Quantifying greenhouse gas emissions from the decomposition of crop residue

The grains baseline Greenhouse Gas (GHG) report (Sevenster et al., 2022) indicated on-farm (Scope 1) GHG emissions from crop residues account for 20.4% of total emissions, of which 89% is due to decomposition of residues left in the field. In the most recent (April 2023) National Inventory Report (NIR) submitted to the UN Framework Convention on Climate Change (UNFCCC), agricultural soils accounted for 15% (CO2-e) of total emissions and were the second largest source within the agricultural sector. Data show an upward trend in agricultural soil emissions over time largely due to an increase in fertiliser Nitrogen (N) use and an increasing trend of retention of crop residues. The lack of country specific crop residue decomposition data is a major deficiency in the NIR which may lead to an overestimation of N2O emissions from agricultural soils. At present, the IPCC (2006) default value for N2O Emissions Factor (EF) from crop residues (1%) is used in the NIR, a value based on a large global dataset (Bouwman et al., 2002) which is biased towards mid-latitude, temperate regions, with numerous studies showing deviations from the default under different conditions (Rochette et al., 2018). The disaggregated EF value for dry climates (0.5%) is due to be adopted in the 2024 NIR, however, there are opportunities for this value to be further disaggregated (IPCC 2019). This disaggregation can be based on environmental factors (e.g., climate, soil C content, soil pH, etc) and management-related factors (e.g., N application rate per fertiliser type, fertiliser type, crop type – with differences between legumes and non-legume crops, etc). For EF revisions to be considered and included in the NIR, they must be underpinned by sound scientific evidence, including robust field-based measurements.

The Australian Government reports on its emissions inventory to the UN on an annual basis and is free to use country specific data in its calculations. References to published literature are critical, as values can be questioned by agricultural sector expert panel members. There is a dearth of information or field-based data, particularly within the Australian context, on N2O emissions from crop residue decomposition, meaning there remain critical knowledge gaps. This investment will generate the underpinning scientific evidence necessary for a revision of EFs to be considered and potentially accepted for use in the NIR. Quantification of GHG emissions from crop residues using field-based measurements and crop attribute data that feed into GHG inventory calculations across a cross-section of crop types (cereals, oilseeds, legumes), climates and soils will better reflect the Carbon footprint of Australian grain growers and the sustainability credentials of the Australian grains industry.

References:

Bouwman et al (2002) Emissions of NO and NO from fertilized fields: summary of available measurement data. Global Biogeochemical Cycles, 16: 1058.

IPCC (2019) Chapter 11: NO emissions from managed soils, and CO emissions from lime and urea application. In ‘2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories’, Calvo Buendia, E., Tanabe, K., Kranjc, A., Baasansuren, J., Fukuda, M., Ngarize, S., Osako, A., Pyrozhenko, Y., Shermanau, P. and Federici, S. (eds). Published: IPCC, Switzerland.

Rochette et al (2018) Soil nitrous oxide emissions from agricultural soils in Canada: Exploring relationships with soil, crop and climatic variables. Agriculture, Ecosystems and Environment, 254: 69-81.

Sevenster et al (2022) Australian Grains Baseline and Mitigation Assessment. Main Report. CSIRO, Australia