Benefits and limitations of white clover N₂ fixation as the main N input into mixed cropping rotations and dairy pastures: A case study of the recent agricultural intensification in the Canterbury Plains, New Zealand

On a ‘traditional’ mixed cropping farm in Canterbury, NZ, annual cash crops are grown for 2–5 years followed by sheep-grazed perennial ryegrass/white clover pastures for 2–5 years. Generally, soil nitrogen (N) fertility can be maintained with this system via white clover N2 fixation, N fertiliser is not used and nitrate (NO3-) leaching from the system complies with environmental standards. From 1990–2010, the trend for mixed cropping farms in Canterbury was to either prolong the number of years under arable crops or change to dairy systems. In both cases, large inputs of N fertiliser were used to increase and maintain productivity, and dairy systems were irrigated. Increased N and water inputs resulted in increased yield. For example, for wheat, average yield was 5.1 t ha-1 from 1990–1992 with little use of N fertiliser but 8.0 t ha-1 from 2007–2009 with added N while dairy pastures commonly produced around 15 t dry matter ha-1 annum-1. However, increased use of N fertiliser and irrigation has resulted in increased NO3- leaching from soils and this has raised concerns about water availability and degradation of lowland streams and the aquifers which supply this water. Land use controls are being implemented over the area where NO3- could enter the aquifers. It is concluded that use of white clover N2 fixation as the N input into mixed cropping rotations and dairy pastures is likely to give yields 60–70% of that achievable with use of inorganic N fertiliser. Nitrate leaching from these systems is likely to comply with environmental standards. However, unless mitigation strategies are put in place, N losses from dairy systems are likely to increase substantially with increased applied N above that required to give pasture production achievable with white clover N2 fixation as the N input into the system