CHARACTERIZATION OF RHIZOBIA AND ARBUSCULAR MYCORRHIZAL FUNGI IN AREAS IMPACTED BY GRAVEL MINING IN BRAZIL

ABSTRACT On-shore oil exploration is one of the main economic activities in the semiarid region (Caatinga biome) of the state of Rio Grande do Norte (RN), Brazil. Gravel mining is an activity associated with oil exploration that causes environmental impacts. Gravel is a base-material for constructions, such as roads and pumpjack bases. The areas of gravel mining and other decommissioned areas where the gravel has been deposited must be revegetated with species native to the biome at end of the activity in the site. An efficient strategy for revegetation of degraded areas has been the planting of leguminous species that can associate with rhizobia and arbuscular mycorrhizal fungi (AMF). Nevertheless, the impact of mining activities on the autochthonous populations of these microorganisms is unknown. The objective of the present work is to characterize the density of rhizobia and AMF spores in four areas impacted by the oil exploration in RN when compared to non-impacted adjacent areas. Gravel samples were collected in dry and rainy seasons in two mining areas: one in a pumpjack base, and one in a waste disposal area. Surface soil (topsoil) samples were collected in adjacent areas with native Caatinga vegetation. Assays were carried out to evaluate the most probable number (MPN) of rhizobia and to identify the type and density of arbuscular mycorrhizal fungi (AMF), using rhizobia trap plants Mimosa tenuiflora (Mart.) Benth. and Mimosa caesalpiniifolia Benth. The MPN of rhizobia was low in all areas, but higher in the rainy season. The highest amount and diversity of AMF spores were found in the dry season. Among the symbiotic microorganisms studied, the AMF presented lower densities in the evaluated areas when compared to those in the adjcent native vegetation areas, denoting the impact of this activity.

摘要 陆上石油勘探是巴西北里奥格兰德州（RN）半干旱区域（卡廷加生物群系（Caatinga biome））的主要经济活动之一。砾石开采作为与石油勘探配套的产业，会对环境造成负面影响。砾石是道路、抽油机基座等建筑工程的基础基材。在场地作业结束后，砾石开采区及其他砾石堆积的废弃区域，需采用该生物群系的本土物种开展植被修复工作。修复退化区域的高效策略之一，是种植可与根瘤菌（rhizobia）及丛枝菌根真菌（arbuscular mycorrhizal fungi，AMF）形成共生关系的豆科物种。然而，采矿活动对这些微生物的本土种群所产生的影响仍未明确。本研究旨在表征北里奥格兰德州4处受石油勘探影响区域的根瘤菌密度与丛枝菌根真菌孢子密度，并与未受干扰的毗邻对照区域进行对比。研究人员于旱季与雨季在两处采矿区域采集砾石样本：一处位于抽油机基座，另一处为废弃物堆放区；同时在毗邻的原生卡廷加植被区域采集表层土壤（topsoil）样本。本实验采用根瘤菌诱捕植物细叶含羞草（Mimosa tenuiflora (Mart.) Benth.）和 Caesalpinia叶含羞草（Mimosa caesalpiniifolia Benth.），以测定根瘤菌的最大可能数（most probable number，MPN），并鉴定丛枝菌根真菌（AMF）的种类与密度。结果显示，所有区域的根瘤菌最大可能数均处于较低水平，但雨季的数值显著更高；丛枝菌根真菌孢子的数量与多样性则在旱季达到峰值。在所研究的共生微生物中，受评估的受影响区域内丛枝菌根真菌的密度相较于毗邻的原生植被区域更低，这表明该采矿活动确实对环境产生了干扰。