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Rice yield increases in response to improvements in crop management, but the impact on greenhouse gas (GHG) emissions in the subtropical region of Southern Brazil remains unknown. A three-year field study was developed aiming to evaluate the impact that an increase in crop management levels (high and very high) has on soil methane (CH 4) and nitrous oxide (N 2 O) emissions, as compared to the level (medium) currently adopted by farmers in Southern Brazil. Differences in crop management included seed and fertilizer rates, irrigation, and pesticide use. The effect of crop management levels on the annual partial global warming potential (pGWP = CH 4 × 25 + N 2 O × 298) ranged from 7,547 to 17,711 kg CO 2 eq ha-1 and this effect was larger than on the rice grain yield (9,280 to 12,260 kg ha-1), resulting in approximately 60 % higher yield-scaled GHG with the high crop management level compared to the current level. Soil CH 4 emissions accounted for 98 % of pGWP in the flooded rice season, whereas N 2 O prevailed during the drained non-rice season (≈65 %). Although it was impossible to relate emissions to any individual input or practice, soil CH 4 emissions in the rice season were linearly related to the biomass produced by the rice crop (p<0.01) and by ryegrass in the previous non-rice season (p<0.1), both of which were possibly related to the supply of labile C for methanogenesis. A future increase in rice yield as a result of the adoption of improved crop management may require additional agricultural practices (e.g., intermittent irrigation) to offset the increased GHG emissions.

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Flooded rice fields are a significant anthropogenic source of methane (CH 4) and nitrous oxide (N 2 O) from agriculture in Asia, Latin America, and the Caribbean regions. In this work, we comparatively assessed the potential of intermittent irrigation and continuous rice flooding for reducing soil CH 4 and N 2 O emissions, partial global warming potential (pGWP), and its yield-scaled version (YpGWP) in northwestern Japan and southern Brazil. Seasonal CH 4 emissions under continuous flooded soils were slight higher in Japan (738 ± 87 kg ha −1) than in Brazil (623 ± 197 kg ha −1), and they were probably related to the higher level of soil organic C and the longer period under flooding in the seedling transplanting system in the Japanese site. Intermittent irrigation had similar efficiency in decreasing soil CH 4 emissions in both study areas, with the maximum mitigation potential of 71% in northwestern Japan and of 62% in southern Brazil. No significant difference in seasonal soil N 2 O emissions (−0.17 ± 0.05 to −0.24 ± 0.06 kg N 2 O ha −1 in Japan and 0.32 ± 0.08-1.16 ± 0.40 kg ha −1 in Brazil) or rice yield (7328-8064 kg ha −1 in Japan and 9391-10,231 kg ha −1 in Brazil) between irrigation systems was observed in either area. The potential of intermittent irrigation for reducing pGWP was around three times higher than that of continuous flooding in both sites. Thus, a reduction by 47-63% and 62-80% in yield-scaled pGWP was observed in southern Brazil and northwestern Japan, respectively. Like the well-established labor-intensive rice transplanting systems used in Asia, the introduction of feasible irrigation suppression systems in mechanized direct seeding rice fields in southern Brazil and other countries of Latin America and the Caribbean region is an effective choice for reducing greenhouse gas emissions with no adverse impact on rice yield.

Resumo -O objetivo deste trabalho foi avaliar o impacto das plantas de cobertura de inverno azevém (Lolium multiflorum) e azevém+cornichão (Lotus corniculatus), bem como da drenagem parcial do solo, nas emissões de CH 4 e N 2 O em um Planossolo cultivado com arroz irrigado. Amostras de ar foram coletadas semanalmente com câmara estática fechada, parcialmente inserida no solo (0,05 m). Calcularam-se o potencial de aquecimento global parcial (PAGp=CH 4 ×25+N 2 O×298) e o PAGp por unidade de produtividade de grãos (PAGpPG). A drenagem parcial foi estabelecida pela suspenção temporária da irrigação por períodos de 8 a 15 dias, durante três ocasiões ao longo do ciclo da cultura. O aporte de resíduos pelas plantas de cobertura intensificou os fluxos de CH 4 do solo, que excederam em dez vezes a emissão acumulada do solo em pousio invernal. As emissões de N 2 O foram maiores com o consórcio azevém+cornichão, em comparação ao azevém e ao pousio. O cultivo de plantas de cobertura elevou o índice PAGpPG (0,35 kg de CO 2 eq por quilograma de arroz), comparativamente ao pousio (0,06 kg kg -1 ). Nos tratamentos com cobertura de inverno, as estratégias de mitigação devem focar principalmente no CH 4 , que foi responsável pela maior porção (80%) do PAGp. A drenagem parcial reduziu em até 50% o PAGp e apresenta grande potencial para mitigar as emissões de gases em sistemas de produção de arroz irrigado.Termos para indexação: Oryza sativa, gases de efeito estufa, irrigação intermitente, manejo da água, potencial de aquecimento global, resíduos culturais. Impact of cover crops and soil drainage in CH 4 and N 2 O emissions under irrigated rice cultivationAbstract -The objective of this work was to evaluate the effect of the winter cover crops ryegrass (Lolium multiflorum) and ryegrass+birdsfoot trefoil (Lotus corniculatus), as well as of partial soil drainage, on CH 4 and N 2 O emissions in an Alfisol cultivated with paddy rice. Air samples were collected weekly using static closed chambers partially inserted in the soil (0.05 m). Partial global warming potential (pGWP= CH 4 ×25+N 2 O×298) and yield-scaled pGWP were calculated. Partial soil drainage was established with temporal suspensions of the irrigations for periods of 8 to 15 days, during three occasions throughout the crop cycle. Biomass input by cover crop residues increased CH 4 fluxes, which exceeded ten times the accumulated emissions of fallow soil. N 2 O emissions were higher in the ryegrass+birdsfoot trefoil treatment, compared with ryegrass and fallow. Cover crops increased yield-scaled pGWP (0.35 kg CO 2 eq per kilogram of rice) compared with fallow (0.06 kg kg -1). In the treatments with winter crops, mitigation strategies should focus mainly on CH 4 , which was responsible for the greater portion (80%) of pGWP. Partial drainage reduced pGWP up to 50% and has great potential for mitigating gas emissions in subtropical irrigated rice production systems.

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