Effect of the number of tillages in fallow season and fertilizer type on greenhouse gas emission from a rice (Oryza sativa L.) paddy field in Ehime, southwestern Japan Agricultural fields, including rice (Oryza sativa L.) paddy fields, constitute one of the major sources of atmospheric methane (CH 4 ) and nitrous oxide (N 2 O). Organic matter application, such as straw and organic fertilizer, enhances CH 4 emission from paddy fields. In addition, rice straw management after harvest regulates CH 4 emissions in the growing season. The interaction of tillage times and organic fertilizer application on CH 4 and N 2 O emissions is largely unknown. Therefore, we studied the effects of fallow-season tillage times and fertilizer types on CH 4 and N 2 O emissions in paddy fields in Ehime, southwestern Japan. From November 2011 to October 2013, four treatments, two (autumn and spring) or one (spring) in the first year, or two (autumn and spring) or three (autumn, winter, and spring) in the second year times of tillage with chemical or organic fertilizer application, were established. Gas fluxes were measured by the closed-chamber method. Increasing the number of tillage times from one to two decreased succeeding CH 4 emission and the emission factor for CH 4 (EF CH4 ) in the rice-growing season, suggesting that the substrate for CH 4 production was reduced by autumn and spring tillage in the fallow season. Higher EF CH4 [1.8-2.0 kg carbon (C) ha] was observed when more straw was applied (6.9-7.2 Mg ha −1 ) in the second year. Organic fertilizer application induced higher CH 4 emission just after the application as basal and supplemental fertilizers, especially at a lower straw application rate. This indicated that EF CH4 in the organically managed fields should be determined individually. Organic fertilizer application with two tillage times induced N 2 O efflux during the rice-growing season in the second year, but N 2 O emissions were not affected by winter tillage. Although paddy fields can act as an N 2 O sink because of reduced soil conditions when straw application was high, application of organic C and nitrogen as fertilizer can enhance N 2 O production by the denitrification process during the growing season, especially in the ripening stage when soil anaerobic conditions became moderate. These results suggest that negative emission factors for N 2 O (EF N2O ) can be applied, and EF N2O of organic fertilizer should be considered during the estimation of N 2 O emission in the paddy field.ARTICLE HISTORY

Green manure application helps maintain soil fertility, reduce chemical fertilizer use, and carbon sequestration in the soil. Nevertheless, the application of organic matter in paddy fields induces CH4 and N2O emissions. Prolonging mid-season drainage reduces CH4 emissions in paddy fields. Therefore, the combined effects of green manure application and mid-season drainage prolongation on net greenhouse gas emission (NGHGE) were investigated. Four experimental treatments were set up over a 2-year period: conventional mid-season drainage with (CMG) and without (CM) green manure and prolonged (4 or 7 days) mid-season drainage with (PMG) and without (PM) green manure. Astragalus sinicus L. seeds were sown in autumn and incorporated before rice cultivation. No significant difference in annual CH4 and N2O emissions, heterotrophic respiration, and NGHGE between treatments were observed, indicating that green manure application and mid-season drainage prolongation did not influence NGHGE. CH4 flux decreased drastically in PM and PMG during mid-season drainage under the hot and dry weather conditions. However, increasing applied carbon increases NGHGE because of increased CH4 and Rh. Consequently, combination practice of mid-season drainage prolongation and green manure utilization can be acceptable without changing NGHGE while maintaining grain yield in rice paddy fields under organically managed rice paddy fields.

Biochar application can reduce global warming via carbon (C) sequestration in soils. However, there are few studies investigating its effects on greenhouse gases in rice (Oryza sativa L.) paddy fields throughout the year. In this study, a year-round field experiment was performed in rice paddy fields to investigate the effects of biochar application on methane (CH 4) and nitrous oxide (N 2 O) emissions and C budget. The study was conducted on three rice paddy fields in Ehime prefecture, Japan, for 2 years. Control (Co) and biochar (B) treatments, in which 2-cm size bamboo biochar (2 Mg ha −1) was applied, were set up in the first year. CH 4 and N 2 O emissions and heterotrophic respiration (Rh) were measured using a closed-chamber method. In the fallow season, the mean N 2 O emission during the experimental period was significantly lower in B (67 g N ha −1) than Co (147 g N ha −1). However, the mean CH 4 emission was slightly higher in B (2.3 kg C ha −1) than Co (1.2 kg C ha −1) in fallow season. The water-filled pore space increased more during the fallow season in B than Co. In B, soil was reduced more than in Co due to increasing soil moisture, which decreased N 2 O and increased CH 4 emissions in the fallow season. In the rice-growing season, the mean N 2 O emission tended to be lower in B (−104 g N ha −1) than Co (−13 g N ha −1), while mean CH 4 emission was similar between B (183 kg C ha −1) and Co (173 kg C ha −1). Due to the C release from applied biochar and soil organic C in the first year, Rh in B was higher than that in Co. The net greenhouse gas emission for 2 years considering biochar C, plant residue C, CH 4 and N 2 O emissions, and Rh was lower in B (5.53 Mg CO 2 eq ha −1) than Co (11.1 Mg CO 2 eq ha −1). Biochar application worked for C accumulation, increasing plant residue C input, and mitigating N 2 O emission by improving soil environmental conditions. This suggests that bamboo biochar application in paddy fields could aid in mitigating global warming.

Nitrous oxide (N 2 O) emissions from agricultural fields are an important source of the increasing atmospheric N 2 O concentration. We conducted a two-year investigation of soil N 2 O emissions induced by the application of combined organic and synthetic fertilizer (COS) and distilled silage waste (DSW). Three experimental treatments were applied to a Citrus unshiu field in January 2013 in Ehime, Japan: no fertilizer (NF), COS, and DSW. The applied nitrogen (N) from DSW was 192 and 244 kg N ha −1 in the first and second years, respectively, although the N application in COS was 192 kg N ha −1 in both years. The main N forms in COS and DSW were ammonium-and nitrate-N, respectively. Soil N 2 O and carbon dioxide fluxes, soil chemical properties, and mineral N leaching from topsoil were measured. The soil N 2 O flux increased after fertilization in COS and DSW, and a higher N 2 O efflux after supplemental fertilization was induced by warm and wet soil conditions. The emission factor of N 2 O was higher in COS (2.02%) than in DSW (1.18%), while N leaching was higher in DSW than in COS. The organic materials remaining after the application possibly increased the N 2 O emissions in the summer season. Therefore, to mitigate N 2 O emissions in citrus orchards, fertilizer containing organic materials should be applied during a cool and dry season.

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