Incentives for Change
Reasons for Water Quality Improvement
Investigating the reasons for decreasing phosphorus concentrations in many waterways
What Did We Do?
Fertiliser and farm dairy effluent contain phosphorus, which can enter freshwater via runoff or leaching from agricultural land. In freshwater, phosphorus can stimulate algal growth, leading to impairment of water for swimming, fishing, drinking, and reduced biodiversity.
Algal growth in New Zealand streams and rivers is widespread due to agricultural phosphorus, but in many areas the concentration of phosphorus in our waterways is decreasing.
Phosphorus has decreased at over 40% of measured sites in streams and rivers since 1994, and 65% of sites since 2004, despite an increase in national dairy cow numbers by 26% and the expansion of dairying into areas previously used for sheep farming.
The research investigated the possible factors contributing to reductions in phosphorus concentrations at these sites. We found the 3 most probable causes for improvement were that on-farm strategies were mitigating phosphorus loss from land, industry guidelines were directing where to best use strategies (for example, in critical source areas), and phosphorus was being mentioned more in policy instruments.
How Can The Research Be Used?
- This research tells us that farmers and growers should continue to implement strategies to mitigate phosphorus (P) losses.
- Previous research indicates that strengthened strategies to mitigate P losses will be required as temperatures increase under climate change, because of the decreased rate and magnitude of phosphorus uptake by stream bed sediment at higher temperatures.
- This research found little evidence that decreasing P concentrations are caused by a decrease in fertiliser use or change in fertiliser form. However, this doesn’t mean these practices won’t help – more likely, it’s because these practices aren’t yet widely adopted across whole catchments. Using low-water-soluble-P fertilisers, such as reactive phosphate rock, in critical source areas remains one of more than 40 mitigations Our Land and Water research recommends to decrease nutrient losses.
- Industry guidelines can help direct where to best use on-farm mitigation strategies. Advances in farm mapping (eg MitAgator) can allow these practices to be targeted even more precisely to critical source areas of phosphorus loss, improving their cost-effectiveness.
- Where losses haven’t been caught in time or where changes have not been made through voluntary means, policy instruments can help in decreasing losses.
In the Media
Farmers Weekly, February 2019
“We are now at a stage where we can provide some guidance on the strength of factors such as land use practices put forward in the scientific and public literature as likely causes of phosphorus trends in stream flow” – Rich McDowellVIEW ARTICLE →
Dairy Farmer, 18 February 2019
Probable causes are that on-farm strategies are stopping phosphorus loss from land and that together with policy, the actions are being directed to areas on-farm or in the catchment where phosphorus losses are most concentrated. They are called critical source areas and farm environment plans are a way to find and manage them.VIEW ARTICLE →
Royal Society research update, 28 February 2019
The newness of national policy direction for phosphorous in Aotearoa such the National Policy Statement for Freshwater Management (NPS-FM) and the lead-in time for its implementation suggest that the changes seen in water quality are not likely to be stemming from the NPS-FMVIEW ARTICLE →
Irrigation NZ News, Autumn 2019 (page 36)
“Sales of ‘alternative’ fertilisers are still low,” says Professor McDowell, “but targeted use of low-water-soluble-P fertilisers, such as reactive phosphate rock, can decrease phosphorus losses."VIEW ARTICLE →
Professor Rich McDowellAgResearch Profile →
Professor Mike HedleyMassey University Profile →
Peter PletnyakovAgResearch Profile →
Clint RissmannLand and Water Science Profile →
Warwick CattoBallance Agri-Nutrients Profile →
Wesley PatrickMinistry for the Environment Profile →
Ross MonaghanAgResearch Profile →
Warwick DoughertyNSW Department of Primary Industries Profile →
Cameron GourleyVictoria Department of Primary Industries Profile →
Ronaldo VibartAgResearch Profile →
Mark ShepherdAgResearch Profile →
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