CH4 uptake and N availability in forest soils along an urban to rural gradient

Concern about increases in atmospheric CH4 concentrations has resulted in investigations of the magnitude of and the factors that control aerobic soils as a sink for CH4. N additions decrease CH4 consumption in temperate forest, prairie and agricultural soils, suggesting that low rates of CH4 consumption are associated with high concentrations of available N. We have observed the opposite pattern in a series of oak-dominated forest sites of similar age on similar soils located along an urban to rural land-use gradient. Low rates of CH4 consumption were observed in urban forest sites with low amounts of available N relative to rural sites that had high concentrations of available N. In situ consumption rates ranged from 6.9 to 2.1 mg CH4m−2d−1. Mean rates of consumption were 30% lower in urban than in rural and suburban forest sites, suggesting that factors associated with urbanization may decrease rates of CH4 consumption in temperate forest soils. We hypothesize that low consumption rates of CH4 in urban forest sites are a product of low rates of organic matter degradation and nutrient cycling caused by air pollution (especially ozone) damage to forest tree leaves. Reduced rates of decomposition may result in lower rates of C and N flow from litter to soil microbial populations responsible for consumption of CH4.