Retention of deposited NH4+-N and NO3--N in coniferous forest ecosystems in southern Sweden

The main aim of the study was to investigate the retention pattern of atmospherically deposited NH4+ plus NO3- in podsolized forest soils in southern Sweden during the period 1985-1994. Nitrogen budgets were calculated for 37 coniferous forest sites dominated by Norway spruce (Picea abies (L.) Karst.) or Scots pine (Pinus sylvestris L.). Total deposition of NH4+ plus NO3- was obtained by adding bulk deposition and dry deposition. Dry deposition was calculated by using regional data on air concentrations of particulate and gaseous N compounds and previously established deposition velocities in stands of Norway spruce and Scots pine. Nitrogen leaching was obtained by combining measured NH4+ and NO3- concentrations with the amount of percolating soil water. The latter was chiefly estimated from the Cl- balance, assuming that the flux in throughfall was equal to the Cl- flux in the soil water. Nitrogen uptake by the trees was estimated by inserting data on diameter growth in empirical biomass functions and then applying typical N concentration values gained from the literature. The average N deposition was 13.3 +/- 7.4 kg N ha(-1) yr(-1) (+/-SD). Nitrogen leaching, estimated annual soil N retention (maximum N immobilization), ecosystem N retention and bole N accumulation (N uptake in bole wood + bole bark) were positively correlated with N deposition. Bole N accumulation and ecosystem N retention were also negatively correlated with stand age. The seven stands that showed a measurable amount of N leaching had a total N deposition of >15 kg N ha(-1) yr(-1) and were all situated in the southernmost part of the investigated area. The average value of the estimated soil N retention, 8.5 +/- 6.0 kg N ha(-1) yr(-1), was close to the range of default values used in estimates of critical N loads to European forest ecosystems.