Guideline Index

Chapter 12: Nitrogen and Nitrogen Fertilisers

12.3 Losses of N to the environment

Significant amounts of N either from the soil or from fertiliser may be lost to the environment under certain climatic, soil and management conditions. There are four main processes that can cause N to be lost from pasture systems. These processes are denitrification, ammonia volatilisation, surface runoff losses following irrigation or rainfall and leaching losses.

12.3.1 Denitrification

Denitrification in soils mainly occurs when soil oxygen concentrations become very low following heavy rainfall, flood irrigation or poor drainage. Under these conditions, micro-organisms convert soil nitrate into gaseous forms of N called nitrous oxide (a harmful greenhouse gas) and di-nitrogen gas, which are lost to the atmosphere.

Denitrification losses from dryland pastures at Ellinbank were measured at less than 10% of N applied, or between 6 and 17 kg N/ha/year, in non-waterlogged soil (Eckard et al. 2003). However, denitrification losses increased greatly when the soil was saturated (i.e. greater than field capacity).

Losses of 10 – 30% have been measured from nitrate-based fertilisers (i.e. Calcium Ammonium Nitrate – CAN), particularly when applied to warm, wet soils. These losses decline rapidly once soils are no longer saturated. However, denitrification losses are negligible with N fertilisers such as urea, DAP or ammonium sulphate. Consequently, nitrate forms of fertiliser are often not recommended when pastures are flood irrigated in the warmer months. Urea or DAP are safer and more efficient sources of N to use under these conditions. During the wetter periods of the year, it is advisable to watch the weather reports and allow soils to drain before applying N fertiliser, but also to avoid N sources containing nitrate. This strategy will also minimise nitrate leaching losses and surface runoff of N.

Denitrification losses don’t just occur after N fertiliser application, but also occur as a result of grazing animals and under legume based pastures. Grazing animals can reduce soil aeration through soil compaction, increasing nitrous oxide emissions. In northern Victoria, nitrous oxide emissions from grazing cow urine deposition was 4.2-4.5 kg nitrous oxide-N/ha over a 2 year period (Galbally et al. 2005). Gaseous losses under legume based systems have been measured at 8 kg N/ha/yr from a pasture that was fixing approximately108 kg N/ha/yr.

Recommendations to reduce denitrification losses

  • Do not apply N fertiliser to saturated soils. If N fertilisation is necessary, apply urea or ammonium based fertilisers.
  • Avoid applying N fertiliser to warm (>10°C) waterlogged soils, as this increases the rate of denitrification.

12.3.2 Ammonia volatilisation

Ammonia volatilisation is a process where N is lost to the atmosphere as a gas. Ammonia volatilisation following recently applied urea or ammonium based fertilisers (i.e. DAP, ammonium sulphate) or urine deposition tends to increase when conditions are warm and dry. When ammonium based fertilisers or urine is deposited on soil, they undergo a process called ammonification which increases the soil pH to >7.5 (more alkaline) and results in ammonia volatilisation to the atmosphere. Urine patches are the major source of ammonia volatilisation from dairy pasture systems, however Australian and New Zealand research has shown that urea fertiliser is more likely to result in ammonia volatilisation compared to DAP or ammonium sulphate.

12.3.2.1 Nitrogen fertiliser and acid soils

Research at Ellinbank in Victoria has shown that ammonia volatilisation losses from urea fertiliser applied between May and November are usually below 8% of the N fertiliser applied and are thus of little environmental or economic concern (Eckard et al. 2003). However, ammonia volatilisation losses in summer may average 14% of the N applied as urea, with losses as high as 22% likely where urea is applied after a light rainfall, followed by hot and dry weather.

