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  Modeling soil organic carbon and yam yield under different agronomic management across spatial scales in Ghana

  Yam, a major food crop for West Africa, has not been managed to reach its potential productivity. The current practice of planting yam continuously for years after clear-cutting a field is not sustainable and has led to deforestation and nutrient depletion. By examining the effect of improved management on yam cultivation in Ghana, this study aimed to solve the tradeoff between improving yam yield and sustaining soil organic carbon (SOC). We used a calibrated and validated process-based crop simulation model, Systems Approach to Land Use Sustainability, to assess the impact of four management treatments: continuous unfertilized rainfed yam (control), pigeonpea-yam rotation, yam with 3 Mg/ha pigeonpea residue incorporated and yam with 23−23 N-P2O5 kg/ha fertilizer added. We modeled 10 years of yam yield and SOC across cropland in Ghana with varying levels of soil carbon, rainfall amount, and precipitation pattern. On average, simulated yam tuber yield was the highest with a pigeonpea residue incorporation treatment (4.1–11.9 Mg/ha, average of 7.5 Mg/ha). The rotation (average yield of 6.4 Mg/ha) and fertilizer (average of 7.0 Mg/ha) treatments produced comparable increases in yam yield over the control treatment (1.9–9.2 Mg/ha, average of 4.9 Mg/ha). The low yam yield of the control treatment was mostly attributed to nutrient deficiency (nitrogen and phosphorus). Drought also limited yam growth, particularly in northern Ghana. The three improved management treatments increased soil nutrient availability and thus improved yield. SOC declined under all four tested treatments over the simulated 10 years, but declined least with residue incorporation (average rate -0.3 Mg/ha/year), followed by fertilizer addition (-0.43 Mg/ha/year), rotation (-0.42 Mg/ha/year), and the control (-0.51 Mg/ha/year) management. Our work provides a benchmark for yam yield response to alternative management across Ghana, and highlights pigeonpea’s contribution to sustainable intensification of yam. Further research is needed to untangle the interacting effects of land use and agronomic management on SOC.
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  The Rock-Eval® signature of soil organic carbon in arenosols of the Senegalese groundnut basin. How do agricultural practices matter?

  Soil organic carbon (SOC) ensures soil quality and productivity of cultivated systems in the Sahelian region. This study uses Rock-Eval® pyrolysis to examine how cultural practices impact the quantity of SOC and quality of SOM in cultivated sandy soils in the Senegal groundnut basin. This cost-effective method provides information on SOC thermal stability, which has been shown to be related qualitatively to biogeochemical stability of SOC. We sampled soils within two villages in agricultural plots representative of local agricultural systems, and in two local preserved areas (tree plantation and shrubby savanna). SOC concentrations ranged from 1.8–18.5 g.kg−1 soil in the surface layer (0−10 cm) and from 1.5–11.3 g.kg−1 soil in the 10−30 cm layer. SOC contents of cultivated soils decreased significantly (p-value < 0.0001) according to field amendment, in the following order: addition of organic wastes> addition of manure > millet residues left after harvest > no organic input. We found that the quantity and the quality of SOC are linked, and that both depend on land-use and agricultural practices, especially upon the type of organic inputs. Quantity of SOC and quality of SOM are correlated strongly in the tree plantation (R² = 0.98) and in the protected shrubby savanna (R² = 0.97). They are also correlated significantly in cultivated soils receiving organic wastes (R² = 0.82), manure (R² from 0.74 and 0.91), or millet residues (R2 = 0.91) but not in soils that receive no organic inputs. Indexes based upon Rock-Eval® pyrolysis were represented in an I/R diagram that illustrates the level of SOC stabilization. The indexes of the studied soils were plotted against comparable results from literature. Thermal signatures of the Senegalese Arenosols show an inversion of I and the R indexes compared to data from the literature. This result highlights SOC stabilization as a function of soil depth. Indeed, the refractory pool in the studied soils (where refractory pool ranged from 7.7–21.3 % in the 0−10 cm layer, and from 12.5–24.3 % in the 10−30 cm) was more abundant than in Ferralsols in natural conditions, where refractory pool ranged from 2 to 9%. The soil organic matter in these Arenosols while positively affected by organic inputs, is dominated by more or less labile forms that mineralize quickly: a quality that is excellent for productivity of these agrosystems, but not for mitigation of climate change in the long term.