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Tracing the fate of mineral N compounds under high ambient N deposition in a Norway spruce forest at Solling/Germany

Feng, Z., Brumme, R., Xu, Y.-J., Lamersdorf, N.
Forest ecology and management 2008 v.255 no.7 pp. 2061-2073
nitrogen, environmental fate, coniferous forests, Picea abies, ammonia, nitrates, stable isotopes, forest ecosystems, forest soils, depth, trees, plant tissues, conifer needles, branches, stemwood, roots, labeling techniques, soil-plant-atmosphere interactions, leaching, throughfall, Germany
The fate of high and equally distributed ammonium and nitrate deposition was followed in a 72-year-old roofed Norway spruce forest at Solling in central Germany by separately adding ¹⁵NH₄ ⁺ and ¹⁵NO₃ ⁻ to throughfall water since November 2001. The objective was to quantify the retention of atmospheric ammonium and nitrate in different ecosystem compartments as well as the leaching loss from the forest ecosystem. δ¹⁵N excess in tree tissues (needles, twigs, branches and bole woods) decreased with increased tissue age. Clear ¹⁵N signals in old tree tissues indicated that the added ¹⁵N was not only assimilated to newly produced tree tissues but also retranslocated to old ones. During a period of over 3-year ¹⁵N addition, 30% of ¹⁵NH₄ ⁺ and 36% of ¹⁵NO₃ ⁻ were found in tree compartments. For both ¹⁵N tracers, 15% of added ¹⁵N was found in needles, followed by woody tissues (twigs, branches and boles, 7-13%) and live fine roots (7%). The recovery of ¹⁵NH₄ ⁺ and ¹⁵NO₃ ⁻ in the live fine roots differed with soil depth. The recovery of ¹⁵NH₄ ⁺ tended to be higher in the live fine roots in the organic layer than in the upper mineral soil. In the live fine roots in deeper soil, the recovery of ¹⁵NO₃ ⁻ tended to be higher than that of ¹⁵NH₄ ⁺. Soil retained the largest proportion of ¹⁵N, accounting for 71% of ¹⁵NH₄ ⁺ and 42% of ¹⁵NO₃ ⁻. Most of ¹⁵NH₄ ⁺ was recovered in the organic layer (65%) and the recovery decreased with soil depth. Conversely, only 8% of ¹⁵NO₃ ⁻ was found in the organic layer and 34% of ¹⁵NO₃ ⁻ was evenly distributed throughout the mineral soil layers. Nitrate leaching accounted for 3% of ¹⁵NH₄ ⁺ and 19% of ¹⁵NO₃ ⁻. Only less than 1% of the both added ¹⁵N was leached as DON. These results suggested that trees had a high contribution to the retention of atmospheric N and soil retention capacity determined the loss of atmospheric N by nitrate leaching.