Impact of crop rotation on mycorrhizal fungi in irrigated soils of the Doukkala (Morocco)
Abstract
This study has been conducted on the soils of irrigated perimeter of Doukkala. It is to assess the impact of the rotation of a species not mycotrophic (Beta vulgaris) with cereals on the natural resources of the mycorrhizal arbuscular fungi whose profits for the growth of plants are known. The results obtained show that this type of culture has a negative effect on the richness of the soil in spores and diversity of mycorrhizal fungi, and on the content of arbuscules which are the places of exchanges between the partners. The authors offer recommendations on cultivation practices which can be modulated in order to preserve this natural resource.
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Introduction
In Morocco, the irrationalisation of cropping practices, agricultural intensification, inappropriate exploitation of nonrenewable natural resources and irrigation are the origin of many soil degradation processes, which are likely to limit their productivity and hinder a sustainable management of this natural resource. Indeed, the work of (Naman et al., 2001) on soils in irrigated areas semi-arid, with agricultural intensification, has highlighted an important decrease of the organic matter and nitrogen component. In addition, Badraoui and al., (1998) have demonstrated that irrigation in the semi-arid areas caused a significant increase of the electrical conductivity annual rate, an increase in the exchangeable sodium and a salinization of soils. In the irrigated perimeter of the Doukkala, despite the efforts made by the guardianship organisms especially after the introduction of the sugar beet, the intensification of the development of agricultural land has generated a deterioration of the quality of the soil and groundwater (Naman and Soudi, 1999; Rahoui et al., 1999; Naman et al., 2001; Naman, 2003). As well, the major worry concern is no longer only the improvement and stabilization of yields, but also the preservation and improvement of the quality of soils; the only guarantee of a sustainable agriculture.
This problem has had the merit of implementing several methods of analysis mainly physico-chemical to assess the state of health of soils. However, the biological component which plays a vital role in the process of mineralization and transformation of organic matter (Chaussod, 1996) has been poorly studied in the soils degraded of semi-arid Moroccan (Nouaim, 1994; Naman, 2003). Thus, the microbial biomass of the soil, mainly the mycorrhizal fungi that form symbioses with the majority of plant species with a few exceptions such as the beet, it played an important role in the transfer of the mineral elements to the plant (Boudarga, 1989; Chaussod, 1996). In effect, the mycelium of these fungi may exceed tens of kilometers per gram of soil and thus represent an extensive trail allowing the flow of nutrients through the soil to the host plant that benefit as well of mineral elements located beyond the area of exhaustion. The researchs of Brundett (1991), Requena et al., (1996), Graf (2004) and Planchette (2005) have demonstrated that these fungi have a very important role in the aggregation and regeneration of semi-arid soils. In addition, it recognizes their role bio-fertilizer (Gianinazzi-Pearson and Gianinazzi, 1986; Strulu, 1991; Müller A.M.et al., 2013), bio-protector (Sylvia and Williams, 1992; Charest et al., 1993; Paradis et al., 1995) and their participation in the regulation of the pool of carbon in the atmosphere and in the soil (Söderström, 2002). It is therefore biological agents, which represent an important component of fertility (Müller et al., 2013), and allow improving the quality of organic soils while ensuring the sustainability of agrosystem.
Except our preliminary work on the mycorrhizae of agricultural soils in the Doukkala area (Boudarga 2004; 2007; 2009) who study the richness in mycorrhizal fungi, we had no reference concerning the biological quality of these soils and more particularly the mycorrhizae which are today considered as biological indicators of the agricultural soils quality (Chaussod, 1996; Kling and Jakobsen, 1998; Egli, 2001).
The aim of this work is to study the impact of sugar beet culture on the richness and the mycorrhizal potential of these soils. The beet is a not mycotrophic plant cultivated in the irrigated area of the semi-arid zone of the Doukkala,The evaluation will focus on the quantification of the number of spores as well as their identification and on the mycorrhizal activity in the root system of cultivated plants. In parallel, and with the aim of having a control to observe the effect of the culture of the beet on the biodiversity of mycorrhizal fungi arbuscular, a study on the diversity in these fungi is carried out in a region not irrigated (bour) never cultivated by the beet.
Conclusion
In light of these results, it clearly appears that the cultivation of sugar beet, which is a non mycotrophic plant and is widely cultivated in the Doukkala region, acts negatively on the rate of spores of mycorrhizal fungi and this was confirmed in the two regions. A similar result was found by Black and Tinker (1979) on the rotation Barley- Beet.Number of spores quantified showed dominance spores whose size is between 50-100μm in both regions, indicating these fungi are preferred by this type of rotation.
Although there is no host specificity in arbuscular mycorrhizal symbiosis, these results show the existence of an ecological specificity consistent with results of Gollotte et al. (2002) in prairie soils. Similarly, Oehl and al. (2003) showed that agricultural intensification causes a significant reduction in species diversity of arbuscular mycorrhizae. This leads us to assume that the cereal-beet rotation practiced over many successive years in the study area or the conduct of monocultures, will probably lead to a decrease in species diversity. In both regions, the analysis of the roots of cereals grown after beet showed a reduction in the amount of arbuscules, indicating a disturbance in the relationship between the two partners and affects the physiology and thus the growth of the plant. This similar behavior in the two regions shows that the plant reacts in the same way.
Concerning the biodiversity our results are consistent with other studies which have demonstrated the predominance of species of the genus Glomus in arid and semi-arid areas that adapt to water stress and saline.The Glomus species of were often cited as abundant species in the cultivated soil (Kachkouch et al., 2012, Sghir et al., 2015). Other authors also reported the presence of Glomus in arable soils, is one of the reasons for their dominance and their adaptation to disturbed environment (Bakkali Yakhle et al., 2011).
The results suggest that the intensive culture of species non-mycotrophic such that the beet associated to other factors (physico-chemical, work the ground etc.) significantly reduces the diversity of arbuscular fungi which is reflected on the total number of spores, on diversity and on the mycorrhizal activity (Zhao and Zhao, 2007, Maurer et al., 2014). Although there is not a specificity host-fungus in the arbuscular symbiosis, the existence of an ecological specificity as highlighted by Gollotte et al. (2002) in grassland soils expressed in our case through the variations of the biodiversity of the mycorrhizal profile.
The results obtained in this work have shown the impact of the culture of a species not mycotrophic on the number of spores of mycorrhizal fungi, their diversity and on the potential of functioning of the mycorrhizae within the roots. In the light of these results, we can propose certain practices in order to preserve the strains mycorrhizae who inhabit our soils:
• Alternate lines of mycotrophic species such as leeks with beets. for maintaining mycorrhizal strains in soil.
• Avoid making the same rotations on the same plots, which may favor certain species of mycorrhizal fungi and thus reduce diversity.
• Avoid rotation (beet- beet ) which leads to further reduce mycorrhizal population..
A rational management of our agricultural soils would keep this fundamental heritage necessary for sustainable agriculture.