Nitrogen and your carbon footprint

Take home message

• The Australian grains industry applies nearly 1M t of nitrogen (N) fertiliser per annum, about two-thirds of the total consumption of N fertiliser by the country. Nitrogen fertiliser usage has increased by 36% over the past decade (FAOSTAT, 2022). In direct contrast, total production of grains across the country has levelled off, or slightly declined.
• A detailed multi-season, multi-farm study of N fertiliser recovery in sorghum grown on the Darling Downs (2015-18), found that on average 26% of the N fertiliser was permanently lost and only 39% was recovered in the crop biomass (Rowlings et al. 2022). In addition, only 21% of the N applied was actually recovered in the crop at harvest with soils supplying nearly 80% of the N for growth, a prime example of N fertiliser inefficiency.

Background

Denitrification is the primary source of gaseous N loss in the high clay Vertosol which is the dominant soil type used for grain production in northern New South Wales and Queensland. The gaseous products include the inert di-nitrogen (N₂) and nitrous oxide (N₂O), the latter being only a small fraction of the total N loss (e.g., 1 kg per 100 kg N applied is emitted as N₂O compared to 10+ kg N as N₂). Nitrous oxide is a potent greenhouse gas, contributing 4% of Australia’s national greenhouse gas emissions in terms of its Global Warming Potential (GWP). Australia’s agricultural sector is the primary emitter of N₂O, producing approximately 60% of Australia’s total emissions per annum. Within the agriculture sector of the National Greenhouse Gas Inventory (NGGI), soils produce 95% of the N₂O, mainly from emissions associated with application of N fertilisers as well as N₂O emitted from residue decomposition.

The current NGGI (2019) estimates 8 Mt of carbon dioxide-equivalents (CO₂e) emissions are directly emitted from Australian soils from the application of N fertilisers, across all commodities (grains, livestock, horticulture, cotton, cane). One N₂O molecule is equivalent to the climate warming effect (GWP)of 273 CO₂ molecules. The GWP conversion factor is used to ensure we can account for all greenhouse gases (CO₂, methane (CH₄) and N₂O) into a single currency (CO₂e) to enable  standardised reporting of greenhouse gas emissions. There is a direct relationship between the amount of N applied as fertiliser and N₂O emissions. In the majority of cases, the emissions are a proportion of the N rate which is known as the emission factor (EF) (i.e., the amount of N applied emitted as N₂O-N).  The EF varies by commodity sector and currently ranges from 0.2% for dryland grain cropping to 2% in sugar cane.

For example, in sugar cane 2% of the applied N rate is emitted as N₂O-N. If 200 kg N is applied, 4 kg N is directly emitted as N₂O-N which is 6.3 kg N₂O (converting it to the N₂O molecule itself). This emission is then converted to a value which allows us to determine its impact on the atmosphere in terms of its warming effect. So, if 200 kg N is applied in sugar cane production, the N₂O emissions are equivalent to 1716 kg CO₂ in terms of its warming impact on the atmosphere. As a means of comparison, this amount of nearly 2 t CO₂ is the same emissions as if you drove a motor vehicle for 10,000 km.

In dryland grains there are soil and climatic considerations which have increasing relevance on the magnitude of N₂O emissions. The 0.2% is a weighted average of the low N₂O emitting Western Grains Zone (and its sandier soils) and Northern and Southern Grains Zones. In particular, the Northern Grains Zone with its high clay soils has a higher potential compared to the west.

Your carbon footprint

In developing a carbon footprint for your farm, all sources of N₂O emissions must be estimated. The N₂O emissions are normally calculated using equations outlined in the NGGI. These include ‘direct’ emissions - inorganic and organic N fertilisers, urine and dung from grazing animals, crop residue decomposition, the mineralisation of soil organic matter and the cultivation of organic soils (peats) which is rarely the case in grains. Also included are ‘indirect’ emissions i.e., N₂O emitted from atmospheric deposition (of NH₃) from N fertilisers and manures or from nitrate leaching. The primary calculation is the direct emissions of N₂O from the application of N fertilisers. Through research funded by GRDC and associated Rural Development Corporations and the federal government’s Carbon Farming Initiative, the Australian grains industry has one of the most comprehensive data sources for N₂O emission from N fertilisers in the world. This data collected using automated measuring field systems (Grace et al. 2020) has been used to calibrate and validate the APSIM soil-crop simulation model for grains systems of Australia (Mielenz et al. 2016a, 2016b, 2017).

