The Near East North Africa (NENA) region spans over 14 % of the
total surface of the Earth and hosts 10 % of its population. Soils of the
NENA region are mostly highly vulnerable to degradation, and future food security
will much depend on sustainable agricultural measures. Weather variability,
drought and depleting vegetation are dominant causes of the decline in soil
organic carbon (SOC). In this work the status of SOC was studied, using a
land capability model and soil mapping. The land capability model showed that
most NENA countries and territories (17 out of 20) suffer from low productive lands
(> 80 %). Stocks of SOC were mapped (
The Near East North Africa (NENA) region spans over 14 % of the total surface of the Earth and hosts 10 % of its population (Elhadi, 2005). The largest importer of wheat in the world, this region is also one of the poorest (FAO, 2015). A recent assessment of global hunger index (GHI), based on four indicators – undernourishment, child wasting, child stunting and child mortality – showed that most of the NENA countries and territories reflect a low to moderate GHI. Countries suffering from armed conflicts, Syria, Iraq and Yemen, are at a serious risk (von Grebmer et al., 2017). With the scarce natural resources and difficult socioeconomic conditions, it is questionable whether food security will be reached by 2030, unless a significant change in agricultural practices and governance occurs (FAO, 2017).
Most of the land area of the NENA region falls in the hyper-arid, arid and
semi-arid climatic zones. Climate change is expected to exacerbate drought and the
scarcity of water. Weather variability, drought and depleting
vegetation are major concerns in the loss of soil productivity and
agricultural sustainability. Changes in soil organic carbon (SOC) can affect
the emission of greenhouse gases to the atmosphere and negatively influence
the global climate (Lal, 2003). In fact, destructive land management
practices are impacting soil functions. Land use change, mono-cropping and
frequent tillage are considered to cause a rapid loss of SOC (Guo et al.,
2016). These agricultural practices disturb the stability of soil
characteristics, employed under local land cover (LC) and climate conditions (Bhogal et al.
2008). Thus, most NENA lands contain
NENA pedo-climatic conditions present major constraints on carbon sequestration (Atallah et al., 2015). To maintain soil productivity and land quality, several technical and socioeconomic measures need to be adopted. Additional efforts oriented to maintaining and increasing SOC can contribute to poverty reduction and achieve food security (Plaza-Bonilla et al., 2015). Good agricultural practices, based on low tillage or no tillage, may result in the reduction of SOC breakdown and the enhancement of the soil carbon pool (Atallah et al., 2012; Cerdá et al., 2012; Boukhoudoud et al., 2016).
In order to compare situations and problems, global soil organic carbon maps
are a priority. As recently as December 2017, the GSP-FAO ITPS launched
version 1 of the global soil organic carbon map, showing the SOC stock in
topsoil (
SOC content (%) in topsoil and subsoil in the major soil groups of the NENA region with standard deviation related to soil class.
Data on SOC and soil inorganic carbon (SIC) contents in soils were retrieved
from the soil database of the FAO-UNESCO Digital Soil Map of the World (DSMW)
at
In terms of area, the largest soil units are Yermasols (4670.6 km
Soil inorganic carbon level and the resistance to land degradation in the major soil units of the Near East North Africa region (Source: DSMW, FAO, 2007).
The SOC content can vary depending on soil type, topography, land cover, erosion–sedimentation and soil management. Within the topsoil (0–0.30 m), the SOC contents are between 0.13 % and 1.74 %, while in the subsoil (0.30–1 m) values range between 0.16 % and 0.9 % (Fig. 1). Two out of the three predominant soil classes (Xerosols and Aridisols) have SOC contents below 0.5 %. Overall, the NENA soils are poor in SOC, as less than 20 % of soil resources have SOC contents above 1.0 %.
The NENA soil map was prepared using the topographic map series of the
American Geographical Society of New York, as a base, at a nominal scale of
The land cover map was that of the European Space Agency (ESA), at 300 m spatial resolution. The reference coordinate system used was a geographic coordinate system based on the World Geodetic System 84 (WGS84) reference ellipsoid. The legend assigned to the global LC map was based on the UN Land Cover Classification System.
Total SOC and SIC stocks were calculated separately for the topsoil (0–0.3 m)
and subsoil (0.3–1.0 m) using the following equations:
Stocks of SOC under different land cover/land use were evaluated as well.
