Consequences of nitrogen fertilization on soil methane consumption in a productive temperate deciduous forest

To investigate the consequences of long-term N additions on soil CH₄ dynamics, we measured in situ CH₄ uptake rates, soil profiles and kinetics parameters during the growing season in a temperate deciduous forest in northwestern Pennsylvania (Allegheny College Bousson Environmental Forest). Measurements were made in control and adjacent plots amended with 100 kg N ha^–1 year^–1 for 8 years. We found that the in situ consumption rates were 0.19±0.02 (mean±SE) for the control and 0.12±0.01 mg CH₄–C m^–2 h^–1 for the N treatment, indicating that consumption had been reduced by 35% after 8 years of N amendments. Despite the large difference in rates of consumption, there were no differences in the CH₄ concentration profiles between the control and N-amended plots. Laboratory incubations of CH₄ consumption throughout the soil column (organic horizon and mineral soil depths) showed that rates were greatest in the organic horizon of both control and N-amended soils, although consumption was reduced by 42% in the N-amended plot. However, the rate in the organic horizon was only about 50% the rate measured in organic horizons at other temperate forests. The apparent K_m K_m(app)] value in the organic horizon of the control plot was fourfold less than the K_m(app) value in the organic horizon of another temperate forest, but similar to the K_m(app) values in adjacent plots amended with N for a decade. Unlike results for other temperate forests, K_m(app) values at Bousson generally did not decrease with soil depth. These results indicate that N cycling strongly controls the CH₄-consuming community, and suggest that alterations of the N cycle due to N deposition or addition may alter rates and the location of CH₄ consumption by soils, even in soils with high N content and cycling rates.