Soil property and soil organic carbon pools and stocks of soil under oases in arid regions of Tunisia

In Tunisia, the lowest soil organic carbon stocks are attributed to arid soils. This study was conducted to assess and to quantify soil organic carbon (SOC) stocks and pools under oases agrosystems in arid regions of Tunisia. Soil samples were collected from 0- to 5-cm and from 0- to 30-cm layer depth from two principal types of soils (Lithosols and Solonchaks) under three oases. Total SOC, particulate organic carbon (POC) and SOC < 50 µm content and stocks were calculated. The results showed that Lithosols contain the highest SOC content (1.11%). However, Solonchaks accumulate the highest POC. Both Lithosols and Solonchaks store large amount of SOC < 50 µm. Particularly in 0–5-cm layer, negative and significant correlation was found between POC and bulk density (BD) in Lithosols but in Solonchaks POC was positively correlated with BD. On average SOC stocks in Lithosols in 0–5-cm layer are above those calculated for Solonchaks and not exceed 0.81 kg C/m². The greater SOC stocks in 0–30 cm depth characterize Lithosols (4.33 kg C/m²). SOC stocks for Solonchaks were below 3.19 kg C/m². Otherwise, differences in POC stocks were observed in 0–30-cm layer between soils. Solonchaks store between 0.98 and 1.24 kg C/m² of POC. The proportion of POC stocks relative to the total SOC stock is <25% in two types of soil. Organic carbon stocks are mostly stable, as SOC < 50 µm stocks reached over 70% of SOC. We can suggest that arid soils have high capacity to store organic carbon under oasis agrosystems.