Mitigation Options to Remove Non-Point Source Phosphorus From Runoff in New Zealand and Australia.

The loss of phosphorus (P) from land to water is detrimental to surface water quality in many parts of New Zealand and Australia. Farming, especially pasture-based dairying, can be a source of that P. Strategies to mitigate the P loss from farms vary in their effectiveness and costs due to differences in farm management systems, topography, stream density and climate. In the absence of significant financial assistance, a range of fully-costed strategies are required to ensure that the best mix of mitigation measures can be found for each farm. We present a summary of the efficacy and cost of a range of mitigation options available now, or under development, to lessen P loss from grazed grassland farms and catchments. Mitigations are classified as those that intervene in on-farm management (e.g., optimum soil test P, low solubility P fertilizer, effluent spreading), amendments such as alum, and tile drain filters, and edge of field (in-stream sorbents, buffer strips, constructed wetlands, and dams for water recycling). We considered components of cost-effectiveness such as material, labour and opportunity costs to assess the best options for a “typical” dairy farm in Southland, New Zealand. Although many P mitigation options are available, an analysis of strategies that target on-farm management options such as effluent and fertilizer management were the most cost-effective way of decreasing P losses (cost beneficial to 65 USD $/kg P conserved) although their overall impact is limited. Field amendments that may capture more P than preventing P loss by on-farm management were not as cost-effective (25-500 USD $/kg P conserved), but better than edge-of-field strategies, such as wetlands (250->500 USD $/kg P conserved). However, it is important to note that these latter measures often have other benefits, such as N removal and/or modification of peak flows.