Short and Long-Term Effect of Land Use and Management on Soil Organic Carbon Stock in Semi-Desert Areas of North Africa-Tunisia

Soil organic carbon (SOC) plays an important role in the global C cycle, as well as in the maintenance and improvement of the soil quality. Over time, special attention has been paid to it in the study of the SOC reserves worldwide; however, reduced attention has been given to assessing the spatial patterns of SOC stock (SOCS) in semi-desert ecosystems. In this line, there are no conclusive studies in drylands of Africa affected by aeolian processes (semi-desert conditions) mainly due to the complexity of sample collection, and this is especially significant in some soil types such as Arenosols (AR) and Calcisols (CL). This study evaluated the spatial variability of SOC and SOCS in AR and CL with woody crops in relation to land use and management (old plantations > 100 years: centenary olive grove; new plantations < 12 years: young olive grove, almond, and pistachio) in semi-desert conditions. For this purpose, 16 soil profiles (for 0–40 and 40–100 cm depth) were selected and studied in an experimental area of Menzel Chaker-Sfax in southeastern Tunisia (North Africa). The main results indicated that the SOCS on average was higher in Old Cultivated AR (OC-AR) with 41.16 Mg ha−1 compared to Newly Cultivated AR (NC-AR) with 25.13 Mg ha−1. However, the SOCS decreased after a long period of cultivation in CL from 43.00 Mg ha−1 (Newly Cultivated CL: NC-CL) to 32.19 Mg ha−1 (Old Cultivated CL: OC-CL). This indicates that in the long term, CL has more capacity to store SOC than AR, and that in the short term, AR is more sensitive to land management than CL

Tunisian Soil Organic Carbon Stock – Spatial and Vertical Variation

Communication présentée au 24th DAAAM International Symposium on Intelligent Manufacturing and Automation, 23-26 October 2013, Zadar, Croatia.Soil organic carbon (SOC) is of big importance in the global carbon cycle. Distribution patterns of SOC in various regions of Tunisia constitute a baseline for studies on soil carbon changes. This paper presents Tunisian SOC stock calculated using soil profile descriptions defined by FAO/UNESCO classification, and the digital soil map 1:500 000. A soil database has been compiled, containing data from 5024 horizons and 1483 profiles. SOC stocks have been calculated for each profile by a classical method for a given depth, it consists of summing SOC stocks by layer determined as a product of bulk density (Db), organic carbon (OC) content, and layer thickness. Db values were calculated from pedotransfer functions when we have missing values. SOC stocks by profiles were calculated and linked by soil type to polygons of a digital soil map of Tunisia. In total, Tunisian SOC stocks are 1.006 Pg C in the 0 to 100 cm soil depth, and 0.405 Pg C in the upper layer 0-30 cm. The surface horizon (0 – 30 cm) stored 40% of the soil organic carbon stock. OC stocks were higher in Luvisols 71.6 and 159.2 t/ha in 0 – 30 and 0-100 cm soil depth, respectively. In Podzoluvisols there are 6.19 and 138.8 t/ha, but amounts are lower in Lithosols at 18.4 and 40.4 t/ha

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

Soil organic matter amendments in date palm groves of the Middle Eastern and North African region: a mini-review

Countries in the Middle Eastern and North African (MENA) region are among the most water-scarce regions in the world, and their dryland soils are usually poor in organic carbon content (<0.5%). In this study, we summarize examples of how people in the few oases of the MENA region overcome environmental challenges by sustainably managing economically important date production. On the basis of the limited studies found in the existing literature, this mini-review focuses on the role of traditional soil organic matter amendments beneath the soil surface as a key tool in land restoration. We conclude that soil organic matter amendments can be very successful in restoring soil water and preventing the soil from salinization

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

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

Mixed organic and bentonite based amendments improve soil hydrological conditions and irrigation efficiency in oasis soils

