The excessive phosphorus (P) concentrations in surface waters is an important environmental issue. Phosphorus loss has the potential to be higher on dairy farms (1-10 kgP/ha) compared with sheep and beef farms (0.1-2.2 kgP/ha). This summary covers a few known strategies to reduce P loss that have been studied, however, the area that they are to be implemented in needs to be trialled first to ensure it will result in a reduction in nutrient losses to water.
The wide range of geographic features and climate within New Zealand and Australian dairy farms means that P can be lost by many pathways. These pathways from farm to water can be in dissolved (in the surface and ground water) and particulate (attached to soil particles or in dung) forms.
The diagram below visualises the potential pathways for P. In the dissolved form it can move through the soil matrix and enter the ground water. Attached to soil particles or in dung, P can move across the land via surface runoff and overland flow when there is heavy rainfall.
This paper examined a range of mitigation strategies to reduce the P entering waterways from pastoral dairy farms in New Zealand and Australia. The strategies that are more relevant to New Zealand will be focused on in this summary.
Key Points
On-farm management was the most cost effective for reducing P loss.
Constructing wetlands cost the most but also had the greatest potential benefit.
Constructed wetlands can have other benefits that need to be taken into consideration such as N removal and modification of peak flow.
The loss of phosphorus (P) from land to water is detrimental to surface water quality in many parts of New Zealand.
Farming can be a source of P loss, but preventing it requires a range of fully costed strategies because little or no subsidies are available in New Zealand.
The effectiveness of mitigation strategies is dependent on farm management systems, topography, stream density, and climate.
In this paper the mitigation strategies were separated into 3 categories:
Management (e.g. decreasing soil test P, fencing streams off from stock, or applying low-water-soluble P fertilizers)
Amendments (soil additives e.g. alum or red mud)
Edge-of-field mitigations (e.g. natural or constructed wetlands)
Management
Optimum soil test phosphorus concentration
This is a simple approach where the aim is to maintain the soil test level for P at optimum for pasture production. Maintaining a soil test P level above this causes unnecessary P loss. However, maintaining optimum P levels does not prevent P loss, it just decreases the risk and the amount lost in a runoff event as a result of rainfall.
The optimum soil test level for P, measured as Olsen P, is dependent on the soil type. For ash and sedimentary soils an Olsen P of 20-30 is optimal for pasture production, however, on pumice and peat soils the optimum is 35-45.
Phosphorus fertilizer management
Under best management practices, loses of P from fertilizers have been estimated to be around 10% of total farm losses. Best practice requires the P fertilizer to be applied when there is a low chance of surface runoff as a result of heavy rainfall. This is during months when the weather is less variable so the future rain events can be predicted. This is required as most P fertilizers are highly water soluble.
The use of reactive phosphate rock (RPR) can be used as a mitigation as the amount of soluble P is significantly lower than other P fertilizers while still having a similar amount of total P.
Stream Fencing
Livestock access to streams can damage the stream bank and bed and allows for the direct deposition of P into the streams. Excluding livestock by fencing off waterways removes the direct deposition of dung into the water and reduces the amount of sediment entering the water. Resulting in reduced P loss to the waterways.
Restricted grazing of cropland
In areas of New Zealand where pasture growth is low in winter (e.g. Southland), around 10% of the farm is sown in a forage crop to allow stock to remain on the farm throughout the year.
The grazing of crop during winter has been shown to have greater P losses than winter-grazed pasture. Restricting cattle and sheep to their maintenance feed requirements and only allowing them 3-4 hours grazing time instead of 24 hours. This reduces the treading damage to the soil and removes the dung from the cropped area.
Irrigation management
Border-check (dyke) irrigation is the practice of periodically flooding an area of land with water from one end. The water exiting the area can enter stream networks and with it carrying P. This type of irrigation is common in dry and flat areas. Alternatives to border-check irrigation such as spray irrigation and subsurface drip irrigation can minimise P loss, however, it often is a result of improved water efficiency that results in the best reduction in P losses.
