ABSTRACT. Gliricidia (Gliricidia sepium) and sombreiro (Clitoria fairchildiana) have been recommended for agroforestry systems and reforestation of degraded areas due to their fast growth and symbiosis with rhizobia. However, little is known about native populations that nodulate these species. The objective of this study was to evaluate the phenotypic and genetic diversity of nitrogen-fixing bacteria isolated from nodules of gliricidia and sombreiro in alley cropping systems located in the pre-Amazon region of Maranhão State and to confirm their nodulation ability. Nodules were field collected from 20 plants of each species. The isolated strains were characterized morphologically, their 16S rRNA gene was partially sequenced, and their symbiotic ability was authenticated in siratro (Macroptilium atropurpureum). Despite being in the same climate and soil conditions, gliricidia and sombreiro are nodulated by different rhizobia genera, with Rhizobium predominant in gliricidia and Bradyrhizobium in sombreiro. Endophytic strains also colonized nodules in the field. Approximately 60% of Rhizobium strains did not nodulate siratro, whereas all Bradyrhizobium strains did. Native strains isolated from gliricidia nodules had low efficiency, and only four strains isolated from sombreiro nodules were efficient in siratro. These results highlight the importance of symbiotic relationships in the regulation of biological nitrogen fixation.Keywords: Bradyrhizobium; Rhizobium; Papilionoideae; tree legumes; acid soil. Simbioses de rizóbios comGliricidia sepium e Clitoria fairchildiana em um Oxisol na região da Pré-Amazônia do Estado do Maranhão RESUMO. Gliricídia (Gliricidia sepium) e sombreiro (Clitoria fairchildiana) têm sido recomendadas para uso em sistemas agroflorestais e reflorestamento de áreas degradadas devido ao seu rápido crescimento e simbiose com rizóbios. No entanto, pouco é conhecido em relação às populações nativas que nodulam estas espécies. O objetivo deste estudo foi avaliar a diversidade genética e fenotípica das bactérias fixadoras de nitrogênio isoladas de nódulos de gliricídia e sombreiro em sistemas alley cropping localizados na região pré-Amazônica do Estado do Maranhão, e autenticar sua capacidade de nodulação. Foram realizados: coleta dos nódulos em campo, isolamento, caracterização cultural, sequenciamento parcial do gene 16S rRNA e autenticação da capacidade simbiótica das estirpes em siratro (Macroptilium atropurpureum). Apesar de estarem sob as mesmas condições edafoclimáticas, gliricídia e sombreiro são colonizados por distintos gêneros de rizóbios, com predominância de Rhizobium em gliricídia, e Bradyrhizobium em sombreiro. Estirpes endofíticas também foram encontradas colonizando os nódulos. Cerca de 60% das estirpes de Rhizobium não nodularam siratro e todas de Bradyrhizobium nodularam esta espécie. Todas as estirpes nativas isoladas de nódulos de gliricídia apresentaram baixa eficiência, e somente quatro de sombreiro foram eficientes em siratro. Estes resultados ressaltam a importância das rela...
SUMMARYA sustainable management of soils with low natural fertility on family farms in the humid tropics is a great challenge and overcoming it would be an enormous benefit for the environment and the farmers. The objective of this study was to assess the environmental and agronomic benefits of alley cropping, based on the evaluation of C sequestration, soil quality indicators, and corn yields. Combinations of four legumes were used in alley cropping systems in the following treatments: Clitoria fairchildiana + Cajanus cajan; Acacia mangium + Cajanus cajan; Leucaena leucocephala + Cajanus cajan; Clitoria fairchildiana + Leucaena leucocephala; Leucaena leucocephala + Acacia mangium and a control. Corn was used as a cash crop. The C content was determined in the different compartments of soil organic matter, CEC, available P, base saturation, percentage of water saturation, the period of the root hospitality factor below the critical level and corn yield. It was concluded that alley cropping could substitute the slash and burn system in the humid tropics. The main environmental benefit of alley cropping is the maintenance of a dynamic equilibrium between C input and output that could sustain up to 10 Mg ha -1 of C in the litter layer, decreasing atmospheric CO 2 levels. Alley cropping is also beneficial from the agricultural point of view, because it increases base saturation and decreases physical resistance to root penetration in the soil layer 0 -10 cm, which ensures the increase and sustainability of corn yield.Index terms: corn, alley cropping, litterfall, rootability.(
The sustainable management of soils has proved a key challenge for the smallholder agriculture in southeastern Amazonia, Brazil. We assessed the capacity of an alley cropping system to sustain corn productivity. The experiment included six treatments: Clitoria ? Pigeon Pea; Leucaena ? Pigeon Pea; Acacia ? Pigeon Pea; Clitoria ? Leucaena; Leucaena ? Acacia and Control treatment (no legumes). We determined chemical and physical indicators of soil quality. Leucaena had the highest macronutrient concentrations (40.17 g N kg -1 ), except for P. All legumes had high Ca (13.82-17.84 g kg -1 ) and very low P (0.51-2.83 g kg -1 ) and Mg (1.73-2.92 g kg -1 ) concentrations. Acacia had the lowest N, P, K and Mg concentrations. Pre-planting soil analysis indicated that soil quality indicators were below the critical levels needed for a productive agricultural system, especially for phosphorus, sum of bases and base saturation. Physical indicators of quality, such as bulk density (1.40-1.30 Mg m -3 ), total porosity (0.46-0.50 m m -3 ) and soil aeration capacity (0.10-0.17 m m -3 ), were substantially improved as a result of the surface application of residues. There was a cumulative effect of residue application on corn crop productivity. Because of its capacity to recycle nutrients and improve soil quality over the period of 3 years, alley cropping in association with no-tillage, can be an efficient strategy for maintaining productivity in the low-fertility soils of the humid tropics.
Symbiotic biological nitrogen fixation (BNF) is a complex process that involves rhizobia, a diverse group of α and β-proteobacteria bacteria, and legume species. Benefits provided by BNF associated with legume trees in tropical environments include improvements to efficiency of nitrogen (N) use, increase of soil carbon sequestration, stabilization of soil organic matter, decrease of soil penetration resistance, and improvement of soil fertility. All these benefits make BNF a crucial ecosystem service to the sustainability of tropical agriculture. Due to the importance of this ecological process and the high diversity of rhizobia, these bacteria have been extensively characterized worldwide. Currently, over 400 species of rhizobia are known, distributed into seven families. In the humid tropics, Leucaena leucocephala, Acacia mangium, Gliricidia sepium, and Clitoria fairchildiana are four of the most common species used by family farmers to create sustainable agricultural systems. These four legumes perform symbiosis with different groups of rhizobia. Exploring BNF could help to enable sustainable intensification of agriculture in the humid tropics, mainly because it can increase N use efficiency in an environment where N is a limiting factor to plant growth.
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