A no-tillage soybean-wheat cropping system has been practiced for many years on the "Terra Rossa" soils of eastern Paraguay. Soil compactness and soil erosion have recently been identified as potential problems. This study examines the effect of replacing unprofitable wheat by green-manure turnip (Raphanus sativus L. var. oleiferus Metzg.) on soil properties and soybean production. Gaseous phase, porosity, bulk density, water saturation, cone index, pH, exchangeable-cations, available-phosphorus (P) and aggregate size distribution of the soil were measured. Contrary to initial expectations, turnip did not reduce soil compactness. Instead, turnip stabilized the aggregate structure of the surface soil. Positive effects of turnip on subsequent soybean growth and yield were detected in a rather dry year but not in an exceptionally wet year. In a second part of this study, nutrient return from turnip and wheat residues were compared. Turnip produced 10.7 t ha -1 of shoot dry matter, and absorbed 294, 27, 302, 175, and 33 kg ha -1 of N, P, K, Ca, and Mg, respectively. Wheat absorbed 98, 11, 67, 11, and 7 kg ha -1 of N, P, K, Ca, and Mg, respectively. About 75% of the N absorbed by wheat was removed from the field at harvest whereas most nutrients in the turnip residue were returned to the soil before planting of soybeans with positive effects on soil fertility. Additional benefits of green-manure turnip would include a reduced chance for erosion through improvements in aggregate structure and through a more complete soil cover. Key words: soil compactness, aggregate structure, crop residue, nutrient return ADUBAÇÃO VERDE COM NABO PARA SOJA EM SISTEMA DE PLANTIO DIRETO NO LESTE PARAGUAIO RESUMO: Um sistema de plantio direto de rotação soja-trigo foi praticado por muitos anos em uma "Terra Roxa" no leste do Paraguai. A compactação do solo e a erosão foram reconhecidas recentemente como problemas potenciais. Este estudo examina o efeito da substituição do trigo antieconômico pela adubação verde com nabo (Raphanus sativus L. var. oleiferus Metzg.) sobre as propriedades do solo e a produção de soja. A fase gasosa, porosidade, densidade do solo, saturação em água, índice de cone, pH, cátions trocáveis, fóforo disponível (P) e distribuição de agregados foram medidos. Ao contrário das expectativas iniciais, o nabo não reduziu a compactação do solo. Ao contrário, ele estabilizou a estrutura dos agregados na superfície do solo. Efeitos positivos do nabo no crescimento e produtividade da soja plantada em seguida, foram detectados em um ano muito seco, mas não em um ano excepcionalmente úmido. Em uma segunda parte desse estudo foram comparados os retornos de nutrientes do nabo e do trigo. O nabo produziu 10.7 t ha -1 de matéria seca da parte aérea e absorveu 294, 27, 302, 175, e 33 kg ha -1 de N, P, K, Ca, e Mg, respectivamente, enquanto o trigo 98, 11, 67, 11, e 7 kg ha -1 de N, P,K,Ca, e Mg, respectivamente. Aproximadamente 75% do N absorvido pelo trigo foi removido do solo por ocasião da colheita, enquanto que a maio...

Effects of an agropastoral system on the production of soybean and wheat were investigated by comparing, from 2003 to 2007, agropastoral plots that had been converted from 7-year pasture (guinea grass: Panicum maximum; 1996 to 2003) to crop cultivation, with control plots that had been continuously cropped over 10 years with soybean at the Japan International Cooperation Agency's Paraguay Agricultural Technology Center (CETAPAR-JICA). Soybean productivity in this area increased from 1979 until 1993, when yield peaked at 3.39 t/ha and then declined. In this study, soybean yields ranged from 1.48 to 3.56 t/ha in agropastoral plots and from 0.63 to 2.47 t/ha in control plots. In each year, the yield in the agropastoral plots was 1.1 to 2.4 times more than in the control plots. Wheat yields were also higher in agropastoral plots (1.59 to 3.17 t/ha) than in control plots (1.18 to 2.31 t/ha). In each year, yield in the agropastoral plots was 1.2 to 1.8 times more than in the control plots. Thus, soybean and wheat yields were sustained by introducing an agropastoral system. We also examined the chemical and physical properties of the soil under initial conditions in both plot types. The concentrations of phosphate, potassium and magnesium in surface soil in agropastoral plots were significantly lower than in control plots. The content of organic matter at soil depths of 0 to 60 cm in the agropastoral plots was significantly higher than in the control plots, and the physical properties of the agropastoral plot soil (gaseous phase, bulk density, and soil aggregates) were improved. We conclude that the agropastoral system positively affected all these properties.

