Long-term effects of enhanced nitrogen and sulphate additions on soil acidification and nutrient cycling in a Norway spruce stand

This thesis describes the long-term effects of enhanced inputs of ammonium and sulphate (NS) in a spruce stand in SW Sweden on the accumulation and fluxes of nutrients in above and below ground biomass, soil chemistry and leaching of nutrients. Ammonium and sulphate were added annually at a rate of 100 kg N and 114 kg kg S ha'1, which was five to six times higher than the deposition rate of the study area. The main nutrients studied were N, P, K, Ca, Mg and S. Other important parameters were pH, NH4, N 0 3 and A1 in soil, soil solution and deposition. The addition of NS decreased the pH in the mineral soil by 0.4 units and in the soil solution by 0.3 units. The increased acidity of the soil was almost totally buffered by dissolution of solid aluminium compounds (gibbsite). The input of inorganic nitrogen by deposition was totally retained in the ecosystem, as the stand was unsaturated with respect to nitrogen. The high additional input of nitrogen soon turned the stand into a nitrogensaturated system. About 40% of the Mg was leached from the humus layer. The NS treatment increased the leaching of all nutrients except P and K from the root zone and the water soluble concentrations of all nutrients except NH4 and S04 increased in the rhizosphere soil. The amount of fine roots (<2mm) decreased in the humus layer but increased in mineral soil and was unaffected overall when both humus and mineral soil were taken into consideration. The uptake of Ca and Mg by fine roots was blocked by ammonium in the forest floor and by ammonium and aluminium in the mineral soil. The concentrations of these nutrients decreased in the fine roots and also in the needles. The annual accumulation of N during the first six years (1988-1993) in the above and below ground biomass increased by a factor of 9 compared to control (C ) plots. However, the accumulation of P, K, Ca, Mg and S was less than double the C level. During the six year treatment, the above ground biomass increased by 47% relative to C plots. During the following years (until 1998) the yearly basal area increment in NS plots decreased and in 1997 it was lower than in C plots. Changes in the nutrient concentration in needles indicated that deficiency of P, K and later Mg could be the reason for the growth decline. It is suggested that high NS addition may increase biomass production in a short-term perspective, although not without negative effects on soil and soil solution. In a long-term perspective, it will lead to a strong disturbance both of the stand and its environment in the form of acidified soils and high leaching rate of nutrients and aluminium to groundwater