NITROGEN CYCLING IN ACID FOREST SOILS SUBJECT TO INCREASED ATMOSPHERIC NITROGEN INPUT

Nitrogen cycling was studied in four forest ecosystems in the Netherlands, characterized by increased atmospheric nitrogen input. The study involved in situ incubation experiments to measure nitrogen transformation rates in the ectorganic (L+F+H) layer and the top 5 cm of the mineral soil. Throughfall and lysimeter data were collected from two of these sites to determine nitrogen solute fluxes and the proton production related to nitrogen cycling. The annual nitrogen input by throughfall ranged from 22 to 55 kg N ha-1 year-1. On the basis of the nitrogen budget, three of the forests could be characterized as nitrogen saturated. In two of these three forests high nitrification rates (16 kg N ha-1 year-1) were measured in the ectorganic layer and in the top of the mineral soil. At the other sites relatively low nitrate production rates (5-6 kg N ha-1 year-1) were measured. Net mineralization rates ranged from 26 to 71 kg N ha-1 year-1 in the investigated layers. A strong positive relationship between net nitrogen mineralization and nitrification rate was found at the two sites with high nitrification rates. Nitrification rate relative to net mineralization rate ranged from 22 to 29%. A comparison between incubation and lysimeter data revealed a quantitative discrepancy. It was concluded that the in situ incubation technique underestimates nitrogen transformation rates. Nitrogen solute fluxes indicated that in the nitrifying site, high nitrate production rates in the ectorganic layer coincided with large nitrate fluxes underneath this layer, while the nitrate produced in the top of the mineral soil had no net effect on the nitrate flux underneath that layer. As a result, high proton production rates (4.7 kmol H+ ha-1 year-1) as a result of nitrogen transformations, were calculated for the ectorganic layer of this forest.