Grazing-induced floristic changes in plant communities may accelerate or reduce plant and soil processes through changes in litter quality. Here, we intended to elucidate if the joint action of live and senescing plant tissue of palatable and non-palatable species differentially influences soil processes and properties. We conducted a 1-year experiment with mesocosms from a subhumid grassland. Mesocosms were monocultures of palatable or non-palatable species and a multispecific control. Palatable species included a legume and annual and perennial grasses, whereas non-palatable species included a perennial grass and annual and perennial forbs. Palatable monocultures showed greater soil mineral nitrogen, soil bacterial diversity, and lower soil pH than nonpalatable monocultures. These differences were not accounted for by differences in plant biomass. The multispecific control treatment only exhibited greater shoot biomass than the monocultures, and lower root biomass than the palatable monocultures. Our results suggest that the whole (live + dead plant tissue) had a specific imprint on soil system even when variation was not very apparent in terms of plant biomass, and that this effect was associated with plant palatability to domestic large herbivores.

<p>El presente artículo surge de la revisión de la teoría y temas prácticos desarrollados durante el curso”Caracterización y contribución de las plantas que promueven el crecimiento de microorganismos en la sostenibilidad de la agricultura”, llevado a cabo en el Laboratorio de Microbiología de Suelos de la Corporación Colombiana de Investigación Agropecuaria (Corpoica), ubicado en Mosquera (Cundinamarca), Colombia, en julio de 2010. Esta actividad fue desarrollada en el marco de la Red Dimiagri que incluye a investigadores de Argentina, Brasil, Colombia, España, Guatemala, México y Uruguay, reunidos en una acción de coordinación financiada por el Programa Iberoamericano de Ciencia y Tecnología para el Desarrollo (Cyted). Los aspectos inherentes al crecimiento y la sanidad vegetal, el sistema radical, el suelo circundante (rizósfera), los microorganismos asociados en ese sistema y su contribución al manejo sustentable del complejo suelo-planta fueron analizados en este trabajo. También se abordan temas como la biodiversidad microbiana y su efecto en la calidad del suelo; el ciclado de nutrientes del suelo por acción microbiológica; la importancia de los microorganismos en la promoción del crecimiento vegetal y su utilización biotecnológica como alternativa para favorecer la sustentabilidad y calidad de los suelos. Además se pretende interiorizar en los conceptos relacionados con el consorcio suelo-planta-microorganismo y el objetivo de mitigar el impacto ambiental negativo causado por el uso excesivo de insumos químicos en los cultivos agrícolas, mediante la utilización de microorganismos promotores del crecimiento vegetal, que incluyen tanto a bacterias como a hongos benéficos asociados con las raíces de las plantas.</p><p> </p><p><strong>Microorganisms that enhance plant growth and soil quality. Review</strong></p><p>The present article of revision arise from theory and practical subjects developed during the course “Characterization and contribution of plant growth-promoting microorganisms in the agricultural sustainability” carried out in the Laboratory of Soil Microbiology of the Colombian Corporation of Agricultural Research (Corpoica) located in Mosquera (Cundinamarca), Colombia, in July 2010. This activity is in the framework of the Dimiagri network that includes researchers from Argentina, Brazil, Colombia, Spain, Guatemala, Mexico and Uruguay, gathered in a Coordination Action funded by the Iberoamerican Program of Science and Technology for the Development (Cyted). Aspects inherent to the growth and plant health, root system, the surrounding soil (rhizosphere), microorganisms that system partners and their contribution to sustainable management of soil-plant were analyzed in this work. Topics related to the microbial biodiversity and its effect on soil quality; nutrient cycling in the soil by microbiological activity; the importance of microorganisms in plant growthpromotion and their biotechnological application as an alternative to favor sustainability and soil quality were presented. The aim of this review is to show important concepts related to the soil-plant-microorganism system, which will allow to achieve the general objective: to mitigate the negative environmental impact due to the excessive use of chemical products on agricultural crops by using plant growth-promoting microorganisms, including bacteria and beneficial fungi associated to plant roots.</p>

Plant growth-promoting bacteria of the genus Azospirillum are present in the rhizosphere and as endophytes of many crops. In this research we studied 40 Azospirillum strains isolated from different plants and geographic regions. They were first characterized by 16S rDNA restriction analysis, and their phylogenetic position was established by sequencing the genes 16S rDNA, ipdC, hisC1, and hisC2. The latter three genes are involved in the indole-3-pyruvic acid (IPyA) biosynthesis pathway of indole-3-acetic acid (IAA). Furthermore, the suitability of the 16S-23S rDNA intergenic spacer sequence (IGS) for the differentiation of closely related Azospirillum taxa and development of PCR protocols allows for specific detection of strains. The IGS-RFLP analysis enabled intraspecies differentiation, particularly of Azospirillum brasilense and Azospirillum lipoferum strains. Results demonstrated that the ipdC, hisC1, and hisC2 genes are highly conserved in all the assessed A. brasilense isolates, suggesting that these genes can be used as an alternative phylogenetic marker. In addition, IAA production determined by HPLC ranged from 0.17 to 98.2 μg mg(-1) protein. Southern hybridization with the A. brasilense ipdC gene probe did not show, a hybridization signal with A. lipoferum, Azospirillum amazonense, Azospirillum halopreferans and Azospirillum irakense genomic DNA. This suggests that these species produce IAA by other pathways. Because IAA is mainly synthesized via the IPyA pathway in A. brasilense strains, a species that is used worldwide in agriculture, the identification of ipdC, hisC1, and hisC2 genes by PCR may be suitable for selecting exploitable strains.

Azospirillum has been one of the most studied genera of plant growth promoting rhizobacteria (PGPR) worldwide over the past 50 years. The use of these microorganisms in agriculture practices has been adopted due to their ability to associate in rhizospheric, epiphytic, or endophytic ways with roots and promote whole plant growth or crop productivity. The biological treatment of seeds (inoculation) in more