Nitrogen losses in runoff waters from a loamy soil treated with sewage sludge

Sludge from the same waste-water treatment plant, processed in different ways, was applied superficially to plots of a soil developed on marls (Udic calciustept) at doses equivalent to 10 t ha−1 of dry matter. The concentration of mineral nitrogen (ammonium, nitrate) in runoff waters was measured to assess the effects of composting and thermally drying of sludge on the pollution of surface waters. Significant differences of NH4-N and NO3-N concentration in both runoff waters and soil only appeared during the first five runoff events after sludge application. Thereafter, runoff volume decreased remarkably, possibly due to vegetation growth. The treatment with composted sludge contributes mainly to NO3-N runoff, whereas the NH4-N was predominant in runoff from plots of thermally-dried sludge. The treatment with fresh sludge provided NH4-N and NO3-N to surface runoff in similar amounts. The electrical conductivity of the runoff was not greatly affected by the surface application of the sewage sludges.