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Phosphorus Best Practice

Investigating whether current fertiliser and effluent guidelines and policy are strong enough to prevent phosphorus loss

Rich McDowell photo by Dairy Farmer

PROJECT DETAILS

Challenge funding: $50,000

Research duration: June 2016 – December 2019

What Are We Doing?

Adding phosphorus to New Zealand’s soils can help grow plants – but unfortunately it does the same thing in our water.

Intensive pastoral agriculture in New Zealand has expanded into areas at high risk of phosphorus entering freshwater, such as land with a moderate to high slope, regular and frequent rainfall, or with soil that has a low capacity to absorb phosphorus.

This research investigated whether current practice and policy for fertiliser and farm dairy effluent (FDE) application are sufficient to minimise phosphorus loss.  The research also explored the factors affecting phosphorus loss after the application of fertilisers.

A key global challenge is to meet rising worldwide food demand while protecting water and managing uncertainty around potential future phosphorus price or supply shocks. Research contributed to by this team has compared approaches to phosphorus mitigation and policy in New Zealand and overseas. It forms part of a global effort to understand how to improve efficiency, facilitate change, incentivise phosphorus recovery, and conserve phosphate rock reserves.

How Can The Research Be Used?

  • Our research has found that when fertiliser is managed badly, it’s responsible for 30 to 80% of the phosphorus that drains away from a farm. When it’s managed well, that can decrease to less than 10%.
  • This research has provided a scientific basis for New Zealand and Australian farmers to follow the fertiliser industry-developed ‘4R guidelines’:  Apply fertiliser in the Right place, at the Right time, at the Right rate, and in the Right form.
    • Right place: Avoid critical source areas (CSAs) that have runoff more frequently than other parts of the farm or soil with poor P sorption capacity (eg anion storage capacity <10%). This includes near stream areas. While low water-soluble products may help avoid an initial burst of P loss, frequent runoff events or an inability of the soil to sorb P will mean that even low water-soluble P will be lost in time. It is best to avoid or minimise P applications to CSAs.
    • Right time: Avoid applying a highly water-soluble P fertiliser if rainfall is scheduled within 7 days that will likely to induce runoff. This might mean applying at a different time of year to avoid excessively wet (eg northerly aspects) or dry (spring instead of summer to avoid soil hydrophobicity) conditions.
    • Right rate: The quantity of P loss increases with the rate of P applied so avoid applying more P than is necessary – that is, no more than agronomically required. Where the soil is unable to hold onto much P (eg many Podzol, Organic and Semi-Arid soils) consider reducing the rate and altering the crops grown. For instance, many ryegrass monocultures do not require the high Olsen P required for mixed clover-ryegrass swards.
    • Right form: Water solubility influences the availability of fertiliser-P for loss in surface runoff or leaching. Superphosphate is highly water soluble (90%), while other forms such as serpentine super or reactive phosphate rock are less water soluble (~15 and <5% respectively) – resulting in a slower availability over time.
  • This research indicates that despite adhering to regional regulatory rules, significant phosphorus losses still occur when farm dairy effluent (FDE) is applied at low depths (<2mm) to stony free-draining soils, such as those common in the Canterbury region, the second-largest dairy-producing region in New Zealand.
    • Industry bodies and regional and central government could use these results to strengthen guidelines and regulations regarding the use of FDE on soils of low sorption capacity.
    • Farmers should avoid applying FDE to freely draining soil under irrigation, or use solids removal technology that reduces phosphorus to very low concentrations.

In the Media

Mitigating P-loss on sensitive areas

Ground Effect, February 2020

For the most efficient and cost-effective reductions in nutrient loss, McDowell recommends farmers understand where the CSAs are on their farm and target mitigation strategies to those areas.

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Challenge looks at how to build trust and capture more of our agricultural earnings

Irrigation NZ News, Winter 2019 (page 19)

“More research needs to be done to determine whether applying less FDE at lower rate would decrease phosphorus losses,” says lead researcher Rich McDowell, chief scientist of the Our Land and Water National Science Challenge. “Until we know this, the research suggests that farmers should try to avoid applying FDE to freely draining shallow stony soils under irrigation.”

