The effect of increased n deposition on nitrous oxide, methane and carbon dioxide fluxes from unmanaged forest and grassland communities in Michigan

Atmospheric nitrogen deposition is anticipated to increase over the next decades with possible implications for future forest-atmosphere interactions. Increased soil N2O emissions, depressed CH4 uptake and depressed soil respiration CO2 loss is considered a likely response to increased N deposition. This study examined fluxes of N2O, CH4 and CO2 over two growing seasons from soils in unmanaged forest and grassland communities on abandoned agricultural areas in Michigan. All sites were subject to simulated increased N-deposition in the range of 1-3 g N m(-2) annually. Nitrous oxide fluxes and soil N concentrations in coniferous and grassland sites were on the whole unaffected by the increased N-inputs. It is noteworthy though that N2O emissions increased three-fold in the coniferous sites in the first growing season in response to the low N treatment, although the response was barely significant (p < 0.06). In deciduous forests, we observed increased levels of soil mineral N during the second year of N fertilization, however N2O fluxes did not increase. Rates of methane oxidation were similar in all sites with no affect of field N application. Likewise, we did not observe any changes in soil CO2 efflux in response to N additions. The combination of tillage history and vegetation type was important for the trace gas fluxes, i.e. soil CO2 efflux was greater in successional grassland sites compared with the forested sites and CH4 uptake was reduced in post-tillage coniferous- and successional sites compared with the old-growth deciduous site. Our results indicate that short-term increased N availability influenced individual processes linked to trace gas turnover in the soil independently from the ecosystem N status. However, changes in whole system fluxes were not evident and were very likely mediated by competitive N uptake processes.