In the semi-arid regions of Africa, agriculture provides food, income, power, stability and resilience to rural livelihoods (Challinor et al. 2006). In Senegal, public policies and private initiatives were carried out after independence for sustaining farmers in order to achieve food security. During the 1970s, the company of BUD-Senegal and the Centre pour le Développement de l’Horticulture initiated the intensification of the horticulture sector in the Niayes region (Wade, Benz & Egg 2004). The Niayes region is the eco-geographical area of the Senegalese coast, of about 180 km long and 25 km wide. It is bounded to the west by the Atlantic Ocean and to the east by the Dakar/Saint-Louis national highway (Figure 1). The landscape components include depressions and dunes with shape process going back to the periods of quaternary and tertiary. The dominant soil types are sandy soils, vertisols and hydromorphic soils (Ndao 2012). Moisture conditions allowed the accumulation of soil organic matter in the depressions. Natural vegetation is characterised by wetland species (Eleais guineensis, Prosopis africana) and savanna species (Adansonia digitata, Acacia spp.; Faye 2010). Market gardening, arboriculture and fishing are the main socio-economic activities. Until 2004, 60% of the national needs for consumption of vegetables were produced from the Niayes region, as well as 80% of the exports of horticultural products (Seck et al. 2005). The richness of the biodiversity and the proximity to the sea give the Niayes area a microclimate with more moderate temperatures compared to the climate in the regions located in the mainland. Such biophysical conditions have created a favourable living environment in the Niayes region and therefore have led to ineluctable change in a diverse range of land uses. Consequently, agricultural lands tend to decrease in favour of the construction of buildings and industrial units. This situation was a serious concern because the Niayes region, intended primarily for market gardening activities, is the smallest of the six eco-geographical zones of Senegal. It represents about 2% of the surface area of the country. These spatial dynamics tend to affect the performance of horticultural activities. In the Sahel, the risk of food insecurity related to climate conditions was emphasised (Benson & Clay 1998; Cooper et al. 2008; Giannini, Biasutti & Verstraete 2008; Leroux 1995; Toupet 1995). Such a situation resulted from the drought of the 1970s that revealed the vulnerability of agricultural production systems in the Sahel (FAO 2012). However, human activities are often factors of imbalance in the agro-system organisation. Because of favourable climatic conditions for human development, construction of buildings tends more and more to monopolise land use in the Niayes region. Industrial activities are also expanding. This urbanisation and industrialisation process causes the loss of agricultural land and constitutes a threat to food security because of a high contribution of market gardening and horticultural products in food demand in Senegal. Certainly, rice and millet are the basic cereals for local consumption. Nevertheless, onion, carrot, cabbage, eggplant, tomato, pepper, beans, okra and cassava, also produced in the Niayes region, are various condiments widely consumed by people in Senegal. Watermelon, melon, mango and citrus fruits, which come from the rural Niayes areas, are also consumed nationally. In this regard, the Niayes region is the field of predilection for vegetable and fruits production, therefore justifying the analysis of the risk in relation to the dynamic of land use. The risk analysis uses the concepts of hazard, vulnerability and challenge (Brooks 2003; Cummins & Mahul 2008; Veyret & Reghezza 2005). In this study, urbanisation and industrialisation represent the hazard. Agricultural areas reflect the vulnerability and their dynamics lead to food insecurity (Brooks 2003; World Bank 2010b). Furthermore, the construction of an urban pole in Diamniadio by public authorities, with the implementation of major infrastructures like the Centre International de Conferences Abdou Diouf (CICAD), the Université Amadou Moctar MBow of Diamniadio or the turnpike road, has markedly increased the loss of arable land. Thus, through a linear and prospective analysis of the dynamics of agricultural lands of the study area and its potential for agricultural productivity, it is intended to provide indicators of the level of risk for food insecurity. The objective of this study is to determine the level of risk of food insecurity with regards to the dynamics of land use and soil fertility within the Niayes region.

