ABSTRACT 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 (CH4) and nitrous oxide (N2O) 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 = CH4 × 25 + N2O × 298) ranged from 7,547 to 17,711 kg CO2eq 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 CH4 emissions accounted for 98 % of pGWP in the flooded rice season, whereas N2O prevailed during the drained non-rice season (≈65 %). Although it was impossible to relate emissions to any individual input or practice, soil CH4 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.

ABSTRACT The DayCent ecosystem model, widely tested in upland agroecosystems, was recently updated to simulate waterlogged soils. We evaluated the new version in a paddy rice experiment in Southern Brazil. DayCent was used to simulate rice yield, soil organic carbon (SOC), and soil CH4 fluxes. Model calibration was conducted with a multiple-year dataset from the conventional tillage treatment, followed by a validation phase with data from the no-tillage treatment. Model performance was assessed with statistics commonly used in modeling studies: root mean square error (RMSE), model efficiency (EF), and mean difference (M). In general, DayCent slightly underestimated rice yields under no-tillage (by 0.07 Mg ha-1, or 9.2 %) and slightly overestimated soil C stocks, especially in the first years of the experiment. A comparison of observed and simulated CH4 daily fluxes showed that DayCent could simulate the general patterns of soil CH4 fluxes with slight discrepancies. Daily soil CH4 fluxes were overestimated by 0.43 kg ha-1 day-1 (12 %). Growth-season CH4 emissions under no-tillage were also somewhat overestimated (11 % or 45.29 kg ha-1). We conclude that DayCent simulated SOC, rice yield, and CH4 with some inaccuracies, but the overall performance was considered adequate. However, the model failed to represent the observed potential of no-tillage to mitigate CH4 emissions, possibly because model algorithms could not capture the actual field conditions derived from no-tillage management, such as soil redox potential, plant senescence, and surface placement of plant residue.

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 CH4 e N2O 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=CH4×25+N2O×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 CH4 do solo, que excederam em dez vezes a emissão acumulada do solo em pousio invernal. As emissões de N2O 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 CO2eq 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 CH4, 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.

Abstract 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 CH4 and N2O 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= CH4×25+N2O×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 CH4 fluxes, which exceeded ten times the accumulated emissions of fallow soil. N2O emissions were higher in the ryegrass+birdsfoot trefoil treatment, compared with ryegrass and fallow. Cover crops increased yield-scaled pGWP (0.35 kg CO2eq per kilogram of rice) compared with fallow (0.06 kg kg-1). In the treatments with winter crops, mitigation strategies should focus mainly on CH4, 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.

O cultivo de arroz irrigado pode contribuir com a emissão de metano (CH4), em razão das condições anaeróbicas, após o alagamento do solo. Alternativas ao alagamento contínuo da irrigação na cultura do arroz têm sido desenvolvidas a fim de mitigar o efluxo de CH4 para a atmosfera. O objetivo deste estudo foi verificar o efeito do regime de irrigação contínua, intermitente e por banhos no efluxo de CH4 em arroz irrigado. Para tal, foi conduzido um experimento no Departamento de Fitotecnia da Universidade Federal de Santa Maria, durante os anos agrícolas 2007/2008 e 2009/2010. Os tratamentos consistiram em irrigação contínua e intermitente, em 2007/2008, e irrigação contínua, intermitente e por banhos, em 2009/2010. A prática da irrigação intermitente é eficiente em abrandar o efluxo de CH4 no cultivo do arroz irrigado, quando as condições climáticas permitem a ausência da lâmina de água durante o cultivo. A eficiência em suavizar o efluxo de CH4 pelo manejo intermitente da irrigação na cultura do arroz depende das condições eletroquímicas do solo, durante a intermitência dos ciclos de alagamento da cultura.

