Runoff Phosphorus Losses from Surface‐Applied Biosolids

Runoff losses of dissolved and particulate phosphorus (P) may occur when rainfall interacts with manures and biosolids spread on the soil surface. This study compared P levels in runoff losses from soils amended with several P sources, including 10 different biosolids and dairy manure (untreated and treated with Fe or Al salts). Simulated rainfall (71 mm h −1 ) was applied until 30 min of runoff was collected from soil boxes (100 × 20 × 5 cm) to which the P sources were surfaced applied. Materials were applied to achieve a common plant available nitrogen (PAN) rate of 134 kg PAN ha −1 , resulting in total P loading rates from 122 (dairy manure) to 555 (Syracuse N‐Viro biosolids) kg P ha −1 Two biosolids produced via biological phosphorus removal (BPR) wastewater treatment resulted in the highest total dissolved phosphorus (13–21.5 mg TDP L −1 ) and total phosphorus (18–27.5 mg TP L −1 ) concentrations in runoff, followed by untreated dairy manure that had statistically ( p = 0.05) higher TDP (8.5 mg L −1 ) and TP (10.9 mg L −1 ) than seven of the eight other biosolids. The TDP and TP in runoff from six biosolids did not differ significantly from unamended control (0.03 mg TDP L −1 ; 0.95 mg TP L −1 ). Highest runoff TDP was associated with P sources low in Al and Fe. Amending dairy manure with Al and Fe salts at 1:1 metal‐to‐P molar ratio reduced runoff TP to control levels. Runoff TDP and TP were not positively correlated to TP application rate unless modified by a weighting factor reflecting the relative solubility of the P source. This suggests site assessment indices should account for the differential solubility of the applied P source to accurately predict the risk of P loss from the wide variety of biosolids materials routinely land applied.