Rice FACE Studies for Improving Assessment of Climate Change Effects On Rice Agriculture.

Impacts of climate change on the crop production are results of the combined effects of climate factors and management options including selection of varieties and nutrient managements. Many of the existing crop models are capable of simulating the effects of climate and managements options, but have not been fully tested for their ability to simulate the combined effects in the open fields. Testing the crop models at multiple sites is sometimes useful for the general crop responses to climate but does not allow for the effects of atmospheric CO₂ concentration ([CO₂]), which is projected to increase to 470-570 µmol mol^-1 by the middle of the century. Free-air CO₂ enrichment (FACE) is so far the best approach in determining the CO₂fertilization effect under open field conditions. For rice (Oryza sativa L.), the first FACE experiment was started at Shizukuishi, the northern part of Japan in 1998. While Shizukuishi FACE showed important interaction between elevated [CO₂] and other factors such as genotype, nitrogen and temperature, the results obtained in one environment must be confirmed by multi-environment experiments. We have therefore established a new FACE site in Tsukuba, Ibaraki, Japan, about 430 km south of the Shizukuishi FACE site. The annual mean temperature at the Tsukuba FACE site is more than 4°C warmer than at the Shizukuishi site, so FACE experiments using the same set of cultivars at both sites provide a unique opportunity to test whether the CO₂ responses of the cultivars are similar over widely different environments. The comparison across different FACE sites using a common variety (Akitakomachi) suggested that yield enhancement was limited under both warm and cool conditions; Under global warming, yield advantage due to higher [CO₂] may diminish. Opportunities exist, however, for improving rice productivity under future [CO₂]; yield enhancements due to elevated [CO₂] differed largely between cultivars ranging from 3 to 36%. Higher-yielding cultivars with a large sink size showed a greater [CO₂] response. We also tested the effects of soil and water warming inside the FACE treatment and showed that substantial increases in methane emission from the paddy fields. Rice models need to account for these important interactions to predict likely impacts of climate change on rice agriculture.