Since 1990, the ESA (Climate Change Initiative project) has started to produce LC maps of the NENA region. The version used
in the study corresponds to the second phase of the 2015 global LC
(
According to the results based on the land capability model, 40 %–100 % of soils in the region fall within the low, very low and non-arable classes (Fig. 2). Thus, the proportion of highly and medium productive soils varies between 0 % (Bahrain, Qatar, Oman and UAE) and 60 % (Jordan). Countries and territories like Iraq, Lebanon, Morocco, Palestine, Somalia, Syria and Tunisia have between 9 % and 20 % highly to medium productive soils. The remaining NENA countries and territories have less than 5 % high and medium productive lands. Some of these countries and territories belong to the food-insecure nations.
Distribution of land capability classes (% of total national area) for 20 countries and territories of the NENA region, based on the USDA model (1999) and Digital Soil Map of the World (FAO, 2007).
The soil productivity concept is based strictly on soil properties. But, with the lack of water in dry lands and the prevalence of rainfed agriculture, the soil cannot show its full potential for food production. Similarly, irrigation with brackish water restricts crop productivity due to the development of secondary soil salinity. With properly managed irrigation, the medium productive lands can provide moderately good harvests. For instance, our field observation in Jordan showed that a large area of productive lands was cropped with barley, not because of land suitability, but due to low rainfall (< 200 mm). In drought-affected years, the land is converted into a grazing area for small ruminants following crop failure to make maximal profit from the exploitation.
The low productivity of the soil is reflected in the SOC contents. The
accumulation of SOC in the NENA region is limited by the high mineralization
rate (Bosco et al., 2012). Climate change and recurrent drought events affect
SOC sequestration in the soil. It is estimated that a rise in temperature of
3
Among the soil properties affecting SOC, the clay and calcium carbonate contents are most relevant. High clay content tends to counteract the decomposition of SOC, as found in clay soils of Morocco and in Vertisols of northern Syria (FAO and ITPS, 2015). But, the dominant soil classes (Table 1), characterized by sandy and sandy loam textures, are subject to fast decomposition. Next to the clay texture, the presence of calcium carbonate decreased the decomposition of composted organic material in subhumid coastal Lebanon (Al Chami et al., 2016). This slower turnover of organic matter was explained by the low porosity and prevalence of micropores in soil macroaggregates (Fernãndez-Ugalde et al., 2014).
Based on the mapping of SOC density (ton ha
Spatial distribution of SOC density (ton ha
This is especially relevant to the Gulf countries, Iran, Tunisia and Morocco,
with values below 221 Mt (Fig. 4). Such low OC sequestration potential can
be explained by the sparse natural vegetation and the reliance on irrigation
to produce food and feed crops. A regional implementation plan for
sustainable management of NENA soils appeared in 2017
(
Spatial view of total soil organic carbon stock (Mt) across the
countries and territories of the NENA region (
A comparison has been undertaken between the FAO methodology, adopted in the
present work (two soil layers: 0–0.3, 0.3–1 m), and a previous study
whereby the soil profile was divided into six depths down to 2 m (Hengl et
al., 2014). Both approaches agree about the NENA region (SOC content:
1 %–2 % and SOC stock: 20–204 ton ha
Proportion of main land cover (% total land area) and land use in the NENA region (
The choice of scale when using or producing soil maps may lead to uncertainty
in small countries and places with fragmented land use (Darwish et al., 2009). The coarse
scale adopted in this work (
Another source of error can be associated with the SOC content in soil classes or major groups from the DSMW (FAO, 2007). The level of uncertainty in the assessment of the SOC density in NENA countries and territories depends on the variability of SOC, as suggested by the standard deviations of the means (Fig. 1). Therefore, the SOC content is the major source of variability for the SOC density at the soil class level. Cambisols and Fluvisols are shown to have the largest standard deviation, caused by a long land use history and large anthropogenic impact. Subsoil is less subject to pedoturbation and direct human influence; thus SOC content has lower variability.
