Nitrogen nutrition of soybean in Brazil: Contributions of biological N 2 fixation and N fertilizer to grain yield. Can. J. Plant Sci. 86: 927-939. There has recently been concern in Brazil whether biological N 2 fixation (BNF) is capable of meeting the increased N needs of newly released more productive cultivars, as well as doubts about the advantages of annual reinoculation of seeds. Forty experiments were performed over 3 yr in oxisols containing at least 10 3 cells of Bradyrhizobium g -1 in the State of Paraná, southern Brazil to estimate the contributions of BNF and of N fertilizer. The experiments were performed at two sites, Londrina and Ponta Grossa, under conventional (CT) or no-tillage (NT) systems, with two cultivars or BRS 134 (medium-maturity group)]. Treatments included non-inoculated controls without or with 200 kg of N ha -1 , and inoculation without or with N fertilizer applied at sowing (30 kg of N ha -1 ), or at the R2 or R4 stage (50 kg of N ha -1 ). Compared with the non-inoculated control, reinoculation significantly increased the contribution of BNF estimated by the N-ureide technique (on average from 79 to 84%), grain yield (on average 127 kg ha -1 , or 4.7%) and total N in grains (on average 6.6%). The application of 200 kg of N fertilizer ha -1 drastically decreased nodulation and the contribution of BNF (to 44%), with no further gains in yield. Application of starter N at sowing decreased nodulation and the contribution of BNF slightly and did not increase yields, while N fertilizer at R2 and R4 stages decreased the contribution of BNF (to 77%) and also yields. Estimates of volatilization of ammonia ranged from 15 to 25% of the N fertilizer applied, and no residual benefits of the N fertilizer in the winter crop were observed. The results highlight the economical and environmental benefits resulting from replacing N fertilizer with inoculation in Brazil, and reinforce the benefits of reinoculation, even in soils with high populations of Bradyrhizobium. Les expériences ont été réalisées dans deux localités, Londrina et Ponta Grossa, sous une régie des sols conventionnelle ou sans labour, avec les cultivars Embrapa 48 (précoce) ou BRS 134 (intermédiaire). Les traitements ont inclus des témoins non inoculés avec ou sans 200 kg N ha -1 , et des traitements inoculés avec ou sans fertilisant azoté appliqué au semis (30 kg N ha -1 ) ou aux stades R2 ou R4 (50 kg N ha -1 ). En présence d'une population établie de Bradyrhizobium, des augmentations significatives ont été observées dans la contribution de la FBN (de 79 à 84% en moyenne), dans le rendement (de 127 kg ha -1 , ou 4,7%, en moyenne) et dans l'azote des grains (6,6% en moyenne) pour la réinoculation. L´application de 200 kg de fertilisant azoté ha -1 a diminué considérablement la nodulation et la contribution de la FBN (à 44%), sans permettre d'augmentation du rendement. L'application de N au semis a diminué légèrement la nodulation et la contribution de la FBN, sans conduire à des rendements plus élevés, tandis que la fertilisation a...

No abstract

Nowadays, in a context of climate change, economical uncertainties and social pressure to mitigate agriculture externalities, farmers have to adopt new cropping systems to achieve a sustainable and cost-effective grain production. Conservation agriculture consists of a range of cropping systems based on a combination of three main principles: (1) soil tillage reduction, (2) soil protection by organic residues and (3) diversification in crop rotation. Conservation agriculture has been promoted as a way to reduce production costs, soil erosion and soil fertility degradation under both tropical and temperate conditions. Conservation agriculture-based cropping systems have diffused widely under Brazilian large-scale farms' conditions and more recently in Europe in the context of medium-size mechanized farms. Their diffusion, however, is still limited under small-scale non-mechanized farms' conditions of tropical countries. To assess the advantages and limits of such new cropping systems, this article compares experiences with conservation agriculture from the tropical Cerrado region of Brazil and from temperate conditions of Europe. It focusses on agronomic performances, environmental impacts and economical results. Conservation agriculture systems appear to be interesting options to achieve sustainable and intensive crop production under different agroecological environments because they use efficiently available resources and maintain soil fertility. However, this mostly results from the permanent presence of an organic mulch on the soil surface and the incorporation of cover crops in the rotations. Such modifications require a significant reorganization of the production process at farm level, and when facing technical or socioeconomic constraints, most farmers usually opt for applying only partially the three main principles of conservation agriculture. Investigating more fully the consequences of such partial implementation of conservation agriculture principles on its actual efficiency and assessing the most efficient participatory approaches needed to adapt conservation agriculture principles to local conditions and farming systems are top priorities for future research. (Résumé d'auteur

