EFFECT OF IRRIGATION AND HYBRIDS ON MICROBIAL POPULATION DYNAMICSIN THE RIZHOSPHERE OF OIL PALM ȍELAEIS GUINEENSIS JACQ.Ȏ* BOLANOS-CARRIEL C*, ORELLANA H, BERNAL G Department of Agronomy, Faculty of Agriculture, Central University of Ecuador, Quito 170150, Ecuador. Email: carlos.bolanos@upr.edu Received: 17 December 2015, Revised and Accepted: 23 February 2016 ABSTRACT Objective: This research was carried out to evaluate the effect of irrigation and hybrids on the population dynamics of microorganisms associated with the rhizosphere of oil palm (Elaeis guineensis Jacq.). Methods: Plots were installed under irrigated and non-irrigated conditions using three hybrids (Dura × Pisifera) of oil palm. A split-plot design was used with three replications. Samples of soil were extracted and processed to determine the populations of functional groups of heterotrophic bacteria, actinomycetes, fungi, phosphorus (P) solubilizing microorganisms, and vesicular-arbuscular (VA) mycorrhizae. Samples were taken during the wet (June) and the dry season (August). Results: No significant differences were detected for bacterial populations (heterotrophic and actinomycetes) for irrigation and hybrids. Populations recorded for heterotrophic bacteria when water stress accounted for 28 centibars (cbs) were 7.43 × 10 6 colony forming units per g of dry soil (CFU/g) in irrigated plots, versus 5.86 × 10 6 CFU/g in non-irrigated plots. Fungal populations were 4.49 × 10 3 CFU/g in irrigated plots, while in non-irrigated plots were 1.81 × 10 4 CFU/g. Mycorrhizae levels ranged between 13.6% and 21.9% in irrigated and non-irrigated plots, respectively. The populations of P-solubilizing microorganisms ranged from 6.64 × 10 5 CFU/g in irrigated plots to 2.91 × 10 4 CFU/g in plots without irrigation. Conclusions: Under the conditions of this study, in the oil palm rhizosphere, there were not significant differences between the bacterial populations (heterotrophs and actinomycetes) for irrigation and hybrids. Under the same conditions, significant differences were found among fungal populations, P-solubilizing microorganisms, and VA mycorrhizae, in the irrigation factor. The major bacterial populations were recorded in the rainy season 7 cbs; meanwhile, the higher fungal populations were recorded in the dry season (28 cbs). Hybrids not exert a major influence on microbial populations under the conditions of this study. With some of the microorganisms isolated in this study, starting tests are needed to identify potential bio-fertilizers, and/or bio-enhancers of soil properties. Keywords: Vesicular arbuscular mycorrhizae, Phosphorus-solubilizing microorganisms, Oil palm, Heterotrophs, Irrigation. INTRODUCTION Oil palm (Elaeis guineensis J.) is one of the most important crops in Ecuador with an area of 207.285 ha. It is also a crop of high socio-economic impact, and major source for production of clean and renewable biofuels. The main objective of the oil palm growers in Ecuador is increase yield; for this purpose, they apply good management practices for higher profitability [1]. However, there is a considerable reduction of yield due rapid change and impairment of the physical, chemical, and biological properties of soils. The latter has been largely unexplored; nevertheless, it is considered fundamental to study the soil biological community to solve the problem of exhaustion of soils and low production of plantations [2]. Oil palm demands large amounts of water, from germination to harvest of the last bunch at the end of the productive cycle. In Ecuador rainfall is irregular, therefore, it is necessary to keep the soil humidity in optimal status. Considering the soil as a major water reservoir and the depletion of this reserve for crop consumption and it will be required artificial recharge by irrigation [3]. On the other hand, oil palm extracts significant amounts of macro and micronutrients, for the production of an enormous quantity of biomass. Therefore, the exploitation of plant growing promoting rhizobacteria (PGPR’s) or mycorrhizae could contribute to the nutrient acquisition and nutrient recycle. The present study was carried out to identify and contrast the magnitude of different soil microorganisms under three hybrids of oil palm in irrigated and non-irrigated conditions during the dry and wet season. The final objective of the project will be to generate bio-inoculants and/ or bio-enhancers of soils that contribute to crop nutrition and crop protection in adverse seasons. This research was carried out in the “Centro de Investigaciones de Palma Aceitera” situated in the province of Esmeraldas – Ecuador, at 264 m above the sea level, latitude 0°2’29”; longitude 79°24’54”. The climate in this area is tropical with a temperature of 20-24°C, 626 hrs lux, relative humidity 87.8%, and annual precipitation of 2881 mm. Intraspecific hybrids correspond to the crosses Dura × Pisifera of E. guineensis: Hybrid from ASD Costa Rica (Deli × Ghana), hybrid from “Instituto Nacional de Investigaciones Agropecuarias” (INIAP) (Deli × Yangambi) and hybrid from “Centre de Coopération Internationale en Recherche Agronomique pour le Développement” (CIRAD) (Deli × La Mé 2501). Oil palms of 5 years after transplanting were used during this research. Factors in study were arranged in a split plot design with three replications, where irrigation received the main plots, and hybrids the subplots. The experimental area consisted of 18 plots with 30 oil palms each. The area of the main plots were 6230 m 2 (70 m × 89 m), and the subplot’s area was 2047 m 2 (23m × 89m). The experimental unit consisted of eight palms. Soil samples were collected using a disinfected drilling bore of 4 cm diameter in eight random locations at 1 m distance of the trunk of the palm. Samples were placed in Ziploc ® bags and transported on ice. In Short Communication Vol 4, Issue 2, 2016 ISSN - 2321-6832