Based on the land cover map of the ESA, the bare lands correspond to 80 % of
the whole NENA region (Fig. 5). Grassland, sparse vegetation cover and
rainfed agriculture represent 4.39 %–5.27 %. The irrigated crops do
not exceed 0.66 % of the total area, distributed in a limited cultivated
area (Fig. 5). The NENA region possesses a land area of about
15 million km
The combination of the SOC stock map with the land cover map showed a significant
effect of land cover on SOC stocks in the NENA region. As can be expected,
shrublands, sparse vegetation and bare lands gave the smallest values,
between 14 and 26 ton ha
Despite the expected impact of frequent plowing, the soils under mixed trees
and irrigated crops have higher SOC density than rainfed crops. This could be
linked to the higher biomass produced under irrigated conditions in these
water-limited areas. The highest SOC stock is observed under evergreen
forests, whose area is very limited (3380 km
SOC density (tons ha
In addition to the stocks of SOC in relation to land cover–land use, the
stocks of SOC and SIC were established per country/territory (Fig. 7). The range of SIC
stocks is very wide, from less than 25 tons ha
Climatic conditions characterized by wetting–drying cycles and a long dry and hot season (Boukhoudoud et al., 2016) promote the decomposition of SOC. Further, frequent cultivation, irrigation with saline water and soil salinity rise in coastal areas exert significant effects on soil microbial functional properties. For instance, 3 months after the application of glyphosate-based herbicide to the soil under olive trees in coastal Lebanon, lipase activities significantly decreased (Boukhoudoud et al., 2017). Soil classification and SOC mapping help to identify hotspots that need to be improved or require special management measures and bright spots with satisfactory C accumulation levels that need to be protected. In this section, major practices affecting SOC will be presented, followed by a discussion of preventive and remediation measures.
Tillage practices contribute to the vulnerability of soils to water erosion. If not properly managed, some 41 million hectares in the NENA region would be affected by water erosion (FAO and ITPS, 2015). The erosion of soil surface layers can affect the soil carbon in two possible ways. The greater exposure of carbonates to climatic elements could increase the loss of SIC to the atmosphere and ground water. Compared to stable soils, the higher decomposition of SOC in eroded soils decreases the productivity of cultivated crops and can reduce SOC stock, if not properly managed (Plaza-Bonilla et al., 2015).
A possible measure to reduce the risk of erosion is no-tillage practice. No-tillage coupled with mulching, to reduce weed development and omit herbicide application, as part of conservation agriculture (CA), aims to return more plant residues to the soil, enhance C sequestration, increase soil aggregates, improve water infiltration and protect soil carbon from decomposers (Palm et al., 2014). Through a modification of common practices, such as the frequency and depth of tillage, changes in the SOC could be promoted in most soils. Experiments conducted by ICARDA, Syria, showed that no-tillage performed well in terms of energy and soil conservation (Plaza-Bonilla et al., 2015). Elsewhere, in Palestine, soil conservation was found to pay, with a net profit 3.5 to 6 times higher than without conservation measures (FAO and ITPS, 2015). In dry land regions, agricultural activities based on CA practices are beneficial as crop residues are left on the soil surface (Plaza-Bonilla et al., 2015). The presence of residues protects the soils from high evaporation, water and wind erosion. This is especially relevant to soils that are sensitive to degradation, such as the very shallow Lithosols, the periodically swelling and shrinking Vertisols, Gypsic Yermasols (Aridisols), the poorly structured Solonchaks and Solonetz, the sandy-textured Arenosols and the desert soils (Xerosols).
Major constraints facing soil conservation measures, in the eastern Mediterranean, were due to knowledge and perception, prevailing practices of complete removal such as hay or forage and sometimes burning of residues after harvest, land tenure and the type of landscape (FAO, 2012; FAO and ITPS, 2015). These factors are socioeconomic in nature, rather than scientific. They are related to the ability of growers to accept new techniques and adopt them. In many situations, the transfer from the research stations to the farmers was not smooth. For instance, CA was successfully tested in experimental stations in Morocco and Lebanon, but several social and technical barriers prevented it from reaching farmers (Mrabet et al., 2012; FAO, 2012).
