Contribution of trees to soil carbon sequestration under agroforestry systems in the West African Sahel

Consequent to recent recognition of agricultural soils as carbon (C) sinks, agroforestry practices in the West African Sahel (WAS) region have received attention for their C sequestration potential. This study was undertaken in the S,gou region of Mali that represents the WAS, to examine the extent of C sequestration, especially in soils, in agroforestry systems. Five land-use systems were selected in farmers' fields [two traditional parkland systems, two improved agroforestry systems (live fence and fodder bank), and a so-called abandoned land]. Soil samples taken from three depths (0-10 cm, 10-40 cm, and 40-100 cm) were fractionated into three size classes (2,000-250 mu m, 250-53 mu m, and < 53 mu m) and their C contents determined. Whole-soil C contents, g kg(-1) soil, across three depths ranged from 1.33-4.69 in the parklands, 1.11-4.42 in live fence, 1.87-2.30 in fodder bank, and 3.69-5.30 in abandoned land; and they correlated positively with silt + clay content. Using the C-13 isotopic ratio as an indicator of relative contribution of trees (C3 plants) and crops (C4 plants) to soil C, more tree-origin C was found in larger particle size and surface soil and indicated that long-term tree presence promoted storage of protected C in deeper soil. Existing long-standing agroforestry practices of the region such as the parklands seemed to have little advantage for sequestering additional C, whereas improved agroforestry practices such as live fence and fodder bank introduced in treeless croplands seemed to be advantageous.