Fluxes of CO₂ and CH₄ in soil profiles of a mountainous watershed of Nepal as influenced by land use, temperature, moisture and substrate addition

Abstract

Effects of land use, moisture, temperature and substrate on production of CO₂ and consumption of CH₄ were measured in a series of laboratory incubation experiments on bulk soil samples from 0–10, 10–20, 20–40, 40–60, 60–80 and 80–100 cm soil depths under four dominant land uses [forest, grazing land, irrigated rice on level terraces (Khet), and upland maize–millet on sloping terraces (Bari)] of Mardi watershed (area=144 km²), Nepal. In addition, baseline physical and chemical properties of these soils were measured. The production of CO₂-C day⁻¹ per unit soil organic carbon (SOC) content in topsoil was lowest in grazing land, indicating a possibility of higher C sequestration with this land use than with other land uses. There was a decreasing trend of CO₂ emission with soil depth in all land uses, as was also the case with the SOC content. The CO₂ emission was increased by 90, 58, 27 and 23% for Bari, Khet, grazing and forest soil, respectively, with increase in moisture level from 40 to 60% (w/w). The CO₂ release from forest soil went up from 15 to 50 μg CO₂ g⁻¹ dry soil with increase in temperature from 5 to 15 °C and it further increased to 67 μg CO₂ g⁻¹ at 20 °C with estimated Q ₁₀ values of 3.4 and 1.8, respectively. Significantly higher amounts of CO₂ were emitted from all the land use types upon addition of glucose to the soil, indicating high potential of microbial activity. Consumption of CH₄ was more rapid in the soil from 10 to 20 cm depth for all the land use types. There was a 89% increase in CH₄ consumption in forest soil with increase in moisture level from 40 to 60%, while it was decreased by 38, 73, and 40% for Khet, Bari and grazing soil, respectively. Addition of (NH₄)₂SO₄ inhibited CH₄ oxidation in soils of all land uses, indicating a negative effect of N fertiliser input on CH₄ uptake in soil.