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