Recycled organic materials as fertilisers to meet agriculture demand on phosphorus in Mediterranean soils

Phosphorus (P) is essential for plant development and to feed the world population. Most of the P that is used as a fertiliser in agriculture has a mineral origin, whose reserves are scarce as it is a non-renewable resource. Also, international conflicts add pressure to agriculture due to several reasons, for example, increasing the price of the fertilisers (including P fertilisers). At least since 2015 the European Union promotes strategies based on circular economy, and, more recently with the Mission ‘A Soil deal for Europe’, pursues to enhance soil health as this is the base of our food systems, habitats, economy, and prosperity. In Mediterranean areas of Europe, calcareous soils with a low content in organic matter and limitations in the phytoavailability of nutrients such as P and micronutrients (Fe and Zn), are predominant. For these reasons, biobased P fertilisers from wastes/residues could be an alternative to mineral P fertilisers in these soils at the same time as they could enhance soil health as they are rich in organic matter and nutrients. The objective of this study was to assess the potential of different residues (digestate from a biogas plant, compost of olive mill pomace, compost of solid urban waste, vermicompost and vegetal residues) to act as P fertilisers in calcareous soils. Diammonium phosphate (DAP) and the non-application of P were also included in the study to compare with biobased fertilziers. A field experiment was developed (randomized block-4 design) in a calcareous Vertisol in the South of Spain (Córdoba) for two years (in the traditional wheat and sunflower rotation). The different biobased P fertilisers and DAP were applied at the same rate (30 kg P ha^-1) and incorporated into the arable layer of the soil (20 cm depth) before sowing durum wheat. Different analyses were done to evaluate the immediate (durum wheat) and residual (sunflower) effects of the different fertilisers: soil P and micronutrients’ availability, biomass and yield, plant nutrient uptake, soil enzyme activities, and bacterial and fungal composition in soil. The obtained results showed a good performance of the biobased fertilizers in comparison with the mineral fertiliser (DAP), i.e., wheat biomass was not negatively affected due to the application of the biobased fertilisers. In addition, the organic fertilizers had an obvious effect on the activity of soil enzymes, especially in acid phosphatase, finding the highest values in the non-P fertilised soils. The biobased P fertilizers evaluated in this study can be an efficient alternative to mineral P fertilizers in Mediterranean areas to maintain soil P availability, enhance soil functionality, provide organic matter, P and other nutrients to the plant.