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.

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.

Os mecanismos de estabilização da matéria orgânica (MO) têm sido estudados em solos tropicais e subtropicais brasileiros; no entanto, poucos trabalhos avaliaram a influência da parte metodológica do uso das soluções nos resultados obtidos. Objetivou-se avaliar a eficiência de duas soluções salinas (iodeto de sódio - NaI 1,8 kg dm-3 e politungstato de sódio - PTS 2,0 kg dm-3) na separação de frações densimétricas da MO em dois solos (Argissolo Vermelho do sul do Brasil - 220 g kg-1 argila e Latossolo Vermelho do Cerrado - 630 g kg-1 argila) e o reflexo dessa eficiência na magnitude e importância do mecanismo de proteção física por oclusão em agregados no acúmulo de MO em solos brasileiros. Amostras de solo coletadas nas camadas de 0,00-0,05; 0,05-0,10; e 0,10-0,20 m foram submetidas ao fracionamento físico densimétrico e separadas as frações leve livre (FLL), leve-oclusa (FLO) e pesada (FP) da MO do solo. O uso de PTS 2,0 kg dm-3 aumentou o rendimento de carbono orgânico (CO) das FLL e FLO em ambos os solos em relação à solução de NaI 1,8 kg dm-3, sendo o efeito mais pronunciado na FLO. A utilização do sistema plantio direto (PD) aumentou os estoques de CO total na camada de 0,00-0,05 m do Argissolo em relação ao solo em preparo convencional (PC). O mecanismo de proteção física da MO por oclusão em agregados foi efetivo no aumento dos estoques de C do solo, sendo responsável por aproximadamente ⅓ do acúmulo na camada superficial do Argissolo (0,00-0,05 m). Para o Latossolo, não houve diferenças entre o estoque de CO total do PD e do PC, porém o solo sob PD apresentou acúmulo de aproximadamente ⅔ do estoque de C como FLO na camada superficial do solo. A proteção física por oclusão em agregados é um mecanismo expressivo na estabilização e sequestro de C em solos tropicais e subtropicais, cuja importância pode ser mascarada pela baixa eficiência da solução de NaI em estudos de fracionamento densimétrico da MO do solo.

The mechanisms of stabilization of organic matter (OM) have been studied in Brazilian tropical and subtropical soils; however, few studies have evaluated the influence of the methodological part of the use of the solutions in the results. The aim of this study was to evaluate the efficiency of two salt solutions (1.8 kg dm-3 sodium iodide - NaI, and 2.0 kg dm-3 sodium polytungstate - SPT) in separation of densimetric soil organic matter (SOM) fractions in two soils (Acrisol from southern Brazil - 220 g kg-1 clay, and Ferralsol from the Cerrado [Brazilian tropical savanna] - 630 g kg-1 clay) and the impact of this efficiency in interpreting results such as the magnitude and importance of the mechanism of physical protection by occlusion in aggregates in organic matter accumulation in Brazilian soils. Soil samples taken from the 0.00-0.05, 0.05-0.10, and 0.10-0.20 m layers underwent density fractionation, and the free light fraction (FLF), occluded light fraction (OLF), and heavy fraction (HF) of OM were obtained. The use of 2.0 kg dm-3 SPT increased the content of organic carbon (C) in the FLF and in the OLF of both soils in comparison to 1.8 kg dm-3 NaI, and the effect was more pronounced in the OLF. Use of the NT system increased the SOM content in the 0-0.05 m layer compared to CT in the Acrisol. The mechanism of physical protection of SOM by occlusion in aggregates was effective in increasing soil C stocks, accounting for ⅓ of C accumulation in the 0.00-0.05 m layer of the Acrisol. For the Ferralsol, there were no differences between the stock of total organic carbon (TOC) in NT and CT, but the soil in NT showed an accumulation of approximately ⅔ of the C stock as OLF in the topsoil. Physical protection by occlusion in aggregates is a significant mechanism in C stabilization and sequestration in tropical and subtropical soils, but its importance may be masked by the low efficiency of the NaI solution in studies of densimetric fractionation of SOM.