The major constraint for sustainable agriculture in arid regions of the world is the availability of (suitable irrigation) water. The quantity and quality of irrigation water, as well as the application technique and intervals between irrigation are important due to the useful amount of water brought to the soil and their direct influence on crop production. The oasis systems in southern Tunisia also suffer from scarcity of irrigation water resources because of low precipitation and limited available ground water. Organic matter (OM) application to improve soil fertility and enhance water holding capacity (WHC) is the traditional method in Tunisian oasis systems, but because of regional climatic conditions (dry and hot weather) the degradation of applied OM in the soils is very fast and consequently the WHC decreases too. The objective of this study was to examine the WHC of new alternative soil amendments in the oasis systems in a state of degradation. The soil studied is a gypsum soil. We compared these sandy soils amended either with (i) manure (MS), (ii) compost (CS), (iii) manure and bentonite (MBS) and (iv) compost and bentonite (CBS). The treatments were compared with untreated original oasis sandy soil (U). These five treatments were compared with the most degraded regional soil, i.e. that of the salt plain Chott Djerid (CD). To estimate the soil hydraulic parameters (including WHC) retention curves were measured and van Genuchten parameters fitted to the data. The results of soil water retention curves revealed that the untreated soil (U) retained less water at any matric potential compared to the amended soils. The soil hydraulic conductivity decreased with the use of bentonite combined with organic matter. The saturated hydraulic conductivity for untreated soil was higher than the amended soil with CBS and MBS by 11 and 18 times, respectively. These results suggested that the soil amendments significantly improved the soil water retention. Therefore these soil mixed amendments are appropriate for their potential wider use in sandy (oasis) soils to improve irrigation efficiency amounts and to reduce irrigation frequency

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)

Stream sediments geochemistry and the influence of flood phosphate mud in mining area, Metlaoui, Western south of Tunisia

Heavy metals in industrial wastes do worldwide significantly influence surface waters, soils, and human health. The relationships between geochemical stream sediment composition and industrial pollution by phosphate mud flood were examined to assess the severity level of the heavy metal contamination due to the mining activities in the Gafsa-Metlaoui basin (Tunisia). A statistical approach based on linear correlations and principal components analysis of 8 metal contents was used in stream sediment, which was applied to 21 samples of surveyed Metlaoui stream. Within the final model, four Eigen factors did explain almost 90% of the total variance matrix (F1: 42.25%, F2: 19.60%, F3: 15.50 and F4: 10.15%). Three main metal associations were found in the Metlaoui stream sediment. The first was concerned with Titanium (Ti) and Zirconium (Zr) as conservative elements. The second related to the important role of Manganese (Mn) with Iron (Fe) hydroxides in fixing metals. Finally, the third one consisting of Zinc (Zn), Nickel (Ni), Copper (Cu) and Molybdenum (Mo) is all known to be linked to local mining activities sources. The overall degree of contamination was in the following order (high to low): Ti > Zr > Zn > Mo > Mn > Ni > Fe > Cu, but the extent of the contamination did decrease downstream from the mining activity. Phosphate mining activity, especially washing phosphate, leads to an unwanted heavy metals contamination in the nearby surrounding environment

Soil Organic Matter Composition in Coastal and Continental Date Palm Systems: Insights from Tunisian Oases

In Tunisia, the coastal oasis of Chenini is characterized by a lush vegetation cover, while more inland continental oases (e.g. Guettaya oasis) have a very scarce vegetation cover. For sustaining date production, organic fertilizer is applied, either spread on the soil surface (in Chenini) or buried under a sand layer (in Guettaya). We examined on a molecular level how these management techniques affect soil organic matter composition in oasis systems. 13C nuclear magnetic resonance spectroscopy signals indicated for Guettaya a dominance of fresh plant input, which was most pronounced in the uppermost soil close to palms. Evidence of more degraded organic matter was found in deeper soil near the palms, as well as in soil more distant from palms. Amino sugar analysis revealed lower contents in the uppermost Guettaya soil near the palms. The overall microbial amino sugar residue contents were similar in range as those found in other dryland environments. With increased distance from trees the amino sugar contents declined in Guettaya where the palms grow on the bare soil but this was not the case for Chenini with multi-layer vegetation cover under the date palms. In agreement with previous dryland studies, the soil microbial community in both oasis systems was dominated by fungi in topsoil, but shifted towards bacteria-derived residues in subsurface soil. This might be due to higher variability of temperature and moisture in topsoil and/or lower degradability of fungal remains, but here further research is required