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

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.