The nitrogen value of anaerobically digested sewage sludge applied to grassland during winter

The manurial value of liquid anaerobically digested sewage sludge (LAS), surface applied once at four different times during winter to contrasting swards, was assessed over each of three growing seasons by comparison with fertilizer N. Four nominal rates of N (0, 18, 36 and 54 kg ha⁻¹) were used based on the inorganic N content of LAS. Herbage dry matter (DM) and N uptake responses were measured by cutting three times per year. Application of LAS in late winter compared with early winter led generally to greater N uptake and DM production. Herbage production response from LAS ranged from 5·4 to 30·1 kg DM kg⁻¹ N; within this range, larger responses were obtained from a sown sward than from permanent pasture. The apparent efficiency of LAS compared with fertilizer N ranged from 0·41 to 0·88 (permanent pasture) and 0·20 to 0·77 (sown sward) but occasionally mean apparent efficiencies of over 1·0 occurred because fertilizer N was poorly recovered. There was no residual effect on annual N uptake from applying sludge. It is concluded LAS is a useful nitrogenous manure when applied to grassland in late winter/early spring but more research is needed to elucidate the importance of pathways of N losses following the surface application of LAS, and to quantify these losses.