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 N₂ for rice paddy. However, its ecological significance especially on greenhouse gases emissions remains unclear. To investigate the effect of azolla cover on methane (CH₄) and nitrous oxide (N₂O) 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 CH₄ 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 CH₄ flux and night respiration (CO₂ emissions) between the two treatments. The cumulated CH₄ 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 N₂O 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 N₂O emissions were recorded thereafter from both treatments. Azolla cover did not affect N₂O emissions from both treatments.

Hot-water- and water-extractable organic matter were obtained from soil samples collected from a rice paddy 31 years after the start of a long-term rice experiment in Yamagata, Japan. Specifically, hot-water-extractable organic carbon and nitrogen (HWEOC and HWEON) were obtained by extraction at 80°C for 16 h, and water-extractable organic carbon and nitrogen (WEOC and WEON) were obtained by extraction at room temperature. The soil samples were collected from surface (0–15 cm) and subsurface (15–25 cm) layers of five plots that had been treated with inorganic fertilizers alone or with inorganic fertilizers plus organic matter, as follows: PK, NPK, NPK plus rice straw (RS), NPK plus rice straw compost (CM1), and NPK plus a high dose of rice straw compost (CM3). The soil/water ratio was 1:10 for both extraction temperatures. We found that the organic carbon and total nitrogen contents of the bulk soils were highly correlated with the extractable organic carbon and nitrogen contents regardless of extraction temperature, and the extractable organic carbon and nitrogen contents were higher in the plots that were treated with inorganic fertilizers plus organic matter than in the PK and NPK plots. The HWEOC and WEOC δ¹³C values ranged from −28.2% to −26.4% and were similar to the values for the applied rice straw and rice straw compost. There were no correlations between the HWEOC or WEOC δ¹³C values and the amounts of HWEOC or WEOC. The δ¹³C values of the bulk soils ranged from −25.7% to −23.2% and were lower for the RS and CM plots than for the PK and NPK plots. These results indicate that HWEOC and WEOC originated mainly from rice plants and the applied organic matter rather than from the indigenous soil organic matter. The significant positive correlations between the amounts of HWEOC and HWEON and the amount of available nitrogen (P < 0.001) imply that extractable organic matter can be used as an index for soil fertility in this long-term experiment. We concluded that the applied organic matter decomposed more rapidly than the indigenous soil organic matter and affected WEOC δ¹³C values and amounts.

We investigated temporal changes in paddy rice phenology and yield under climatic and varietal changes in Shonai region, Yamagata Prefecture, northeast Japan across a period of 1982–2017. In the study region, the dominant variety shifted from Sasanishiki to Haenuki in mid 1990s. Temporal trends across the study period were significant in mean air temperatures for the months from May to July, and the temperature rise has accelerated heading, which was delayed by the varietal shift. We built models of the effects of climatic and varietal changes on phenological events and yield components: number of grains per land area, percentage ripened grains, and 1000-grain weight. Combining the yield component models, we built a yield model, with which we estimated the effect of the temperature rise from May to July on the yield. The temperature rise in May and June has reduced the yield by accelerating phenology, while that in July has increased the yield by raising 1000-grain weight. The net effect was no change in the yield across the study period, which, however, does not indicate an insensitivity of the yield to the temperature rise. Rather, the finding points to the dependency of the net change in the yield on seasonal pattern of the climate change. Climate change impacts on rice should therefore be quantified on the changes at monthly or shorter time scale rather than a seasonal mean. Our finding also points to the need to pay attention to the changes in agronomic practices such as varieties.