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How to Minimise Phosphorus Loss from Fertilised Pasture

Our Land and Water blog, March 2020

“Managing fertiliser well isn’t expensive,” says Professor Richard McDowell, Chief Scientist for Our Land and Water. “Farmers can halve the amount of phosphorus leaving their farm with minimal impact on farm profitability.”

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Team Snapshot

Research Outputs

PAPERS

Global mapping of freshwater nutrient enrichment and periphyton growth potential

R. W. McDowell, A. Noble, P. Pletnyakov, B. E. Haggard & L. M. Mosley
Scientific Reports, February 2020

Periphyton (algal) growth in many freshwater systems is associated with severe eutrophication that can impair productive and recreational use of water by billions of people. However, there has been limited analysis of periphyton growth at a global level. We combined several databases to model and map global dissolved and total nitrogen (N) and phosphorus (P) concentrations, climatic and catchment characteristics for up to 1406 larger rivers that were analysed between 1990 and 2016. We predict that 31% of the global landmass contained catchments may exhibit undesirable levels of periphyton growth. Almost three-quarters (76%) was caused by P-enrichment and mapped to catchments dominated by agricultural land in North and South America and Europe containing 1.7B people. In contrast, undesirable periphyton growth due to N-enrichment was mapped to parts of North Africa and parts of the Middle East and India affecting 280 M people. These findings can be used to better target investment and actions at finer spatial scales to remediate poor water quality owing to periphyton growth.

The potential for potassium chloride fertiliser applications to leach cadmium from a grazed pasture soil

R. McDowell
Geoderma, November 2019

Cadmium (Cd) is a biotoxic element that can enter the human food chain via plants grown in Cd-enriched soil. Chloride (Cl) can solubilise Cd in soil. Although fertilisers containing Cl are not recommended as they may increase plant uptake of Cd, potassium chloride (KCl) is regularly applied to replenish and maintain K in the soil of grazed pastures. A trial was conducted to see if an autumn application of low or high KCl and the same molar rates of CaCl2 would leach Cd over 4-months of normal or high autumn-winter rainfall without enriching Cd in plant shoots. Applying Cl (e.g. as KCl) in autumn may be a simple strategy to help slow the enrichment of soil Cd and does not increase the uptake of Cd by pasture shoots.

Direct exports of phosphorus from fertilizers applied to grazed pastures in Australasia

D. Nash, R. McDowell, L. Condron, M. McLaughlin
Journal of Environmental Quality - Special Section: Celebrating The 350th Anniversary Of Discovering Phosphorus—For Better Or Worse, August 2019

The 4R concept was developed to help mitigate P exports. This review investigates the factors affecting P exports soon after the application of mineral fertilizers and studies quantifying its potential impact in different systems. Initially, P fertilizers and reactions that might affect their short-term P export potential are reviewed, along with P transport pathways. We then review studies quantifying the short-term impact of fertilizer application in different regions. Under poor management, recently applied fertilizer can contribute a considerable proportion (30–80%) of total farm P exports in drainage, but when fertilizer is well-managed, that figure is expected to be <10%. We use three model systems of varying hydrology common to Australasia to demonstrate the principles for selecting fertilizers that are likely to minimize P exports soon after their application.

A Global Perspective on Phosphorus Management Decision Support in Agriculture: Lessons Learned and Future Directions

R. Drohan, N. Bechmann, F. Djodjic, D. Doody, J. Duncan, A. Iho, P. Jordan, P. Kleinman, R. McDowell, P Melander, I. Thomas, P. Withers
Journal of Environmental Quality - Special Section: Celebrating The 350th Anniversary Of Discovering Phosphorus—For Better Or Worse, August 2019

The evolution of phosphorus (P) management decision support tools and systems has been most strongly affected in developed regions by national strategies (i) to optimize levels of plant available P in agricultural soils, and (ii) to mitigate P runoff to water bodies. Recent developments focused on integrated digital mapping of hydrologically sensitive areas and critical source areas have rapidly advanced runoff modeling and education. Advances in technology will facilitate the development of decision support systems that predict heterogeneity over wider geographical areas. However, significant challenges remain in incorporating “big data” in an acceptable format that accounts for catchment variability, farming systems, and farmer behavior. Future efforts will undoubtedly focus on improving efficiency and conserving phosphate rock reserves in the face of future scarcity or prohibitive cost. The principles reviewed here are critical for sustainable agriculture.

