Long-term reduction in nitrogen and proton inputs did not affect atmospheric methane uptake and nitrous oxide emission from a German spruce forest soil

Since 1991, polluted and ‘cleaned’ (to produce natural, unpolluted solution) throughfall has been applied to soil under roofed plots of the 70-year-old Norway spruce plantation at Solling, Germany. From January 1993 to January 1994 and from April 2000 to April 2001, methane (CH4) and nitrous oxide (N2O) fluxes were measured weekly or biweekly from roofed plots receiving unaltered rain or clean rain, and from an adjacent, ambient unroofed plot. No significant differences in either CH4 uptake or N2O emission rates were found after 7 years of treatments. From 2000 to 2001, cumulative CH4 uptake rates were 1.67, 1.79 and 1.07 kg CH4 ha−1 yr−1 in the clean rain, the control and the ambient plot, respectively. The cumulative N2O emissions were low and in the range of 0.25–0.41 kg N2O-N ha−1 yr−1 for all plots. Our results suggest (1) that the soil CH4 sink was not reduced by high atmospheric nitrogen and acid deposition in the past, or alternatively, (2) recovery of degraded forest soils by methanotrophs may take some decades, (3) soil acidification and nitrogen eutrophication have a negligible effect on N2O emissions of temperate spruce forests.