Re-aligning stream rehabilitation theory and practice to attenuate edge-of-field and in-stream nitrate export in agricultural waterways in Canterbury, New Zealand
In lowland Canterbury, New Zealand, waterways are agricultural drains <2 m wide that have been subjected to decades of intensive farming practices and are currently impacted by aquatic weeds, deposited fine sediments, nutrient levels well above acceptable human health limits, and depauperate freshwater communities. We attempt to overcome the common hurdles in stream rehabilitation practice by incorporating societal, ecosystem service and ecological scales into a single regional-scale experiment called the Canterbury Waterway Rehabilitation Experiment (CAREX). Using a network of nine one-kilometer long waterways across 14 farms, we articulated restoration goals based on the desired freshwater services, including reducing downstream nitrate-N export. Several restoration tools are available to remove N at multiple spatial scales, including managed aquifer recharge at the catchment scale and stock exclusion and riparian plantings at the farm scale. However, evidence of their effectiveness and best practices associated with their use are lacking, particularly in combination with one another.
We characterized ‘hotspots’ and ‘hot moments’ of nitrate-N export using continuous measurements and grab sampling seasonally or more to identify the most appropriate locations N removal tools at the farm scale. To complement existing good management practices, edge-of-field denitrifying bioreactors and in-stream additions of labile carbon were trialled to reduce downstream export of nitrate-N. Bioreactors reduced edge-of-field nitrate-N loss by <10 to >70%, with greater N removal associated with longer water residence time; however, in-stream solutions are needed to address the large downstream export of nitrate-N that is not removed by edge-of-field and riparian nutrient tools. Combining edge-of-field and in-stream tools in a synergistic fashion highlights the suitability a toolbox-based stream rehabilitation approach which addresses the scale and source of nutrient problems in agricultural waterways. Practical, cost-effective stream rehabilitation tools are recommended to address downstream nutrient export while catchment-scale nutrient plans are developed to address losses from land.