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

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 50mgPkg−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 from the 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 well-crystallized 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.