Increased effluent pond storage and low rate effluent application
The land application of farm dairy effluent from the dairy shed is at risk to lose nutrients to ground water if it is not managed effectively. Strategies can be used, such as deferred irrigation, to decrease P loss.
The management strategies mentioned above have a range of effectiveness, measured in percentage of total P reduced, shown below in the graph. Both flood irrigation management improvements and restricted grazing of cropland showed the greatest reductions in P losses.
Amendments
These are things that can be added to the soil to reduce P loss and are more likely to be used in Australia than New Zealand due to the limited availability in New Zealand.
Over the past two decades, by-product materials rich in Ca, Al, and Fe have been identified as decreasing P loss from soils with varied success. These include, but are not limited to, zeolites, aluminum sulfate, water treatment residuals, and fluidized bed bottom-ash and fly ash from coal-fired power plants (e.g., Reichert andNorton, 1994; Sakadevan and Bavor, 1998; Moreno et al., 2001; Callahan et al., 2002).
Application of aluminium to pasture and cropland has been used to increase the ability of phosphorus to attach to the soil particles and decrease the solubility in dung pats.
Amendments, added to tile drains or directly to surface soil, were often constrained by supply or were labor intensive. As the amendments are more applicable to Australian agriculture, they have not been reported in depth in this summary. If you would like to know more then check out the full paper below.
Edge-of-field
Buffer Strips
Grass buffer strips decrease P loss in surface runoff by a combination of filtration, deposition and improving infiltration. Riparian and grass buffer strips have been trailed in NZ and show a decrease in P loss.
Sediment Traps
In-stream sediment traps are useful for the retention of coarse-sized sediment. If the sediment trap was to be cleaned twice a year then around 90% of fine sand could be removed.
Natural and constructed wetlands
Wetlands can either be P sinks or sources depending on a range of factors. Over time the wetlands ability to retain particulate P decreases as it becomes fill of sediment.
The retention of dissolved P by wetlands tends to be poor. Increasing the size of the wetland can increase the retention of dissolved P. When there is P-rich sediment retained within the wetland, it can dissolve and release P which then exits the wetland into the nearby waterway, which is what we don't want to happen!
Constructed wetlands can be designed to remove P by decreasing flow rates and increasing contact with vegetation. They can also improve contact between inflowing water and sediment to encourage P uptake and create environments where bacterial processes are encouraged to remove P.
There have been three experimental wetlands constructed in NZ (in the Waikato, Northland and Southland). These wetlands have shown limited P uptake due to sediment-poor inflow and anoxic conditions in the wetland sediments.
The effectiveness of the edge of field mitigation strategies can be seen below. The negative value indicates an increase in P loss. The huge range in values from the constructed wetlands shows that these must be created carefully to remove P and not increase the P loss to water. When done correctly, these constructed wetlands have the greatest percentage reduction in total P loss.
In the table below are the cost effectiveness of each mitigation strategy. The edge-of-field strategies, which remove P from runoff (i.e., wetlands) or prevent runoff (i.e., irrigation runoff recycling systems), were generally the least cost effective, but their benefits in terms of improved overall resource efficiency, especially in times of drought, or their effect on other contaminants like N need to be considered.
In general, on-farm management strategies were the most cost-effective way of mitigating P exports as can be seen in the table below (cost range, $0 to $200 per kg P conserved).
Having a number of mitigation strategies available and the benefits of each, allow farmers or farm advisors the ability to choose to suit specific farm systems and environments.
Further work should examine the potential for targeting strategies to areas that lose the most P to maximize the cost-effectiveness of mitigation strategies, quantify the benefits of multiple strategies, and identify changes to land use that optimize overall dairy production, but minimize catchment scale nutrient losses.
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Full Paper:
McDowell, R. W., & Nash, D. (2012). A review of the cost-effectiveness and suitability of mitigation strategies to prevent phosphorus loss from dairy farms in New Zealand and Australia. Journal of Environmental Quality, 41(3), 680-693.