The Rock-Eval® signature of soil organic carbon in arenosols of the Senegalese groundnut basin. How do agricultural practices matter?

Agriculture, Ecosystems & Environment

Oscar Pascal Malou et al.

David Sebag

Patricia Moulin

Tiphaine Chevallier

Ndeye Yacine Badiane-Ndour

Abou Thiam

Lydie Chapuis-Lardy

Agrosystems

Organic inputs

Rock-Eval® pyrolysis

Thermal analysis

West Africa

October 1, 2020

10.1016/j.agee.2020.107030

https://www.sciencedirect.com/science/article/pii/S0167880920302152

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.

From the soil plant system to the field