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Phosphorus losses from sewage sludge disposed on a field: evidence from storm event simulations

Sewage sludge is used as fertilizer, owing to its nutrient contents. But little is known about the consequences of sludge disposal on erosion intensity and P losses from soil to runoff. This paper reports on a study of three simulated storm events (40 mm h-1 during 30 min), performed on a field supplied with liquid sludge (23 g L-1 of dry solid). The sludge phosphorus content was 26.5 g kg-1 of dry solid. The runoff and erosion intensity as well as the P losses were monitored during the experiment. The distribution of phosphorus on the soil surface, was investigated after the last simulation. Sludge disposal reduced the total amount of runoff water collected during the experiment from 290 m3 ha−1 to 134 m33 ha−1. It also decreased the quantity of sediment exported from 1200 kg ha−1 to 500 kg ha−1 and the total phosphorus loss from 2.7 kg ha-1 P to 1.4 kg ha−1 P. But, it increased the average concentration of dissolved phosphorus in the overland flow from 0.15 mg L−1 P to 0.57 mg L−1 P. The distribution pattern of P and organic matter, on the soil surface, after the simulations, showed a heterogeneity induced by the runoff processes. This effect is increased by sludge disposal.