Paddy rice fields may contribute to methane (CH4) emission from soil due to anaerobic conditions after flooding. Alternatives to continuous flooding irrigation in rice have been developed to mitigate CH4 efflux into the atmosphere. This study aims to investigate the effects of irrigation managements in the CH4 efflux during the rice growing season. An experiment was carried out at in Santa Maria, Rio Grande do Sul State, Brazil, during 2007/08 and 2009/10 growing seasons. The treatments were continuous flooding and intermittent irrigation in 2007/08 and continuous flooding, intermittent irrigation and flush irrigation in 2009/10. Intermittent irrigation is effective in mitigating CH4 efflux from rice fields when climatic conditions enable water absence during cultivation, but its efficiency depends on the electrochemical soil conditions during the flooding cycles.

Plantas de cobertura de inverno são fontes de C e de N em sistemas de produção de arroz irrigado, mas pouco se conhece sobre o efeito do manejo e da qualidade dos seus resíduos culturais nas emissões de metano (CH4) e de óxido nitroso (N2O) do solo. O estudo consistiu de um experimento conduzido em casa de vegetação e teve como objetivo principal avaliar a influência do manejo (incorporado ou em superfície) e do tipo de resíduos culturais (azevém e serradela) de plantas de cobertura sobre as emissões de CH4 e de N2O de um Planossolo Háplico sob cultivo de arroz irrigado por inundação. Utilizou-se o método da câmara fechada, e as concentrações de CH4 e de N2O nas amostras de ar foram determinadas por cromatografia gasosa. Coletas da solução do solo em duas profundidades (2 e 20 cm) foram realizadas simultaneamente às coletas de ar. A manutenção dos resíduos culturais na superfície do solo resultou em menor emissão de CH4 e de N2O em comparação à sua incorporação. O tipo de resíduo cultural não teve efeito na emissão de CH4, enquanto para o N2O a distinção entre os resíduos ocorreu apenas quando estes foram mantidos na superfície do solo condição na qual as maiores emissões ocorreram com a adição dos resíduos da leguminosa (serradela). O processo mais intenso de redução do solo em subsuperfície (20 cm), evidenciado pelos maiores teores de íons reduzidos (Mn2+ e Fe2+), e a estreita relação entre a emissão de CH4 e os teores de C orgânico dissolvido nessa profundidade indicam que a camada subsuperficial foi a principal fonte de CH4 quando da incorporação dos resíduos culturais. A adoção de estratégias de manejo que promovam a manutenção dos resíduos culturais na superfície do solo é fundamental quando se visa à mitigação das emissões de CH4 e N2O em solos sob cultivo de arroz irrigado. O CH4 compõe mais de 90 % do potencial de aquecimento global parcial (CH4 + N2O) nesses sistemas de produção e, portanto, deve ser o foco principal da pesquisa.

Winter cover crops are sources of C and N in flooded rice production systems, but very little is known about the effect of crop residue management and quality on soil methane (CH4) and nitrous oxide (N2O) emissions. This study was conducted in pots in a greenhouse to evaluate the influence of crop residue management (incorporated into the soil or left on the soil surface) and the type of cover-crop residues (ryegrass and serradella) on CH4 and N2O emissions from a flooded Albaqualf soil cultivated with rice (Oryza sativa L.). The closed chamber technique was used for air sampling and the CH4 and N2O concentrations were analyzed by gas chromatography. Soil solution was sampled at two soil depths (2 and 20 cm), simultaneously to air sampling, and the contents of dissolved organic C (DOC), NO3-, NH4+, Mn2+, and Fe2+ were analyzed. Methane and N2O emissions from the soil where crop residues had been left on the surface were lower than from soil with incorporated residues. The type of crop residue had no effect on the CH4 emissions, while higher N2O emissions were observed from serradella (leguminous) than from ryegrass, but only when the residues were left on the soil surface. The more intense soil reduction verified in the deeper soil layer (20 cm), as evidenced by higher contents of reduced metal species (Mn2+ and Fe2+), and the close relationship between CH4 emission and the DOC contents in the deeper layer indicated that the sub-surface layer was the main CH4 source of the flooded soil with incorporated crop residues. The adoption of management strategies in which crop residues are left on the soil surface is crucial to minimize soil CH4 and N2O emissions from irrigated rice fields. In these production systems, CH4 accounts for more than 90 % of the partial global warming potential (CH4+N2O) and, thus, should be the main focus of research.

