Manure Phosphorus Extractability as Affected by Aluminum‐ and Iron By‐Products and Aerobic Composting

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Abstract
Shifts in manure phosphorus (P) chemical forms and pool sizes induced by water treatment residuals and industrial mineral by‐products are largely undefined. We conducted a manure P fractionation study to determine mechanisms of reduction of dissolved reactive phosphorus (DRP) in poultry manure upon mineral by‐product additions. The effects of composting on the P immobilization efficacy of the by‐products were determined using laboratory self‐heating composting simulators. The mineral by‐products included an aluminum‐water treatment residual (Al‐WTR) and an iron‐rich titanium‐processing by‐product. The noncomposted manure averaged 0.11 g g−1 of total P as DRP forms. The by‐products significantly reduced manure DRP, by an average of 39 and 48% in the Al‐ and the Fe‐treated manure, respectively. The by‐products also reduced the 0.5 M NH4F–extractable phosphorus (FEP) fraction. Shifts in P forms between FEP and 0.1 M NaOH–extractable phosphorus (SHEP) depended upon the Al and Fe contents of the by‐products while the combined FEP + SHEP pool remained constant. Phosphate sorption measurements supported the observations that the Fe‐rich by‐product was more effective at reducing manure DRP and enhancing the formation of SHEP forms at the expense of FEP than the Al‐WTR. Composting had no effect on the efficacy of either by‐product to reduce DRP. Potential mechanisms of enhanced P stabilization in treated manure upon composting included chemical shifts from the DRP and FEP fractions to the citrate–bicarbonate–dithionite extractable P fraction. Thus, the choice of P immobilization agents affected the stability of immobilized P forms and should be taken into consideration in developing manure processing and nutrient stabilization methods.

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