Phosphorus Solubility in Biosolids‐Amended Farm Soils in the Mid‐Atlantic Region of the USA

Abstract

Abstract There have been increased environmental concerns about agricultural P management, including land application of biosolids (municipal sewage sludge). This study investigated the influence of current N‐based land application practices for biosolids on soil P. We collected soil samples from 11 biosolids application sites and from adjacent setback areas that had never received biosolids. These samples were analyzed for oxalate‐extractable phosphorus (P ox ), aluminum (Al ox , and iron (Fe ox ); degree of phosphorus saturation (DPS = (P ox /0.61[Al ox + Fe ox ]) × 100), Mehlich‐1 P; iron‐oxide strip extractable phosphorus (FeO‐P); and water‐soluble phosphorus (WSP). In many cases Mehlich‐1 P was excessive (>50 mg kg −1 ), but this occurred in both application sites and setback areas. Biosolids application sites had significantly greater P ox in the topsoils (0 to 20 cm) when averaged for all sites, with means of 589 and 296 mg kg −1 in the application sites and setback areas, respectively. However, biosolids applications also increased [Fe ox + Al ox ], which meant that the DPS was not always increased. There was a trend for higher P ox and FeO‐P in the biosolids‐amended soil profiles (60‐cm depth) and highest P concentrations were consistently found at the 0‐ to 5‐cm depth. The net effect of biosolids applications on P ox , relative to [Fe ox + Al ox ], was related to FeO‐P. Our data suggest that adding biosolids according to current N‐based guidelines will lead to an accumulation of P in soils, but the release of this P may be mitigated by the associated increases in soil [Fe ox + Al ox ].