Palm et al. 2010. Identifying potential synergies and trade-offs for meeting food security and climate change objectives in sub-Saharan Africa

Palm, C.A., Smukler, S.M., Sullivan, C.C., Mutuo, P.K., Nyadzi, G.I., Walsh, M.G., 2010. Identifying potential synergies and trade-offs for meeting food security and climate change objectives in sub-Saharan Africa. Proceedings of the National Academy of Sciences 107, 19661–19666.

Abstract

Potential interactions between food production and climate mitigation are explored for two situations in sub-Saharan Africa, where deforestation and land degradation overlap with hunger and poverty. Three agriculture intensification scenarios for supplying nitrogen to increase crop production (mineral fertilizer, herbaceous legume cover crops—green manures—and agroforestry—legume improved tree fallows) are compared to baseline food production, land requirements to meet basic caloric requirements, and greenhouse gas emissions. At low population densities and high land availability, food security and climate mitigation goals are met with all intensification scenarios, resulting in surplus crop area for reforestation. In contrast, for high population density and small farm sizes, attaining food security and reducing greenhouse gas emissions require mineral fertilizers to make land available for reforestation; green manure or improved tree fallows do not provide sufficient increases in yields to permit reforestation. Tree fallows sequester significant carbon on cropland, but green manures result in net carbon dioxide equivalent emissions because of nitrogen additions. Although these results are encouraging, agricultural intensification in sub-Saharan Africa with mineral fertilizers, green manures, or improved tree fallows will remain low without policies that address access, costs, and lack of incentives. Carbon financing for small-holder agriculture could increase the likelihood of success of Reducing Emissions from Deforestation and Forest Degradation in Developing Countries programs and climate change mitigation but also promote food security in the region.

Hickman et al. 2011. Current and future nitrous oxide emissions from African agriculture

Hickman, J.E., Havlikova, M., Kroeze, C., Palm, C.A., 2011. Current and future nitrous oxide emissions from African agriculture. Current Opinion in Environmental Sustainability 3, 370-378.

Abstract

Most emission estimates of the greenhouse gas nitrous oxide (N₂O) from African agriculture at a continental scale are based on emission factors, such as those developed by the IPCC Guidelines. Here we present estimates from Africa from the EDGAR database, which is derived from the IPCC emission factors. Resulting estimates indicate that N₂O emissions from agriculture represented 42% of total emissions from Africa (though that rises to 71% if all savannah and grassland burning is included), or roughly 6% of global anthropogenic N₂O emissions (or 11% including burning). Emissions from African agriculture are dominated by grazing livestock; 74% of agricultural N₂O excluding biomass burning was from paddocks, ranges, and pasture. Direct soil emissions represent 15% of agricultural emissions; substantial changes in direct emissions from North Africa helped drive a 47% continental increase in direct soil emissions from 1970 to 2005. Future trends based on the Millennium Ecosystem Assessment scenarios indicate that agricultural N₂O emissions may double in Africa by 2050 from 2000 levels. Any regional or continental estimates for Africa are, however, necessarily limited by a paucity of direct measurements of emissions in sub-Saharan agro-ecosystems, and the heavy reliance on emission factors and other default assumptions about nitrogen cycling in African agriculture. In particular, a better understanding of livestock-related N inputs and N₂O emissions will help improve regional and continental estimates. As fertilizer use increases in sub-Saharan Africa, emission estimates should consider several unusual elements of African agriculture: farmer practices that differ fundamentally from that of large scale farms, the long history of N depletion from agricultural soils, seasonal emission pulses, and emission factors that vary with the amount of N added.

Yohannes et al. 2011. Soil CO2 efflux in an Afromontane forest of Ethiopia as driven by seasonality and tree species

Yohannes, Y., Shibistova, O., Abate, A., Fetene, M., Guggenberger, G., 2011. Soil CO2 efflux in an Afromontane forest of Ethiopia as driven by seasonality and tree species. Forest Ecology and Management 261, 1090-1098.

