Abstract
Aim: The aim of this study was to compare the texture, mineral element and heavy metal content of the soil under the litter of different selected species in relation to the plan of their pedogenesis.
 Methods: The granulometric analysis of the various samples based on laser diffraction, measurements of Total Nitrogen and Total Organic Carbon were performed using the Elemental Analyzer, the Organic Matter content is estimated by the loss of ignition method using a Salvis furnace, In order to estimate the CaCO3 content. The same samples were heated in the oven at 1000°C for 60 minutes and reweighed. The carbonate content is estimated by the loss in mass during this second firing, multiplied by 2.274 which is the molecular weight ratio between CaCO3 and CO2. The heavy metal and mineral composition was evaluated by ICP-AES and AAS.
 Results: The granulometric analyses show a sandy texture, according to the FAO classification. The organic matter content thus observed in the soils under the litter of these four species did not show any significant difference. The concentration of major mineral elements recorded in soils under the litter of Blighia welwitchii, Oncoba welwitschii, Zanthoxylum gilletii and Harungana madagascariensis did not show any significant difference. However, potassium, phosphorus, and magnesium have a high concentration compared to calcium and sodium. This study showed that the levels of cadmium, lead, arsenic and aluminum in the various samples of litter soils of all plant species including nickel for B. welwitschii are above the standards set by the Canadian Council of Ministers of the Environment for soil quality. Pollution factors for aluminum are the highest. This indicates that aluminum is the most polluting metal. The low pollution factors for lead and nickel are due to the low fixation of these metals by soils.
 Conclusions: Our study took place in the Luki Biosphere reserve and showed that the different soils under litter are polluted by very toxic and persistent heavy metals such as cadmium, lead, aluminum, arsenic and nickel. This study has revealed new aspects of heavy metal pollution. This pollution represents a serious threat to the environment in general and to humans in particular through the food chain.
Highlights
Soil is one of the main components of tropical natural resources, and has been considered for several decades as the environmental interface of communities 1
The minerals in this figure showed that the soils underneath the litter of each plant species under study have evolved so much that they are in a very advanced stage of alteration
The concentration of major mineral elements recorded in soils under the litter of each plant species by the spectrometry method Atomic Absorption Spectrometry (AAS) and Inductively Coupled Argon Plasma Optical Emission Spectrometry (ICP-AES) did not show any significant difference
Summary
Soil is one of the main components of tropical natural resources, and has been considered for several decades as the environmental interface of communities 1. Heavy metals are trace metal elements that are highly toxic and persistent in the environment 4,5. Work on heavy metals has begun to attract the attention of scientists because of their non-biodegradability, toxicity, persistence, and their presence in food chain 6. Heavy metals are among the widespread pollutants resulting from anthropogenic activities 7 and given the fact that they do not degrade, can migrate and accumulate in different components of natural ecosystems and eventually be incorporated into the food chain endangering the health of populations 8,9
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