EFFECTS OF WEEKLY NITROGEN ADDITIONS ON N CYCLING IN A CONIFEROUS FOREST CATCHMENT, GARDSJON, SWEDEN

In spite of nitrogen being a major limiting factor for forest growth i n most parts of Scandinavia, increased nitrate levels in surface water s and soil water have been coupled to questions of increased inputs an d decreasing immobilization of nitrogen in watersheds. To study an eco system response to elevated nitrogen deposition, NH4NO3 was added to a 0.52 ha forested headwater catchment in weekly portions by means of s prinklers below the canopy. Total nitrogen input as throughfall increa sed from the ambient 11 kgNha(-1)yr(-1) to approximately 51 kgha(-1)yr (-1) in the treatment years. The catchment, situated in G (a) over cir cle rdsjon, Sweden, is dominated by naturally generated Norway spruce with Scots pine in drier areas. The mean age of the forest is 104 yr. N concentrations in foliage, weight and N concentrations in litterfall , in situ net N mineralization and nitrate transformation (resin core technique), and leaching of N below the organic LFH horizon were studi ed on a catchment (NITREX) or plot (NITREX, CONTROL) level. This was c oupled with input-output budgets to estimate fluxes and cycling of N d uring the pre-treatment year and the third year of treatment. There wa s a significant increase in net N mineralization in the NITREX plots t he 3rd year of treatment, and a significant interaction between plot a nd year indicating a response to N addition for both net mineralizatio n and nitrate transformation. The increased flux of nitrate from incub ated soil cores, as well as the increased concentration in soil water and runoff indicated that the increased nitrification was possibly cou pled with a preferential uptake of NH4-N by the microflora. The spatia l variability of the nitrogen transformation rates was generally large and increased with treatment. The N concentration in needle litter sh owed an increase the 5th year of treatment, but no effect of nitrogen addition was found on the total weight of litterfall or on the N conce ntration of foliage. The key processes that determine the ecosystem re sponse to increased inputs of nitrogen seem to be net mineralization a nd nitrogen transformation rates, together with the mechanism and capa city of N assimilation into the soil pool. The major sink for both ext ernally added and internally produced inorganic N was calculated to be the soil organic pool. N assimilation into the upper organic LF horiz on dominated, and this assimilation is the first of three suggested wa ys for coniferous ecosystems to respond to increased chronic N input. Over time, the system is hypothesized to change towards a system with a reduced assimilation capacity, a possible decreased decomposition ra te, and an increased export of nitrate through leaching. The time-span of these changes is still uncertain. (C) 1998 Elsevier Science B.V.