Quantifying the addition of nitrogen to agricultural land by groundwater via irrigation.

The concentration of nitrate in groundwater in Canterbury has significantly increased primarily due to land use intensification, resulting in reduced water quality. The region therefore is faced with the need to effectively manage land use to improve nutrient management. As the land is primarily used for agriculture, nutrient management is fairly complex. An increase in the use of fertiliser 20 to 30 years ago still affects water quality on the Canterbury Plains, due to the long lag times in the groundwater. Monitoring by Canterbury’s regional council has shown that these effects are now being detected in many groundwater aquifers. With new nutrient load limits currently being enforced across the Canterbury region, farmers are faced with a challenge to reduce nutrient losses from their land. However, this can be difficult to do while still maintaining economic productivity and therefore finding suitable management techniques is an urgent requirement for farmers. This research investigates whether nitrogen in irrigation groundwater can be used to partially replace nitrogen fertilisers, in the Selwyn and Ashburton Districts in Canterbury. Using the nitrogen in the groundwater, as an alternative to fertiliser, could also reduce the amount of nitrogen in groundwater, by recycling nitrate back onto agricultural land and turning an issue into a solution. Sixteen farms were selected to participate in this study and data was primarily collected through informal surveys with farmers. Monthly groundwater samples were collected from each farm’s groundwater well and analysed for nitrate-nitrogen concentrations. The nutrient modelling programme OVERSEER® was used to determine how nitrogen losses vary based on three different nitrogen application scenarios for selected farms. The contribution of groundwater nitrogen to agricultural land is heavily dependent on the nitrate-nitrogen concentration in groundwater, the amount of irrigation water applied and the size of irrigated land. These scenarios modelled through OVERSEER® showed that nitrogen loss estimates can be improved using the actual measured nitrate-nitrogen concentrations in groundwater rather than using OVERSEER® default values. Fertiliser application also adjusted nitrogen losses based on the contribution of nitrogen from irrigation water. By reusing the actual nitrogen data measured on farms in OVERSEER®, more accurate nitrogen losses can be calculated. This will become important when farmers need to comply with nutrient limits set as part of Environment Canterbury’s Land and Water Regional Plan. The amount of solid nitrogen fertiliser used on farm can also be decreased in some cases without compromising pasture growth and allowing for a reduction in fertiliser costs. Results showed that some farmers could reduce nitrogen fertiliser by 21 percent, depending on their nitrogen contribution. Farm Environmental Plans are now a requirement for farmers in the Canterbury region and they outline how environmental issues will be managed on the farm. Nitrogen contributions from groundwater may also be incorporated into the Farm Environmental Plan as a form of Good Management Practice, as nitrogen will be recycled through the farm system by irrigating the nitrogen lost back onto the land