Impacts of climate and land use on N2O and CH4 fluxes from tropical ecosystems in the Mt. Kilimanjaro region, Tanzania

In this study, we quantify the impacts of climate and land use on soil N2O and CH4 fluxes from tropical forest, agroforest, arable and savanna ecosystems in Africa. To do so, we measured greenhouse gases (GHG) fluxes from 12 different ecosystems along climate and land-use gradients at Mt. Kilimanjaro, combining long-term in situ chamber and laboratory soil core incubation techniques. Both methods showed similar patterns of GHG exchange. Although there were distinct differences from ecosystem to ecosystem, soils generally functioned as net sources and sinks for N2O and CH4 respectively. N2O emissions correlated positively with soil moisture and total soil nitrogen content. CH4 uptake rates correlated negatively with soil moisture and clay content and positively with SOC. Due to moderate soil moisture contents and the dominance of nitrification in soil N turnover, N2O emissions of tropical montane forests were generally low (<1.2 kg N ha(-1) year(-1)), and it is likely that ecosystem N losses are driven instead by nitrate leaching (similar to 10 kg N ha(-1) year(-1)). Forest soils with well-aerated litter layers were a significant sink for atmospheric CH4 (up to 4 kg C ha(-1) year(-1)) regardless of low mean annual temperatures at higher elevations. Land-use intensification significantly increased the soil N2O source strength and significantly decreased the soil CH4 sink. Compared to decreases in aboveground and belowground carbon stocks enhanced soil non-CO2 GHG emissions following land-use conversion from tropical forests to homegardens and coffee plantations were only a small factor in the total GHG budget. However, due to lower ecosystem carbon stock changes, enhanced N2O emissions significantly contributed to total GHG emissions following conversion of savanna into grassland and particularly maize. Overall, we found that the protection and sustainable management of aboveground and belowground carbon and nitrogen stocks of agroforestry and arable systems is most crucial for mitigating GHG emissions from land-use change.