Phosphorus solubility in biosolids-amended farm soils in the Mid-Atlantic region of the USA

There have been increased environmental concerns about agricultural P manag ement, including land application of biosolids (municipal sewage sludge). T his study investigated the influence of current N-based land application pr actices for biosolids on soil P. We collected soil samples from 11 biosolid s 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]) x 100), Mehlich-1 P; iron-oxide strip ex tractable phosphorus (FeO-P); and water-soluble phosphorus (WSP). In many c ases Mehlich-1 P was excessive (>50 mg kg(-1)), but this occurred in both a pplication sites and setback areas, Biosolids application sites had signifi cantly greater P-ox in the topsoils (0 to 20 cm) when averaged for all site s, with means of 589 and 296 mg kg-' in the application sites and setback a reas, respectively. However, biosolids applications also increased [Fe-ox Al-ox], which meant that the DPS was not always increased, There was a tre nd 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 ac cording 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 increas es in soil [Fe-ox + Al-ox].