NITROGEN CYCLING IN 2 NORWAY SPRUCE (PICEA-ABIES) ECOSYSTEMS AND EFFECTS OF A (NH4)2SO4 ADDITION

Nitrogen cycling in two Norway spruce (Picea abies (L.) Karst.) ecosystems in the ARINUS experimental watershed areas Schluchsee and Villingen (Black Forest, SW Germany) and initial effects of a (NH4)2SO4 treatment are discussed. Although N reserves in the soils are similar and atmospheric N input is the same low to moderate level characteristic for many forested areas in SW Germany, N export by both seepage and streamwater differs considerably. At Villingen, deposited N is almost totally retained in the ecosystem, whereas at Schluchsee N export is the order of the input. This is explained by differences in forest management history. The Villingen site had been subject to excessive biomass export (e.g., litter raking) leading to unfavorable microbial transformations in the soil. In contrast, as a 'relic' of the former beech stand, the Schluchsee site is characterized by high biological activity in the soil with vigorous nitrification despite low pH values. Accordingly, the two ecosystems responded differently to the additional N input (150 kg NH4+-N ha-1 as (NH4)2SO4). Nitrification starting immediately in the Schluchsee soils led to continued Al mobilization and leaching of basic cations and NO3-. The availability of Mg, already deficient before treatment, further decreased due to Mg leaching and marked N uptake by the stand. In contrast, most of the added N in Villingen was immobilized in the soil. Hence, uptake by the stand and leaching of NO3- and cations was correspondingly lower than at Schluchsee. The results emphasize the problems associated with the definition of generally applicable values for 'critical loads' of N deposition.