Soil quality under food-processing wastewater irrigation in semi-arid land, northern Spain: Aggregation and organic matter fractions

The use of cannery wastewater for irrigation represents a prime solution in the optimization of water resources in semi-arid areas. However, the environmental risks associated to this practice must be evaluated in detail. Soil quality could be affected as a result of the addition of organic residues and some salts and electrolytes to the soil. We evaluated soil aggregation status and organic matter dynamics in three semi-arid agricultural soils in northeast Spain before and after four years of irrigation trials in order to asses the effect of wastewater irrigation on soil physical status. Aggregation indexes (mean weight diameter, dispersible clay), particulate organic matter abundance, fine sand-size organic matter and carbon (C) linked to the mineral fraction were used as soil quality indicators and quantified before and after four years of trials. Four irrigation treatments ('clean' control, two blends of waste and wastewater, and waste water) were applied to three different sites to supply crop water needs. The water application methods, crops and land management varied between sites. A permanent alfalfa crop was planted under sprinkler irrigation on a Xeric Petrocalcid and a Xeric Haplogypsid. A crop rotation (corn, wheat, barley, sunflower) was implemented under flooding irrigation on a Typic Xerofluvent. We speculated that aggregation could be improved as a consequence of the addition of fresh organic residues. However, the simultaneous addition of dissolved salts could counteract for this effect and in fact lead to a destabilization of soil structure. Results differed for different soil characteristics (particle-size distribution, gypsum content) and management regimes. Changes in aggregation and/or organic matter fractions were observed in the non-gypsic soils. These changes were due to soil management and properties and were not affected by the application of wastewater. The addition of organic solids in suspension led to a proportionate increase in labile organic matter in the gypsic soil. This had no effect on soil aggregate stability, which was mostly dominated by soluble calcium. Our study illustrates the absence of negative effects of irrigation with cannery wastewater on soil physical status and organic fractions in any of the three studied semi-arid Mediterranean soils.