Modelling nitrogen balance in two Southern Swedish spruce plantations

Abstract: Emissions from agriculture and the combustion of fossil fuels increase the atmospheric load of reactive nitrogen (N). N deposition is projected to continue posing significant harm both for environmental and human health over the upcoming decades. Eutrophication as a result of N leaching is one of the negative effects of heavy deposition. It occurs when the input of N into an ecosystem exceeds its uptake capacity. This study analysed modelled data series on nitrogen deposition, leaching, and forest growth from 1900 to 2100 for two Southern Swedish spruce forestry stands. Moreover, several modelled N fertilisation schemes were tested. A maximum bearable nitrogen input rate indicating the retainable influx of N was estimated next to a maximum effective fertilisation rate. That threshold gave the highest application of fertiliser at which the expected biomass harvest increased. Both sites receive between 11 and 13 kg N/ha/yr of mean annual nitrogen deposition depending on the chemistry transport model applied. The first site, Hissmossa, can retain 10 kg N/ha/yr and shows harvest increase up to the single application of 20 kg N/ha per rotation period. The other study site, Västra Torup, has a maximum bearable nitrogen input rate of 12 kg N/ha/yr and fertilisers are effective up to the dose of 43 kg N/ha. In conclusion, both sites can be seen to be nitrogen saturated. Leaching should be expected, at least in reaction to biomass removal. Forest fertilisation bears only little potential to increase harvest but will probably increase the eutrophication issue.