Methane oxidation potential of reclaimed grassland soils as affected by management

Reclaimed grassland soils could act as methane (CH4) sinks, but the effect of management on the CH4 oxidation capacity of these soils is not well established. CH4 oxidation was studied using soils (depth, 0-40 cm) from an undisturbed grassland (reference site) and reclaimed minelands managed for 20 to 25 years as hay field, grazed pasture, meadow, and grazed paddock. There was no N fertilization, grazing, or haying at the meadow; the hay field and grazed pasture received 90 kg of urea-N per year, and the paddock received hay input and animal droppings. In incubation with a low CH4 concentration (3 μL L), oxidation rates (μg CH4-C kg h) were negatively related to clay content and were significantly lower in the reclaimed (mean, 0.16) than in the undisturbed (mean, 0.36) grassland soils. Differences in oxidation rates (range, 0.07-0.18) among the hay, grazed pasture, and meadow soils were marginal, suggesting a limited effect of management on CH4 oxidation. When incubation was conducted using a high CH4 mixing ratio (concentration, 45 μL L), oxidation rates were positively correlated with both mineralizable C and NH4. Under this incubation condition, oxidation rates (μg CH4-C kg h) were in this order: hay and grazed pasture (1.4) < undisturbed soil (4.8) < meadow (5.8) < paddock soil (10.1). High rates in the paddock soil may have resulted from in situ exposure of this soil to CH4 released from cow droppings, leading to the evolution of an abundant CH4-oxidizing microflora. The results of this study suggest two modes of CH4 oxidation in the reclaimed soils: (i) high-affinity oxidation, which represented only 33% of the level in the reference soil after more than 2 decades without disturbance; and (ii) low-affinity oxidization, which was highly correlated with the availability of labile substrates and showed varying degrees of recovery from mining disturbance.