To minimise the risk of N loss by volatilisation during the hotter months and where irrigation is not available, urea fertiliser can be applied 2-3 days before grazing, as the pasture canopy reduces the wind speed near the fertiliser granules reducing gaseous loss. However, care needs to be taken to ensure that cows do not ingest lumps of fertiliser as this could lead to ammonia toxicity (see Section 12.4.3.2 to consider other issues associated with applying N fertiliser prior to grazing). However, in this case, it is important to consider if soil moisture and temperature will limit the response to N fertiliser application (see Section 12.4.3.3).

At other times of the year when irrigation is not possible, urea can be applied to moist soil provided that temperatures and evaporation rates are not high. Avoid applying N fertiliser after light summer rainfall if soils are dry and evaporation rates are high, as volatilisation rates can be high. In hot dry conditions, use a weather forecast to time N fertiliser application to occur just prior to a rainfall event. Approximately 8 to 10 mm of rain is required to reduce volatilisation in dry soils. Heavy dew does not provide enough moisture. Ammonium based fertilisers (i.e. DAP) or sulphate of ammonia may be more appropriate to apply when conditions are dry (because they are subject to less volatilisation), but they are often more expensive per unit of N compared to urea, and this cost difference is seldom justified by the environmental benefit.

Although ammonia volatilisation is not an immediate environmental concern, loss of N to the atmosphere reduces the cost effectiveness of applying N fertiliser and should be avoided if possible. The following sets out the best management practices for reducing ammonia volatilisation under dryland and irrigated dairy systems.

Recommendations to reduce volatilisation losses- dryland pastures (Eckard 2001)

  • Between the cooler, wetter months (May to November in south eastern Australia), ammonia volatilisation losses from urea fertiliser are too small to be of economic or environmental concern, and do not justify switching to higher-cost N fertiliser sources.
  • If urea fertiliser is applied in the drier months (November to March in south eastern Australia) without irrigation, apply fertiliser 2 to 3 days prior to grazing to minimise wind speed at ground level and reduce ammonia volatilisation. Care must be taken to avoid cows ingesting lumps of fertiliser as this could lead to ammonia toxicity.
  • In summer, where soils are dry and evaporation is high, avoid applying urea fertiliser after a rainfall event, as this may increase volatilisation losses above 22%.

Recommendations to reduce volatilisation losses – spray irrigated pastures (R. Eckard pers. comm. 2013)

  • Apply N fertiliser within 24 hours prior to spray irrigation.
  • In summer, where evaporation is high, avoid applying urea fertiliser after a spray irrigation as this is likely to increase volatilisation losses.

Recommendations to reduce volatilisation losses – border check irrigation (Mundy 1997; K. Kelly pers. comm. 2013)

  • In summer, apply urea fertiliser after border check irrigation as soil moisture will be adequate to dissolve the urea and minimise volatilisation. Take care not to damage wet soils with fertiliser spreaders.
  • If urea is applied prior to border check irrigation, it is important to apply close to irrigation (within 24 hours), but care is required not to over water and generate surface N runoff into drains.
12.3.2.2 Nitrogen fertiliser and alkaline or heavily limed acid soils

Ammonia volatilisation can occur with other ammonium fertilisers, particularly ammonium sulphate and DAP, when they are used on alkaline or heavily limed acid soils (soil pH>8 in water). Urea should not be applied to pastures that have been recently top dressed with lime. Substantial N may be lost as ammonia due to chemical reactions with the lime on the soil surface. If lime must be applied at a similar time to urea, then apply the urea first and apply the lime at least 2 weeks later.

12.3.2.3 Urine deposition

Recent research conducted in Western Australia as part of the Greener Pastures project (Bennett et al. 2011) has shown that 45% of the N contained in urine volatilised in summer, 20-30% in winter and only 0-5% in spring.

12.3.3 Leaching

When soils are saturated, free draining or artificially drained (e.g. mole or subsurface pipe or tile drains – see Chapter 7.4.5.2) soils may leach nitrate as water drains through the soil. In countries like New Zealand, Europe and the USA, there are stringent controls to minimise N losses by leaching. The major concern is nitrate leaching into ground water that may be used for drinking water and the environmental impact of elevated nitrate in rivers and lakes. Unlike ammonium, nitrate is not held by soil and is readily lost when water drains through soil.

