The primary aim of this project has been to benchmark greenhouse gas emissions across the cotton industry and develop guidelines which potentially reduce nitrous oxide, and associated non-global warming nitrogen gases, which incur a significant penalty on profitability. Due to the episodic nature of emissions, and the difficulty in accessing furrow irrigated field sites, the use of automatic greenhouse gas sampling systems is critical for field studies, however this is not always possible due to both the long-term (1-2 yr) nature of these studies and the expense of the equipment. This project has included a detailed field study on the Darling Downs (Freeser property) to verify previous field studies, and in the final year sought to benchmark field based emissions across the industry and identify industry wide guidelines for reducing emissions.

The Freeser property was specifically selected on the basis that a mixed irrigated cropping rotation (grains-cotton) is representative of diversification in the cotton industry. The mixed cropping system provides a different set of agronomic conditions to those examined in earlier cotton studies using automatic continuous sampling chambers.

Our earlier studies estimated Emission Factors (i.e. % of N emitted as N2O expressed as proportion of N added) from irrigated cotton (only) systems at Narrabri and Dalby ranging from 0.16-0.83%. The current Emission Factor recognised by the Australian Government is 0.5%, however this is based on a limited dataset. Nitrous oxide emission data collected at the Fresser property using the automatic sampling system also suggests an Emission Factor in this range, however there is evidence to suggest that the increased inputs of carbon into soil from grain based system, whilst essential for soil structure and soil carbon sequestration, may also increase the potential for gaseous N losses, therefore N management in mixed cotton-grains system needs to be carefully considered and in many cases, significantly reduced.

Benchmarking studies using a range of cotton monoculture and cotton-grains rotation soils were completed using an intact soil core laboratory incubation approach to estimate both seasonal and maximum potential N2O and total N gas loss (excluding NH3) using fertiliser enriched with the stable isotope 15N. The incubation study also provided information to develop a simple laboratory based diagnostic assay to determine the N2O loss potential and seasonal emissions for widespread use without having to undertake expensive sampling procedures in the long-term.