Effects of storage by freezing on concentrations of molybdate reactive phosphorus in poultry litter extracts

In the laboratory, excessive variability in molybdate reactive phosphorus (MRP) concentrations in water extracts of poultry litter that appeared to be associated with storage by freezing was frequently observed. In one experiment, repeated twice, the effects of a factorial combination of storage temperatures [freezing (-16 to -15 degrees C) or room temperature (21-24 degrees C)] was studied for 60-62 h with poultry litter type (three broiler and one pullet) on MRP concentrations in water extracts. An additional experiment was conducted to compare the effects on phosphorus (P) concentration of a 1 mg P L(-1) standard of spiking with 1 mg L(-1) of Fe(+3) or Al(+3) and storage under room temperature or freezing for two and five days. No statistical differences were observed in MRP levels of extracts kept at room temperature measured immediately after the extraction, after 62 h or eight days. Freezing, however, consistently decreased MRP concentrations measured immediately after thawing compared to measurements in extracts kept at room temperature. Reductions ranged from less than 1% to 46%, depending on poultry litter composition, especially total iron (Fe) and aluminum (Al) concentrations. Freezing effects were reversible and after five days MRP concentrations were comparable to those of extracts kept always at room temperature. Spiking with Al(+3) did not affect P solubility but spiking with Fe(+3) and freezing for five days reduced the MRP concentrations in the P standard when measured immediately or two days after thawing. The results from the P, Fe(+3), and Al(+3) experiment plus the observation of precipitates containing P and Fe in some of the poultry litter extracts suggested complexing between these elements as a partial explanation of these responses. Organic compounds and Al(+3) might also be involved. The solubilization of the floccules upon thawing might be retarded by changes induced by freezing. In consequence, freezing of water extracts of poultry litter can bring about significant, albeit transient, decreases in soluble orthophosphate concentrations.