ALBERT

Description: The formation of nitrate (nitrification) in soils is an important process that influences the form of N available for plant uptake and the potential off-site N losses. Gross nitrification is one of the main sources of nitrous oxide (N2O) and nitric oxide (NO) from soils. A field experiment was designed to verify the idea that gross nitrification rates in soils can be readily predicted by a model approach where seasonal variability is described only by soil moisture and soil temperature and the magnitude of gross nitrification is controlled by the soil organic matter (SOM). Gross nitrification rates were measured by a Barometric Process Separation (BaPS), first described by Ingwersen et al. (1999). The BaPS measurements were validated with the commonly used 15N pool dilution technique measurements at six times. In general, the rates determined from both measurement approaches were in the same order of magnitude and showed a good correlation. The effects of three different fertilisations (mineral fertiliser, manure and the control) over more than 100 years on gross nitrification rates were investigated. During 2004 soil probes from the long-term "static fertilisation experiment" at Bad Lauchstädt were sampled weekly and were measured in the laboratory under field conditions and subsequently under standardised conditions (16°C soil temperature and −30 kPa matrix potential) with the Barometric Process Separation system (BaPS). Gross nitrification rates determined by the BaPS-method under field conditions showed a high temporal variability and ranged from 5 to 77 μg N h−1 kg−1 dry mass, 2 to 74 μg N h−1 kg−1 dry mass and 0 to 49 μg N h−1 kg−1 dry mass with respect to manure, mineral fertiliser and control. The annual average was 0.32, 0.26 and 0.18 g N a−1 kg−1 dry mass for the manure site, mineral fertiliser site and control site, respectively. On all sites gross nitrification revealed a strong seasonal dynamic. Three different methods (a temperature and soil moisture dependency model from Recous et al., 1998, a multiple linear regression and the method proposed in this paper) were applied for reproducing the measured results. On the manure site 78% to 80%, on the mineral fertiliser site 66% to 72% and on the control site 39% to 56% of the observed variations could be explained by the tested models. Gross nitrification rates determined under standardised conditions did not show any seasonal trends but did also however reveal high temporal variability.