Soil organic carbon and soil inorganic carbon density
(tons ha
A debate has been taking place about the effect of no-tillage on SOC. Many
authors agree that under CA, SOC increases near the soil surface, but not
necessarily throughout the profile. A study compared 100 pairs, where
no-tillage has been practiced for over 5 years. The absence of tillage led
to higher C stocks (0–0.3 m soil depth) in 54 % of pairs, while
39 % showed no difference in stocks (Palm et al., 2014). In the absence
of tillage, the slower decomposition of residues results in higher C
accumulation on the soil surface. Over a period of 5 years, zero tillage
promoted an increase in SOC equal to 1.38 Mg ha
Practices such as the application of N fertilizers, organic amendments,
incorporation of residues and crop rotations influence the levels of SOC. In
soil mining practices without minimal input of fertilizers, the lack of
available nutrients makes most crops entirely reliant on the mineralization
of accumulated SOC (Plaza-Bonilla et al., 2015). In East Africa, 14 years of
continuous cultivation without any input decreased SOC from 2 % to
1 % (Sharma et al., 2012). The application of N fertilizers was
associated with increased levels of soil C, as compared to the absence of N
fertilizers (Palm et al., 2014). In a 10-year rotation of wheat–grain legume
in northern Syria, the application of N fertilizers to the cereal
caused a notable increase of SOC in the top 1 m of soil, equal to
0.29 Mg ha
The effects of crop rotations on SOC are related to the amounts of above and
belowground biomass produced and retained in the system. In a study conducted
in semi-arid northern Syria, a 12-year rotation produced higher SOC in
wheat–medic (12.5 g SOC kg
In poor dry land regions, especially in the rainfed agricultural systems, some practices hinder the accumulation of SOC. Overall, crop residues serve as fodder or are used for household cooking or heating, leaving little plant material on the soil surface. Even animal dung is used as cooking fuel in many regions. The low SOC content could be improved by increasing the crop residues produced and incorporated. Such an approach requires the application of fertilizers in order to avoid the depletion of soil nutrients (Plaza-Bonilla et al., 2015). By removing residues, animal dung and crops, no residues are left in the soil except roots. In the absence of fertilizers, these practices can mine the soil N, and over the years the pool of nutrients in the soil can be imbalanced and depleted.
Some authors question the validity of remediation measures to promote SOC accumulation in most of the NENA region. Results from research stations in Egypt and Syria provide evidence to the contrary. In a trial in northeast Cairo, Egypt, the irrigation of a sandy soil with sewage water, for 40 years, changed its texture to loamy sand (Abd el-Naim et al., 1987). This modification of the soil texture led to a significant improvement of the soil physical properties. Further, within the same long-term trial, the irrigation with sewage water, for 47 years, increased SOC to 2.79 %, against 0.26 % in the control experiment (Pescod and Arar, 2013). This rather slow accumulation could be related to the sandy soil texture and to the input of the organic matter in labile, soluble forms.
The addition of more stable composted materials was tested in semi-arid north
Syria. The amount of compost, 10 Mg ha
The irrigated land represents a minor fraction of agriculture in the NENA region, but irrigated crops are essentially found on prime soils (Fig. 4). Frequent wetting of irrigated soils makes them more likely to lose C as compared to dry soils. But, this partial loss is compensated by higher biomass production and greater OM inputs from roots, even if residues are removed. Lack of moisture limits soil mineralization (Sharma et al., 2012). Irrigated soils promote intense microbial activity and a rapid decomposition of SOC. In the fertile region of Doukkala, Morocco, known for producing wheat and sugar beet, a decade of irrigated farming decreased soil organic matter by 0.09 % per year (FAO and ITPS, 2015). This loss could have been reduced through the incorporation of crop residues. But, in these mixed farming systems, aboveground residues are consumed by farm animals.
The irrigation of soils in the NENA region is expected to affect the SIC. Dry land soils were considered to contain an equivalent stock of SIC to SOC (Sharma et al., 2012). But higher SIC than SOC levels were found in this study, notably in the subsoils. Despite this large stock, there is a major knowledge gap regarding the effects of land use and management on the dynamics of SIC. This is especially relevant to the irrigation with calcium- or sodium-enriched groundwater (Plaza-Bonilla et al., 2015). In these conditions, the formation of calcium carbonate could be accompanied by some release of carbon dioxide while the development of sodicity can cause irreversible SOC loss.
The NENA area, consisting of 14 % of the Earth surface, contributes only
4.1 % of total SOC stocks in topsoil. The soil resources of the NENA region
are developed under dry conditions with prevailing rainfed agriculture.
The majority of lands in NENA countries and territories are of low productivity. The current
mapping of SOC density showed that 69 % of soil resources represent a SOC
stock below 30 tons ha
The digital soil map of the world (DSMW) Version 3.6,
completed January 2003 and updated 2007 can be downloaded from FAO, 2017:
(
TD provided the idea and contributed to the concept note, production and analysis of SOC data in NENA countries and territories. AF produced the land cover and SOC maps of the area of study. TA provided the main part of the challenges of the conservation agriculture and edited the paper.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Regional perspectives and challenges of soil organic carbon management and monitoring – a special issue from the Global Symposium on Soil Organic Carbon 2017”. It is a result of the Global Symposium on Soil Organic Carbon, Rome, Italy, 21–23 March 2017.
This paper was supported by the FAO, GSP-ITPS, UNESCWA and CNRS Lebanon within a land degradation assessment and SOC mapping project related to food security. Edited by: Annette Cowie Reviewed by: two anonymous referees