There has been little research on nitrate leaching in Australia, but research conducted at Ellinbank in Victoria has shown when high N rates (above about 70 kg N/ha) are applied to free-draining soils or when heavy rainfall followed N fertiliser application, nitrate leaching losses ranged from 5 kg N/ha/yr in a low rainfall year to 38 kg N/ha/yr in a higher drainage year (Eckard et al. 2004). Leaching is generally not considered to be a major cause of N loss in duplex soils because of low water flow through the subsoil. However, on well-drained soils, as in many dairying areas around Australia, N losses through leaching may be significant.

It is important to understand that nitrate leaching is not only due to fertiliser application. Other factors such as the amount of N in soil which is leachable and urine deposition from grazing animals are also very important. Research has shown pastures to be generally efficient in uptake of N fertiliser, therefore the timing of N fertiliser application to maximise plant uptake and reduce the amount of N that remains in soil, can reduce leaching losses. Nitrate leaching losses under dryland systems are generally lower in spring when pastures are actively growing and rapidly absorbing nitrate. The first leaching event of the season often results in the greatest nitrate leaching losses due to the build-up of soil nitrate over the summer and autumn period when pasture is not actively growing.

The largest source of N leached in dairy pasture systems is from urine deposition.

Grazing animals excrete around 80% of their total N waste as urine and considering cows urinate 10-12 times per day and the urine falls on a small area, the N application rate can be as high as 1000 kg N/ha. This rate of N deposition is much higher than a pastures’ requirement for N, therefore if water drains through the soil, much of this urine-derived N can be leached. Studies in south Australia have shown that 26-33 kg N/ha/year were leached under irrigation and 10-13 kg N/ha under rain-fed dairy pasture following a single application of urine which contained 650 kg N/ha/year and 2 applications totalling 1604 kg N/ha/year (Pakrou and Dillon, 2004). Most urine gets deposited around water troughs, gateways, shelter belts, laneways and milking sheds. Urine spots in paddocks may get additional N inputs from fertiliser and dung and this increases the risk of nitrate leaching further.

Nitrate leaching occurs when excess soil water drains from the soil, taking with it soluble N in the form of nitrate. To minimise nitrate leaching losses, leachable nitrate must be minimised in the soil during times of potential high drainage. Some ways to manage this are listed below.

Recommendations to reduce leaching losses

  • Only apply N fertiliser to actively growing pasture at a rate between 25 and 50 kg N/ha in any single application.
  • New Zealand research suggests that annual applications greater than 200 kg N/ha increase the risk of nitrate leaching (Cameron et al. 2002; Meneer et al. 2004).
  • Avoid grazing until ryegrass or tall fescue pastures have grown at least 2 new leaves, and brome (prairie grass), cocksfoot and kikuyu pastures have grown at least 3 new leaves, to reduce the intake of N in the cows diet and therefore the amount of N excreted.
  • Avoid applying N fertilisers on areas where animals congregate i.e. gateways, water troughs, shelter belts, as these areas will already have high soil N.
  • If N must be applied during the wetter period, use a weather forecast to wait at least 2 days after any significant rainfall event to allow the soils to drain to just below field capacity before applying N.
  • If irrigating, take care to avoid overwatering, as this may result in nitrate leaching.
12.3.3.1 Other management options to reduce N leaching

If nitrate leaching is likely to be an issue, then it may be necessary to stand cows off pasture during the wettest period of the year either on feed pads or agist on to another property. These management practices are currently being used in New Zealand to deal with tighter restrictions on nitrate leaching, however such environmental restrictions are currently not in place in Australia. It is important to consider that N excretion can increase if cows are consuming excessive amounts of N in their diet, so take care not to apply too much N fertiliser or graze before pastures have grown enough leaves to dilute the plant N concentration (see Section 12.1.3 and Section 12.4). If N fertiliser or grazing rotation length is not an issue, then another management option is to feed low protein forages such as maize silage, as a way of reducing the amount of nitrate which is excreted in cattle urine.

