Natural 15N abundance in two nitrogen saturated forest ecosystems at Solling, Germany

Abstract

This research deals with a comparative study of two different N-saturated forests: 1. beech forest and 2. spruce forest at the same locality of Soiling, Central Germany. The present results show that 15N natural occurrence in the rainfall (both above and below canopy) at Soiling site is similar (δ15N = -15‰ to +19‰) to other sites of the world (such as NITREX sites, USA etc.). Furthermore, 15N values in the soil water ranged from -4.32 (±2.09) to +5‰ (±1.47), which also corresponds to NITREX sites and other sites of Europe and USA. In both forests, δ15N enrichment of both NH4-N and NO 3-N showed a decreasing trend of their values from bulk precipitation to the upper soil layer, but increasing in the deeper soil layer again. An increase in the 15N enrichment of soil water from upper soil depth to lower soil depth was observed in our study and it is assumed to be due to the strong net nitrification taking place in the upper layer (organic surface layer) of soil. The soils at both sites showed characteristic low (negative) δ15N values in the upper organic layers, strongly increasing to positive δ15N values in the mineral soil. In the lower depths of mineral soil horizons of both stands, an increase in δ15N values was found to culminate at +3 to +5‰. In contrast to the mineral soil horizon, in the organic soil horizon (0 to 6 cm depth) of both sites there was almost a similar or slight decrease in δ15N values with depth. This is attributed to the high nitrification rate in the organic soil horizon, resulting in excessive seepage water NO3-output at both sites (especially at the spruce site).