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 Km [Km(app)] value in the organic horizon of the control plot was fourfold less than the Km(app) value in the organic horizon of another temperate forest, but similar to the Km(app) values in adjacent plots amended with N for a decade. Unlike results for other temperate forests, Km(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.