Comparison of organic carbon stock of Regosols under two different climates and land use in Tunisia

Drylands are affected by desertification and by a serious process of degradation. Studying the properties of their soils and their carbon sequestration characteristics is a key step as a major contribution for their conservation to cope with the global warming issue. Considering the geographical position of Tunisia, most of its territory is in the dryland zone with a deficit in water balance, and half of its area is only occupied by Lithosols and Regosols. These soils and despite their existence in dry climates are either pastureland or agricultural land, they play a considerable role for the peasants; however, their organic carbon stocks remain little studied and not well known. The present work focuses on the state of the carbon stock in the Regosols of Tunisia. Two study sites were studied; the first was a Regosol in Gafsa under arid climate, the second, another Regosol in Kairouan under semiarid climate. In these two sites, we also have two different modes of land use. In fact, in each site, we have a plot in an olive grove adjacent to another in a pastoral mode. Results showed that soil particle size, pH, and bulk density were very close. The difference was observed in the content of organic carbon and therefore in organic carbon stock. In olive groves, the organic carbon stocks of the soil, 1-m deep, were 8.22 (+/- 0.72) Kg.C.m(-2) in Gafsa and 12 (+/- 0.46) Kg.C.m(-2) in Kairouan. This increase mainly concerned the surface layers (0-20 cm) and the deepest layers (80-100 cm). By comparing the carbon stocks of soils under olive groves with those under pasture, the gain in 15 years varied between 1.48 and 2.46 Kg.C.m(-2). These results prove the low organic matter content of Regosols from North Africa, but on the other hand, their great capacity to sequester organic carbon. These same soils, if organically amended, could be fertile and productive soils, but also, could act as a carbon sink to cope with global warming.