Abstract
Variability of soil CO₂ efflux strongly depends on soil temperature, soil moisture and plant phenology. Separating the effects of these factors is critical to understand the belowground carbon dynamics of forest ecosystem. In Ethiopia with its unreliable seasonal rainfall, variability of soil CO₂ efflux may be particularly associated with seasonal variation. In this study, soil respiration was measured in nine plots under the canopies of three indigenous trees (Croton macrostachys, Podocarpus falcatus and Prunus africana) growing in an Afromontane forest of south-eastern Ethiopia. Our objectives were to investigate seasonal and diurnal variation in soil CO₂ flux rate as a function of soil temperature and soil moisture, and to investigate the impact of tree species composition on soil respiration. Results showed that soil respiration displayed strong seasonal patterns, being lower during dry periods and higher during wet periods. The dependence of soil respiration on soil moisture under the three tree species explained about 50% of the seasonal variability. The relation followed a Gaussian function, and indicated a decrease in soil respiration at soil volumetric water contents exceeding a threshold of about 30%. Under more moist conditions soil respiration is tentatively limited by low oxygen supply. On a diurnal basis temperature dependency was observed, but not during dry periods when plant and soil microbial activities were restrained by moisture deficiency. Tree species influenced soil respiration, and there was a significant interaction effect of tree species and soil moisture on soil CO₂ efflux variability. During wet (and cloudy) period, when shade tolerant late successional P. falcatus is having a physiological advantage, soil respiration under this tree species exceeded that under the other two species. In contrast, soil CO₂ efflux rates under light demanding pioneer C. macrostachys appeared to be least sensitive to dry (but sunny) conditions. This is probably related to the relatively higher carbon assimilation rates and associated root respiration. We conclude that besides the anticipated changes in precipitation pattern in Ethiopia any anthropogenic disturbance fostering the pioneer species may alter the future ecosystem carbon balance by its impact on soil respiration.

Seiler et al. 1984. Field studies of methane emission from termite nests into the atmosphere and measurements of methane uptake by tropical soils

Seiler, W., Conrad, R., Scharffe, D., 1984. Field studies of methane emission from termite nests into the atmosphere and measurements of methane uptake by tropical soils. Journal of Atmospheric Chemistry 1, 171-186. doi:10.1007/bf00053839.

Abstract

The flux of CH₄ and CO₂ from termite nests into the atmosphere has been measured in a broad-leafed-type savannah in South Africa. Measurements were carried out on nests of species of six genera, i.e., Hodotermes, Macrotermes, Odontotermes, Trinervitermes, Cubitermes, and Amitermes. The flux rates of CH₄ relative to the flux rate of CO₂ in terms of carbon obtained for the individual species showed ratios of 2.9×10^-3, 7.0×10^-4, 6.7×10^-5, 8.7×10^-3, 2.0×10^-3 and 4.2×10^-3, respectively. Using data published on the assimulation efficiencies of termites, the flux of carbon as CH₄ accounts for 6.0×10^-5 to 2.6×10^-3 of the carbon ingested which results in a global CH₄ emission by termites of 2 to 5×10¹² g/yr. Methane is decomposed in the soil with average decomposition rates of 52 g/m²/h. The annual CH₄ consumption in the tropics and subtropics is estimated to be 21×10¹² g which exceeds the CH₄ emission rate by termites.

Delmas et al. 1991. Sources and sinks of methane in African Savanna. CH4 emissions from biomass burning

Delmas, R.A., Marenco, A., Tathy, J.P., Cros, B., Baudet, J.G.R., 1991. Sources and sinks of methane in African Savanna. CH4 emissions from biomass burning. J. Geophys. Res. 96, 7287-7299. doi:10.1029/90jd02496.


Abstract

Sources and sinks of atmospheric methane are studied in savanna regions of west and central Africa. Flux measured over dry savanna soils, using static chambers, is always negative the average uptake rate being 2 × 10¹⁰ molecules/cm²/s. In these regions, sources are linked to biomass burning. Methane and CO₂ emission from combustion of savanna plants and wood is studied by both field experiments and laboratory experiments using a combustion chamber. For savanna plants most of the carbon (85%) contained in the biomaterial is volatilized as CO₂ and 0.1 to 0.25% as methane. For graminaceous plants like loudetia simplex the ratio C-CH₄/C-CO₂ is 0.11%; it is 0.28% for hyparrhenia the other main type of savanna plants and it attains 1.4% for the combustion of wood. In natural fire plumes this ratio is around 0.26% for savanna fires and 0.56 to 2.22% for forest fires. These results show that methane release is highly dependent on the type of combustion. Methane to CO₂ ratios are also studied in vertical profiles in the troposphere taken during the TROPOZ I campaign, an aerial research expedition carried out over west Africa during the bushfire period. Within polluted layers, the average ratio of CH₄ to CO₂ excess over ambient air concentration is 0.34%. These results show that biomass burning in tropical Africa constitutes an important source of atmospheric methane estimated to about 9.2 × 10⁶ T(CH₄)/yr.