The primary deficiency in the NGGI is the lack of country specific data for the EF’s associated with the N₂O from the decomposition of crop residues and the partitioning of indirect emissions from atmospheric deposition (volatilisation) and leaching. The use of ‘default’ EFs from the international literature as prescribed by the Intergovernmental Panel of Climate Change (IPCC) may significantly over-estimate the N₂O emissions from the Australian grains industry.

Reducing N₂O emissions

There are four agronomic management interventions that can be used either singularly or in combination to potentially reduce N₂O emissions. Reducing the N rate itself; delayed or split applications of N fertiliser so that it coincides with plant uptake, leaving the applied N less prone to losses (from leaching or denitrification); placement of the N fertiliser in bands below the surface where it is less susceptible to loss; and/or apply a specific N fertiliser or product which slows the production of nitrate (NO₃^-), the primary precursor to N₂O.

In Australia, the preferred N fertiliser is urea. There is ample evidence in grain systems (e.g., De Antoni Migliorati et al. 2014 and 2016; Scheer et al. 2016. Schwenke et al. 2019a, 2019b) that the use of 3,4-Dimethylpyrazole phosphate (DMPP) with urea can significantly reduce the production of N₂O by 79% and not have a significant impact on production (Figure 1). This is not widely adopted due to cost.

Figure 1. Sorghum grain yield and N₂O production in the Northern Grains Zone in response to different N fertiliser products

References

De Antoni Migliorati M, Scheer C, Grace P, Rowlings D, Bell M, McGree J (2014) Influence of different nitrogen rates and DMPP nitrification inhibitor on annual N₂O emissions from a subtropical wheat–maize cropping system. Agriculture, Ecosystems Environment 186, 33-43.

De Antoni Migliorati M, Bell M, Lester D, Rowlings D, Scheer^, C, de Rosa D, Grace P (2016) Comparison of grain yields and N₂O emissions on Oxisol and Vertisol soils in response to fertiliser N applied as urea or urea coated with the nitrification inhibitor 3,4-dimethylpyrazole phosphate. Soil Research 54, 552-564.

FAOSTAT (2022) Statistical Division of the UN Food and Agriculture Organization (FAO). http://faostat.fao.org/

Grace PR, van der Weerden TJ, Rowlings DW, Scheer C, Brunk C, Kiese R, Butterbach‐Bahl K, Rees RM, Robertson GP, Skiba UM (2020) Global Research Alliance N₂O chamber methodology guidelines: Considerations for automated flux measurement. Journal of Environmental Quality 49, 1126-1140.

Mielenz H, Thorburn PJ, Harris RH, Officer SJ, Li G, Schwenke GD, Grace PR (2016a) Nitrous oxide emissions from grain production systems across a wide range of environmental conditions in eastern Australia. Soil Research 54, 659-674.

Mielenz H, Thorburn PJ, Harris RH, Grace PR, Officer SJ (2017) Mitigating N₂O emissions from cropping systems after conversion from pasture− a modelling approach. European Journal of Agronomy 82, 254-267.

Mielenz H, Thorburn PJ, Scheer C, de Antoni Migliorati M, Grace PR, Bell MJ (2016b) Opportunities for mitigating nitrous oxide emissions in subtropical cereal and fibre cropping systems: A simulation study. Agriculture, Ecosystems & Environment 218, 11-27.

Rowlings D, Lester D, Grace P, Scheer C, de Rosa D, De Antoni Migliorati M, Friedl J, Bell M (2022) Seasonal rainfall distribution drives nitrogen use efficiency and losses in dryland summer sorghum. Field Crops Research 283, 108527.

Scheer C, Rowlings DW, De Antoni Migliorati M, Lester DW, Bell MJ, Grace PR (2016) Effect of enhanced efficiency fertilizers on nitrous oxide emissions in a sub-tropical cereal cropping system. Soil Research 54, 544-551.

Schwenke G, Haigh M (2019a) Can split or delayed application of N fertiliser to grain sorghum reduce soil N₂O emissions from sub-tropical Vertosols and maintain grain yields? Soil Research 57, 859-874.

Schwenke G, Haigh B (2019b). Urea-induced nitrous oxide emissions under sub-tropical rain-fed sorghum and sunflower were nullified by DMPP, partially mitigated by polymer-coated urea, or enhanced by a blend of urea and polymer-coated urea. Soil Research 57, 342-356.

Contact details

Peter Grace
Queensland University of Technology
Ph:  07 3138 9283
Email: pr.grace@qut.edu.au