Potential risk of nitrate pollution of the semi-arid zones: Case of the western Mid-Cheliff valley (northern Algeria)
Agriculture is the most important activity in the Western Mid-Cheliff valley (North Algeria). This activity consists of two major types: tree plantations and horticulture. Horticulture requires large amounts of mineral fertilizers, such as nitrogen. This study shows, firstly, the spatial variation of nitrate concentrations for the year 2004 in periods of high water using a map prepared by simple kriging. It is apparent from this map that the areas most affected are those for which the level of intensification of nitrogen fertilization (areas of horticulture) is the strongest. These areas have a more porous soil and are located on the edges of the valley. These same areas are known for intensive farming practices. In contrast, the pollution levels are lower in the fine-textured soil covering the central part of the valley. The lower pollution level is also due to low water infiltration, a feature of semi-arid areas. Secondly, this study was able to estimate, for the year 2004, the total amount of nitrogen present in soils in the valley: 86% of the nitrogen present is due to agricultural activities, most of which, 97%, is attributed to nitrogen fertilizer used extensively in horticulture, for potatoes in particular. Extrapolated to the total irrigated area, this contribution is estimated at 238 kg•ha‐1 for the year 2004. Pedo-climatic conditions seem to encourage the loss of nitrogen through denitrification and evaporation, together with a low rate of natural nitrogen mineralization. In contrast, the losses by leaching at depth appear very low.