The monitoring of the dynamics of agricultural lands in the study area highlights a high level of risk to food security occurrence induced by a significant decrease in surface area for agricultural production. For a Sub Saharan developing country with agriculture as a driving economy, any loss of agricultural land constitutes a serious threat to food production (World Bank 2010a). Indeed, in the Niayes region, agricultural land has decreased from 2384 ha in 1999 to 1541 ha in 2014. The shrinkage rate has rapidly increased between 2009 and 2014. During this period of time, the losses of agricultural land were evaluated to 556 ha, which represents 65% of the total reduction of the whole land because of a new way of using agricultural lands for habitat areas and/or industrial establishments, which ended up decreasing the horticultural crops production (Figure 5). Therefore, it is predicted that the depletion of agricultural land reserves in the study area in the 2050s will lead to a progressive urbanisation front towards the Niayes. Such a phenomenon will, in the end, continue towards other areas such as Mboro and Lompoul, which are adjacent areas in the same geographical zone as Diamniadio. Consequently, the rural areas in the whole coastal region are thus threatened in the long term (Eriksson & Juhl 2012; World Bank 2003). The probability of the risk of food insecurity is high in the extent to which the soils of the studied area present a high potential of fertility, with neutral soil acidity level (pH 7.0–8.0) and a favourable soil pH range for crop production, compared to relatively high soil acidity levels in the other eco-geographical zones in Senegal (Figure 4). For crop production, the lower the soil acidity, the better the soil conditions for higher crop production. (Diack et al. 2016; Lal 1989). Description of the soil profile (Table 3) presents shallow soil depths, indicating that groundwater in the Niayes region is near the soil surface anytime of the year, which is a favourable condition for cropping (Diatta 1996; Sène, Matty & Diatta 2014). Soil colour ranging from light reddish brown to dark greyish brown is an indicator of a good quality soil with hydromorphic features, a relatively high clay content and a stable polyhydric structure (Blavet, Mathe & Leprun 2000; Mauricio & Ildeu 2005; Koné 2007; Koné et al. 2009). The calcium content of the Niayes soils is also high, ranging up to 90 % (Figure 6). At the same time, salinity is detected only in trace in the Niayes area, while in the lower delta sodium levels can be up to 1.5 Cmol kg⁻¹ of soil. In comparison with soils from the Peanut Basin, soils from the Niayes area are higher in organic carbon and calcium contents for all samples. Greater organic matter content was noted in the Niayes region than in the lower delta of the Senegal River (Feller & Beare 1997). While the organic carbon contents of the soils in the Niayes region can be up to 20 g kg⁻¹, the variability of such soil properties of the Peanut Basin and the Lower Delta River was between 2 g kg⁻¹ and 10 g kg⁻¹. This was shown from 80% of the 12 soil samples analysed (Figure 6). When combined, soil pH, clay and fine silt, exchangeable cations and soil organic matter, these physical and chemical properties are considered as pivotal for the sustainability of agricultural production systems (Pieri 1989; Yemefack, Jetten & Rossiter 2006). Indeed, when comparing the soil from the Niayes area (reference area) with soils from the lower delta of the Senegal River (Diack & Loum 2014) and those from the Peanut Basin (Aguiar 2009), results show that soils from the Niayes region have much better soil quality than the others (Feller 1995; Yao-Kouamé et al. 2008). Higher soil quality, referred to as high soil fertility levels, would result in higher soil productivity (Sanchez 2002) which would ultimately enable an achievement of food security with crops commonly grown in these farmlands on the Niayes region. Therefore, a reduction of these farmlands for one reason or another would negatively impact food security achievement. In this regard, using these farmlands, not for crop production (an activity naturally recognised) but to develop urbanisation and to expand industrialisation, would definitely contribute to reducing food insecurity in Senegal, particularly and in the West African region in general.

Application of optical remote sensing through the processing of satellite images of medium and high spatial resolution (Landsat and SPOT) has made it possible to monitor the changes in agricultural land from 1999 to 2014. This quantification highlights the dynamics of the land use in the Niayes area illustrated by a reduction of the surface area, which constitutes a real risk of food insecurity at the national scale. Thus, the loss of agricultural lands of about 100 ha per year in our study area is likely to cause a diminution in the volume of production of fruits and vegetables. The availability of arable lands from the rest of the Niayes region is also under threat with population growth and the resulting urbanisation process. The risk of food disaster is a serious concern in that the Niayes lands have good soil quality for agricultural purpose compared to soil types from other eco-geographical zones of the country (Peanut Basin and Lower Delta of Senegal River). In addition to the quality of soils, water resources availability with a shallow water table militates in favour of maintaining the use of the Niayes area for agricultural purposes. Like the rainfall variability, which is a limiting factor in the performance of agricultural production systems in the Peanut Basin, market gardening and horticulture in the Niayes region becomes vulnerable because of the ongoing urbanisation and industrialisation process. Incomes usually generated from the agricultural activities tend to decrease the level of resilience for rural as well as for urban populations. Furthermore, in terms of sustainability, peri-urban agriculture can help mitigate global warming by increasing sequestration of carbon in the soil while industrialisation tends to favour emissions of greenhouse gases in the atmosphere. Therefore, the principles of sustainable development impose the application of public policies pertaining to the conservation of the agricultural land of the Niayes region.