Solos de várzea sob cultivo de arroz irrigado contribuem com aproximadamente 18 % das emissões totais de metano (CH4) do Estado do Rio Grande do Sul. Entretanto, a liberação de CH4 depende do curso de redução de cada solo. O objetivo do presente estudo foi avaliar a dinâmica da emissão desse gás de efeito estufa (GEE) em seis solos: Gleissolo (2), Planossolo (2), Chernossolo e Neossolo, representativos do cultivo de arroz irrigado no Sul do Brasil, visando identificar também sua relação com propriedades do solo e as alterações eletroquímicas da solução após o alagamento. O experimento foi realizado em casa de vegetação, com três repetições, segundo delineamento de blocos casualizados. Os solos foram dispostos em vasos de PVC mantidos com uma lâmina de água de 10 cm de altura e cultivados com arroz. A avaliação das emissões de CH4 foi realizada semanalmente, do 3º ao 66º dia após o alagamento (DAA) do solo, com o auxílio de uma câmara de PVC acoplada ao topo dos vasos. As amostras de ar foram coletadas em quatro intervalos de 5 min, para estimativa das taxas de emissão de CH4. A solução do solo também foi coletada e caracterizada. O início da emissão de CH4 variou entre os solos e, normalmente, ocorreu após a quase total redução do Fe3+ (em torno de 90 % da maior liberação de Fe2+) e estabilização dos valores de pH e de Eh da solução. A emissão total de CH4 variou de 8,5 a 44,2 g m-2 e apresentou relação sigmoidal com os teores de C orgânico dos solos (r²=0,83, p < 0,05), sugerindo que a disponibilidade de C somente foi limitante para o processo de metanogênese em teores inferiores a 8 g kg-1 de C no solo. Os resultados mostram que a dinâmica e as quantidades totais de CH4 emitidas são influenciadas pelo tipo de solo e que esforços devem ser direcionados para determinação dos fatores de emissão de CH4 para os diferentes solos representativos da produção de arroz no Sul do Brasil, bem como na avaliação do efeito de práticas agrícolas na mitigação das emissões desse GEE nos diferentes solos.

Paddy soils under flooded rice produce nearly 18% of all methane (CH4) emissions in the state of Rio Grande do Sul. However, CH4 release depends on the intensity of soil reduction. The objective of this study was to evaluate the dynamics of emissions of this greenhouse gas from six representative soils [Albaqualf (2), Endoaqualf (2), Udorthent and Hapludoll] of rice cultivation in southern Brazil, to identify the relation with soil proprieties and electrochemical changes in the soil solution after flooding. The experiment was carried out in a greenhouse with three replications in a randomized block design. Rice was grown in PVC pots filled with soil and covered with a 10 cm water layer. CH4 emissions were evaluated weekly from the 3rd to the 66th day after flooding, using a gas trap fixed on top of the pots. The gas was collected every five minutes, four times, to estimate CH4 emission rates. Soil solution was collected and analyzed as well. The beginning of CH4 emissions varied according to the soil type and usually occurred when 90 % of Fe3+ had been reduced to Fe2+ and pH and Eh stabilized. Total CH4 emission varied from 8.5 to 44.2 g m-2 and had a sigmoid relation to soil organic C (r² = 0.83, p < 0.05), suggesting that C availability only limited methanogenesis when levels were below 8 g kg-1 C soil. The results illustrate that the dynamics and total fluxes were strongly affected by the soil type and efforts must be directed toward determining CH4 emission factors for the different representative soil types of rice cultivation in southern Brazil, and agricultural practices to mitigate greenhouse gases from the different soils should be evaluated.