Nitrogen mineralisation in deciduous forest soils in south Sweden in gradients of soil acidity and deposition

Net nitrogen mineralisation was studied in 600 deciduous forests in four geographically separate regions of southern Sweden. The total nitrogen deposition in the forests varied between 7 and 17 kg ha(-1) year(-1). The most frequent canopy-dominant tree species was Quercus robur, others being Acer platanoides, Carpinus betulus, Fraxinus excelsior, Tilia cordata and Ulmus glabra. Soil pH (0.2 M KCl) varied between 3.1 and 7.7 and the C/N ratio between 8 and 50 in soil sampled 5 cm below the litter layer. The potential net mineralisation rate, measured in a 15-week laboratory incubation experiment, could be explained to 20% by soil pH and to < 10% by the C/N ratio. Nitrification occurred at any pH and was almost complete at pH > 4.5. The most striking finding was that rates of net nitrogen mineralisation and nitrification were by far the highest in the southernmost region, especially in the most acid soils, rates about twice as high as in the other regions. Sites that differed in the dominant tree species did not differ in net nitrogen mineralisation. Nitrogen deposition explained 30% of the variation in mineralisation rate. It is suggested that the high nitrogen deposition in recent decades has favoured ammonification through increasing the nitrogen pool in the soil and altering the quality of the organic matter, the microbial community adapting to the acidified soils and the increased nitrification.