Tephra-mulched soils irrigated with reclaimed urban wastewater in former dry-farming systems of Lanzarote (Spain)

On the island of Lanzarote, under extreme conditions of aridity and water scarcity, a certain amount of rainfed agriculture has been possible due to the use of tephra mulching, a system known locally as arenados, which is a very effective water and soil conservation system. The natural arenados are associated to already present tephra from recent volcanic eruptions. In artificial arenados a 5 to 20 cm layer of pyroclasts is placed over the soil. Over the last decade, the availability of urban reclaimed water (RW) has allowed irrigation networks to be established in artificial arenados with significant increases in crop yields. In this work, a preliminary evaluation of the sustainability of this new form of management was performed. Field plots irrigated with tertiary effluent for almost a decade were studied and compared with adjacent dry-farming fields, both cropped with sweet potatoes. In both cases, the soils had a covering of basaltic tephra mulch. The irrigation lines are placed on top of the mulch. Although this was not a controlled field experiment, since water and soil management was not exactly the same in the studied plots, some significant effects of RW irrigation on soil chemical properties were identified. Crop yields in the irrigated fields can be up to three times the yields obtained under rainfed conditions. The RW quality posed some restrictions for irrigation, particularly as regards salinity, sodicity and boron, although their buildup in soils appears to have been reduced by the mulch effect. In some field plots, crop critical threshold values of soil salinity and boron were reached with no harmful consequences. This developing soil degradation suggests that the sustainability of arenados systems under RW irrigation would appear to be under threat in the mid- to long-term based on current management practices. Further research is required on the salt and boron dynamics in this system, which is an example of conservation agriculture that also helps combat desertification.