Phosphorus leaching from liquid digested sewage sludge applied to sandy soils

Abstract

The aim of the experiment was to determine the importance of liquid digested sewage sludge (‘sewage sludge’) as a source of phosphorus leaching from sandy soils in the UK. The sewage sludge was applied to uncropped loamy sand and sandy loam monolith lysimeters (1·2 m deep) annually for 3 years. The application rate was sufficient to supply approximately 60 kg P/ha annually. An application equivalent to four times this amount was also applied to two other loamy sand lysimeters to test the effects of excessive amounts. Total P (TP), total dissolved P (TDP) and molybdate-reactive P (MRP) concentrations were measured in the drainage water and were compared with leaching from untreated control soils.

After two sewage sludge applications, Olsen's extractable P content of the topsoil had increased: a P surplus of c. 100 kg/ha was required to raise the topsoil concentration by 5 mg/kg. Sewage sludge applications at the lower rate did not increase P concentrations in the drainage. Phosphorus concentrations were generally small and were, as an average of both soil types and nil and lower rate sludge applications, 12, 20 and 34 μg/l for MRP, TDP and TP respectively. Phosphorus leaching losses from the higher rate (excessively large sewage sludge applications) were more variable but not significantly (P > 0·05) different from the other treatments. Average concentrations were 9, 23 and 50 μg/l for MRP, TDP and TP respectively. Leaching from sludge biosolid applications at operational rates to sandy soils moderately well supplied with P is not a major P loss pathway.