The effect of nitrogen deposition and seasonal variability on methane oxidation and nitrous oxide emission rates in an upland spruce plantation and moorland

Rates of CH4 oxidation (-) and N2O emission were measured at upland moorland and coniferous forest sites in Southern Scotland to investigate seasonal Variations in flux and the influence of atmospheric N inputs on the land atmosphere exchange of these two gases. CH4 oxidation rates ranged from -0.4 to -16.7 ng CH4 m(-2)s(-1) and showed a strong seasonal response with a summer maxima. Total annual uptake of CH4 was estimated at 1.8, 0.7 and 1.3 kg ha(-1)yr(-1) at the moorland, high altitude forest and low altitude forest sites, respectively. Highly significant correlations were observed between CH4 oxidation rates and soil temperature, with activation energies between 60 and 140 kJ mol(-1). CH4 oxidation rates were on average 46 and 61% smaller al the high altitude forest site than at the the low altitude forest and moorland sites, respectively, coincident with a large concentration of soil NH4+ -N caused by large rates of N deposition at the hill summit. Addition of N as NO3 to soil cores resulted in a 86% reduction in oxidation rates and addition of N as NH4Cl in a 70% reduction. However, even for NaCl treated cores, a 75% reduction in oxidation rates was observed. N2O fluxes ranged from -0.8 to 15.5 ngNm(-2)s(-1) Fluxes were significantly higher in 1994 than in 1995, corresponding to less rainfall during the week prior to flux measurements in 1995. In 1994 significant correlations were observed between N2O emission rates, soil temperature, soil available NH4+ -N concentrations and CH4 oxidation rates. N2O fluxes were greatest at the hill summit. Total annual emissions were estimated at 0.3 kg N ha(-1)yr(-1) at the high altitude forest and moorland site compared to 0.1 kg N ha(-1)yr(-1)at the low altitude forest sites. It was calculated that 1% of the N deposited in the forest at the hill summit was emitted as N2O. (C) 1997 Elsevier Science Ltd.