Sensitive Catchments – Managing Nutrient Pathways and their Attenuation in NZ Agricultural Catchments
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Productive farms and their associated processing industries make a significant contribution to New Zealand’s economic and social welfare. However, grazed pastoral systems and other intensive landuses are inherently leaky with respect to nitrogen (N), the key nutrient implicated in the deterioration of surface and ground water quality in New Zealand’s agricultural catchments. Current N management efforts in sensitive agricultural catchments are focused within the farm boundary and concentrate on identifying and reducing N loss from the root zone of farms. In many regions, the predicted farm rootzone N loss must comply with a set limit or allocation. Farm N loss allowances, as specified in regional council rules, are generally derived using assumptions about the attenuation of nitrate-nitrogen (NO3-N) as it passes from the paddock root zone to rivers and lakes. This approach ignores the spatial and temporal dynamics of the transport and transformations of NO3-N along flow pathways from farms to rivers and lakes as relatively little is known about these processes in NZ agricultural catchments. This information is increasingly being sought to derive a robust understanding of the contribution farming systems make to water quality outcomes, as is required by New Zealand’s National Policy Statement for Freshwater 2014. Our research in the Manawatu River catchment suggests that N loads measured in the river are significantly smaller than the estimates of N leached from the root zone. The on-going field observations, surveys and experiments indicate that denitrification is a key NO3-N attenuation process in the catchment. This N attenuation capacity appears to vary among the sub-catchments within the catchment. We, therefore, suggest that more cost-effective improvements in water quality can be achieved by selecting landuse practises and mitigation options according to the N attenuation capacity in the subsurface environment (below the root zone) in sensitive agricultural catchments. Further research to understand and quantify this N attenuation capacity in NZ agricultural catchments is important for a number of reasons. Firstly, by taking a catchment perspective, we will be able to help redesign landuse practices in a coordinated fashion by spatially aligning intensive landuse practices with high N attenuation pathways, i.e. ‘matching landuse with land suitability’, to increase agricultural production while reducing environmental impacts. Secondly, we will be able to align the spatial and temporal variation in N loss with built or enhanced attenuation to improve water quality outcomes.