Azolla (Azolla filiculoides) is a common aquatic fern that has been used successfully as a dual crop with lowland rice. It grows rapidly and has the ability to fix N 2 for rice paddy. However, its ecological significance especially on greenhouse gases emissions remains unclear. To investigate the effect of azolla cover on methane (CH 4 ) and nitrous oxide (N 2 O) emissions from rice paddy, a pot experiment with two treatments, control (rice plant only) and azolla cover (rice plus azolla covering on the flooding water), was carried out in Tsuruoka, Yamagata, Japan, in 2016. The results showed that the rice growth parameters, like shoot height, maximum and productive tiller numbers, and plant biomass were not significantly different between the two treatments. Dual cropping of azolla with rice significantly suppressed CH 4 emissions, likely due to an increase in dissolved oxygen concentration and redox potential at the soil-water interface between flooding water and soil surface. There were significant (P < 0.05) positive correlations between CH 4 flux and night respiration (CO 2 emissions) between the two treatments. The cumulated CH 4 emissions during the growth period until 106 days after transplanting (DAT) was significantly lower at 36.2 g C m −2 from azolla cover treatment than that from control treatment pot at 55.4 g C m −2 . A prolonged nonsignificant N 2 O emission under the azolla cover treatment after the initial highest peak at 15 DAT was recorded due to denitrification of the nitrate in initial soil. No further N 2 O emissions were recorded thereafter from both treatments. Azolla cover did not affect N 2 O emissions from both treatments. ARTICLE HISTORY

To investigate the influence of Azolla (A. filiculoides Lam.) incorporated as a green manure and its subsequent growth as a dual crop with rice on simultaneous methane (CH 4 ) and nitrous oxide (N 2 O) emissions from a flooded alluvial soil planted with rice, a pot experiment with three treatments, chemical fertilizers (NPK) as the control, incorporation of Azolla as green manure (AGM), and AGM plus basal chemical fertilizers (NPK+AGM) was conducted in Tsuruoka, Yamagata, Japan in 2017. AGM and NPK +AGM treatments significantly increased CH 4 emissions at early rice growth stages before 63 days after transplanting (DAT) by 123.0% and 176.7% compared to NPK, respectively. At late rice growth stages (after 63 DAT), only the NPK+AGM treatment significantly increased CH 4 emission by 22.1% compared to NPK. However, percentage of CH 4 emitted after 63 DAT relative to the seasonal CH 4 emission followed the order of NPK (86.2%) > AGM (76.5%) > NPK+AGM (73.3%). Higher CH 4 emissions from AGM and NPK +AGM before 63 DAT were attributed to the incorporated Azolla, while the higher emissions after 63 DAT in all treatment groups were ascribed to rice photosynthesis. AGM and NPK+AGM treatments significantly decreased N 2 O emissions by 71.6% and 81.1% compared to NPK, respectively, at early rice growth stages. Azolla incorporation may have restricted N 2 O emission from initial soil nitrate before 63 DAT and not have contributed to N 2 O emissions after 63 DAT. Significantly higher grain yields were observed under the AGM (32.5%) and NPK+AGM (36.3%) compared to NPK. Together, AGM and NPK+AGM treatments significantly increased seasonal CH 4 emissions by 31.5% and 43.5%, and decreased seasonal N 2 O emissions 3.4-and 4.6-fold compared to NPK, respectively. There were no significant differences in the CH 4 emissions per grain yield among the treatments; however compared to NPK, AGM and NPK +AGM treatments significantly reduced N 2 O emissions per grain yield by 78.7% and 84.1%, respectively.

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To determine methane (CH 4) emission differences between edible and forage rice cultivars, we conducted a pot experiment in Yamagata, Japan to grow edible rice Haenuki, and forage rice Fukuhibiki (for feed rice) and Tachisuzuka (for whole-crop silage (WCS) rice) under similar soil and meteorological conditions. The total amounts of N, P, and K fertilizers applied for Fukuhibiki and Tachisuzuka were 1.7, 1.3, and 1.3 times, respectively, higher than those of Haenuki. CH 4 fluxes, and rice plant night respirations were measured once weekly or fortnightly. As per the results, for the whole growth period, shoot height, maximum and productive tiller numbers, and plant biomass were significantly different among the three rice varieties. The rice growth period for Haenuki and Fukuhibiki was 107 days after transplanting (DAT), while that for Tachisuzuka was 135 DAT. The highest peak of CH 4 flux occurred around the heading stage for the three varieties. Consistently significant (P < 0.05) or obvious (P < 0.1) positive correlations between CH 4 flux and night respiration among the varieties were observed from 9 weeks after rice transplanting to harvest, indicating that much of the CH 4 flux was from newly produced root exudates and plant debris through plant photosynthesis. The cumulated CH 4 emissions during the same growth period, 106 DAT, from Haenuki, Fukuhibiki, and Tachisuzuka were 55.36, 77.46, and 78.40 g C m −2 , respectively. Additionally, Tachisuzuka emitted 25.11 g C m −2 more CH 4 between 106-134 DAT. The final cumulated CH 4 emissions from Fukuhibiki and Tachisuzuka were 39.9% and 87.0% higher than that from Haenuki, respectively, throughout their growth period.