Nutrient losses by surface run-off following the application of organic manures to arable land. 1. Nitrogen

Research was conducted on nitrogen (N) surface run-off losses following org anic manure applications to land, utilising a purpose-built facility on a s loping site in Herefordshire under arable tillage. Different rates and timi ng of cattle slurry, farmyard manure and inorganic N and phosphorus (P) fer tiliser were compared, over a 4-year period (1993-97). P losses from the sa me studies are reported in a separate paper. The application of cattle slur ries to the silty clay loam soil increased the loss of solids and NH4+-N in surface water flow compared to control plots receiving inorganic fertilize r only, or no treatment, but had little effect on NO3-N losses by this rout e. Results were consistent with other observations that rainfall events imm ediately after manure applications are particularly likely to be associated with nutrient run-off losses. Losses via subsurface flow (30 cm interflow) were consistently much lower than via surface water movement and were gene rally unaffected by treatment. Increasing slurry application rate and, in p articular, slurry solids loading, increased solids and NH4+-N losses via su rface run-off. The threshold, above which the risk of losses via surface ru n-off appeared to be greatly increased, was ca. 2.5-3.0 t/ha slurry solids, which approximates to the 50 m(3)/ha limit suggested for slurry within UK 'good agricultural practice'. Sealing of the soil surface by slurry solids appears to be a possible mechanism by which polluting surface run-off may o ccur following slurry application on susceptible soils. Total losses of NH4 +-N and NO3--N during the 4-year monitoring period were insignificant in ag ronomic terms, but average soluble N con centrations (NH4+-N+NO3--N) in run -off, ranging from ca. 2.0 mg/l, up to 14.0 mg/l for the higher rate slurry treatments. Peak concentrations of NH4+-N > 30 mg/l, are such as to be of concern in sensitive catchments, in terms of the potential for contribution to accelerated eutrophication and adverse effects on freshwater biota. (C) 2001 Elsevier Science Ltd. All rights reserved.