The response of soil nitrogen and 15N natural abundance to different amendments in a long-term experiment at Ultuna, Sweden

In a long-term field experiment on a clay loam soil at Uppsala, Sweden, changes in nitrogen contents, natural abundance of N-15 in the topsoils receiving various organic amendments at the rate of 2 000 kg C ha(-1)year(-1) and different rates of nitrogen mineralisation were studied. Cropping resulted in clearly lower N losses from the topsoil (0-20 cm) compared to the bare fallow plots. Green manure, animal manure and sewage sludge increased the N, inventory significantly; for the latter two treatments an equilibrium of N input and N output was not reached within 35 years of application. The highest N efficiency with respect to plant uptake and the highest N losses were recorded for the mineral N-treated plots. N-15 abundances were clearly affected by N input, differing in isotopic signature introduced through the amendments. This effect was used to derive, the relative input of 'stable' N from green manure and peat in comparison with the difference method. ((C) Inra/Elsevier, Paris.).