Sustainability of Organic Fertilizers Use in Dryland Mediterranean Agriculture

Abstract

Organic fertilization is a key issue in European Union (EU) regulations, particularly in the context of promoting a circular nutrient economy, maintaining soil quality, and sequestering carbon to face climate change. In a rainfed system in Northeastern Spain, an experiment was set up (split-plot design). It included five pre-sowing N fertilization treatments: control, mineral, pig slurry, and composted sewage sludge (two rates). The average N rates were 0, 30, 141, 176, and 351 kg N ha−1, respectively. They were combined with mineral N topdressings (0, 50, and 100 kg N ha−1). Three crops were grown: barley (nine years), wheat (three years), and rapeseed (one year). In the driest years (c. 350 mm rainfall), the yields averaged 2.5, 2.0, and 1.9 Mg ha−1, respectively. The maximum yields were for barley (6.5 Mg ha−1) and wheat (5.5 Mg ha−1). The avoidance of a significant increase in soil residual NO3−-N, plus the control of soil build up of available P, micronutrients, and Cd, defines the fertilization strategies. (i) With a previous spring drought season, no fertilization is needed in the following year, if devoted to winter cereals. (ii) In rainier seasons, pig slurry or composted sewage sludge (lowest rate) applied at sowing is sufficient; however, 50 kg of mineral-N ha−1 at the topdressing can be applied. The study found that pig slurry favors K, Mg, Cu, and Zn availability, while composted sewage sludge enhances Fe availability. Although it is possible to reduce N inputs from organic fertilizers, organic C build-up will be constrained.