Chapter 3

The Environmental Impact of Nitrogen Fertiliser Use on Dairy Pasture

 

Richard J. Eckard

ILFR, The University of Melbourne & Agriculture Victoria, Ellinbank

Malcolm McCaskill, Agriculture Victoria, Hamilton

 

 

With the dramatic increase in the use of nitrogen fertiliser on intensive pasture in Victoria over the past 15 years (Figure 3.1), concern is now being expressed about the potential environmental impact. As part of a recently completed state-wide (Victoria) nitrogen project "Best management practices for nitrogen in intensive pasture systems" an aim was to quantify these losses.

 


Figure 3.1. Trends in nitrogen fertiliser sales to pastoral farmers in Victoria over the past 20 years.

 

International trends

 

In many countries around the world, particularly in Europe, legislation has been introduced to reduce the environmental impact of excess nutrients leaving farms. A good example is the Netherlands, where a nitrogen and phosphorus accounting system (MINAS) was implemented in 1998. With the MINAS system, all farmers have to account for the nitrogen and phosphorus that is entering and leaving the farm, paying a levy for any nitrogen above 180 kg per ha leaving the farm. This system has resulted in a 58 % reduction in nitrogen leaving dairy farms since its implementation.

 

Nutrient budgeting is emerging as the best management practice for nutrient management on dairy farms. It is a useful tool for legislators to use in reducing the environmental impact of fertilisers, but also an important tool for farmers to use in improving the efficiency of nutrient management on farm. A nutrient budget is merely a balance sheet of total nutrients brought onto the farm in fertiliser and feed etc., balanced against the total nutrients leaving the farm in animal product (i.e. meat, milk and hay sold). An example of a nitrogen budget, using data from the research site at Ellinbank, for a low and high nitrogen fertiliser dairy farm, is presented in Table 3.1. At a farm level Part A of Table 3.1 is all that would be required to determine a nutrient surplus and a nutrient conversion efficiency ratio i.e. the animal products produced as a percentage of the nutrients applied. While the figures in Table 3.1 are on a per hectare basis, for a dairy farm a nutrient budget can either be calculated at an individual paddock level or, more likely, at the whole farm level. The main aim is to reduce the nutrient surplus as far as possible and raise the nutrient conversion efficiency ratio as high as possible. Part B of Table 3.1 provides some indication of likely pathways of nitrogen loss. These loses will vary with season and between farms.

 

Nitrogen loss pathways

 

Nitrogen may be lost from a dairy pasture system in many ways. The most important of these would be:

 

·        Dung and Urine are both very high in nitrogen. An average dung patch may be the equivalent of applying between 130 and 220 kg nitrogen per hectare, while an average urine patch applies the equivalent of 800 to 1300 kg nitrogen per hectare, within the patch. More than 60% of the nitrogen deposited in a urine patch may be lost, either through leaching or volatilisation.

 

The only practical management to reduce losses through these pathways would be to minimise the time that cows spent in the laneways, and to ensure efficiency of effluent collection and spreading back on pastures.

 

·        Ammonia volatilisation results from urea applied, either as fertiliser or as urine. During the cooler and wetter months (May to November) these losses are seldom above 8 % and not worth worrying about. However, in the hot and dry summer losses may be as high as 15 %. In the hot summer 10 mm of rain, within 24 hours of urea application, should halve volatilisation loss, while 20 mm rain should eliminate ammonia loss altogether. Applying nitrogen after rainfall in summer will usually result in the highest loss of ammonia; this can be as high as 22% of the nitrogen applied.

 

      Other sources of nitrogen, like DAP, ammonium nitrate and calcium ammonium nitrate, do not volatilise under our acid soil conditions. However, even accounting for the losses, urea remains the cheapest pure source of nitrogen to use at this stage, with DAP being the cheapest source of nitrogen and phosphorus.

 

Table 3.1. An example of a nitrogen budget for a Victorian dairy farm using either no nitrogen fertiliser, or 200 kg N/ha/year (as urea). All figures are reported in kg N/ha

 

Part A

N Inputs

Zero N

Urea

 

N output in products

Zero N

Urea

N Fertiliser

0

200

Milk

67

107

N2 Fixation

120

70

Meat

6

9

Rainfall

3

3

 

 

 

Hay

0

14

 

 

 

Grain

20

42

 

 

 

Total N Inputs

143

329

Total N Outputs

73

116

 

 

 

N 'Surplus'

70

213

     

N use efficiency ratio (%)

51 35
 
 
Part B

 

N Losses

Zero N

Urea

Volatilisation

8

59

Denitrification

8

18

Leaching

14

24

Transfer to lanes/sheds

19

29

Total losses

50

128

Total N Outputs + Loss

123

244

Nitrogen Balance (stored in the soil)

21

85

 

 

·        Nitrate leaching occurs when soil nitrogen, in the nitrate form, washes down the soil profile. This is the process of loss that is of greatest environmental concern, as water entering the streams and aquifers may be polluted with nitrate from dairy pasture. Losses vary depending on rainfall, but may range between 5 to 20 kg N/ha/year with no nitrogen fertiliser applied. Likewise, with the addition of 200 kg nitrogen leaching losses may vary between 12 and 60 kg N/ha/year depending on surplus rainfall, with urea usually leaching less that ammonium nitrate. In Victoria most of this loss occurs between July and October.

