CH₄ oxidation and N₂O emissions at varied soil water-filled pore spaces and headspace CH₄ concentrations

Emission of N2O and CH4 oxidation rates were measured from soils of contrasting (30-75%) water-filled pore space (WFPS). Oxidation rates of C-13-CH4 were determined after application of 10 mu l C-13-CH4 I-1(10 at. % excess C-13) to soil headspace and comparisons made with estimates from changes in net CH4 emission in these treatments and under ambient CH4 where no C-13-CH4 had been applied. We found a significant effect of soil WFPS on C-13-CH4 oxidation rates and evidence for oxidation of 2.2 mu g C-13-CH4 d(-1) occurring in the 75% WFPS soil, which may have been either aerobic oxidation occurring in aerobic microsites in this soil or anaerobic CH4 oxidation. The lowest 13C-CH4 oxidation rate was measured in the 30% WFPS soil and was attributed to inhibition of methanotroph activity in this dry soil. However, oxidation was lowest in the wetter soils when estimated from changes in concentration of C12+13-CH4. Thus, both methanogenesis and CH4 oxidation may have been occurring simultaneously in these wet soils, indicating the advantage of using a stable isotope approach to determine oxidation rates. Application of C-13-CH4 at 10 mu l C-13-CH4 l(-1) resulted in more rapid oxidation than under ambient CH4 conditions, suggesting CH4 oxidation in this soil was substrate limited, particularly in the wetter soils. Application of (NH4NO3)-N-14-N-15 and (NH4NO3)-N-15-N-15 (80 mg N kg soil(-1); 9.9 at.% excess N-15) to different replicates enabled determination of the respective contributions of nitrification and denitrification to N2O emissions. The highest N2O emission (119 mu g N14+15-N2O kg soil(-1) over 72 h) was measured from the 75% WFPS soil and was mostly produced during denitrification (18.1 mu g N-15-N2O kg soil(-1); 90% of N-15-N2O from this treatment). Strong negative correlations between N14+15-N2O emissions, denitrified N-15-N2O emissions and C-13-CH4 concentrations (r=-0.93 to -0.95, N2O; r= -0.87 to -0.95, denitrified N-15-N2O; P < 0.05) suggest a close relationship between CH4 oxidation and denitrification in our soil, the nature of which requires further investigation. (c) 2005 Elsevier Ltd. All rights reserved.