1 University of Amsterdam, The Netherlands
2 Dresden University of Technology, Germany
3 Radboud Universiteit, Nijmegen, The Netherlands
4 University of Copenhagen, Denmark
5 Norwegian Institute of Bioeconomy Research, Oslo, Norway
6 Swedish Environmental Research Institute, Göteborg, Sweden
7 Swiss Federal Research Institute WSL, Birmensdorf, Switzerland
To study the long-term fate of deposited nitrogen in coniferous forest soils, we re-sampled four 15N tracer experiments across Europe at the former NITREX sites: Gårdsjøn (GD), Sweden, Klosterhede (KH), Denmark, Speuld (SP), The Netherlands and Alpthal (AH), Switzerland, more than 20 years after 15N tracer application 1–4. Most soils were classified as Podzols, only site AH contained clay-rich Gleysols. The sites covered a wide range of N deposition, varying between 4.4 to 55 kg N ha-1 yr-1. The experimentally added N deposition was enriched with 15NH4 and/or 15NO3 at varying levels (476–6074 g 15N ha-1 yr-1) for one year in 1992/93 or 1995/96. At most sites, N treatments have continued uninterruptedly and the fate of 15N in most N pools could be investigated after all these years. This was already done at another NITREX site Ysselsteyn (YS), The Netherlands5. Results of that study revealed that the mineral soil had become the major 15N sink 20 years after 15N application. We hypothesize that this long-term retention of 15N in the mineral soil is related to strong binding of organic N with the mineral matrix. Therefore we will focus on 15N recovery in different soil N pools, to find out if this phenomenon is generally valid. The first results from the other experimental field sites show a wide range of 15N recovery in the organic (0–60 %) and mineral (0–40 %) soil layers. The 15N recovery did not show a consistent trend over time in the soil horizons. We will explore our large data set to identify the controls of the 15N recovery, e.g., some specific soil characteristics, such as grain size distribution, moisture content, pH, CEC, C/N ratio, but also vegetation cover or landscape gradients. The most important regulating factors can be used to predict the long-term 15N retention in the mineral soil, which can ultimately be linked to another important process – soil organic carbon stabilization.
References:
1. Tietema A. et al. 1998. The fate of 15N-labelled nitrogen deposition in coniferous forest ecosystems. For Ecol Manage. 101, 19–27.
2. Koopmans C.J.J. et al. 1996. The fate of 15N enriched throughfall in two coniferous forest stands at different nitrogen deposition levels. Biogeochemistry 34,19–44.
3. Kjønaas O.J. et al. 2007. Use of 15N-labelled nitrogen deposition to quantify the source of nitrogen in runoff at a coniferous-forested catchment at Gårdsjön, Sweden. Environ Pollut. 147,791–799.
4. Schleppi P. et al. 1999. Simulation of increased nitrogen deposition to a montane forest ecosystem: partitioning of the added 15N. Water Air Soil Pollut. 116,129–134.
5. Wessel W.W. et al. 2013. The fate of 15NH4+ labeled deposition in a Scots pine forest in the Netherlands under high and lowered NH4+ deposition, 8 years after application. Biogeochemistry 113, 467–479.