Soil solution chemistry during four years of NH 4NO 3 addition to a forested catchment at Gårdsjön, Sweden

Abstract

Nitrogen has been added to a forested 0.52 ha headwater catchment (G2 NITREX) at Gårdsjön, Sweden, to study the ecosystem response to elevated N deposition. The catchment is dominated by naturally regenerated, mixed-age conifers, mainly Norway spruce, with Scots pine dominating in dry areas. After a pre-treatment period of about 1 yr of soil solution sampling, N was added to the whole catchment as an NH 4NO 3 solution by means of sprinklers. Total N input as throughfall to the catchment increased from the ambient 13 kg N ha −1 yr −1 in the pre-treatment year to a total of about 50 kg N ha −1 yr −1 in the 4 treatment years. Soil solution was collected by tension lysimeters at 4 locations in G2 NITREX covering a moisture gradient from the dry upper to the lower wet parts of the catchment, at 2 locations in a nearby control catchment (F1 CONTROL), and at 2 locations in an adjacent catchment (G1 ROOF) at which ambient throughfall is excluded by a roof and replaced by unpolluted throughfall added by sprinklers. After 4 yr of N addition, the volume-weighted average NO 3 concentrations in G2 NITREX were higher than the pre-treatment values. Concentrations showed a progressive increase over time. In the 2 first treatment years this increase occurred only in the rooting zone but during the second 2 treatment years a pronounced increase also came in deeper layers. The lack of these trends in the F1 CONTROL and G1 ROOF catchments precludes natural variations in climatic conditions as the main cause for this increase. Relative to inputs, NO 3 concentrations in soil solution were low and showed large variations between the drier and wetter locations with peak concentrations in late fall and spring. Nitrate in soil solution generally constitutes less than 10% of the inorganic mobile anions and thereby contributes much less to the leaching of H +, Al, and base cations than CI and SO 4, the dominant mobile anions. Soil solution NH 4 has not changed relative to the control and roof catchments. However, the system is changing. Increases in NO 3 leaching indicate reduced immobilization of NO 3 that can be due to episodic excess N supply of the microflora together with episodes of high waterflow.