Increased nitrification in acid coniferous forest soil due to high nitrogen deposition and liming

This study investigated whether liming stimulates the potential nitrification of acid forest soils in southern Sweden, and whether such stimulation (if present) is more pronounced in areas receiving high nitrogen (N) deposition. A short-term (30 h) soil-slurry incubation technique was used, which reduces the risk of bacterial growth, nitrate immobilization and denitrification during the incubation. The nitrate and nitrite produced were measured after biological conversion to nitrous oxide. The investigation was performed 6-7 yrs after the liming at four coniferous forest sites in the central and western parts of southern Sweden, which receive low and high deposition of N, respectively. Overall, liming had increased pH significantly down to 10 cm soil depth, but at 20 cm depth there was no difference between the limed and non-limed soil. In cases when liming had affected the total N pool and the potential nitrification, this was also limited to the uppermost 10 cm. It seems likely that the effects of liming on the potential nitrification were dependent on N availability, which is in turn influenced by N mineralization, trees' demands for N, and atmospheric N inputs. The strongest stimulatory effect of liming on the potential nitrification was seen on the west coast, indicating that these sites had the highest availability of ammonia for nitrifiers. However, liming also increased nitrification at one of the sites in south-central Sweden, which could have been mediated by increased rates of N mineralization.