Loss of soil fertility estimated from sediment trapped in check dams. South-eastern Spain

This study analyses the loss of fertility in eroded soils, whose nutrients are retained behind check dams. The study was carried out in the basin of the River Quipar, which flows into the River Segura (SE Spain). In two projects carried out in 1962 and 1996, 425 check dams were constructed in an attempt to reduce sedimentation. Of these check dams, 18 were chosen as representative of the different basins and soils sources. A variety of physico-chemical determinations were made to study the characteristics of the soils and nutrients contained in the sediments. High fertility losses were observed in all soil types, especially calcaric Regosols, in which the carbon and nitrogen contents were significantly reduced, and all macro and micronutrients were diminished. Since the source soils contained low levels of organic matter, the sediments showed low rates of enrichment. In general, the soils poorest in organic matter showed the greatest losses of the same, resulting in severe degradation and favouring erosion. Unlike observations made in other studies, the organic matter enrichment rate did not increase with the quantity of sediment. A negative correlation was found between the surface area of the basins and soil loss. Similarly, the soils with a lower nutrient content suffered a greater degree of soil loss. Among the findings of the study is that the following factors need to be taken into consideration when estimating the loss of fertility of sediments retained by check dams: the efficiency of the traps for retaining sediments, the type of trap, the characteristics of the basins and the erosion processes that have taken place. ► Runoff leads the loss of organic matter and chemical nutrients. ► The loss of fertility was substantial in the calcaric Regosols. ► In general, the soils with the lowest OM content lose the most. ► A general correlation was observed between the erosion and nutrient loss rates. ► Check dams are a useful tool for analysing the loss of fertility.