J. For. Sci., 2008, 54(8):377-387 | DOI: 10.17221/36/2008-JFS

Natural abundance of ¹⁵N of a spruce forest ecosystem under acid rain and manipulated clean rain field conditions

S. P. Sah, R. Brumme, N. Lamersdorf

Institute of Soil Science and Forest Nutrition, University of Göttingen, Göttingen, Germany

We analysed stable isotopes of N in a spruce forest under ambient rainfall (no further manipulation of the atmospheric input) and clean rain application (10 years of reduced inorganic N- and acid-constituent input). The objectives of the study were to assess whether or not the natural ¹⁵N abundance would function as an indicator for the N-status of our forest ecosystems. For this purpose, natural ¹⁵N abundance values were measured in needles, litter fall, roots, soil, bulk precipitation, throughfall and soil water of both plots. In the bulk precipitation and throughfall the δ¹⁵N values of NO₃-N were in the range reported by other studies (-16 to + 23‰). In both plots, the throughfall was greatly depleted of ¹⁵N compared to the bulk precipitation and this was attributed to nitrification in the canopy leaves, leading to δ¹⁵N-depleted nitrate production in the leaves that leaches down the soil surface. Nitrate in seepage water showed a general increase in δ¹⁵N values when it passes through the upper mineral soil (10 cm soil depth) and infiltrates into deeper mineral soil horizons (100 cm soil depth), similar to the δ¹⁵N enrichment of total nitrogen in the mineral soil. We observed ¹⁵N depletion in both green needles and litter fall at the clean rain plot, compared to the N-saturated control plot. We assumed it to be due to increased mycorrhizal N-uptake under N limited, i.e. clean rain conditions which are indicated by relatively lower N concentrations of green needles. With respect to the vertical gradient of the ¹⁵N abundance in the forest floor, both plots differ from each other, showing an untypical peak of δ¹⁵N depletion in the humus layer, which is more pronounced at the control plot. In contrast to the mineral soil where mineralisation is a dominant process for fractionation we attribute the δ¹⁵N pattern in the forest floor to additional processes like litter input and immobilisation. We conclude that the δ¹⁵N natural abundance analysis is helpful for interpreting the N-status of forest ecosystems but further research is needed especially with respect to the soil-root interface.

Keywords: acid rain; clean rain; δ15N; stable isotopes; nitrogen; precipitation; throughfall; seepage water; needles; litter; spruce forest

Published: August 31, 2008  Show citation