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Predicting nitrous oxide emissions from fertiliser applications, new research released

31st May 2016

This information comes from a paper entitled “Spatially explicit estimates of N2O emissions from croplands suggest climate mitigation opportunities from improved fertiliser management” which has just been published in Global Change Biology journal and was written by Gerber et al.  To access the paper please click here.

The largest source of nitrous oxide (N2O) emissions from agriculture are synthetic Nitrogen fertiliser and manure applications to crops, which is projected to increase by around 50% from 2000 – 2050 (FAO, 2012). Between 2000 and 2011, annual N2O emissions from synthetic and manure fertiliser increased by 37% and 12% respectively.  Consequently reducing N2O emissions from croplands is critical for addressing climate change and ozone depletion concerns.

Whats the deal with N2O?

N2O is produced naturally from the processes of denitrification and microbally mediated nitrification in soils leading to emissions rates that depend on soil, weather and cropping conditions and are also highly variable over time. These emissions are classified as direct N2O emissions and are different from ‘indirect’ N2O emissions where N2O is formed from Nitrogen that is leached or volatilised from managed soils, and N2O emissions associated with land use change.

How does this link with reporting data and emissions factors?

Emissions factors are often used to relate applied nitrogen to N2O emissions across broad spatial scales.  Despite the relative ease of applying linear emissions models to estimate N2O emissions from crops, recent synthesis of field observations suggest a highly non-linear response.

What happens in the field is that nitrous oxide emissions accelerate with increased N applications. This superlinear response is due to the relatively greater excess nitrogen which is unused by the crop at higher fertilisation levels, and this extra nitrogen is therefore available to be emitted as nitrous oxide.  What has been missing from the models until now is any sub-national data or crop specific data that would allow a further understanding of what the effect is in different regions and with different crops.  This research therefore aims to try and fill some of these gaps by generating relatively accurate and crop specific nitrous oxide emissions data from global croplands as well as crop specific estimates of manure applications.  

This then provides an updated model and nitrogen fertiliser application rate to allow the researchers to calculate spatially explicit, crop specific global N2O emissions and contrast the results with what we currently have (IPCC Tier 1 accounting data). The aim (for practical reasons) should then allow for the identification of crops and regions where small changes in nitrogen application would generate large changes in nitrous oxide emissions.

What did they find?

By pinpointing crops and regions that were associated with disproportionally high or low N2O emissions it provides an opportunity to ask questions as to what the difference in management is between the two areas or whether it is  a cropping or climate issue.  

A geographical example that this model highlighted is below.

Shandong province in China emits around 4% of global cropland N2O yet comprises just 1% of crop harvested area.  Reducing nitrogen application rates by 5% in this province would cut provincial crop N2O emissions by 9% and global crop emissions by 0.35%.  Comparing this to increasing N fertilise application by 50% over sub Saharan Africa would increase N2O emissions by 2.7%.

Although there are always limitations with modelling studies, this research does provide some interesting policy recommendations.  It does show that increased fertiliser application is not strongly coupled to increased nitrous oxide emissions at low Nitrogen application rates, a major opportunity, given increased crop production is necessary to meet growing food demand. It also indicates that in areas with low N application rates, small fertiliser additions generate the most substantial yield improvements.  Conversely small reduction in fertiliser applications in high nitrogen input regions may result in substantially reduced nitrous oxide emissions from that cropland system.

Full reference

Gerber, J. S., Carlson, K. M., Makowski, D., Mueller, N. D., Garcia de Cortazar-Atauri, I., Havlík, P., Herrero, M., Launay, M., O'Connell, C. S., Smith, P. and West, P. C. (2016), Spatially explicit estimates of N2O emissions from croplands suggest climate mitigation opportunities from improved fertilizer management. Glob Change Biol. Accepted Author Manuscript. doi:10.1111/gcb.13341

To access it online, please click here