Suppression of Peatland Methane Emission by Cumulative Sulfate Deposition in Simulated Acid Rain

This field manipulation study tested the effect of weekly pulses of solutions of NH4NO3 and (NH4)2SO4 salts on the evolution of CH4 and N2O from peatland soils. Methane and nitrous oxide emission from a nutrient-poor fen in northern Minnesota USA was measured over a full growing season from plots receiving weekly additions of NH4NO3 or (NH4)2SO4. At this relatively pristine site, natural additions of N and S in precipitation occur at 8 and 5 kg ha-1 y-1, respectively. Nine weekly additions of the dissolved salts were made to increase this to a total deposition of 31 kg N ha-1 y-1 on the NH4NO3-amended plots and 30 and 29 kg ha-1 y-1 of N and S, respectively, in the (NH4)2SO4-amended plots. Methane flux was measured weekly from treatment and control plots and all data comparisons are made on plots measured on the same day. After the onset of the treatments, and over the course of the growing season, CH4 emission from the (NH4)2SO4-amended plots averaged 163 mg CH4 m-2 d-1, significantly lower than the same-day control plot mean of 259 mg CH4 m-2 d-1 (repeated measures ANOVA). Total CH4 flux from (NH4)2SO4 treatment plots was one third lower than from control plots, at 11.7 and 17.1 g CH4 m-2, respectively. Methane emission from the NH4NO3-amended plots (mean of 256 mg CH4 m-2 d-1) was not significantly different from that of controls measured on the same day (mean of 225 mg CH4 m-2 d-1). Total CH4 flux from NH4NO3 treatment plots and same-day controls was 16.9 and 15.1 g CH4 m-2, respectively. In general, stable, relatively warm and wet periods followed by environmental 'triggers' such as rainfall or changes in water table or atmospheric pressure, which produced a CH4 'pulse' in the other plots, produced no observable peak in CH4 emission from the (NH4)2SO4-amended plots. Nitrous oxide emission from all of the plots was below the detection limit over the course of the experiment.