Arbuscular mycorrhizal fungi (AMFs) are soil microorganisms belonging to the Glomeromycota phylum and develop a symbiotic relationship with more than 80% of cultivated vascular plants, including cereals, legumes, root and tuber crops and forest and agroforestry trees. The symbiosis established between the two partners determines the functioning and development of the plant and the fungus (Větrovský et al., 2019). Various strains of AMF are distributed in several terrestrial ecosystems, from hostile to more favourable environments, and play different functions (Willis et al., 2013; Rillig et al., 2019). Especially in agroécosystems, the role of AMFs has been widely reported to enhance crop growth and development through biofertilization, bioprotection, improvement of phytoremediation efficiency of polluted soils and improvement of soil quality (Lenoir et al., 2016). As for biofertilization, the symbiosis formed by AMFs with the roots of plants can improve the plants' ability to absorb water and nutrients from the soil, particularly phosphorus (Tran et al., 2019 ; Chandrasekaran., 2020). Their role in improving the soil physical and chamical properties has long been documented by various researchers around the world (Rillig et al., 2006 ; Willis et al., 2013). Thus, through their services, mycorrhizal fungi are a key element in many biological processes in plants and soil. However, despite their importance in agriculture, there is too little informations on the influence that environmental factors, and more particularly soil properties, may have on virtually all aspects of mycorrhizal performance of a target crop in South Kivu province. Furthermore, we do not know how soil properties might influence the distribution and abundance of fungal propagules in the soil.
Changes in environmental parameters mainly affect the performance of established crops and indirectly the carbon cycle (Gavito et al., 2005; Gavito & Azcon-Aguilar, 2012). These changes impose pressure on the composition, abundance and activity of symbiotic microorganisms in the soil, particularly AMFs (Compant et al., 2010). It has been shown that the intensity of mycorrhizal infection is strongly influenced by climatic and edaphic factors such as soil acidity, carbon/nitrogen ratio and rainfall seasonality (Soudzilovskaia, 2015). Other research reported that high levels of available phosphorus and high electrical conductivity in the soil reduce the AMF richness and diversity (Abdedaiem et al., 2020). On the other hand, the cropping system has a significant influence on the efficiency of AMFs. Plants grown in low-input farming systems show higher mycorrhizal colonization than those grown in conventional soils (Mäder et al., 2000). In addition, the AMF diversity and abundance are strongly influenced by plant cover and phosphate fertilization, which determine their efficiency in agrosystems (Requena et al., 2001). Furthermore, the functioning of the symbiosis between plants and AMF strains is determined from different parameters, the main ones being the mycorrhization frequency and intensity (Soudzilovskaia et al., 2015; Rillig et al., 2019). Mycorrhization intensity reflects the level of mycorrhization and is typically expressed as the percentage of root length colonized by fungi. This index remains the best available quantitative measure of the degree of plant-fungi interaction in the field. It determines also the functioning of the symbiosis (Liu et al., 2019).
The response of plants to mycorrhizal inoculation is affected by several parameters, the main ones being the degree of mycorrhizal dependency of the host plant (Azcon-Aguilar et al., 2003), the density of propagules of native fungal strains in the soil as well as other edaphic conditions such as soil pH, nutrient concentration, soil structure, moisture, temperature and other abiotic stresses (Soudzilovskaia., 2015). The soil mycorrhizal potential (MP) is an important parameter that determines the abundance of fungal propagules in the soil including spores, fungal hyphae and fragments of mycorrhized roots. It also reflects the capacity of a given soil to induce mycorrhisation (Azcon-Aguilar et al., 2003). Furthermore, these parameters show a high susceptibility to land use change and soil degradation by ploughing, overuse of chemical fertilisers, use of chemical pesticides (insecticides and fungicides) and change in soil nutrient balance (Bainard et al, 2017; Borriello et al., 2012; Jansa et al., 2003). If environmental and edaphic parameters can influence the behaviour of AM fungal strains and the density of fungal propagules, then soil management practices, including poor cultivation practices and fertilization, can negatively influence the development of AM fungi by creating inappropriate conditions, which could impair their contribution to plant growth (Melo et al., 20219).
In the South-Kivu, farmers engage in various soil fertility management practices to maximise crop yields. Some opt for good practices such as organic fertilisation, mulching, etc., while many practise bush farming and cultivation on sloping land (Chuma et al., 2022). In addition, it has been shown that different farming practices have a significant influence on soil properties and affect the soil microbial diversity (Fasusi et al., 2021). However, a systematic analysis of the mycorrhizal status and the natural mycorrhization of crops has not yet been performed in South-Kivu. Furthermore, the impact of environmental parameters on the mycorrhizal status of crops has not yet been evaluated. The aim of this study was therefore to (i) determine the mycorrhizal potential of soils collected in two territories of South-Kivu province (Walungu and Kabare) in the rhizosphere of bean (Phaseolus vulgaris L), (ii) to determine the natural mycorrhization rate of bean plants, (iii) to determine the AMF spores density and finally (iv) to evaluate the impact of soil properties on the mycorrhizal potential and natural mycorrhization of bean in Walungu and Kabare. For this study, bean was selected because it is the main legume crop in South-Kivu (Mushagalusa et al., 2020).