In a field experiment, lasting two years, an intensive monitoring of the nitrate reductase activity (NRA) of Norway spruce fine roots was used to investigate the question of whether the NRA is suitable as an indicator for nitrogen availability in the soil. The treatments were a liquid complete fertilisation containing 70-100 kg N (64% as nitrate), simulating a high N input to the soil, a non-treated control, and a water treatment as an additional control. Another treatment was the application of wood ash, which recycled mainly basic cations back into the forest soil (Ca, Mg, K). This treatment caused a mineralisation pulse leading to a shift in nitrate concentration in the soil solution.
The NRA of the fine roots was increased by the liquid fertilisation (averages 34% compared to control), but also by the water treatment (averages 28% more than control) and most by the wood ash treatment (averages 82% more than control). Nitrate concentrations in the soil solution were enhanced during the irrigation with the fertiliser. The pH was distinctly elevated in the soil solution by the ash, but also by the liquid fertiliser treatment. The soil solution applied to the water treated plots was not monitored, but results from an investigation of soil extracts revealed an elevated pH in the soil of the water treated plots as well. The pH of the soil solution was significantly correlated with the NRA in the fine roots, while a correlation between the nitrate concentration in the soil solution and the NRA was not significant. When taking the spatial heterogeneity of the soil nitrate into account, a correlation between the nitrate concentration of the soil extract and the root NRA was found. Although only a weak correlation between the NRA and the actual soil nitrate was observed, the NRA is assumed to reflect the nitrate conditions in the soil, possibly only in a time scale of months. As shown with the wood ash and water treatments, an elevated NRA can also be caused from other environmental parameters which can change the nitrate availability in the soil and uptake properties of the roots. The conclusion is that the NRA of fine roots as a marker for nitrate concentrations in the soil is not suitable on a regional and short-term scale in the field if other environmental parameters (e.g. the soil pH) are subjected to pronounced changes.