Conventional methods of rice cultivation involve continuous flooding of the fields for the majority of the growing season. This practice suffocates the soil, and produces an environment where methane-producing archaea thrive, resulting in significant methane emissions. Because of the high methane emissions of this conventional growing method, and the widespread nature of rice farming, rice agriculture is responsible for 10% of global methane emissions.
Although rice agriculture contributes significantly to global warming, there are simple solutions available to mitigate these emissions - AWD and SRI are two of the best solutions.
AWD, or Alternate Wetting and Drying address the root cause of rice methane emissions - the flooding of the rice fields and the suffocation of the soil. Practising AWD means that the farmer alternates between flooding the rice field, and leaving it to dry throughout the growing season. This allows the soil to breathe, which reduces the methane-producing archaea and the methane emissions that they cause. The irrigation regime is the only practice that is changed.
SRI - the System of Rice Intensification, is a more holistic approach to climate change mitigation through rice agriculture. Farmers that grow rice using SRI utilise AWD for irrigation, but also utilise three other pillars to bring an array of benefits; optimising spacing between plants to minimise competition, transplanting only young seedlings, and building up healthy and fertile soil.
The majority of greenhouse gas emissions from rice agriculture come from the unnatural, continuous flooding of the fields. If this component of rice agriculture is removed, as it is with both SRI and AWD, then methane emissions are significantly reduced, by between 22% and 64%.
However, it should be noted that when AWD is used, either on its own, or as part of SRI, the aeration of the soil causes nitrous oxide emissions to increase. This is because nitrous oxide cannot be produced in low-oxygen soil but is produced in soil that has a high oxygen content. It is important that this increase in nitrous oxide is kept as small as possible so that the methane reductions are not offset by the nitrous oxide increases. Studies have shown that as SRI involves a reduction in fertiliser input, these nitrous oxide emissions can actually be reduced as there is less nitrogen in the soil for nitrous oxide to be produced from. This is not the case with AWD alone, as this method does not dictate a change in fertiliser applications.
SRI and AWD produce roughly the same reduction in greenhouse gas emissions emitted from the field, but when one begins to consider carbon sequestration, the numbers change dramatically. As we said earlier, SRI is a holistic approach to rice growing that actually changes the physiology of the rice plant. This change in physiology is realised in part, by an expansion of the root systems of each rice plant. These expanded root systems take carbon from the atmosphere and deposit it in the soil - increasing the soil organic carbon content, and health of the soil.
Quantifications show that while AWD (and SRI too) can reduce emissions from rice agriculture by nearly 5 tonnes of CO2 equivalent emissions per hectare, per year, the increase in soil organic carbon observed under SRI can sequester 75 tonnes of CO2 equivalent emissions per hectare per year more than AWD and other conventional rice growing practices thanks to the increased root network of SRI. This increase in sequestered emissions not only takes more harmful CO2 out of the atmosphere, but increases the health of the soil, as well as increasing protection against floods and droughts.
As discussed above, the emissions reduction seen under SRI management are largely the same as those seen under AWD, with the main difference being seen in carbon sequestration in agricultural soils. However, SRI also brings a host of other benefits when compared to AWD that are unrelated to climate and benefit the local farmers themselves. Yields are increased typically by 20-50%, and often by 100% or more. As a result of this, and reduced seed costs, farmer income is increased.
The expanded root systems also produce higher quality grain and plants that are more resistant to both biotic and abiotic stressors. This same expanded root network is also responsible for SRI rice crops being more climate change resilient due to their resistance to extreme weather events. These extreme weather events are projected to become increasingly common with climate change.
AWD brings global benefits but SRI brings global and local benefits.
Multiple organisations have recognised the need for more climate friendly rice agriculture and have begun to promote change through a number of different methods. The major focus for these organisations is often climate mitigation and therefore they are most conscious of reducing the methane emissions from rice agriculture as quickly and as widely as possible. When focussing solely on this metric, the implementation of AWD can seem more attractive than SRI - methane savings are broadly the same between the two practices, and AWD is seen as the simpler option.
However, it is this focus on a singular metric that means so many potential benefits are lost. Even when looking solely at climate change mitigation, through spreading AWD only, and not SRI, the huge potential of soil carbon sequestration via SRI is not achieved. In addition to this, all the other potential social and developmental benefits that could be achieved through the spread of SRI rather than AWD are lost.
Climate altering emissions from rice agriculture are an issue that the world needs to address. However, any change to rice agriculture needs to be done carefully so that reducing the harmful climate impacts does not cause a food security crisis. In the quest for rice agriculture change for global climate change impacts it seems unfair that the local farmers that implement new rice growing practices would not benefit from it - this is where SRI comes in - providing global change whilst also benefiting those who do the work.
AWD is a good climate solution, but SRI is the greatest.