Nitrous oxide emissions and methane oxidation by soil following cultivation of two different leguminous pastures

Nitrous oxide (N2O) emissions and methane (CH4) consumption were quantified following cultivation of two contrasting 4-year-old pastures. A clover sward was ploughed (to 150-200 mm depth) while a mixed herb ley sward was either ploughed (to 150-200 mm depth) or rotovated (to 50 mm depth). Cumulative N2O emissions were significantly greater following ploughing of the clover sward, with 4.01 kg N2O-N ha(-1) being emitted in a 48-day period. Emissions following ploughing and rotovating of the ley sward were much less and were not statistically different from each other, with 0.26 and 0.17 kg N2O-N ha(-1) being measured, respectively, over a 55-day period. The large difference in cumulative N2O between the clover and ley sites is presumably due to the initially higher soil NO3- content, greater water filled pore space and lower soil pH at the clover site. Results from a denitrification enzyme assay conducted on soils from both sites showed a strong negative relationship (r = -0.82) between soil pH and the N2O:(N2O + N-2) ratio. It is suggested that further research is required to determine if control of soil pH may provide a relatively cheap mitigation option for N2O emissions from these soils. There were no significant differences in CH4 oxidation rates due to sward type or form of cultivation.