Greenhouse gas emissions from rice systems
Chris van Kessel and Bruce Linquist.
Department of Plant Sciences, University of California, Davis.
Next to wheat, rice is grown on more land than any other crop in the world and often ranks highest in worldwide annual production. Rice systems are unique from other agricultural systems in a number of ways. First, most rice is grown in flooded soils. Flooding soils alters soil chemistry which impacts the production of greenhouse gases (GHG) and nutrient use-efficiency. Secondly, rice systems use a significant amount of water. Much of the water, however, leaves the system into surface waters where it is used for drinking water, or it becomes part of the habitat for water plants and animals. Therefore, not only is the amount of water used a concern but also its quality. Third, rice lands are often used as overwintering grounds for water fowl and are important habitats during migratory seasons. Thus, rice provides other eco-system services.
Given the significance of rice it is important to develop management practices that are able to maintain high yields while at the same time minimizing adverse affects to the environment and optimizing its positive benefits. The broad issues of water use, nutrient use efficiency and GHG are all important drivers of the long-term sustainability of rice systems. Furthermore, these factors are all interrelated. For example, how one manages water affects nutrient use efficiency, water quality and GHG production.
Over the past 10 years, our research group in rice agro-ecosystems has focused on how to maximize rice yields in an efficient way. Straw burning has been largely replaced with incorporating straw which then led to a reduction in the rate of fertilizer-N input and improved water fowl habitat. Alternative seeding practices are being explored that reduce herbicide use. Questions are addressed how straw and fertilizer management practices affect the quality of run-off water, in particular water soluble organic compounds, a key concern when the water is subsequently used for drinking water. Another research question has been to explore new seeding practices to reduce weed pressure and herbicide use. Our ultimate goal is to produce rice with the least environmental impact without jeopardizing the economic livelihood of rice farmers. Components of a comprehensive framework on sustainability include soil fertility issues, emissions of GHG, water use and quality, achieving maximum yield, use of pesticides and its residual effect, and the suitability of rice land for wildlife use during the winter months.
One key component of a comprehensive framework on sustainability is the emissions of GHG. Currently, there is only limited information available on the annual amount of GHG emitted from rice fields as most GHG measurements are taking during short periods in the spring and early summer. This may give inaccurate estimates of GHG emission when extrapolated across the year. The two main GHG of interest are methane and N2O. As N2O is about 10 times more potent than methane, reducing N2O emissions could have a large effect on reducing the greenhouse gas warming potential from rice fields. However, management practices that reduce methane emissions will likely lead to an increase in N2O emissions and vice versa as they are controlled by opposing conditions related to the availability of O2.
Here we propose to develop an annual GHG budget for methane and N2O in wet and dry seeded fields. Wet and dry seeding are the two primary means of establishing rice in the US and they differ substantially in terms of early season water management and thus the potential to emit GHG. We are also looking at means to reduce water use in rice and when developed, the impact on GHG needs to be determined. Finally we are strong believers that the value of our research findings is controlled on how well the findings are applicable and implemented. Therefore, our research is conducted in farmerís fields to reflect farmerís management practices.