The capacity of soils to sequestrate carbon (C) is mainly related to the formation of organo-mineral complexes. In this study, we investigated the influence of soil management systems on the C retention capacity of soil with an emphasis on the silt and clay fractions of two subtropical soils with different mineralogy and climate. Samples from a Humic Hapludox and a Rhodic Hapludox, clayey soils cultivated for approximately 30 years under no-tillage (NT) and conventional tillage (CT) were collected from six layers distributed within 100-cm soil depth from each site and from an adjacent native forest. After the removal of particulate organic matter (POM), the suspension (<53 µm) was sonicated, the silt and clay fractions were separated in accordance with Stokes' law and the carbon content of whole soil and physical fractions was determined. In the Humic Hapludox, the clay and silt fractions under NT showed a higher maximum C retention (72 and 52 g kg-1, respectively) in comparison to those under CT (54 and 38 g kg-1, respectively). Moreover, the C concentration increase in both fractions under NT occurred mainly in the topsoil (up to 5 cm). The C retention in physical fractions of Rhodic Hapludox varied from 25 to 32 g kg-1, and no difference was observed whether under an NT or a CT management system. The predominance of goethite and gibbsite in the Humic Hapludox, as well as its exposure to a colder climate, may have contributed to its greater C retention capacity. In addition to the organo-mineral interaction, a mechanism of organic matter self-assemblage, enhanced by longer periods of soil non-disturbance, seems to have contributed to the carbon stabilization in both soils.

Conhecer como os caracteres agronômicos se correlacionam nas populações segregantes é primordial ao melhorista de plantas, principalmente quando o objetivo for seleção indireta, por meio de um caráter de fácil mensuração ou de maior herdabilidade correlacionado ao caráter de interesse de menor herdabilidade. Dessa forma, a partir da variabilidade observada em caracteres agronômicos nas gerações F2 e F3 de quatro cruzamentos de trigo, avaliados no ano agrícola de 2004, no Centro Agropecuário da Palma, do Centro de Genômica e Fitomelhoramento - FAEM/UFPel, localizado no município de Capão do Leão (RS), este trabalho teve como objetivos estimar: a) coeficientes de correlação genotípica, fenotípica e de ambiente entre os caracteres; b) coeficientes de trilha sobre o rendimento de grãos. Em todas as populações avaliadas, por meio das correlações simples, a seleção de plantas mais produtivas pode ser realizada pela seleção indireta sobre número de afilhos férteis por planta. Mediante a análise de trilha foi possível identificar o efeito indireto do caráter número de afilhos férteis como causador da formação da correlação entre outros caracteres (comprimento da espiga x rendimento de grãos, número de espiguetas da espiga x rendimento de grãos e número de grãos da espiga x rendimento de grãos), enfatizando que o número de afilhos férteis é o principal caráter a ser considerado na obtenção de ganhos em produção por planta.

The knowledge of how the agronomical traits are correlated in segregating populations is of primary importance for plant breeders, specially when one considers indirect selection through a character easily measurable or with high heritability correlated to the trait of interest with lower heritability. Agronomical traits in F2 and F3 generations of four wheat crosses, were evaluated on 2004 agricultural year in the Centro Agropecuário da Palma, of the Centro de Genômica e Fitomelhoramento - FAEM/UFPel, located in Capão do Leão country in Rio Grande do Sul State. The present work had the objectives to estimate for evaluated traits: a) genotypic, phenotypic and environmental correlation coefficients between the traits; and b) path coefficients for grain yield. For all populations, the single correlations indicated that the selection of higher yielding plants can be performed by indirect selection for number of fertile tillers per plant. The path analysis showed that it was possible to identify the indirect effect of the trait number of fertile tillers the as cause of the correlation between other traits (ear length x grain yield, spikelet number per ear x grain yield and number of ear grain x grain yield). Thus suggesting that the number of fertile tillers is the major trait to select for in order to obtain gains on grain yield.