Solubility and mobility of phosphorus recycled from dairy effluents and pig manures in incubated soils with different characteristics

Phosphorus (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 A degrees C. After incubation, the amounts of phosphate ions (iP) in solution (Q(W)) and isotopically exchangeable iP (E) in soils were quantified using an isotopic labeling (P-32) and dilution procedure. In each soil, Q(W) 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 Q(W) 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.