The effect of nitrogen deposition on nitrification, carbon and nitrogen mineralisation and litter C:N ratios in oak (Quercus robur L.) forests

The present study addresses the question why there is a positive relationship between nitrogen deposition and potential net nitrogen mineralisation and nitrification in oak (Quercus robur L.) forest soils in south Sweden (Falkengren-Grerup et al., 1998), and how this is related to the carbon mineralisation. We tested three hypotheses based on European studies ( Persson et al., 2000a and Persson et al., 2000b) that postulate lower availability of carbon due to chemical binding of nitrogen to lignin remains and phenolic compounds or a more decomposed, recalcitrant organic matter due to faster initial decomposition rates of fresh litter. This in turn leads to increased net nitrogen mineralisation, and nitrifiers that may adapt to acid soils when ammonium availability increases. We used soils from two regions exposed to a total deposition of 17 and 10 kg N ha−1 per year and incubated the soils in the laboratory separately as well as in mixtures between the regions. To be able to evaluate how the microbial communities and organic matter interacted in the soil mixtures, we divided the observed values of the net carbon and nitrogen mineralisation and nitrification for the soil mixtures by the calculated expected values. C:N ratios of litter, fresh leafs of Deschampsia flexuosa and microbial biomass were also measured. Contrary to the assumptions in the hypothesis, the soil respiration was somewhat higher in soils subjected to high nitrogen deposition. Furthermore, the observed rate of nitrogen mineralisation was higher than expected in the majority of soil mixtures, while observed rates of carbon mineralisation only showed a weak tendency to be higher than expected. All the results taken together indicate that there has been a positive change in litter quality that leads to increased carbon and nitrogen mineralisation. This conclusion is supported by the C:N ratio of oak litter and fresh leaves of Deschampsia flexuosa that was lower in the most nitrogen-exposed sites and which might indicate an increase in decomposability. The observed values of nitrification were significantly higher than the calculated expected values. Thus, the increased net nitrogen mineralisation in the region with high nitrogen deposition seems to allow nitrifiers to adapt to these acid soils when they are no longer limited by ammonium.