Dick et al. 2006. Effect of N-fixing and non N-fixing trees and crops on NO and N2O emissions from Senegalese soils

Dick, J., Skiba, U., Munro, R., Deans, D., 2006. Effect of N-fixing and non N-fixing trees and crops on NO and N2O emissions from Senegalese soils. Journal of Biogeography 33, 416-423. doi:10.1111/j.1365-2699.2005.01421.x.

Aim Agroforestry systems incorporating N-fixing trees have been shown to be socially beneficial and are thought to be environmentally friendly, both enriching and stabilizing soil. However, the effect of such systems on the emissions of the important greenhouse gas nitrous oxide (N₂O) and the tropospheric ozone precursor nitric oxide (NO) is largely unknown.

Location Soil was collected from the research plots of Institut Sénégalais de Recherches Agricoles at Bandia and Bambey, Senegal, West Africa, and from neighbouring farmers’ fields. Trace gas flux measurements and chemical analysis of the soil were carried out at the Centre for Ecology and Hydrology (CEH), Edinburgh, UK.

Methods Nitric oxide (NO) and nitrous oxide (N₂O) emissions were measured following simulated rainfall events (10 and 20 mm equivalents) from repacked soil cores collected under two tree species (Acacia raddiana) and Eucalyptus camaldulensis) in each of two provenance trails. In addition, soil samples were collected in local fields growing peanut (Arachis hypogaea) and Sorghum (Sorghum vulgare), close to the species trials in Bambey. NO was measured using a flow through system and was analysed by chemiluminescence. Nitrous oxide was measured from the repacked soil core headspace and was analysed by electron capture gas chromatography. Soil mineral N was extracted with KCl and analysed by colorimetric methods on separate soil columns.

Results Light rainfall, which increased the gravimetric soil moisture content to 20%, stimulated an increase in NO emission but there was no detectable N₂O emission. A heavy rainfall event, which increased the gravimetric soil moisture to 30%, stimulated N₂O emission with a subsequent peak in NO emissions when the soils became drier. Soil collected under the N-fixing tree species emitted significantly more N₂O than soil collected under the N-fixing crop species (P < 0.01). NO and N₂O emissions significantly correlated with soil available N (NH₄ and NO₃) (P < 0.05).

Main conclusions Rainfall intensity, supply of mineral N from organic matter and N fixation were the prime drivers of NO and N₂O emissions from seasonally dry tropical soils. The improved soil fertility underneath the trees provided a larger pool of mineral N and yielded larger rates of NO and N₂O emissions.

Dick et al. 2008. The contribution of agricultural practices to nitrous oxide emissions in semi-arid Mali

Dick, J., Kaya, B., Soutoura, M., Skiba, U., Smith, R., Niang, A., Tabo, R., 2008. The contribution of agricultural practices to nitrous oxide emissions in semi-arid Mali. Soil Use and Management 24, 292-301. doi:10.1111/j.1475-2743.2008.00163.x.

Abstract

The yield and flux of nitrous oxide (N₂O) emitted from continuous cereals (with and without urea), legumes/cereal in rotation and cereal/legume in rotation all with or without organic manure was monitored from January 2004 to February 2005. All treatments except continuous cereals had phosphate added. The cereal grown July–October in 2003 and 2004 was pearl millet (Pennisetum glaucum) and the legume was a bean (Phaseolus vulgaris). The 10 m × 10 m plots were established in a semi-arid climate in Mali. The addition of organic manure and both inorganic fertilizers increased yield and N₂O emissions. Continuous cereals treated with both organic manure and urea emitted significantly less N₂O (882 g N/ha per year) than plots receiving no organic manure(1535 g N/ha per year). Growing N-fixing crops in rotation did not significantly increase N₂O emissions. This study supports the new practice of growing cereal and legumes in rotation as an environmentally sustainable system in semi-arid Mali.