Mycorrhizal roots in a temperate forest take up organic nitrogen from 13C- and 15N-labeled organic matter
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Background and aims
The importance of the uptake of nitrogen in organic form by plants and mycorrhizal fungi has been demonstrated in various ecosystems including temperate forests. However, in previous experiments, isotopically labeled amino acids were often added to soils in concentrations that may be higher than those normally available to roots and mycorrhizal hyphae in situ, and these high concentrations could contribute to exaggerated uptake.
We used an experimental approach in which we added 13C-labeled and 15N-labeled whole cells to root-ingrowth cores, allowing proteolytic enzymes to release labeled organic nitrogen at a natural rate, as roots and their associated mycorrhizal fungi grew into the cores. We employed this method in four forest types representing a gradient of soil pH, nitrogen mineralization rate, and mycorrhizal type.
Intact uptake of organic nitrogen was detected in mycorrhizal roots, and accounted for at least of 1–14 % of labeled nitrogen uptake. Forest types did not differ significantly in the importance of organic uptake.
The estimates of organic N uptake made here using 13C-labeled and 15N-labeled whole cells are less than those reported in other temperate forest studies using isotopically labelled amino acids, and likely represent a minimum estimate of organic N-use. The two approaches each have different assumptions, and when used in tandem should complement one another and provide upper and lower bounds of organic N use by plants.
KeywordsOrganic nitrogen uptake Ingrowth core Dual label Temperate forest
dissolved organic nitrogen
fraction of N uptake in organic form
We thank R. Mixon, Z. McAvoy, N. Gagnon, and M. Day for assistance in the field and laboratory. J. Hobbie, S. Ollinger, A. Finzi, J. Aber, M. Ducey, J. Bryce, and two anonymous reviewrs provided thoughtful discussion on experimental design and helped to improve the manuscript. This work was funded by a Switzer Environmental Fellowship, a UNH Dissertation Year Fellowship, the NRESS graduate program, and NSF DEB0614266. We thank the UNH Office of Woodlands and Natural Areas, the Town of Strafford Conservation Commission, and the NH DRED Division of Forests and Lands for field site access.
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