Zinc Availability Following Slurry Applications in Highly Calcareous Soils: Implications for Sustainable Management

A 13-year field experiment was conducted on a highly calcareous soil (>0.3 kg kg–1 of equivalent CaCO3) to evaluate the sustainability of pig slurry (PS) application based on N criteria, with a particular focus on Zn concentrations in barley. Two PS N-rates before sowing, combined with nine topdressing N rates (from 0 to 381 kg N ha–1, using PS and/or mineral fertilizers), were assessed for their effects on Zn availability, barley Zn content, soil chemical properties and barley composition. Increasing PS rates led to higher total Zn concentrations in the topsoil and greater availability of Zn and other nutrients (P, Cu, Fe, Mn and Ni). As a result, Zn and P concentrations in barley tissues (grain and straw) increased. In grain, this represents a maximum average increase in the concentration of Zn and P of 140% and 68%, respectively. Soil diethylenetriaminepentaacetic acid-extractable Zn and Mn, along with available P (Olsen method), were the best predictors of Zn accumulation in both grain and straw (R2 = 0.64–0.65). In the short term, PS applications within the current legal N limits (170–190 kg N ha–1) enhanced the Zn nutritional value of barley (grain Zn-biofortification). In the long term, regular monitoring of Zn and P levels in calcareous soils is recommended. Overall, sustainable PS management within a circular nutrient economy framework cannot be based exclusively on N.