Soil nitrogen turnover in proximal and distal stem areas of European beechtrees

In European beech (Fagus sylvatica L.) forests, a large proportion of the w ater and ion input to the soil results from stemflow which creates a soil m icrosite of high element fluxes proximal to the tree trunk. The soil proxim al to the stem is considered to have different rates of nitrogen turnover w hich might influence the estimation of N-turnover rates at the stand scale. In a previous study we reported high nitrate fluxes with seepage proximal to the stems in a forest dominated by European beech in Steigerwald, German y. Here, we investigated the soil nitrogen turnover in the top 15 cm soil i n proximal (defined as 1 m(2) around beech stems) and distal stem areas. La boratory incubations and in situ sequential coring incubations were used to determine the net rates of ammonification, nitrification, and root uptake of mineral nitrogen. In the laboratory incubations higher rates of net nitr ogen mineralization and nitrification were found in the forest floor proxim al to the stem as compared to distal stem areas. No stem related difference s were observed in case of mineral soil samples. In contrast, the in situ i ncubations revealed higher rates of nitrification in the mineral soil in pr oximal stem areas, while net nitrogen mineralization was equal in proximal and distal areas. In the in situ incubations the average ratio of nitrifica tion/ammonification was 0.85 in proximal and 0.34 in distal stem areas. The net nitrogen mineralization was 4.4 g N m(-2) 90 day(-1) in both areas. Mi neralized nitrogen was almost completely taken up by tree roots with ammoni um as the dominant nitrogen species. The average ratio of nitrate/ammonium uptake was 0.69 in proximal and 0.20 in distal areas. The higher water cont ent of the soil in proximal stem areas is considered to be the major reason for the increased rates of nitrification. Different nitrogen turnover rate s in proximal stem areas had no influence on the nitrogen turnover rates in soil at the stand scale. Consequently, the observed high nitrate fluxes wi th seepage proximal to stems are attributed to the high nitrogen input by s temflow rather than to soil nitrogen turnover.