Inhibition of phosphorus sorption on calcite by dairy manure-sourced DOC

•Dairy manure added to calcareous soils releases dissolved organic carbon (DOC) and phosphate to the soil solution.•Dairy-manure sourced DOC greatly reduces P sorption.•High DOC levels promote formation of more soluble Ca-Mg-P minerals.•Mg substituted amorphous hydroxyapatite minerals occur on calcite minerals exposed to dairy manure extract.

In confined animal feeding operations, such as dairies, manure is amended to soils at high rates leading to increases in P and organic matter in the soils. Phosphorus reacts with soil-Ca to form Ca-P minerals, which controls P availability for leaching and transport through the watershed. In this research, the effects of manure sourced dissolved organic matter (DOM) on P sorption on calcite were measured at different reaction times and concentrations. Reactions were monitored in 1% and 10% manure-to-water extract solutions spiked with P. When manure-DOM was present, a significant reduction in P sorption occurred (2–90% absolute decrease) compared to samples without manure-DOM. The greatest decrease occurred in the samples reacted in the 10% manure solution. XANES spectroscopic analysis showed that at 1% manure solution, a Ca-P phase similar to hydroxyapatite formed. In the calcite samples reacted in the 10% manure solution, K-edge XANES spectroscopy revealed that P occurred as a Ca-Mg-P phase instead of the less soluble hydroxyapatite-like phase. Results from this study suggest that in manure-amended calcareous soils, increased DOM from manure will decrease P sorption capacity and increase the overall P concentration in solution, which will increase the mobility of P and subsequently pose greater risks for impairment of surface water quality.

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