Land application of aerobic sewage sludge does not impair methane oxidation rates of soils

The aim of this study was to measure and compare methane oxidation rates of arable and grassland soils that received 7.5t ha⁻¹ y⁻¹ of noncontaminated aerobically treated sewage sludge for ten years. Arable soils showed generally lower methane oxidation rates (from 6 to 15∗10⁻³ h⁻¹) than grassland soils (from 26 to 33∗10³ h⁻¹). Oxidation rate constants (k) of soils amended with sewage sludge were remarkably close to their respective untreated controls, but a soil, that had received a tenfold sewage sludge application (i.e. 75 t ha⁻¹ y⁻¹), showed a statistically significantly higher k-value. Laboratory addition of up to 1000 mg Pb g⁻¹ soil to this soil did not cause any significant change in methane oxidation, but caused a decrease from 13.9 to 10.9×10⁻³ h⁻¹ in the control soil. Addition of Zn was much more toxic than Pb, with a significant decrease at 300 μg g⁻¹ soil rate and an almost complete inhibition at 1500 μg g⁻¹ soil rate. Higher resistance was evident of sewage sludge treated soil in comparison to control soil, for both biomass C and CH₄ oxidation activity.