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Agricultural soil is the largest source of nitrous oxide (N2O), a greenhouse gas. Soybean is an important leguminous crop worldwide. Soybean hosts symbiotic nitrogen-fixing soil bacteria (rhizobia) in root nodules. In soybean ecosystems, N2O emissions often increase during decomposition of the root nodules. Our previous study showed that N2O reductase can be used to mitigate N2O emission from soybean fields during nodule decomposition by inoculation with nosZ++ strains [mutants with increased N2O reductase (N2OR) activity] of Bradyrhizobium diazoefficiens. Here, we show that N2O emission can be reduced at the field scale by inoculation with a mixed culture of indigenous nosZ+ strains of B. diazoefficiens USDA110 group isolated from Japanese agricultural fields. Our results also suggested that nodule nitrogen is the main source of N2O production during nodule decomposition. Isolating nosZ+ strains from local soybean fields would be more applicable and feasible for many soybean-producing countries than generating mutants.

A laboratory study was conducted to study the effects of liming and different biochar amendments on N2O and CO2 emissions from acidic tea field soil. The first experiment was done with three different rates of N treatment; N 300 (300 kg N ha-1), N 600 (600 kg N ha-1) and N 900 (900 kg N ha-1) and four different rates of bamboo biochar amendment; 0%, 0.5%, 1% and 2% biochar. The second experiment was done with three different biochars at a rate of 2% (rice husk, sawdust, and bamboo) and a control and lime treatment (dolomite) and control at two moisture levels (50% and 90% water filled pore space (WFPS)). The results showed that dolomite and biochar amendment significantly increased soil pH. However, only biochar amendment showed a significant increase in total carbon (C), C/N (the ratio of total carbon and total nitrogen), and C/IN ratio (the ratio of total carbon and inorganic nitrogen) at the end of incubation. Reduction in soil NO3--N concentration was observed under different biochar amendments. Bamboo biochar with the rates of 0.5, 1 and 2% reduced cumulative N2O emission by 38%, 48% and 61%, respectively, compare to the control soil in experiment 1. Dolomite and biochar, either alone or combined significantly reduced cumulative N2O emission by 4.6% to 32.7% in experiment 2. Reduction in N2O production under biochar amendment was due to increases in soil pH and decreases in the magnitude of mineral-N in soil. Although, both dolomite and biochar increased cumulative CO2 emission, only biochar amendment had a significant effect. The present study suggests that application of dolomite and biochar to acidic tea field soil can mitigate N2O emissions.

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Water-intensive systems of rice cultivation are facing major challenges to increase rice grain yield under conditions of water scarcity while also reducing greenhouse gas (GHG) emissions. The adoption of effective irrigation strategies in the paddy rice system is one of the most promising options for mitigating GHG emissions while maintaining high crop yields. To evaluate the effect of different alternate wetting and drying (AWD) irrigation strategies on GHG emissions from paddy rice in dry and wet seasons, a field experiment was conducted at the Tamil Nadu Rice Research Institute (TRRI), Aduthurai, Tamil Nadu, India. Four irrigation treatments were included: One-AWD (one early drying period), Two-AWD (two early drying periods), Full-AWD (wetting and drying cycles throughout the rice season), and CF (continuous flooding). Different rice varieties were also tested in the experiment. In this study, we emphasized one factor (irrigation effect) that affects the dependent variable. The results show that early AWD treatments reduced methane (CH 4 ) emissions by 35.7 to 51.5% in dry season and 18.5 to 20.1% in wet season, while full-AWD practice reduced CH 4 emissions by 52.8 to 61.4% compared with CF. Full-AWD in dry season not only significantly reduced CH 4 emission during that season, it also resulted in the decline of the early season emission in the succeeding wet season. Global warming potential (GWP) and yield-scaled GWP were reduced by early or full season AWD in both rice seasons. The GWP value from nitrous oxide (N 2 O) was relatively low compared to that from CH 4 in both rice seasons. Rice yield was not affected by irrigation treatments although varietal differences in grain and straw yields were observed in both rice seasons. This study demonstrated that early season water managements are also effective in reducing CH 4 and total GHG emissions without affecting rice yield.

Lime-nitrogen (calcium cyanamide, CaCN 2 ) is used as a nitrogenous fertilizer, pesticide, and herbicide. During the process of decomposition of lime-nitrogen in the soil, dicyandiamide (DCD), a nitrification inhibitor, is formed. Therefore, lime-nitrogen application may mitigate nitrous oxide (N 2 O) emission from the soil. We conducted a field experiment to investigate the effect of lime-nitrogen on nitrification and N 2 O emission in fertilized soils, and a soil incubation experiment for further analysis of the effect of the limenitrogen. In a field experiment we compared four nitrogen (N) fertilizer treatments: CF (chemical fertilizer), LN100 (application of all N fertilizer as lime-nitrogen), LN50 (application of 50% of N as lime nitrogen and the remainder as chemical fertilizer), and CFD (chemical fertilizer with DCD). In a soil incubation experiment, we also studied two nitrogen treatments: CF and lime-nitrogen. Soil nitrification activity was lower in the LN100, LN50, and CFD plots than in the CF plot. The duration of this reduction in soil nitrification activity was longer in the LN100 plot than in the other plots. We found an apparent decrease in the N 2 O emission rate between 7 and 14 days after fertilization in the LN100, LN50, and CFD plots compared with that in the CF plot. This period of decreased N 2 O emission paralleled that when DCD was detected in the topsoil layers of the former three plots. Moreover, in the soil incubation experiment, cumulative N 2 O emission was significantly lower in the lime-nitrogen treatment than in the CF treatment, although the difference in cumulative N 2 O emission among the plots was not significant in the field experiment. Correlation analysis suggested that application of lime-nitrogen affects N 2 O emission by controlling both the first (ammonium to nitrite) and the second (nitrite to nitrate) soil nitrification reactions, whereas DCD blocks only the first nitrification reaction.

Small single-celled Chaetoceros sp. are often widely distributed, but frequently overlooked. An estuarine diatom with an extremely high growth potential under optimal conditions was isolated from the Shinkawa-Kasugagawa estuary in the eastern part of the Seto Inland Sea, western Japan. It was identified as Chaetoceros salsugineum based on morphological observations. This strain had a specific growth rate of 0.54 h(-1) at 30°C under 700 μmol · m(-2)  · s(-1) (about 30% of natural maximal summer light) with a 14:10 L:D cycle; there was little growth in the dark. However, under continuous light it grew at only 0.35 h(-1) or a daily specific growth rate of 8.4 d(-1) . In addition, cell density, chlorophyll a, and particulate organic carbon concentrations increased by about 1000 times in 24 h at 30°C under 700 μmol · m(-2)  · s(-1) with a 14:10 L:D cycle, showing a growth rate of close to 7 d(-1) . This very rapid growth rate may be the result of adaptation to this estuarine environment with high light and temperature. Thus, C. salsugineum can be an important primary producer in this estuary in summer and also an important organism for further physiological and genetic research.