Long-term effects of pig slurry on barley yield and N use efficiency under semiarid Mediterranean conditions

Intensive pig farming is a relevant economic activity in Mediterranean areas, which generates large amounts of pig slurry (PS) as a by-product. Pig slurry represents a valuable resource for low-fertility Mediterranean soils, capable of supplying organic matter and plant nutrients, particularly N. The cumulative and residual effects of PS on winter barley (Hordeum vulgare L.) yield and N use efficiency were investigated in a seven-year field experiment under semiarid Mediterranean conditions. Treatments included five rates of PS ranging from 30 to 150m3ha−1 applied either every year or only once at the beginning of the experiment, an annual mineral N fertilization (traditional in the study area), and a control with no fertilization. Plant-available N content in soils after harvest increased with the annual application of PS with respect to the mineral fertilization and the control treatment, especially in dry years. Yearly variations in water availability also shaped the response of barley to PS fertilization. In general, the annual application of PS affected positively grain yield and biomass production. Residual effects were evident for the second and third barley crop. Cumulative N uptake by the barley crop increased with increasing the rate of PS (up to 120tha−1), while N use efficiency decreased, which suggested an increasing risk of leaching and contamination of ground water. As a whole, our data indicate that the annual application of 30m3ha−1 of PS (equivalent to an average of 67kgNha−1) may result in barley yields similar to those obtained with the traditional mineral fertilization, while safely adhering to European regulations for the use of N in agriculture. The application of PS at rates higher than 60m3ha−1 may result in better yields, but also in significantly lower N use efficiency and a higher potential environmental impact.