NITROGEN UPTAKE IN A NORWAY SPRUCE STAND FOLLOWING AMMONIUM-SULFATE APPLICATION, FERTIGATION, IRRIGATION, DROUGHT AND NITROGEN-FREE-FERTILIZATION

The above-ground accumulation of N, N uptake and litter quality resulting from improved or deteriorated availability of water and nutrients in a 25 year old Norway spruce stand in SW Sweden (as part of the Skogaby project) is presented. Treatment include irrigation; artificial drought; ammonium sulphate addition; N-free-fertilisation and irrigation with liquid fertilisers including a complete set of nutrients according to the Ingested principle (fertigation). At start of the experiment the stand contained 86.5 t dry mass and 352 kg N ha(-1). The following three years the annual N uptake in untreated trees was 32 kg N ha(-1) to be compared with the annual N throughfall of 17 kg ha(-1). Simultaneously, the treatment with ammonium sulphate and liquid fertilisation resulted in 48 and 56 kg ha(-1) y(-1), respectively, in treatment specific N-uptake following an application of 100 kg N ha(-1) y(-1). Addition of a N-free fertiliser resulted in improved N-uptake by 19 kg N ha(-1) y(-1) and irrigation by 10 kg N ha(-1) y(-1), compared to control. A linear relation between total above-ground dry mass production and N-uptake was found for trees growing with similar water availability. Dry mass production increased with increased water availability given the same N-uptake. It is concluded that the studied stand this far is not N saturated', as N fertilisation resulted in both increased N uptake and increased growth. Addition of a N-free-fertiliser resulted in increased uptake of N compared to the control, indicating an increased mineralisation rate or uptake capacity of the root system. The linear relation between N uptake and biomass production shows that at this study site N is a highly limiting factor for growth.