No abstract

RESUMO Primavera do Leste é um dos pólos de produção de grãos e fibras do MatoGrosso, com lavouras altamente tecnificadas. Este estudo foi realizado num Latossolo Vermelho-Amarelo da região de Primavera, com objetivo de avaliar a biomassa e a atividade microbiana de solos sob vegetação nativa e sistemas agrícolas anuais e perenes. As amostras de solo foram coletadas em duas profundidades (0-5 e 5-20 cm), no início da estação chuvosa, em áreas sob cultivo de videira (Vitis vinifera), entrelinha e linha, cultivos anuais (soja) e em uma área de vegetação nativa de Cerradão. Foram avaliados o carbono da biomassa microbiana (CBM), carbono prontamente mineralizável e as atividades das enzimas β β β β β-glucosidase, fosfatase ácida e arilsulfatase. Nas duas profundidades avaliadas, os sistemas de uso do solo com culturas perenes e anuais apresentaram reduções médias de 70 % no CBM, em relação à área sob vegetação nativa. O manejo diferenciado na entrelinha do parreiral e a utilização do capim-pé-degalinha (Eleusine indica), como cobertura viva, proporcionaram aumentos no C mineralizável e na atividade das enzimas β β β β β-glucosidase e arilsulfatase nas duas profundidades. Os níveis médios de P no solo sob Cerradão resultaram em valores de atividade da fosfatase ácida inferiores aos dos observados em outros locais do Cerrado. Mesmo assim, na profundidade de 0-5 cm, a atividade da fosfatase ácida no Cerradão foi superior à da entrelinha do parreiral (VE) e à da área com culturas anuais, demonstrando a sua importância na mineralização do fósforo orgânico em áreas sob vegetação nativa. Os resultados obtidos confirmaram a sensibilidade dos parâmetros microbiológicos e bioquímicos para identificar alterações no solo de acordo com os diferentes sistemas de uso da terra.

Winter cover crops may increase soil organic matter (SOM) and improve soil structure in intensively managed summer vegetable cropping systems. Our study examined the influence of three cover crop treatments (fallow, cereal, and legume within a summer crop rotation of sweet corn and broccoli) on microbiological properties associated with five soil aggregate‐size classes (<0.25, 0.25 to 0.5, 0.5 to 1.0, 1.0 to 2.0, and 2.0 to 5.0 mm) of a Willamette silt loam (Pachic Ultic Argixerolls). The distributions of total organic C (TOC), total Kjeldahl N (TKN), soil microbial biomass C (SMBC), readily mineralizable C and N, and two enzyme activities (β‐glucosidase and fluorescein diacetate [FDA] hydrolysis) were compared among aggregate‐size classes of surface (0–20 cm) soils at seedbed preparation (June 1996) and after harvest of either broccoli (September 1995) or sweet corn (September 1996). Although cover cropping did not significantly influence aggregate‐size distribution, TOC, nor TKN on any sampling date, it did significantly (P < 0.05) influence SMBC in September 1995, mineralizable C and N and FDA hydrolysis and β‐glucosidase activities in June 1996, and levels of mineralizable C and N and β‐glucosidase activity in September 1996. Significant (P < 0.01) interactions occurred among the cover crop treatments and aggregate‐size classes for SMBC in September 1995, and among mineralizable C and N and FDA hydrolysis in June and September 1996. Mineralizable C and N and FDA hydrolysis differed (P < 0.01) among aggregate‐size classes between June and September 1996; these changes were significantly (P < 0.01) affected by cover crop treatment. These studies identified focal points for further research on the effects of aggregate‐size classes and cover cropping on the activities and dynamics of their respective microbial communities.

The plasticity of rhizobial genomes is far greater than previously thought, with complex genomic recombination events that may be accelerated by the often stressful environmental conditions of the tropics. This study aimed at evaluating changes in soybean rhizobia due to adaptation to inhospitable environmental conditions (high temperatures, drought, and acid soils) in the Brazilian Cerrados. Both the host plant and combinations of four strains of soybean Bradyrhizobium were introduced in an uncropped soil devoid of rhizobia capable of nodulating soybean. After the third year, seeds were not reinoculated. Two hundred and sixty-three isolates were obtained from nodules of field-grown soybean after the seventh year, and their morphological, physiological, serological, and symbiotic properties determined, followed by genetic analysis of conserved and symbiotic genes. B. japonicum strain CPAC 15 (same serogroup as USDA 123) was characterized as having high saprophytic capacity and competitiveness and by the seventh year represented up to 70% of the cultivable population, in contrast to the poor survival and competitiveness of B. japonicum strain CPAC 7 (same serogroup as CB 1809). In general, adapted strains had increased mucoidy, and up to 43% of the isolates showed no serological reaction. High variability, presumably resulting from the adaptation to the harsh environmental conditions, was verified in rep-PCR (polymerase chain reaction) profiles, being lower in strain CPAC 15, intermediate in B. elkanii, and higher in CPAC 7. Restriction fragment length polymorphism (RFLP)-PCR types of the 16S rDNA corresponded to the following: one type for B. elkanii species, two for B. japonicum, associated to CPAC 15 and CPAC 7, and unknown combinations of profiles. However, when nodC sequences and RFLP-PCR of the nifH region data were considered, only two clusters were observed having full congruence with B. japonicum and B. elkanii species. Combining the results, variability was such that even within a genetically more stable group (such as that of CPAC 15), only 6.4% of the isolates showed high similarity to the inoculant strain, whereas none was similar to CPAC 7. The genetic variability in our study seems to result from a variety and combination of events including strain dispersion, genomic recombination, and horizontal gene transfer. Furthermore, the genetic variability appears to be mainly associated with adaptation, saprophytic capacity, and competitiveness, and not with symbiotic effectiveness, as the similarity of symbiotic genes was higher than that of conserved regions of the DNA.

No abstract