Stream water quality in intensive cereal cropping catchments with regulated nutrient management

Regulating farm nutrient management is a European Union policy approach to meet water quality targets. In two intensively cropped catchments (9 km2 and 11 km2) in Ireland, high resolution monitoring was undertaken over two years to investigate interactions between the regulations and nutrients in streams.In a catchment with predominantly well-drained soils (Arable A) farmed for spring barley, high phosphorus (P) concentrations and turbidity during peak river flows indicated high connectivity of nutrient sources with streams. High baseflow volumes, however, maintained mean stream P concentrations below the environmental quality standard (EQS). Total oxidised nitrogen (TON) concentrations decreased during storms and baseflow concentrations remained relatively stable and below drinking water standards.In a contrasting catchment on moderate to poorly drained soils (Arable B) farmed for winter wheat, dairy and beef, the EQS for P was never met. High baseflow P concentrations during summer were attributed largely to loss of dilution in low discharge rates. Spatial surveys of water quality indicated baseflow P concentrations were influenced by persistent point sources. Baseflow TON concentrations were lower during summer than winter and this was attributed to seasonal disconnection and attenuation of N sources.Average annual streamflow total P loads were higher from Arable B (0.72 kg ha−1) than from Arable A (0.27 kg ha−1) whereas TON loads were higher from Arable A (28 kg ha−1) than from Arable B (17 kg ha−1). Suspended sediment losses, after a first approximation, were less than 0.18 t ha−1 yr−1. The upper quartile of stream discharge dominated annual loads of P, TON and suspended sediments. Of most concern were stream P concentrations in Arable B and potentially P and N loads from both catchments given both catchments contribute to eutrophic coastal waters. Compliance with farm nutrient management regulations should mitigate persistent agricultural point sources but will not affect non-agricultural point source influences on stream P concentrations. Under current regulations, decreased N surplus at source and subsequent depletion of nitrate from vadose and groundwater stores may decrease TON loads over time.

► We present the first analysis of catchment-scale water quality responses to intensive cereal cropping in Ireland. ► We evaluated the efficacy of farm nutrient regulations using high resolution data in hydrologically contrasting catchments. ► Stream nutrient concentrations met standards except for phosphorus in a catchment with low baseflow and high peak flow. ► Nutrient and suspended sediment loads in streamflow were comparatively low. ► Critical source areas and times for hydrological connectivity of nitrogen sources were important.