Transforming phosphorus use on the island of Ireland: A model for a sustainable system

K. Macintosh, J. Chin, B. Jacobs, D. Cordell, R. McDowell, P. Butler, P. Haygarth, P. Williams, J. Quinn, V. O’Flaherty, J. McGrath
Science of the Total Environment, March 2019

Phosphorus is an essential part of the world food web and a non-substitutable nutrient in all biological systems. A key global challenge is to meet rising worldwide food demand while protecting water and environmental quality, and seeking to manage uncertainty around potential future phosphorus price or supply shocks. This paper presents a stakeholder-generated conceptual model of potential transformative change for implementing phosphorus sustainability on the island of Ireland. Key transition pathways identified included: incentivising phosphorus recovery, developing collaborative networks to facilitate change, developing markets and value chains for recovered products; implementing data-informed practices on-farm to prevent losses and increase efficiencies, and harmonisation of technologies with end-user needs. A comparable model was previously produced for the North American region. We describe consensus and differences around key priorities between the two regions' conceptual models.

The efficacy of good practice to prevent long-term leaching losses of phosphorus from an irrigated dairy farm

R.W.McDowell, C.W.Gray, K.C.Cameron, H.J.Di, R.Pellow
Agriculture, Ecosystems & Environment, March 2019

We measured P fractions in leachate at 70cm depth from two soils of low-P sorption capacity in an intensively grazed dairy farm. In response to best-practice annual applications of P as either fertiliser or a lower rate of fertiliser plus FDE, dissolved and particulate P concentrations increased annually 4% to 7%. Mean total P load over 14 years (2001–2015) from the FDE-treated, free-draining shallow soil was 1.46 kg ha−1 yr−1, much greater than the same soil without FDE (0.25 kg ha−1 yr−1) or the moderately well-drained soil with or without FDE applied (0.12 kg ha−1 yr−1, for both treatments).

Transforming soil phosphorus fertility management strategies to support the delivery of multiple ecosystem services from agricultural systems

K. Macintosh, D. Doody, P. Withers, R. McDowell, D. Smith, L. Johnson, T. Bruulsema, V. O’Flaherty, J. McGrath
Science of the Total Environment, February 2019

Despite greater emphasis on holistic phosphorus (P) management, current nutrient advice delivered at farm-scale still focuses almost exclusively on agricultural production. This limits our ability to address national and international strategies for the delivery of multiple ecosystem services (ES). As soil P fertility plays a central role in ES delivery, we argue that soil test phosphorus (STP) concentration provides a suitable common unit of measure by which delivering multiple ES can be economically valued relative to maximum potential yield. This value can then be traded, or payments made against one another, at spatio-temporal scales relevant for farmer and national policy objectives. Implementation of this framework into current P fertility management strategies would allow for the integration and interaction of different stakeholder interests in ES delivery on-farm and in the wider landscape.

Managing diffuse phosphorus at the source versus at the sink

K. Macintosh, B. Mayer, R. McDowell, S.Powers, L. Baker, T. Boyer, B. Rittmann
Environmental Science and Technology, September 2018

Judicious phosphorus (P) management is a global grand challenge and critical to achieving water quality objectives while maintaining food production. The management of point sources has been successful in lowering P inputs to aquatic environments, but more difficult is reducing P discharges associated with diffuse sources. This critical review describes how the metrics of P quantity in kg ha–1 yr–1 and P form can influence decision-making and implementation of diffuse-P management strategies. The review draws upon a number of “current practice” case studies that span agriculture, cities, and aquatic sectors. These diverse examples from around the world highlight different diffuse-P management approaches and workable options for achieving water quality improvement and wider P sustainability. The diffuse-P management options discussed in this critical review are transferable to other jurisdictions at the global scale.

