Relative contribution of trees and crops to soil carbon content in a parkland system in Burkina Faso using variations in natural 13C abundance

The origin of organic matter was studied in the soils of a parkland of karite (Vitallaria paradoxa C.F. Gaertn) and nere (Parkia biglobosa (Jacq.) Benth.), which is extensively cultivated without the use of fertilisers. In such systems, fertility (physical, chemical and biological) gradients around trees have been attributed by some authors to a priori differences in fertility, allowing for better tree establishment on richer sites. In reverse, other workers believed that these gradients are due to the contribution of trees to the formation of soil organic matter through litter and decay of roots. Measurements of the variations in the C-13 isotopic composition allowed for a distinction between tree (C-3) derived C and crop and grass (C-4) derived C in the total soil organic C content. The organic carbon contents of the soils were recorded under the two species at two soil depths and at five distances going from tree trunk to the open area and their C isotopic signatures were analysed. The results showed that soil carbon contents under karite (6.43 +/- 0.45 g kg(-1)) and nere (5.65 +/- 0.27 g kg(-1)) were significantly higher (p < 0.01) than in the open area (4.09 +/- 0.26 g kg(-1)). The delta C-13 of soil C was significantly higher (p < 0.001) in the open area (-17.5 +/- 0.3 parts per thousand) compared with the values obtained on average with depth and distance from tree under karite (-20.2 +/- 0.4 parts per thousand) and nere (-20.1 +/- 0.4 parts per thousand). The C-4-derived soil C was approximately constant, and the differences in total soil C were fully explained by the C-3 (tree) contributions to soil carbon of 4.01 +/- 0.71, 3.02 +/- 0.53, 1.53 +/- 0.10 g kg(-1), respectively under karite, nere and in the open area. These results show that trees in parklands have a directly positive contribution to soil carbon content, justifying the need to encourage the maintenance of trees in these systems in semi-arid environments where the carbon content of soil appears to be the first limiting factor for crop growth.