THE INFLUENCE OF TREES ON NITROGEN DYNAMICS IN AN AGRISILVICULTURAL SYSTEM IN SWEDEN

During 1992 and 1993, nitrogen dynamics and microbial activity were investigated in an agrisilvicultural system consisting of oats or barley cultivated along the sides of a poplar plantation in Sweden, At each of three experimental sites (two silt loams and one silty clay loam), sampling for mineral nitrogen was carried out in three layers down to 90 cm at two distances from the trees, A (0.5-1.5 m) and B (4.0-5.0 m), two times each year (spring and autumn). Sampling of soil for organic matter, carbon and nitrogen, potential nitrification, N mineralization, basic respiration and substrate-induced respiration was carried out in the 0-10 cm layer at three distances from the trees: A (0.5-1.5 m), B (2.5-3.5 m) and C (4.0-5.0 m). Significantly larger amounts of organic matter, total carbon and nitrogen at A than at B and C, indicated increased inputs from the trees through litter, decaying roots and root exudates. This could explain that the rates of nitrogen mineralization, potential nitrification and respiration were significantly higher at A than at B and C. The presence of trees resulted in a better utilization of nitrogen and moisture in the soil, reducing the potential for nitrate leaching and accumulating nitrogen close to the trees. The higher concentration of ammonium, lower concentration of nitrate and the consistently lower NO3--N/NH4+-N-ratios observed at A than at C might be explained by a combined effect of increased nitrogen mineralization and efficient nitrate uptake by the trees.