Effects of nitrogen load to the forest floor in sitka spruce stands (Picea sitchensis) as affected by difference in deposition and spruce aphid infestations

Abstract

Studies of biogeochemical cycling and soil acidification have been carried out in even aged stands of Norway spruce, sitka spruce, Douglas fir, beech and oak under the frame of “The Element Cycling Project”. Deposition of excess nitrogen to forests is important as a potential acidifying input. In Denmark, reduced vitality in Norway spruce has promoted extensive planting of sitka spruce. However, several spruce aphid infestations have caused defoliation in many sitka spruce stands. The objectives of this study were to evaluate the effects of deposition and increased litterfall due to spruce aphid infestations on nitrogen transformations in the forest floor in sitka spruce stands on different soil types. The deposition of throughfall nitrogen range from 19 to 35 kg/ha/year. Fluxes of nitrogen in litterfall ranged from 21 to 77 kg/ha/year, whereas nitrogen leaching range from 1 to 57 kg/ha/year. Leaching was lowest at the infertile sites, but increased with magnitude of deposition and aphid infestations. Proton production according to the nitrogen transformations was largest at the fertile site most often affected by infestations. Huge amounts of bird droppings, honey dew and input of easily available nutrients by canopy leaching probably induced litter decomposition and formation of NO ⁻₃ in the soil water.