Measurement of methane and nitrous oxide emissions from Australian rice grown under conventional and water saving irrigation practises.

Rice production feeds > 50% of the world population with 250 million tonnes consumed in 2022 and is expected to continue to rise by a further ~6% by 2030. Favourable climate and soil conditions for growing temperate rice, together with low disease pressure and advanced irrigation systems enables Australia to achieve some of the highest rice yields in the world with low resource inputs. However, currently there remains a lack of complete season baseline datasets for greenhouse gas emissions from Australian rice crops. National and regionally specific greenhouse gas accounting and the global warming potential and mitigation strategies for these cropping systems remain unclear.  Furthermore, recent innovative irrigation and water management practices utilizing low-cost, technology driven irrigation automation in Australia now indicates the potential to further significantly change how rice is grown. Practical implementation of alternate rice growing irrigation techniques, in which the soil is kept between 0 to -20 kPa, without water being permanently ponded during the growing season have been enabled, producing commercial crops of > 13 Mg ha^-1. However, these conditions may lead to ‘tradeoff’ emissions of nitrous oxide (N₂O), an even more potent greenhouse gas than the more ubiquitous methane (CH₄) emissions commonly associated with rice crops. Two rice water management techniques have been compared in the 2023-2024 Austral Summer: i) Conventional drill sown (DIR) in which planted seeds are flushed with water 3-4 times until the crop has developed to the 4^th leaf stage (up to 50 days after the first irrigation) followed by continuous flooding until drainage pre-harvest. ii) Water saving practise, locally known as aerobic (AER) in which the crop is flushed intermittently throughout the entire season with standing water being avoided. Methane and N₂O emissions have been monitored in commercial fields using non-steady state closed chambers followed by gas chromatography (GC) and a newer laser-based method, optical feedback-cavity enhanced absorption spectroscopy (OF-CEAS). The AER system reduced seasonal CH₄ emissions to 1.3 kg CH₄-C ha^-1 from 32 kg CH₄-C ha^-1 that were determined in the DIR system. Although, high N₂O-N emission peaks of up to 1043 µg m^-2 h^-1 were recorded, associated with rainfall and fertilizer application events, total seasonal fluxes suggest that the adoption of this alternative irrigation practise can reduce the global warming potential of rice crops by 51% compared with conventional management. Because both crops were managed for yield potential, when gas emissions were related to rice productivity, yield scaled emissions were 97 kg CO₂eq Mg^-1 season^-1 (DIR) and 47 kg CO₂eq Mg^-1 season^-1 (AER), the lowest that have ever been recorded globally.