An integrated assessment of nitrogen source, transformation and fate within an intensive dairy system to inform management change

From an environmental perspective optimised dairy systems, which follow current regulations, still have low nitrogen (N) use efficiency, high N surplus (kg N ha^-1) and enable ad-hoc delivery of direct and indirect reactive N losses to water and the atmosphere. The objective of the present study was to divide an intensive dairy farm into N attenuation capacity areas based on this ad-hoc delivery. Historical and current spatial and temporal multi-level data-sets (stable isotope and dissolved gas) were combined and interpreted. Results showed that the farm had four distinct attenuation areas: high N attenuation: characterised by ammonium-N (NH₄ ⁺-N) below 0.23 mg NH₄ ⁺-N l^-1 and nitrate (NO₃ ^--N) below 5.65 mg NO₃ ^-N l^-1 in surface, drainage and groundwater, located on imperfectly to moderately-well drained soils with high denitrification potential and low nitrous oxide (N₂O) emissions (av. 0.0032 mg N₂O-N l^-1); moderate N attenuation: characterised by low NO₃ ^--N concentration in drainage water but high N₂O production (0.0317 mg N₂O-N l^-1) and denitrification potential lower than group 1 (av. δ¹⁵N-NO₃ ^-: 16.4, av. δ¹⁸O-NO₃ ^-: 9.2), on well to moderately drained soils; low N attenuation—area 1: characterised by high NO₃ ^--N (av. 6.90 mg NO₃ ^--N l^-1) in drainage water from well to moderately-well drained soils, with low denitrification potential (av. δ¹⁵N-NO₃ ^-: 9.5, av. δ¹⁸O-NO₃ ^-: 5.9) and high N₂O emissions (0.0319 mg N₂O l^-1); and low N attenuation—area 2: characterised by high NH₄ ⁺-N (av. 3.93 mg NH₄ ⁺-N l^-1 and high N₂O emissions (av. 0.0521 mg N₂O l^-1) from well to imperfectly drained soil. N loads on site should be moved away from low attenuation areas and emissions to air and water should be assessed.