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Since rotation of irrigated rice (Oryza sativa L.) and upland crop cultivations is a major land use management widely conducted in Japan, dynamics of greenhouse gas emission from drained paddy fields are required to be clarified. In the present study, the seasonal courses of [N.sub.2]O flux from a drained paddy field were measured continuously over one and a half years with an automated monitoring system at an interval of six times per day. The experimental paddy field with Gray lowland soil was drained and cultivated with two upland cropping systems: single cropping of upland rice and double cropping of soybean [Glycine max (L.) Merrill] and wheat (Triticum aestivum L.). Unlike many of the previous studies, the increase in [N.sub.2]O flux after fertilizer application was not distinctive, which suggests that the contribution of nitrification to the [N.sub.2]O flux is relatively small in this field. Temporal peaks of [N.sub.2]O flux were often observed after heavy rainfalls and harvest of the crops, which lasted for a few to approximately 15 d. In all the crop cultivations, high peaks of [N.sub.2]O flux were observed when the crops were in the flowering to ripening stages, which suggests significant influence of crop development on [N.sub.2]O flux. [N.sub.2]O production in the rhizosphere or possible pathway for transport through the plant body is possible loss mechanisms. In particular, the amounts of [N.sub.2]O flux during the flowering and ripening stages of the summer crops were significant and were most responsible for the cumulative [N.sub.2]O emissions. Cumulative [N.sub.2]O emissions increased 4.0 to 5.3 times by the land use change from paddy rice cultivation to upland crop cultivations.

Our previous pot experiments showed that using Azolla either or both as dual and green manure with rice increases its yield or significantly reduces either or both methane (CH4) and nitrous oxide (N2O) emissions. To confirm these findings in an actual field, Azolla was either grown as a dual crop (herein Cover) or incorporated as green manure plus dual cropping (herein AGM + Cover) at the beginning of the experiment along with rice. Compared with the control (chemical fertilizer; herein NPK), NPK + Cover and AGM + Cover treatments did not influence cumulative CH4 emissions throughout the rice growth period. However, AGM + Cover treatment affected significant CH4 emissions at early, middle, and later rice growth stages by 140.6%, 24.6%, and 33.1%, respectively, compared with NPK + Cover treatment. These emissions were attributed to the readily available carbon substrate for methanogens following the incorporation of Azolla as green manure. Compared with NPK, NPK + Cover and AGM + Cover significantly increased N2O emissions by 645.9% and 816.2%, respectively, during the middle rice growth stage. No significant N2O emission differences were observed in the three treatments in the early or later rice growth stages. The higher N2O emissions from the middle rice growth stage were ascribed to high substrate availability from the dead Azolla by higher summer air temperature in the 2019 season. AGM + Cover significantly decreased rice yield by 37.5% (NPK) and 35.3% (NPK + Cover), with no significant differences between NPK and NPK + Cover. This reduction was attributed to nitrogen immobilization from the incorporated Azolla during the early stage. Therefore, to ascertain the potential of Azolla in paddy fields to address environmental safety while sustaining yield, emphasis on the interaction of different application methods with various management practices is necessary. [ABSTRACT FROM AUTHOR]

Composts are an attractive organic-farming alternative to chemical fertilizers and are used for carbon sequestration to increase soil carbon storage. In Japan, a new type of mixed compost fertilizer called compost-compound fertilizer mixture (CCFM) has been registered. It exhibits several merits for organic farming; that is, it is odorless, easy to handle, and has a controllable nutrient content. CCFM consists of compost mixed with chemical fertilizer, such that nutrient content can be effectively adjusted. Although it has been pointed out that pellet composts generate more greenhouse gas emissions (GHGEs), such as carbon dioxide (CO₂) and nitrous oxide (N₂O), the mechanism involved is not clearly understood. To investigate the effect of CCFM on GHGEs, soil microbial properties and plant growth, incubation and pot experiments were conducted in a Hungarian Chernozem and a Japanese Andosol. Both soils were incubated with 1) CCFM prepared using pig manure mixed with chemical fertilizer, 2) chemical fertilizer only, or 3) no fertilizer. A pot experiment was conducted using Komatsuna (Brassica rapa). In the Chernozem, the maximum N₂O production rate was observed after chemical fertilizer treatment. The emission factors of chemical fertilizer and CCFM were 0.25% and 0.19%, respectively. In contrast, N₂O production was not detected in the Andosol. Meanwhile, CO₂ production was higher (P < 0.05) in the Chernozem than in the Andosol; furthermore, it was higher under CCFM than under chemical fertilizer treatment, with 7.5% of amended carbon changing to CO₂, thus suggesting that dominant carbon remained in the soil. Microbial biomass increased faster in the Andosol than in the Chernozem and the change was faster under the CCFM than under the chemical fertilizer or control treatments. In the pot experiment, CCFM tended to enhance plant growth in both soils. These experiments showed that CCFM has a soil-carbon storage effect with less resultant GHGEs, especially in Andosols, thereby leading to a more sustainable agriculture. Nonetheless, further studies are required for long-term experiments with other crops. [ABSTRACT FROM AUTHOR]

Biochar can reduce N2O emissions and it can be added to the soil once, whereas fertilizers are often applied every cultivation season. The aging of biochar in soil affects its functioning but it is unclear whether palm shell biochar (PSB) could still mitigate N2O emissions even when additional basal N fertilizers are applied 1 year after the initial biochar application. We studied the impact of fresh and aged PSB (0%, 6%, 12%, and 18% w/w of dry soil) on N2O emissions, soil properties, nutrient content and yield of Komatsuna (Brassica rapa var. perviridis) under sandy soil conditions. The aged PSB non-significantly reduced N2O emissions but significantly offset soil acidification, and maintained a high soil nutrient status. Biochar application with fertilizer significantly increased plant tissue K and Ca content but decreased N, P and Mg content compared to the treatments without biochar. At higher application rates, biochar had negative effects on crop yield but as it aged, the negative effects were offset as a result of the similar variation in plant N uptake. Since seasonal N fertilizer application seems to be inevitable in Komatsuna cultivation, addition of biochar could be a possible way of counteracting the effects of excessive fertilizer use. Further research is needed to assess the feasible biochar application rates for Komatsuna fields in various soil types under field conditions. [ABSTRACT FROM AUTHOR]