A review of regulations and guidelines related to winter manure application

Jian Liu , Peter J. A. Kleinman, Helena Aronsson, Don Flaten, Richard W. McDowell, Marianne Bechmann, Douglas B. Beegle, Timothy P. Robinson, Ray B. Bryant, Hongbin Liu, Andrew N. Sharpley, Tamie L. Veith Ambio
A Journal of the Human Environment – February 2018

Winter manure application elevates nutrient losses and impairment of water quality compared to applications in other seasons. We reviewed worldwide mandatory regulations and voluntary guidelines on efforts to reduce off-site nutrient losses associated with winter manure applications. Most of the developed countries implement regulations or guidelines. Developing countries lack such official directives, despite increasing concern over water quality. An analysis of five case studies reveals that directives are affected by local socio-economic and biophysical considerations. Successful programs combine site-specific management strategies along with expansion of manure storage to offer farmers greater flexibility in winter manure management.

Impacts of long-term biomass management on soil phosphorus under temperate grassland

Boitt G, Black A, Wakelin SA, McDowell RW, Condron LM
Plant and Soil - Plant Science, September 2017

We assessed and quantified the cumulative impact of 20 years of biomass management on the nature and bioavailability of soil phosphorus (P) accumulated from antecedent fertiliser inputs. Contemporary plant production and P uptake were over 2-fold higher for the biomass retained after mowing compared with the biomass removed regimes. Soil C, total P were significantly higher under biomass retention. Reserves of soluble and labile inorganic P in soil were significantly depleted in response to continued long-term removal of P in plant biomass compared to retention. However, this was only sufficient to sustain plant production at half the level observed for the biomass retention after 20 years, which was partly attributed to limited mobilisation of organic P in response to P removal.

Temperature and Nitrogen Effects on Phosphorus Uptake by Agricultural Stream-Bed Sediments

McDowell RW, Elkin, KR, Kleinman PJA
Journal of Environmental Quality, March 2017

Climate change will likely increase the growing season, temperatures, and ratio of nitrogen (N) to phosphorus (P) loss from land to water. However, it is unknown how these factors influence P concentrations in streams. We sought to evaluate differences in biotic and abiotic processes affecting stream sediment P dynamics under different temperature and N-enrichment regimes. Analysis of sediment samples indicated that P uptake via abiotic processes was greater at 19 than at 26°C. The addition of N stimulated P uptake by the microbial biomass at 19°C, but microbial uptake was potentially inhibited at 26°C. Because microbial biomass is a temporary store of P, these data suggest that more P may be available with increasing temperatures during the growing season, especially under baseflow, implying that strategies to mitigate P losses from land to water should be strengthened to prevent potential water quality impairment.

Guiding phosphorus stewardship for multiple ecosystems services

MacDonald G, Jarvie H, Withers P, Doody D, Keeler B, Haygarth P, Johnson L, McDowell R, Myyittah M, Powers S, Sharpley A, Shen J, Smith D, Weintraub M, Zhang T.
Ecosystem Health and Sustainability, December 2016

We propose a conceptual framework—the “phosphorus‐ecosystem services cascade” (PESC)—to integrate the key ecosystem processes and functions that moderate the relationship between P released to the environment from human actions and ecosystem services at distinct spatial and temporal scales. Phosphorus stewardship can have emergent ecosystem service co‐benefits. Applying the PESC framework, we identify key research priorities to align P stewardship with the management of multiple ecosystem services, such as incorporating additional services into agri‐environmental P indices, assessing how widespread recycling of organic P sources could differentially impact agricultural yields and water quality, and accounting for shifting baselines in P stewardship due to climate change. Ultimately, greater precision in targeting stewardship strategies to specific locations would help optimize for ecosystem services and more effectively internalize the downstream costs of farm nutrient management.

The effect of irrigation and urine application on phosphorus losses to subsurface flow from a stony soil

Gray CW, McDowell RW, Carrick, S, Thomas, S
Agriculture, Ecosystem & Environment, October 2016

Factors such as irrigation and soil pH can increase the solubility of P and its loss to water. Urine patches are known to alter pH but it is unknown if this increases P loss. This study investigated subsurface P loss from a stony soil under irrigation with or without the application of urine. Results showed that despite receiving urine average concentrations were significantly (P < 0.05) lower than non-urine treatments. Increasing the rate (not amount) of irrigation did not affect the load of P lost, thought to be due to attenuation. Lower P losses following urine application were due to the buffering effect of the soil and enhanced P uptake as dry matter production increased in response to N in urine. However, concentrations still exceeded guidelines. Strategies should be considered to decrease losses.

