Submitted to: The Journal of the Torrey Botanical Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/4/2010
Publication Date: 10/15/2010
Publication URL: http://hdl.handle.net/10113/48753
Citation: Jin, V.L., Romanek, C.S., Donovan, L.A., Sharitz, R.R. 2010. Soil nitrogen availability and in situ nitrogen uptake by Acer rubrum L. and Pinus palustris Mill. in the southeastern U.S. Coastal Plain. The Journal of the Torrey Botanical Society. 137(4):339-347. Interpretive Summary: Plants commonly take up inorganic nitrogen (N) from soils to support their growth. Recent studies have shown that plants can also use soil organic N forms. The importance of plant uptake of soil organic N in many ecosystems, however, remains unclear. In this study, we measured plant uptake of organic and inorganic N in the field for two tree species (Acer rubrum L., Pinus palustris Mill) growing in the Coastal Plain region of the southeastern United States. We used stable isotope labeling techniques to trace nutrient movement into plant tissues (15N, 13C). We also measured soil organic and inorganic N concentrations to assess soil N availability for plant uptake. We found that neither species took up isotopically labeled organic N. More than 85% of the labeled organic N was bound to the soil, but the remaining organic N was converted to inorganic N which was then used by both plant species.
Technical 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 forest, 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 2M KCl-extractable N in floodplain (57 +/- 3%) and sandhills soils (75 +/- 3%), followed by NH4+ then NO3- in both soils. Label recovery from both sites indicated that up to 13% of glycine label was mineralized to NH4+, 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 intact organic N uptake did not occur in either species examined in this study, soluble organic N provided easily mineralizable substrate in these two contrasting temperate ecosystems, consistent with the important role of free organic N in forest soil N cycling.