1. Responses of ecosystem carbon stocks to changes in land use, management, and climate in Ghana Climate Scenarios for the 21st Century 1.No Climate Change (NCC): average precipitation and mean monthly minimum and maximum temperature from 1972 to 2000 are used for the 21st century. 2. Low Climate Change (LCC): by the year 2100 the annual precipitation decreases by 110 mm and mean monthly minimum and maximum temperatures increase by 2.7 o C and 2.2 o C, respectively. 3.High Climate Change (HCC): by the year 2100 the annual precipitation decreases by 234 mm and mean monthly minimum and maximum temperatures increase by 4.3 o C and 3.4 o C, respectively. Nitrogen Fertilization Scenarios for the 21st Century 1.N04: the average nitrogen (N) fertilizer level of 4 kg N ha -1 yr -1 continues 2.N3: the nitrogen fertilizer level increases to 30 kg N ha -1 yr -1 3.N6: the nitrogen fertilizer level increases to 60 kg N ha -1 yr -1 Modeling System The General Ensemble biogeochemical Modeling System (GEMS) (Liu et al., 2004) was used to simulate C dynamics in vegetation and soils. GEMS was developed for integrating well-established ecosystem models (e.g., CENTURY) with various spatial databases to provide simulations of biogeochemical cycles over large areas. Results Carbon Dynamics during the Past Century The ecosystem C stock decreased from 135 Mg C ha -1 in 1900 to 75 Mg C ha -1 in 2000. Accordingly, the SOC stock reduced from 27 to 22 Mg C ha -1 (fig. 2). However, the dynamics of SOC and the magnitude of SOC loss varied significantly with land use/land cover types (fig. 3). There was a reduction of 26% SOC for the open forest until the 1940s due probably to preferable deforestation for cultivation and 43% for all cropping systems. Dynamics of Carbon Stocks as Related to Nitrogen Fertilization Figure 6 shows that the study area has been a C source at a rate of 54 kg C ha -1 yr -1 during the past century, and remains a C source for this century at a rate of 10 kg C ha -1 yr -1 in the absence of climate change. The ecosystem could turn out to be a small C sink at 7 kg C ha -1 yr -1 under LCC, and 1 kg C ha -1 yr -1 under HCC with fertilization of 60 kg N ha -1 yr -1. Acknowledgements This is a contribution of SEMSOC funded by the US Agency for International Development (USAID)/West Africa Regional Program. Authors thank Zhengpeng Li for assistance in model implementation. Fig. 2. Temporal trends of ecosystem C and SOC stocks in the Ejura-Sekyedumasi district in the past century. Take Home Messages 1.Deforestation of open forest and other woodlands for agricultural use has resulted in a substantial reduction in ecosystem and soil carbon stocks across the Ejura-Sekyedumasi district of Ghana. 2.The dynamics of soil organic carbon depend also on land management and climate variables. 3.Low nitrogen fertilization level is the principal constraint on current crop production and poses a progressive threat to food security under the changing climate. 4.To increase nitrogen fertilization would be a critical measure to achieve food security and agricultural sustainability during the 21st century. Zhengxi Tan 1,*, Larry L. Tieszen 2, Emmanuel Tachie-Obeng 3, Shuguang Liu 1,4, and Amadou M. Dieye 4 1 Science Applications International Corporation (SAIC), contractor to USGS Center for EROS, Sioux Falls, SD 57198, USA. Work was performed under USGS contract 03CRCN0001. 2 USGS Center for Earth Resources Observation and Science, Sioux Falls, SD 57198, USA 3 Environmental Protection Agency, Accra, Ghana. 4 Geographic Information Science Center of Excellence, South Dakota State University, Brookings, SD 57007, USA. * Corresponding author at ztan@usgs.gov Fig. 3. Soil organic carbon (SOC) dynamics linked to major land use/land cover types from 1900 to 2000. Fig. 5. Average grain yield in the past century and its response to nitrogen fertilization in the 21st century. Carbon Budgets Associated with Progressive Climatic Warming in the 21st Century If only low climate change (LCC) is projected for the 21st century and no changes in land use and land management practices take place, the temporal trends of ecosystem C and SOC stocks for selected time points are presented in figure 4. Responses of Grain Yields to Nitrogen Fertilization Fig. 4. Temporal trends of ecosystem carbon and SOC stocks under low climate change scenario (LCC) across the Ejura-Sekyedumasi district during the 21st century. Fig. 6. Responses of soil organic carbon stock (SOC) to nitrogen fertilization levels averaged for all cropping systems under climate change scenarios from 2000 to 2100. Introduction The African continent is one of the weakest links in our understanding of the global carbon (C) cycle because of the sparse observation network (Williams et al., 2007). The C budgets at regional and global scales are probably better constrained in Sub-Saharan Africa than in any other region (Houghton and Hackler, 2006). Ghana is in Sub-Saharan Africa; therefore, we use a biogeochemical model to: 1. estimate the spatial and temporal variations in C stocks during the past century within the Ejura-Sekyedumasi district of Ghana, and 2.simulate the sensitivity of C stocks to changes in land use/land cover and management with climate change scenarios during the 21st century. Materials and Methods Fig. 1. Study area – Ejura-Sekyedumasi district in central Ghana, in contrast to Assin forest zone in the south and Bawku savanna zone in the north of Ghana. References Houghton RA and JL Hackler (2006), Emissions of carbon from land use change in sub-Saharan Africa. J. Geophys. Res., 111, G02003. Liu S, M Kairé, E Wood, O Diallo, and L Tieszen. 2004. Impacts of land use and climate change on carbon dynamics in southcentral Senegal. J. Arid Environments, 59:583–604. Williams, CA, NP Hanan, JC Neff, RJ Scholes, JA Berry, AS Denning, and DF Baker. 2007. Africa and the global carbon cycle. Carbon Balance and Management, 2:3. Study Area The Ejura-Sekyedumasi district represents the forest/savanna transitional zone in Ghana (fig. 1). The mean annual minimum and maximum temperature between 1972 and 2000 was 21.4 o C and 31.2 o C, respectively, and annual precipitation was 1,226 mm. Land cover during that period was dominated by the cultivated savanna derived from open forest and closed savanna woodlands. Cumulative change in land use/land cover from 1972 to 2000 was 13.5%. Major crop species include cassava, improved maize, plantain, and cocoyam. Compared with that under NCC-N04 scenario, the yield under climate change scenarios would increase 18% with 30 kg N ha -1 yr -1 and 42% with 60 kg N ha -1 yr -1 for maize, and accordingly 25% and 60% for cocoyam, and 7% and 25% for cassava (see fig. 5). U.S. Department of the Interior U.S. Geological Survey November 2007