Sustainable management of biosolid phosphorus: a field study

Stabilized wastewater biosolids are a renewable resource for improving soil and crop quality, but their regular application can rapidly elevate soil available phosphorus (P) (e.g. Olsen-P[OP]) leading to increased transfer of soluble reactive P (SRP) in land run-off causing eutrophication. To assess the eutrophication risk of biosolid P recycling, 143 arable fields with different biosolid P input histories, and representing different soil type and biosolid type combinations, were sampled and their P chemistry determined. Differences in soil total P between the surveyed fields were used as the best measure of net biosolid P inputs. Rates of soil OP increase (range 3-11% of soil total P, mean 7%) were lowest on a P-fixing soil and tended to increase more on sandy soils (6-9%) and on loamy soils receiving biosolids dosed with both iron (Fe) and lime (9-11%). Potential release of SRP to run-off was measured as water-extractable P (WEP) and varied widely (6-52% of soil OP). WEP concentrations were lowest on clayey soils, and where biosolids containing P-binding elements [calcium (Ca) or Fe, but not together] were applied. The relationship between OP and WEP was therefore highly dependent on both soil type and biosolid type. However, analysis of soil P sorption parameters indicated that eutrophication risk on loamy and clayey soils was significantly reduced when OP represented <20% of the soil P sorption capacity. Our results suggest that biosolids could be more sustainably managed by matching biosolid type to soil type and P fertility status.