Short- and long-term effects of nitrogen fertilization on methane oxidation in three Swedish forest soils

Under normal conditions, CH4, one of the most important greenhouse gases, is subject to biologi cal oxidation in forest soils. However, this process can be negatively affected by N amendment. The reported experiment was conducted in order to study the short- and long-term effects of N amendment on CH4 oxidation in pine (Pinus sylvestris L.) forest soils. Soil samples were taken from three experimental sites, two of which had been amended with N once, over 20 years earlier, while the third had been amended 3 weeks earlier. The soil samples were incubated fresh at 15 degrees C at ambient CH4 concentrations (ca. 1.8 ppmv CH4). The variation in CH4-turnover rates was high within the treatments: CH4 was produced [up to 22.6 pmol CH4 g dry wt, soil(-1) h(-1)] in samples from the recently amended site, whereas it was consumed at high rates (up to 431 pmol CH4 g dry wt. soil(-1) h(-1)) in samples from the plot that had received the highest N amendment 27 years before sampling. Although no significant differences were found between N treatments, in the oldest plots there was a correlation between consumption of atmospheric CH4 and the total C content at a depth of 7.5-15 cm in the mineral soil (r(2) =0.74). This indicates that in the long-term, increased C retention in forest soils following N amendment could lead to increased CH4 oxidation.