Global Landscape of Organic Carbon and Total Nitrogen in the Soils of Oasis Ecosystems in Southern Tunisia

The oasis soils of Tunisia face several climatic and soil constraints. Trying to have cultures that are profitable and beneficial in terms of soil C and N sequestration in such environments is already a challenge. To conduct this, we tested under identical conditions four types of occupation in sub-plots adjacent to the crops; barley alone, alfalfa alone, intercropping barley/alfalfa, and a control fallow in a saline gypseous desert soil poor in organic matter. Field experimentation was carried out in the oasis of Degache, which is very representative of other Tunisian oases. The stocks of C and N of the plot were calculated from the start in September 2019 before the installation of the different crops. After 21 months, the control plot shows a decrease of −41% in its stock of C and −25% in its stock N. However, the best result is that of the barley/alfalfa intercropping with an increase of +126.46% in the C stock and +178.67% in the N stock. After almost two years of experience, the beneficial effect of the intercropping system in the oasis is clear. These results are very motivating and seem to be a solution to the rapid decline in soil organic stocks

Improved estimation and prediction of the wind-erodible fraction for Aridisols in arid southeast Tunisia

Wind erosion is a serious environmental threat. However, for Northern Africa estimates or predictions of the wind erodible fraction (WEF) in arid soils are rare. This study was conducted to (i) compare the flat sieve (WEFfs) and the Fryrear equation (WEFc) in order to estimate WEF (ii) fuse the directly measured WEFfs values to other measured soil parameters, i.e. soil organic matter (SOM), carbonate (CaCO3), sand, silt and clay content, to obtain a new improved WEF equation prediction. Samples were collected at ten locations near Gabès (Southern Tunisia) at three depths (0–2.5, 2.5–5 and 5–15 cm) in cultivated or native soils.The obtained WEFfs were>80% and therefore the soils would be classified as highly wind erodible (WEF > 50%). The lowest WEFfs values were found in the cultivated soils (82.5%), ranging from 82.5 to 96.9%. Our WEFfs estimates were positively correlated to sand (r = 0.78; p < 0.001), but negatively correlated to CaCO3 content (r = -0.82; p < 0.001). Interestingly, neither SOM nor clay content was strongly related to WEFfs. However, the WEFfs in cultivated soils was significantly linked to ∑(SOM + clay content). The Fryrear estimation predicted all WEFc values to be less than 60%. This clear mismatch to our data relates to CaCO3 presence, high sand and low SOM content in our Tunisian soils, when compared to US soils used to establish the Fryrear method. Therefore, we established a new equation for the wind erodible fraction, WEFmod (%) = 125.83–0.59*Clay(%) − 0.22*Silt(%) − 0.25*Sand(%) + 0.29*SOM(%) −0.69*CaCO3(%) (r = 0.85; p < 0.001). In the studied regions of Tunisia, this equation predicts much better soil WEF. The present paper is a first study to improve WEF estimation and prediction in arid North Africa. Better predictions of WEF are crucial in combating wind erosion of soils both in North Africa and in comparable soils globally

Soil OC and N Stocks in the Saline Soil of Tunisian Gataaya Oasis Eight Years after Application of Manure and Compost

Soil organic matter plays an important role in improving soil properties, crop productivity and is a key constituent and driver of the global carbon cycle. Nevertheless, relatively limited quantitative information is available on the organic carbon (OC) stocks and the actual potentials for OC and total nitrogen (N) sequestration under arid cropping systems. In this study, we evaluated the immediate and long-term (after eight years) effects of compost or manure additions, at a rate of 100 t ha−1, on the soil OC and N stocks in the Gataaya oasis in Southern Tunisia. The oasis had been abandoned and no additions had taken place in the 10 years prior to experiment. Soil samples were taken systematically every 10 cm up to a depth of 50 cm. After adding compost (CMP) and manure (MAN) in 2013, the bulk density (BD) decreased in the surface layers, especially at the 0–10 cm soil layer where it declined from 1.53 g cm−3 to 1.38 g cm−3 under compost and 1.41 g cm−3 under manure. Soil OC and N stocks, however, increased after adding compost and manure. Manure contributed more to OC stock increase than compost, with +337 and +241%, respectively. Correspondingly, the N stock increased by + 47 and +12%, respectively, due to manure and compost. After four years, compared to 2013 stocks, the decrease in OC stock was almost identical with −43 (CMP) and −41% (MAN). However, N stock seemed more stable under compost compared to manure, with −2 and −19%, respectively. After eight years, the N stock remained higher in the deepest layer 30–50 cm compared to other layers. This suggested that high gypsum application can inhibit N mineralization. The initial enhanced OC stock after the organic amendment, both for compost and for manure, was very quickly lost and after eight years had virtually returned to the initial OC state by the end of the eight years. Therefore, these oasis ecosystems require a near annual supply of exogenous organic material to maintain OC at an enhanced level. After eight years, manure amendment was found to be better than compost for increasing soil OC (3.16 against 1.86 t/ha, respectively) and for increasing N (0.35 against 0.18 t/ha, respectively). However, the cost and availability make the amendment with compost more interesting in oasis (400 Tunisian dinars/t for compost against 1016 Tunisian dinars/t for manure)