SUMMARYNitrogen removal in soybean grains at harvest may exceed biological N 2 fixation, particularly if grain yields are as high as typically achieved on "Terra Rossa" soils of Eastern Paraguay. Applying N fertilizer or coating seeds with rhizobial inoculants that enhance nodulation may represent a way of balancing the N budget. However, the effects of such treatments appear to be highly sitespecific. The objective of this study was to examine the effects of N application (N) and rhizobial inoculation (I) on nodulation, N accumulation and soybean yields in Eastern Paraguay. Field experiments were conducted in two consecutive soybean seasons. Dry conditions in the first year delayed sowing and reduced plant number m -2 and pod number plant -1 . Grain yields were generally below 2 t ha -1 but the +N+I treatment increased yields by about 75%. In the second year favorable conditions resulted in yields of around 4 t ha -1 and the treatments had no effect. Nitrogen accumulation was higher in the first year and could therefore not explain the observed yield differences between years and treatment combinations. The positive effect of the +N+I treatment in year one was associated with a more rapid root growth which could have reduced susceptibility to intermittent drought stress. Nodule biomass decreased between flowering and pod setting stages in the +I treatment whereas further increases in nodule biomass in the -I treatment may have led to competition for assimilates between nodules and developing pods. Based on these preliminary results we conclude that N application and seed inoculation can offer short-term benefits in unfavorable years without negative effects on yield in favorable years.Index terms: Glycine max, inoculum, nitrogen, nodulation, rhizobium. RESUMO: EFEITO DA APLICAÇÃO DE FERTILIZANTE-NITROGENADO E DA INOCULAÇÃO DE SEMENTES COM RIZÓBIOS NO RENDIMENTO DA SOJA NO PARAGUAI ORIENTALA remoção do N em grãos de soja na colheita pode exceder a fixação biológica do N 2 , particularmente se os rendimentos dos grãos forem tão elevados quanto os obtidos tipicamente nos solos "Terra Roxa" da região oriental do Paraguai. A aplicação do fertilizante nitrogenado ou a inoculação das sementes com rizóbios que estimulem a nodulação podem representar uma maneira de equilibrar o balanço de N, porém o efeito de tais tratamentos parece ser altamente específico do local. O objetivo deste estudo foi examinar os efeitos da aplicação do nitrogênio (N) e da inoculação com rizóbios (I) na nodulação, na acumulação de N e nos rendimentos de soja sob condições da região oriental do Paraguai. A pesquisa em campo foi conduzida durante duas safras consecutivas de soja. As condições de seca ou estiagem no primeiro ano atrasaram a semeadura e reduziram o número de plantas por m 2 e o número de vagens/planta. Os rendimentos de grãos foram geralmente inferiores a 2 t ha -1 ; no entanto, o tratamento + N + I aumentou o rendimento em aproximadamente 75 %. Nas condições favoráveis do segundo ano, os rendimentos foram em torno de 4 t ha -1 e...

maintain constant soil temperatures (Benegas, 1998), and improve soil structure and water infiltration rates The introduction of no-tillage farming stabilized soybean [Glycine (Gill, 1998). On the other hand, some studies indicated max. (L.) Merr.] yields on "Terra Rossa" soils of eastern Paraguay at a yield level that is among the highest in the world. The objective that long-term no-tillage farming might also cause sevof this study was to clarify the effect of long-term no-tillage in a eral problems. These are accumulation of applied nutrisoybean-based farming system on physical properties of "Terra ents in the surface soil (Seki et al., 2001; Pierce et al., Rossa" soils with different texture. Four farmers' fields of different 1994), high bulk density (Benegas, 1998; Betz et al., soil texture were studied in 1990 and 2000. Two fields had fine-textured 1998; Pierce et al., 1994), low water permeability (Besoil and other two had coarse-textured soil. A 1-m deep pit was negas, 1998), and high soil penetration resistance (Betz prepared at each site at the exact same location in 1990 and 2000,

The effects of an agropastoral system on the production of soybean and wheat were investigated by comparing (from 2007 to 2009) agropastoral plots that had been reconverted from 4-year pasture (Guinea grass: Panicum maximum; 2003 to crop cultivation, with control plots that had been continuously cropped over 13 years with soybean and wheat at CETAPAR-JICA. In 2003, Guinea grass pastures were converted from 10-year continuous cultivated soybean and wheat fields, and then grazed intensively from 2004 to 2007, while the control plots continued to be cultivated with soybean and wheat. Animal production in converted pastures was very high and the weight gain per hectare was 1,113 to 1,500 kg/ha over 3 years after introducing an intensive grazing system with supplemental feeding in dry season. The average daily gain was maintained at 0.554 to 0.621 kg/head/day despite a high stocking rate (4.5-6.0 UA/ha). However, after the reconversion in 2007, the soybean yields from 2008 to 2009 were 3.71 and 1.24 ton/ha, which were 1.02 and 1.43 times higher than those in the control plots, respectively. In 2008, the wheat yield was 2.72 ton/ha, the same as that in the control plots. Thus, we concluded that the agropastoral effects on the soybean and wheat yield were small after reconversion to intensive grazing. The reason for the small effect is likely to be the low supply of Guinea grass litter under high grazing pressure, which did not result in any accumulation of the organic matter in the soil.