The effect of soil moisture extremes on the pathways and forms of phosphorus lost in runoff from two contrasting soil types

Simmonds B, McDowell RW, Condron LM
Soil Research, September 2016

Soil moisture and Olsen P concentrations play an important role in phosphorus (P) losses in runoff. Our hypotheses were as follows: (1) rainfall applied to a dry soil would cause greater particulate P losses in surface runoff due to hydrophobicity; (2) P losses from a wet soil would be dominated by drainage and filtered P; and (3) both runoff processes would result in environmentally unacceptable P losses at agronomically productive Olsen P concentrations depending on the sorption capacity of the soil. Compared with dry Organic soil, the wet Brown soil lost a greater proportion of TP as particulate via surface runoff. However the most important pathway for the Organic soil, wet or dry, was filtered P loss in drainage. These data can be used to more effectively target strategies to mitigate P losses.

Municipal composts reduce the transfer of Cd from soil to vegetables

Al Mamun S, Chanson G, Muliadi, Benyas E, Aktar M, Lehto N, McDowell R, Cavanagh J, Kellermann L, Clucas L, Robinson B
Environmental Pollution, June 2016

Cadmium (Cd) is a non-essential trace element that accumulates in agricultural soils through the application of Cd-rich phosphate fertiliser. Vegetables can accumulate Cd to concentrations that sometimes exceed food safety standards. We investigated the potential of low-cost soil amendments to reduce Cd uptake by spinach, lettuce and onion. Lignite and compost had the greatest ability to sorb Cd and were subsequently selected for pot trials. The addition of 2.5% (dry w/w) municipal compost reduced the Cd concentration in onions, spinach and lettuce by up to 60% in both soils. The addition of lignite gave variable results. This Cd immobilisation was offset by soil acidification caused by the lignite. The results indicate that municipal compost is a low-cost soil conditioner that is effective in reducing plant Cd uptake.

A review of the policies and implementation of practices to decrease water quality impairment by phosphorus in New Zealand, the UK, and the US

McDowell RW, Dils RM, Collins AL, Flahive K, Sharpley AN
Nutrient Cycling in Agroecosystems, September 2015

We outline policy in New Zealand, the UK, and the US who have imposed limits for P. We contrast the strategies used and discuss their likelihood of being able to decrease P losses and improve water quality. A poor understanding of catchment processes and critical source areas of P loss has resulted in some areas where regulation has had minimal effect on P discharges. Furthermore, we give several examples where social and economic challenges are of equal if not greater relevance to P discharges (e.g. subsidies). Other policy instruments (e.g. trading schemes) may be needed at larger scales, but should be flexible and encourage innovation. There is increasing recognition among all 3 countries that while targeting good management practices can substantially decrease P losses from existing land use, to achieve ‘good’ water quality in catchment, policy may have to consider land use change.

A national assessment of the potential linkage between soil, and surface and groundwater concentrations of phosphorus

R.W. McDowell, N. Cox, C.J. Daughney, D. Wheeler and M. Moreau
Journal of the American Water Resources Association – August 2015

A meta-analysis of three national databases determined the potential linkage between soil and surface and groundwater enrichment with phosphorus (P). A subset of sites dominated by aquifers with gravel and sand lithology showed increasing P concentrations with as little as 10 years data. These data raise the possibility that groundwater could contribute much P to surface water if: there is good connectivity between surface and groundwater, intensive landuse occurs on soils prone to leaching, and leached-P is not attenuated through aquifers. To avoid poor surface water quality, management and planning may need to consider the connectivity and characteristics of P in soil-groundwater-surface water systems.

CONFERENCE PRESENTATIONS

Science to policy and back again: compliant land use practices still cause P leaching

Rich McDowell
NZ Freshwater Sciences Conference – December 2018

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