Plant-availability of phosphorus recycled from pig manures and dairy effluents as assessed by isotopic labeling techniques

[Departement_IRSTEA]Ecotechnologies [TR1_IRSTEA]TED [Axe_IRSTEA]TED-SAFIR <br/>[Departement_IRSTEA]Ecotechnologies [TR1_IRSTEA]TED [Axe_IRSTEA]TED-SAFIR

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anglais. Deposits of phosphate rocks are non-renewable and the only fossil resource for the production of phosphate fertilizers. The presence of phosphorus (P) in animal and domestic wastes provides an alternative opportunity to recycle P for use as P fertilizer. Hence, the objective of the present study was to assess the plant availability of recycled P products derived from dairy effluents (one product, hereafter named as "RPDE") and pig manures (four products, "RPPM"), through bio- and chemical precipitation processes, respectively. The RPDE product is composed of Ca–P (partly as hydroxyapatite, HA) and RPPM products contain recovered struvite (ST) and Ca–P. Plant-availability of recycled P was compared to that of commercial triple superphosphate (TSP), reference HA, and reference ST. To this end, pot and soil incubation experiments were used. A pot experiment with a mixture of ryegrass and fescue was carried out using a P-deficient and slightly acidic (pH = 6.49) soil, at 50 mg P kg-1 application rates of the different products. The 32P-labeling of soil P was used to determine the L-value (i.e. plant-available soil P) and to accurately quantify the P taken up by plants fromthe different P sources. Shoot and root biomass productions, plant P nutrition and L-value increased owing to application of P products. There were no significant differences between RPDE and RPPM products with regard to plant P nutrition. All recycled products were as effective as TSP and reference ST. By contrast, the P-equivalence of HA was only 22% of TSP in the slightly acidic soil. Thus, plant availability of P in RPDE product was higher than that of wellcrystallized HA (synthetic product). Product application to incubated soils induced an increase in the amounts of phosphate ions in soil solution and isotopically exchangeable P (E-values), which were overall correlated to the L-values and plant P uptake. Plant-availability of recycled P may thus be inferred appropriately from simple soil incubations. In conclusion, this study shows that it is possible to substitute commercial fertilizers (such as TSP) by P recycled from pig manures and dairy effluents.