Wetlands and Agriculture in Africa: Major Sources of N2O?

Abstract:

Papyrus wetlands in East Africa are rapidly being converted to agricultural production in an effort to increase food security. This conversion is often seasonal, with wetlands being used for grazing and crop production of maize, sugarcane, and rice during dry seasons, and flooding occurring during wet seasons. An important question with respect to greenhouse gas production is whether wetland conversion to agriculture increases N₂O fluxes. This trend has been shown in temperate regions where increased N₂O fluxes are positively related to low soil C:N ratios, especially when soil moisture content remains high. In order to examine whether denitrification contributes to N₂O flux, we measured potential denitrification rates (PDR by acetylene block method) in intact papyrus wetlands and agricultural converted wetlands in Kenya, Tanzania, Uganda, and Rwanda, and also performed multivariate analysis to relate soil characteristics to PDR. Agricultural land-cover types included maize, sugarcane, rice, and grazing. Results showed that intact wetlands are potentially important sources of N₂O, as PDR in papyrus vegetation were consistently the highest (p<0.05; 128 - 601 μg N₂O g DW^-1 hour^-1) while grazing sites showed the lowest (0.1 - 0.5 μg N₂O g DW^-1 hour^-1). Rates were second highest in rice fields (2.3 - 303 μg N₂O g DW^-1 hour^-1), and intermediate in maize and sugarcane (6.5 - 75 μmg N₂O g DW^-1 hour^-1 and 5 - 30 μg N₂O g DW^-1 hour^-1 respectively). PDR across all sites was inversely related to soil C:N ratio, with nitrate consistently limiting PDR in the wetland sites while soil carbon limited PDR in agricultural sites. This is seemingly in contrast with other findings that show that lower C:N ratios result in high N₂O fluxes from drained wetland sites. However, flux measurements along with more realistic process-based measurements of denitrification are urgently needed to more fully understand the effect of agricultural conversion of wetlands in East Africa.