CARBON SEQUESTRATION POTENTIAL IN SMALLHOLDER FARMING SYSTEMS IN NORTHERN GHANA Jawoo Koo 1, J.B. Naab 2, J.W. Jones 1, W.M. Bostick 1 and K.J. Boote 3

INTRODUCTION Agriculture is thought to play a critical role in sequestering carbon in soils. Increases in soil C sequestration may be achieved by adoption of best management practices such as no tillage, fertilization, irrigation, residue management. Determination of the best management practices that can increase soil carbon will require long term field experimentation and evaluation relative to soil type and climate.

Introduction….cont. In developing countries, such long term experiments are difficult to conduct because of socio-economic constraints. An alternative approach is to use simulation models to quantify the potential of different management options used by farmers over long term, across soil types and weather to sequester soil carbon. However, to provide useful information to researchers and policy makers, the models must be capable of quantifying crop performance in environments where they are to be used. Thus, it is necessary to adapt the models to soil, climate, and cropping systems of interest and to evaluate predictions from the model relative to local data.

Introduction…cont. Gijsman et al. (2002) modified the DSSAT crop simulation models to incorporate a soil organic matter- residue module from the CENTURY model. The combined model, DSSAT-CENTURY, has become a more suitable model for simulating low-input systems and conducting long-term sustainability analyses. However, if model parameters for specific field situations are uncertain, estimations of soil carbon evolution are inevitably uncertain as well.

OBJECTIVE To estimate the regionally aggregated soil carbon sequestration potential in smallholders’ cropping systems in Northern Ghana using DSSAT-CENTURY cropping systems model and scenario analyses.

Materials and Methods Study Area Covered and area of about 18 km Annual rainfall of about 1000 mm Guinea savanna vegetation Surveyed 132 farmers’ fields in 2004 and 2006 Predominantly sandy soil texture (78% sand) Depleted soil organic carbon (median = 0.44%) Cropping history and management practices

Simulation Model & Input Data Used the DSSAT-CENTURY model to simulate crop growth and soil carbon dynamics Simulation time period – 20 years (2006 – 2025) Weather data: stochastically generated using Weatherman program based on 8 yrs daily data Soil data: soil profile (1 m depth) for each field was generated using the SBuild program based on the soil carbon & texture measured in 2006 Nitrogen in rainfall: annual additions of 5 kg [N] ha -1 to soils in each cropping season was assumed Manure addition

Simulation Model & Input Data..cont. Genetic coefficients Sorghum (CSM388): Folliard et al (2004) Maize (cv. Obatanpa): J.B. Naab (unpublished) Millet: ICRISAT-Mali Peanut (cv. Chinese): Naab et. al. (2004) Bush fallow: V.K. Walen (unpublished) Soil Organic Matter Pools 3 soil organic matter pools - SOM1:SOM2:SOM3 Estimated using data from long term experiments in Burkina

Simulation Model & Input Data…cont. Cropping sequence Projected cropping sequence in each field created for the simulation period One step transition matrix of crops calculated from surveyed cropping history Stochastically created cropping sequences for all fields using Markov Chain Monte Carlo method Scenario Analyses 5 scenarios 1 representative field was chosen and continuous cultivation of each crop simulated for each scenario

Table 1: Simulated management practices Scenario (Abbreviation) TillageFertilization Residue Removal Cereal s Legume s Bush Fallow Business-As- Usual (BAU) Hand-hoeing at 20 cm No fertilization100%75%100% No-Till (NTL) No-TillNo fertilization25% Fertilization (FRT) Hand-hoeing at 20 cm Sorghum: 20 kg[N]/ha/yr Maize: 40 kg[N]/ha/yr Millet: 20 kg[N]/ha/yr Peanut: No fertilization Bush Fallow: No fertilization 100%75%100% Leaving Residues (RSD) Hand-hoeing at 20 cm No fertilization25% Recommended Management Practice (RMP) No-TillMaize: 40 kg[N]/ha/yr Sorghum: 20 kg[N]/ha/yr Millet: 20 kg[N]/ha/yr Bush Fallow: No fertilization 25%

Soil Carbon Sequestration Rate The yearly soil carbon sequestration rate for each scenario in each field was calculated based on the soil carbon evolution under the BAU scenario as follows: where n is the number of years simulated, TRT n is the amount of SOC for each scenario in year n (kg/ha), and BAU n is the amount of SOC for the BAU scenario in year n (kg/ha).

Figure 1.Simulated aboveground vegetative biomass from continuous mono- cropping systems at a representative field in the study area for different management scenarios

Figure 2. Average aboveground crop vegetative biomass enhancement under the simulated management scenarios for the continuous mono-cropping system of each crop for 20 years. Error bar represents one standard deviation.

Figure 3. Simulated evolution of soil organic carbon to 20 cm depth for continuous mono-cropping systems under different management scenarios at a representative field in the study area for 20 years

Figure 4. Average soil organic carbon enhancement under the simulated management scenarios for the continuous mono-cropping system of each crop for 20 years. Error bar represents one standard deviation

Figure 5. Evolution of simulated soil carbon in three representative fields with the maximum, median, and minimum soil carbon sequestration rates.

Figure 6. Evolution of simulated soil carbon aggregated over 132 farmers’ fields in the study area in Wa, Ghana, for 20 years

Figure 7. Mean and standard deviation of the aggregated soil carbon sequestration rates and the crop biomass increase rates under different management scenarios

Summary and Conclusions Scenario analyses showed the potential of soil carbon sequestration when recommended management practices such as fertilization, conserving residues, and no-till are adopted When recommended practices were adopted, the regionally aggregated soil carbon sequestration rate was estimated to be 173 kg [SOC]/ha/yr This value was within the upper range of published estimates (50 to 200 kg [SOC]/ha/yr made by Lal (2003a)

Summary and Conclusions Small doses of fertilizer applications showed significant aboveground biomass production in cereals, but they did not benefit soil carbon sequestration without changes in residue management and tillage practices. Solely relying on fertilizer showed the least potential in sequestering soil carbon. When fertilization is not a feasible option, the most effective management practice to sequester soil carbon would be no-till farming.

Summary and Conclusions Although average crop biomass production under no-till was less than a tilled system, the soil carbon sequestration potential under no-till was significantly higher than a tilled system (α=0.05). Simulated results showed that elimination of bush fallows may be more beneficial to increase soil carbon sequestration, when crop residues are left and no-till adopted. Although overall results showed positive potential of adopting conservation tillage and/or residue management practices, adopting them in real smallholder farming systems will be difficult unless negative barriers are removed.