12.3.3.2 Reducing N leaching from effluent application

It is important to reduce the risk of nitrate leaching following effluent application, by making sure that effluent is not irrigated on to soils which are at field capacity (i.e. very wet). If soils are at field capacity, then effluent application should be delayed until a soil water deficit is created. Soil water deficit is the amount of water removed from the surface soil where pasture has active roots (root zone). It is also the amount of water required to refill the root zone back to field capacity. Effluent should only be applied to the point where the soil water deficit is back to 0, otherwise any additional effluent is prone to leaching or runoff. Applying effluent at low application rates using K-lines or over-head sprinklers and not exceeding the soil water deficit, is recommended in New Zealand as an effective way of reducing leaching and runoff (DairyNZ 2012).

12.3.4 Loss of N in surface runoff after rainfall or irrigation

Surface runoff can occur when rainfall or irrigation falls on saturated soils, or rainfall or irrigation rate is extremely high and soils are unable to absorb all of the water. Most forms of N fertiliser are readily soluble and dissolve in surface water runoff, but soils also release N to runoff water as it moves across the soil surface. Nutrient loss in surface runoff water means a potential loss of valuable nutrients from the farm and can lead to nutrient enrichment of rivers and streams.

Although Australian research on the loss of N in surface water runoff following N fertiliser application shows that losses are generally low (<5 kg N/ha/yr), losses under border check irrigation or hump and hollow drainage (i.e. parts of western Victoria or north west Tasmania) have ranged from 3-23 kg N/ha/yr.

Management practices which minimise surface N runoff losses should be incorporated into farm practice and include the following recommendations:

Recommendations to reduce surface runoff losses

  • Do not apply N fertiliser near (generally within 20 metres) drains, channels, dams, lakes or riparian areas.
  • The volume of water lost as runoff has a big influence on the amount of nutrient lost in runoff, so avoid overwatering and generating surface runoff.
  • Use a weather forecast to avoid runoff within days of N fertiliser application, to allow N time to be absorbed by plants.
  • Where possible, re-use drainage water.
  • Maintain good ground cover and avoid soil erosion.
  • If re-sowing pasture, consider minimum tillage practices.

12.3.5 Use of nitrification inhibitors to reduce N loss to the environment

Nitrification inhibitors are chemical products which can reduce the conversion of ammonium to nitrate and therefore reduce the rate of denitrification and greenhouse gas losses and nitrate leaching following urea or urine application. Dicyandiamide (DCD) has been used to reduce N losses in New Zealand. The effectiveness of DCD is affected by the soil and climatic conditions and New Zealand research has shown that DCD works best to reduce nitrate leaching in late autumn through to early spring when average soil temperatures are below 10°C and drainage is high (Di and Cameron 2004). Another New Zealand study found that DCD application slowed nitrification and reduced soil nitrate concentration and reduced nitrous oxide emissions by an average of 50% from urine patches (Gillingham et al. 2012). In some regions, DCD application resulted in more N being available for pasture growth and pasture production increased.

However, Western Australian studies have found that their higher soil temperatures and soil types reduced the usefulness and cost effectiveness of DCD use (Fillery and Bolland 2007). A northern Victorian study under border check irrigation found that DCD was effective at reducing nitrous oxide emissions for approximately 50 days in mid-spring and 25 days in mid-summer (Kelly et al. 2008), but like the Western Australian study, concluded that higher soil temperatures in northern Victoria were likely to reduce the effectiveness of DCD in this region.

It is important to note that DCD is currently not being sold in New Zealand due to the recent detection of low level traces of DCD in dairy products.