Vortrag (eingeladen), BIOGEOMON, Santa Cruz, USA: 26.06.2006 - 29.06.2006
Abstract:
In many forest ecosystems chronically high atmospheric N deposition has caused considerable losses of inorganic N with seepage. Freezing and thawing of soil may alter the N turnover in soils and thereby the interannual variation of N seepage fluxes, which in turn makes it difficult to assess N status of forest ecosystems. Here, we present data that indicate freeze/thawing effects from 1) laboratory experiments, 2) experimental field manipulations and 3) from long-term monitoring of soil solution chemistry, solute fluxes and soil temperatures in a Norway spruce stand at the Fichtelgebirge, during the years 1993 to 2004. The long term monitoring data revealed that despite constant or even slightly increasing N inputs by throughfall, N losses with seepage in 90 cm declined from 15-32 kg N ha-1 a-1 in the first years of the study period (1993-1999) to 3-10 kg N ha-1 a-1 in 2000 to 2004. The large N losses in the first years could be partly attributed to extreme soil frost in the winter of 1996, ranging from -3.3 to -1.0°C at 15 and 35 cm soil depth, respectively. Considering the entire observation period, maximum fluxes of nitrate and ammonium were observed in the mineral soil following thawing of the soil. The elevated ammonium and nitrate fluxes resulted apparently from increased net ammonification and nitrification rates in the mineral soil, because concentrations and fluxes of nitrate and ammonium were less affected in the O horizon. Our data suggest that (1) extreme soil frost can cause substantial annual variations of nitrate fluxes with seepage and that (2) the assessment of the N status of forest ecosystems requires long periods of monitoring.