THE EFFECT OF AGRICULTURE ON METHANE OXIDATION IN SOIL

Aerobic soils are an important sink for methane (CH4), contributing up to 15% of global CH4 destruction. However, the sink strength is significantly affected by land management, nitrogen (N) fertilizers and acidity. We tested these effects on samples taken from the Broadbalk Continuous Wheat, Park Grass permanent grassland and Broadbalk and Geescroft Wilderness Experiments at Rothamsted. The rates of uptake from the atmosphere of both enhanced (10 ppmv) and ambient (2 ppmv) concentrations of CH4 were measured in laboratory incubations of soil cores under controlled conditions. The most rapid rates of uptake were measured in soil from deciduous woodland at pH 7 (measured in water); acidic (pH 4) woodland soil showed no net CH4 oxidation. While disturbance of the cores used in the experiments did not affect the rate of CH4 uptake, extended (150 years) cultivation of land for arable crops reduced uptake rate by 85% compared to that in the soil under calcareous woodland. The long-term application of ammonium (NH4)-based fertilizer, but not nitrate (NO3)-based fertilizer, completely inhibited CH4 uptake, but the application for the same period of farmyard manure that contained more N than the fertilizer had no inhibitory effect. Autoclaving showed that the uptake of CH4 was microbially mediated. The most likely causes of the inhibitory effects seen are (i) insufficient concentrations of CH4 in, situ to activate methane monooxygenase; (ii) the direct inhibition of CH4 oxidation by NH4+ ions as the methanotrophs become adapted to oxidizing NH4+ ions; (iii) the suppression of methanotrophs by NH4-based fertilizers and acidity; (iv) the requirement of methanotrophs for a stable soil architecture which is incompatible with the disturbance caused by regular arable cultivation. These explanations are not mutually exclusive; several may operate concurrently.