Evaluating Phosphorus Release from Biosolids and Manure‐Amended Soils under Anoxic Conditions

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Abstract
The solubility of P in biosolids and manures has been shown to influence the potential for dissolved P losses in runoff and leachate when these materials are land applied. As a result, some Mid‐Atlantic US states have developed P source coefficients (PSCs) to account for differences in P solubility between fertilizers, manures, and biosolids in P risk assessment tools. The reliability of these PSCs has not been evaluated under anoxic conditions, where environmental changes may affect the P solubility of biosolids or manures. The objective of this study was to assess the effects of anoxic conditions on the release of P from a range of Mid‐Atlantic soils amended with manures and biosolids. The concentration of dissolved P released into solution (0.01 mol L−1NaCl) from the Pamunkey, Berks, and Manor soils was significantly lower under reducing conditions than under oxidized conditions (median ΔP= −0.70, −0.49, and −0.07 mg L−1, respectively; all significant at the 0.001 probability level). There was no significant P source effect on dissolved P released into solution after anoxic incubation of soils. Calculated solubility diagrams and increases in oxalate‐extractable Fe and P sorption index under reducing conditions for all soils suggest the precipitation of (i) an Fe(II)‐oxide that increased the P sorption capacity of the soils or (ii) an Fe(II)‐phosphate that decreased the solubility of P. We propose that current PSCs do not need alteration to account for differences in P solubility of organic sources under reducing conditions under relatively static conditions (e.g., seasonable high water table, periodically submerged soils, stagnant drainage ditches).
Funding Information
  • Municipal Washington Council of Governments
  • Institute of Soil and Environmental Quality at the University of Delaware