 

       Usually nitrate concentrations are highest with the first rains after the autumn break, declining through the winter as the volume of water dilutes soil nitrate. However, this may still mean that total nitrate leached (in kg loss per hectare) actually increases as rainfall increases. Care should, therefore, be taken with nitrogen applications during these periods of high rainfall.

  

To minimise nitrate leaching from pasture:

·        Nitrogen fertiliser rates should not exceed 50 kg N/ha in any single application. Figure 3.2 shows the trend in overall nitrogen loss as nitrogen fertiliser applications are increased.

·        Ammonia sources of nitrogen (DAP and urea) leach less nitrate in cold, wet soils, than nitrate (ammonium nitrate and Calcium Ammonium Nitrate) sources of nitrogen.

·        Apply nitrogen only when the pasture is actively growing and can utilise the fertiliser, especially during periods of high rainfall.

·        Avoid applying nitrogen fertiliser to well drained soils if heavy rains are predicted.

·        Avoid excessive pugging of pasture in winter.

·        Nitrogen and Phosphorus fertilisers should not be applied near streams or in drainage lines entering streams.

 

·        Denitrification occurs when nitrate in waterlogged soils is converted to nitrous oxide, a greenhouse gas 310 times more powerful than carbon dioxide.  Denitrification is more likely to occur under warm than cold conditions.   Fortunately, in south eastern Australia, soils are usually either waterlogged and cold in winter, or warm and dry in summer.  Denitrification losses, measured in the current research project, are small (3 - 4% of nitrogen inputs) in terms of kg nitrogen loss per hectare, and are thus not of economic significance. Greatest losses would be from waterlogged soils when soil temperatures are above 10 °C.

 

Nitrate sources of nitrogen (ammonium nitrate and calcium ammonium nitrate) are likely to lose more to denitrification than ammonia sources (DAP and Urea), particularly on colder, wet soils. In waterlogged and cold soil temperatures (below 10 °C) the conversion of ammonia to nitrate, the substrate required for denitrification, is greatly retarded.

 

To minimise denitrification losses from pasture:

·        Avoid high rates of nitrogen fertiliser on waterlogged soils, particularly during period when soil temperatures are high.

·        While soils are near field capacity, avoid applying nitrogen fertiliser before heavy rainfall and for at least 2 to 5 days after heavy rains depending on how readily the soils drain.

 

Nitrate Leaching : a Case Study from Western Victoria

 

As part of the nitrogen grazing study at DemoDairy, soil water samplers were installed below the root zone on two of the nitrogen treatments (0 and 3x75 kg N/ha). The site had virtually no slope, and surplus water must leave by infiltrating through the subsoil.  This water would eventually join the groundwater aquifer. Nitrate-N concentrations during the 1998 growing season, and streamflow in nearby Brucknell Creek, are plotted in Figure 3.3.  Stream flow peaks give an indication of when nitrate-N could leave the soil profile.

 

 

  

 

 

 

The following points should be noted:

·       nitrate-N concentrations were generally well below maximum allowed for drinking water of 10 mg N/L.

·       nitrate levels were highest early in winter (the start of the drainage season), and declined rapidly during the growing season.

·       nearly all the streamflow (and hence water movement) occurred when nitrate-N concentrations were extremely low.

·       there was little difference in nitrogen concentration between high and low nitrogen treatments (only 0.6 mg N/L).

 

The above example should be seen one close to “best practice” for nitrogen fertiliser management, where nitrogen application resulted in minimal environmental effects.  However, other data from the site showed high nitrate-N concentrations in the topsoil, averaging 26 mg/L with no nitrogen fertiliser to 70 mg/L at the high nitrogen application rate. At this particular site, surplus nitrogen-rich water is cleaned during its passage through the subsoil, presumably through uptake by roots of pasture.  However, on sites with lighter soils or steeper slopes, there is potential for this nitrogen-rich water to move directly into a stream, causing quite high nitrogen losses.

 

Concluding Comments

 

The single largest source of nitrogen loss on dairy farms through all these processes remains from animal urine. Little can be done to reduce this urinary loss apart from reducing stocking rate, reducing time in laneways and improved effluent management. However, there are simple management practices that can improve nitrogen efficiency and reduce environmental impacts of nitrogen losses. This chapter has provided some simple management recommendations that will help reduce losses of nitrogen where nitrogen fertiliser is applied.

 

Data from a completed project funded by DPI and the Dairy Research & Development Corporation

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Last modified:29 January, 2002                         Please Note: Disclaimer             Authorised and maintained by:Richard Eckard
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