Methane oxidation in a temperate coniferous forest soil: effects of inorganic N

Methane oxidation rates were measured in soils obtained from a coniferous forest in northern England. The effects of depth and added K+ (K2SO4), NH4+ ((NH4)(2)SO4) and NO3- (KNO3) on potential CH4 oxidation were investigated in a series of laboratory incubations. The humus (H) layer soil showed much greater CH4 oxidation rates than the other soil layers, with maximal rates of 53 and 226 ng CH4 gdw(-1) h(-1) when incubated with initial 10 and 1000 mul CH4 l(-1), respectively. Additions of the solutes K+, NH4+ and NO3- showed differing degrees of inhibition on CH4 oxidation, which varied with the initial CH4 concentration, the ion added, and the ion concentration. In general, inhibition by the ions was slightly greater for incubations with an initial concentration of 1000 mul CH4 l(-1) than for 10 mul CH4 l(-1) under otherwise identical conditions. For K+ and NH4+ treatments, inhibitory rates were usually less than 15%, but at high K+ and NH4+ concentrations inhibition could reach 50%, the inhibitory effects of NH4+ were consistently slightly greater than those of K+ at the same concentration. In marked contrast to NH4+, NO3- showed a very strong inhibitory effect. Added NO3- and NO2- produced via added NO3- reduction in anaerobic 'microsites' are probably toxic to CH4-oxidizing bacteria. These results, together with those from other reports, suggest that NO3- may have a greater importance in the inhibition of CH4 Oxidation in forest soils than that attributed to NH4+ and needs to be investigated in a wide range of soil types from various forests. (C) 2003 Elsevier Science Ltd. All rights reserved.