Solubility and mobility of phosphorus recycled from dairyeffluents and pig manures in incubated soils with differentcharacteristics

International audiencePhosphorus (P) nutrition of plants in croplands is managed by fertilization. Commercial P fertilizers are manufactured from phosphate rocks, which are non-renewable and the only fossil resource of P. As an alternative, P in human and animal wastes can be recovered and concentrated in products that can be used as P fertilizers. Here, we studied four recycled P products derived from pig manures (hereafter referred as 'RPPM') and another one derived from dairy effluents ('RPDE'). The RPDE product is composed of Ca-P (partly as hydroxyapatite, HA), while RPPM products include recovered struvite (ST) and Ca-P in variable proportions. The objective was to assess the ability of RPPM and RPDE products to increase available P in a range of soils differing in their characteristics (seven soils used), and to compare these recycled P products with a standard fertilizer [commercial triple super phosphates (TSP)], reference HA and reference ST. To this end, products were mixed to the soils and the mixtures were incubated at 75 % water holding capacity and 28 C. After incubation, the amounts of phosphate ions (iP) in solution (QW) and isotopically exchangeable iP (E) in soils were quantified using an isotopic labeling (32P) and dilution procedure. In each soil, QW and E were significantly affected by treatments (control and P-treated soils) and increased due to the application of the different products. However, reference HA and RPDE products were generally less effective than TSP, reference ST and RPPM products. The soil response (variation in QW or E) in TSP treatment was compared to those in other treatments. It enabled the calculation of a relative effectiveness index. Relative effectiveness of HA and RPDE varied among soils (from 5 to 124 %) and increased with decreasing soil pH. Results however showed that the RPDE product tends to be more effective than reference HA, probably due to different degrees of crystallization of Ca–P. Relative effectiveness of RPPM products (80–116 %) was high in all soils and was similar to that of reference ST (90–104 %). To conclude, the present study suggests that RPDE products are effective only in acidic or slightly acidic soils. In contrast, P recycling from pig manures through chemical precipitation can provide effective P fertilizers, independently on soil conditions

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

International audienceDeposits 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