Biosolids Impact Soil P Recovery, Fractionation, and Potential Environmental Risk.
Jim Ippolito, Kenneth Barbarick, and Kate Norvell. Colorado State Univ, Dept of Soil and Crop Sciences, Fort Collins, CO 80523-1170
Biosolids land application typically occurs at agronomic rates based on crop N requirements, but this practice can lead to excess P addition. The Littleton/Englewood (L/E), Colorado wastewater treatment facility has supported biosolids beneficial-use research on dryland wheat-fallow agroecosystem site since 1982. We found soil P increases as repeated biosolids application increased from 0, 6.7, 13, 27, and 40 Mg ha-1. The final study year was 2003, after which P accountability, fractionation, and potential environmental risk were assessed. Between 93 and 128% of the added biosolids-borne P was accounted for when considering conventional tillage practices soil displacement, grain/straw removal, and soil adsorption. The Fe-P fraction dominated all 0-20-cm soil depth biosolids-amended fractions, likely due to amorphous Fe-oxide addition since Fe2(SO4)3 was added at treatment facility inflow for H2S reduction. The Ca-P phase dominated subsoil fractions due to calcareous soil conditions. Of the two organic fractions studied, labile Po and biomass Po were dominant in the soil surface and subsurface, respectively. A combination of drought from 1999 to 2003, conventional tillage, repeated biosolids application, crop failure in 2000 and 2002, and soil sampling timing, may have forced soil surface microorganism dormancy, reduction, or mortality. Subsurface microbial biomass was greater than the surface, possibly due to protection against environmental and anthropogenic variables such as drought and conventional tillage practices, or was increased due to increased dissolved organic carbon inputs. Biosolids-application regulations based on the Colorado Phosphorus Index would not impede current practices at the research site. Proper monitoring, management, and other best management practices are needed for continued assurance that P movement off-site does not become a major issue.