, University of Georgia, Athens, GA 30702
⁴Savannah River Ecology Laboratory, Aiken, SC 29802

Abstract: 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-[2]-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-[2]-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.