Measuring the Stability of Soil Organic Carbon in Arenosols in the Senegalese Groundnut Basin

Soil organic carbon (SOC) contributes to agrosystem productivity. Understanding how farming practices implemented by smallholders affect the levels and distribution of SOC in carbon (C) pools with different stabilities is essential in sub-Saharan Arenosols where SOC mineralization is intense. The stability of SOC was studied by thermal (Therm-C), physical (particulate organic matter >50 mu m, POM-C and fine soil fractions <50 mu m, FF-C), chemical (permanganate-oxidizable carbon, POX-C) and biological (mineralizable C, Min-C) approaches. Soil samples were collected at depths of 0-10 and 10-30 cm in cultivated fields (out- or home-fields) without any input, with millet residues, amended with manure, or with household organic wastes. Globally, average SOC contents were low (<6 g C kg-1). The variability in SOC and C pool contents was sensitive to field management. The different approaches to measuring the stability of SOC did not measure the same fraction of SOC. POM-C and Therm-C were correlated and both explained Min-C similarly, thus suggesting that in these sandy soils, POM-C or Therm-C probably measured comparable properties of the stability of C. The lack of relationships between POX-C and other pools suggested that POX-C encompassed a different nature of SOC while providing complementary information on the biogeochemical stability of SOC.