Factors Regulating Net Methane Flux by Soils in Urban Forests And Grasslands

Long‐term studies in the Baltimore, MD USA metropolitan area have found higher CH 4 uptake by soils in rural versus urban forests and no net uptake in urban lawns. Here, we investigated three factors that could be causing this inhibition; reduced diffusion of CH 4 into soils, production of CH 4 in anaerobic microsites and N additions from fertilizer or atmospheric deposition. Patterns of consumption in homogenized soils in the laboratory were comparable with previous field results, eliminating diffusion as a possible inhibiting mechanism. No CH 4 was produced in anaerobic incubations, signifying that microsite production was not important. Nitrogen additions had no effect on consumption of ambient CH 4 , suggesting that N inputs do not immediately inhibit uptake. However, there were strong negative relationships between CH 4 uptake and nitrification, suggesting that long‐term differences in N cycling associated with urban land use change may have led to a reduction in the microbial populations responsible for methane uptake.