Best Management Practice guidelines for minimising Nitrous Oxide
(N2O) emissions from pasture and cropping systems
(Guidelines listed are based on research work conducted to date
and reviews of published literature)
Minimise surplus nitrate in the soil as this will decrease potential
losses through both nitrate leaching and denitrification
1. Fertiliser Management
Source, rate and timing (split) of N applications
a) Nitrate N sources (ie, ammonium nitrate, potassium nitrate,
calcium ammonium nitrate) may result in greater denitrification
and leaching than ammonia-based sources of nitrogen (ie, urea, DAP,
ammonium sulphate) if applied under cold, wet and waterlogged (soils
close to field capacity or above) conditions. However, ammonia-based
sources could lose high amounts of ammonia gas if top-dressed under
warmer and windy conditions, especially on alkaline soils. Urea
is currently the cheapest straight source of nitrogen, while DAP
is the cheapest mixed source of nitrogen.
b) Match crop or pasture demand – Only apply nitrogen when crop
or pasture is actively growing and can utilise the nitrogen. Nitrogen
is always more efficiently utilised when applied strictly according
to growth potential ie, only apply the highest recommended rates
when no other limiting factors are restricting yield potential.
c) Avoid excessive nitrogen fertiliser rates. For pastures, do
not apply above 50 to 60 kg nitrogen/ha in any single application
and do not apply nitrogen closer than 21 (30 kg nitrogen/ha in spring)
to 28 (50 kg nitrogen/ha) days apart, as this will increase nitrogen
losses dramatically.
d) Warm and waterlogged soils – Avoid high nitrogen rates on waterlogged
soils, particularly if soil temperatures are above 10 °C, as this
will increase denitrification losses. Denitrification is highest
under anaerobic soil conditions, particularly when these conditions
are coupled with warmer soil temperatures.
Coated/chemically treated fertilisers
There are a number of coatings that can be applied to nitrogen
fertilisers that will eliminate nitrous oxide losses directly from
fertiliser. However, these coatings have no effect on losses of
nitrogen derived from legumes and urine. In the future it is likely
that most nitrogen fertiliser sold will be in some form of controlled
release or inhibited form. At this stage these products are too
expensive to justify their commercial use in broad acre agriculture
and require further research to evaluate performance under Australian
conditions.
a) Nitrification inhibitors – this coating inhibits the conversion
of ammonia to nitrate in the soil, thus reducing the chance of both
nitrate leaching and denitrification loss. An example of such a
compound is dicyandiamide (DCD), proven effective in many studies.
b) Controlled-release – A range of polymer-coated / impregnated
fertiliser products are available, releasing their nitrogen according
to the predicted crop growth pattern. This controlled release significantly
improves fertiliser efficiency. However, if the onset of conditions
favourable to denitrification coincides with nitrogen release form
the coated fertilisers, denitrification may still result albeit
at a lower rate than would have occurred using conventional forms
of fertiliser nitrogen.
2. Crop & Pasture management
a) Reduce fallow – During the fallow period the soil continues
to break down organic soil nitrogen into nitrate (mineralisation
followed by nitrification) but there is no crop to utilise this
nitrate; as a result this nitrate is susceptible to nitrate leaching
and denitrification loss following heavy rainfall.
b) Cover crops – Where possible use non-leguminous cover crops
to use residual nitrate nitrogen in soil such as in cotton cropping.
c) Water use efficiency – Use efficient soil and pasture management
practices, including nutrition, to make the best use of water; unused
water if left in excess creates conditions for future runoff from rainfall, waterlogging for denitrification or leaching of nitrates.
d) Other nutrients – If there are other nutrients limiting the
growth potential of the crop or pasture, nitrogen fertiliser use
will be less efficient leading to greater loss potential.
d) Subsoil limitations such as transient salinity, sodicity, acidity,
restrict the ability of crops to effectively utilise soil nitrogen.
Nitrogen inputs (from either fertiliser or legumes) should be adjusted
(reduced) to reflect the true yield capacity of crops where subsoil
limitations are present.
e) Animal stocking rate – The higher the stocking rate the higher
the volume of nitrogen deposited in dung and urine per unit area.
Dung and especially urine are very inefficiently recycled in the
soil plant system, with up to 60% of the nitrogen in a urine patch
being lost to the environment. Higher stocking rate systems demand
a higher nitrogen input regime (either fertiliser or imported feed)
and thus result in a higher nitrogen content excreted in urine.
A urine patch from dairy cow commonly contains between 800 and 1400
kg N/ha effective application rate with the patch. A higher stocking
rate also leads to greater pugging (hoof compaction) of the soil;
pugged soils tend to be more anaerobic due to hoof compaction leading
to higher nitrous oxide losses.
f) Plant breeding – A longer term strategy is breeding plants that
are less nitrophylous ie, a ryegrass plant that does not require
as much nitrogen fertiliser, or plants with a deeper rooting system
to extract nitrate from a greater volume of soil.
3. Soil management
a) Reduced tillage – Soil disturbance such as a tillage operation
breaks up soil organic matter, stimulating greater mineralisation
of organic nitrogen. This leads nitrate becoming available in the
soil at a greater rate following tillage and thus a greater loss
potential. It also reduces soil structure, leading to poorer plant
growth and greater potential for temporary water logging.
b) Irrigation and drainage – Irrigation aims to maintain the soil
above wilting point and below field capacity. Poorly drained soils
are anaerobic thus promoting denitrification of soil nitrate. If
soil nitrate is in excess of crop growth, nitrous oxide loss can
be high in both cases.
c) Soil compaction – The more compact a soil, the more anaerobic
it becomes, leading to higher nitrous oxide loss through denitrification.
Soil is commonly compacted through wheel traffic in cropping systems
and through treading from animal hooves, especially under wet conditions,
in grazing systems.
Authors
Dr Richard Eckard, Dr Roger Armstrong
Contributing referees
Dr Jeff Baldock, Dr Ram Dalal, Mr Neil Young, and Dr Peter Grace
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