The Carbon Farming Knowledge Project involves a series of workshops to increase the understanding of 30
independent agricultural advisers in south-east Australia on reducing greenhouse gas emissions, carbon in
farming systems and the Carbon Farming Initiative – where farmers can earn credits for storing carbon or
reducing greenhouse gas emissions on their properties. The project helps advisors prepare their clients for
potential environmental, economic and social benefits of future carbon management policy.
Session 1: Reduce nitrogen losses and increase nitrogen use efficiency
Summary of August 2014 workshop presentation by Peter Grace, Queensland University of Technology
Background
Nitrous oxide (N2O) is a potent greenhouse gas (GHG). When it comes to reducing global warming, removing
one N2O molecule from the atmosphere is equivalent to removing 300 CO2 molecules. About 1% of applied
nitrogen fertiliser is emitted as N2O, depending on the farming system, climate, and management. There are
two main ways that N2O is emitted – nitrification and denitrification, although more is lost through
denitrification (saturated/water-logged soils). Denitrification occurs in southern Australia in winter.
Why is it important …
Most of Australia’s N2O comes from agricultural soils. Losses of nitrogen (N2) are difficult to measure because it
is quickly diluted in the atmosphere. However, N2O is easy to measure and is a good indicator of nitrogen use
efficiency. High losses of N2O indicates inefficiency in a farming system:
 Soils with high N2O emissions mean there is excess N in the system, and/or poor soil physical condition.
 Soils low in CO2 have low microbial activity and low carbon inputs. A productive soil emits a high rate of
CO2 because its soil microbes are decomposing carbon in residues.
 Soils with high methane (CH4) emissions means the soil is in poor physical condition.
Building soil organic matter – carbon
in soil – is critical in terms of soil
structure and fertility and can reduce
reliance on N fertiliser.
In terms of carbon farming, strategies
to reduce N2O emissions will mean
the gas is not emitted. If this
corresponds to a Carbon Farming
Initiative methodology Australian
carbon Credit Units (ACCUs) can be
earned on that reduction. Farmers
who wish to earn ACCUs by
sequestering carbon in soil have to
maintain the carbon they sequester
100 years. N2O reductions are an
avoided emission so is permanent.
Factors in nitrogen loss
The key factors influencing gaseous nitrogen loss (N2 and N2O) are saturated and water-logged soils, the
presence of soil nitrate, easily degradable forms of soil organic carbon levels – often called labile carbon –
temperature and pH.
The two key factors are water content and soil nitrate – a biological reaction influenced by temperature. For
every 10°C the temperature increases, the rate of denitrification potentially will double. Denitrification occurs
when atmospheric temperatures are from 5-10°C through to 30-40°C but it peaks at 35°C.
Keys to reduce losses
Nitrogen fertiliser is lost by gaseous emissions and leaching. In sandy soils, N mostly leaches, but in clays and
duplex soils, gas is emitted. Yields can be sustained or increased by reducing N losses using these strategies:

Tillage: Reducing tillage and retaining residues improves drainage and aggregation (the way soil
particles bind together) and reduces the risk of waterlogging. Tillage exposes soil aggregates and makes
carbon more accessible to micro-organisms. There are higher CO2 emissions after tillage events.
 Carbon inputs: Retaining residues will improve soil organic matter, important for soil structure and
fertility.
 Liming: Liming helps to achieve a favourable pH for microbial activity – the more neutral the better.
 Cover crops: A short-term cover crop grown at the end of the season can soak up any excess nitrogen
for later release.
 Irrigation: Applying small amounts of water more often promotes plant growth and better matches
water demand to crop growth and uptake of water and nutrients, reducing water-logging.
 Drainage: Improving drainage in high rainfall areas reduces opportunities for waterlogging and nitrogen
losses. This can be through farming system practice, such as no-till.
 Crop type: Crops which require narrow row spaces will ensure that fertiliser N is taken up more
effectively.
 Fallow: Reducing long fallow periods will help reduce opportunities where excess N can be produced
and left in the profile and potentially lost through denitrification.
 Fertilisation: Match fertiliser N supply with plant N demand. This improves nitrogen use efficiency
(NUE). Reducing the nitrogen rate by taking into account residual soil N and the mineralisation of soil
organic matter and residues, deep banding and applying later in the season as crops really need it all
increase NUE. Nitrification inhibitors and enhanced efficiency products will slow the production of
nitrate
Improving NUE
Building soil organic carbon means nitrogen
is being built up as well. There is an optimal
rate of nitrogen that maximises yield and
minimises nitrogen loss (see figure right).
Nitrification inhibitors can potentially reduce
N loss, but they are not cost-effective at
present. Organic sources of N (legumes) that
mineralise slowly are better than large
amounts of mineral N in soil that could be
lost at any point if the crop has not taken it
up. Fertiliser N can be kept for later in the
season when plants have used available
sources. In soils that regularly waterlog,
‘drip-feeding’ N by regular application is the best way to ensure it is not lost.
Useful resources
National Agricultural Nitrous Oxide Emissions Research in Australia – www.n2o.net.au
The Carbon Farming Initiative – www.mycfi.com.au
The Climate Institute – www.climateinstitute.org.au
Building Farmer and Advisor Knowledge in Carbon Farming Project – www.carbonfarmingknowledge.com.au
This project is supported with funding from the Australian Government
More information: Peter Grace, 07 3138 9283, pr.grace@qut.edu.au
www.carbonfarmingknowledge.com.au