Greenhouse gas emissions from cultivated dambos from Central Zimbabwe

Abstract

Nitrous oxide (N2O), carbon dioxide (CO2) and methane (CH4) are potent greenhouse gases (GHGs) which cause climate change. The major anthropogenic sources of these GHGs are industrial production, burning of fossil fuel and agriculture, while the major natural sources include wetlands. However, the contribution of tropical wetlands, particularly seasonal wetlands (dambos) which occupy an estimated 20% of the elevated central African plateau towards GHG emissions are currently unknown as there has been few measurements in the region. Dambos contain large stocks of soil organic carbon (SOC), and they are a potential source of GHGs in their natural state. When dambos are disturbed through practices such as burning, or when vegatation is removed through clipping or clearing and cultivation with the application of both organic and inorganic fertilisers, there is likely to be a change in the SOC and GHG emissions, but currently there is little data on the effect of these various practices on SOC dynamics and GHG emissions. The objective of this study was to quantify SOC and GHG emissions from dambos in their natural state and determine how dambo distubances, and fertiliser application affect SOC and GHG emissions. Soil samples were collected from; a dambo transect, farmers’ gardens, plots that were subjected to different disturbance regimes, and plots that were planted to rape (Brassica napus). Soil organic carbon was determined using Walkely-Black procedure, while GHG emissions were measured using static chambers. Along the transect, the average SOC stocks were 3.3, 4.5, 30,4, 7.2 and 4.4 Mg ha-1 for the upland, margin, mid-slope and bottom catena positions respectively for the 0-40 cm depth. Methane emissions were -0.3, 29.5 and -1.3 mg m-2 hr-1, N2O emissions were 40.1, 3.9, and 5.5 μg m2 hr-1, while CO2 emissions were 2648.9, 896.2, 590.1 mg m-2 hr-1 for upland, mid-slope and bottom catena positions respectively. The average emissions for dambos alone (mid-slope and bottom catena) were 14.6 mg m-2 hr-1 for CH4, 4.7 μg m2 hr-1 for N2O and 744 mg m-2 hr-1 for CO2. Dambo cultivation, clearing and clipping resulted in a 32%, 25 and 16% reduction in dambo SOC, though burning resulted in a 28% increased SOC. Burning significantly (p<0.05) increased CH4 emissions, compared to cultivation, clipping and clearing. Methane emissions were 10.1, 7.0, 7.6, 6.7 and 5.5 mg m-2 hr-1, CO2 emissions were 2541, 3543, 4209, 6081 and 2425 mg m-2 hr-1, while N2O emissions were 30.7, 56.6, 1.8, 23.9 and 57.4 μg m-2 hr-1 for burning, cleared, control, clipping and ploughing respectively. The application of fertilisers in cultivated dambos planted to rape resulted in increased N2O and CO2 emissions. The results showed that the application of cattle manures resulted in N2O emissions that ranged between 218-894 μg m-2 hr-1, while with inorganic N fertilizers N2O emissions ranged from 555 -5186 μg m-2 hr-1. However, for integrated nutrient management, where low amounts of inorganic fertilizers are mixed with cattle manures, N2O emissions ranged from 106 to 2826 μg m-2 hr-1. The use of inorganic fertilisers resulted in higher N2O emissions per kg yield obtained (6-14 g N2O kg-1 yield), when compared to 0.7-4.5 g N2O kg-1 yield and 1.6-4.6 g N2O kg-1 yield for organic manures and for mixtures of organic and inorganic N. Dambos planted to rape were weak sources of CH4 and average annual emissions were between -0.02-0.9 mg m-2 hr-1. It was concluded that dambos exhibit high variability in emissions depending on catenal position along a transect, nature of disturbance and the type of GHG. High inorganic N fertiliser application in cultivated dambos increased N2O emissions, while organic manures and integrated nutrient management resulted in lower emissions and are thus recommended for mitigating GHG emissions in cultivated dambos.