Sanâa Wahbi

Impact of Wheat/Faba Bean Mixed Cropping or Rotation Systems on Soil Microbial Functionalities

Cropping systems based on carefully designed species mixtures reveal many potential advantages in terms of enhancing crop productivity, reducing pest and diseases, and enhancing ecological services. Associating cereals and legume production either through intercropping or rotations might be a relevant strategy of producing both type of culture, while benefiting from combined nitrogen fixed by the legume through its symbiotic association with nitrogen-fixing bacteria, and from a better use of P and water through mycorrhizal associations. These practices also participate to the diversification of agricultural productions, enabling to secure the regularity of income returns across the seasonal and climatic uncertainties. In this context, we designed a field experiment aiming to estimate the 2 years impact of these practices on wheat yield and on soil microbial activities as estimated through Substrate Induced Respiration method and mycorrhizal soil infectivity (MSI) measurement. It is expected that understanding soil microbial functionalities in response to these agricultural practices might allows to target the best type of combination, in regard to crop productivity. We found that the tested cropping systems largely impacted soil microbial functionalities and MSI. Intercropping gave better results in terms of crop productivity than the rotation practice after two cropping seasons. Benefits resulting from intercrop should be highly linked with changes recorded on soil microbial functionalities.

Managing the Soil Mycorrhizal Infectivity to Improve the Agronomic Efficiency of Key Processes from Natural Ecosystems Integrated in Agricultural Management Systems

Phosphorus (P) is a major nutrient limiting plant growth in many soils. To reduce P deficiencies and ensure plant productivity, large quantities of soluble forms of P fertilizers are applied worldwide every year. However up to 80 % of P chemical fertilizer amendments are lost as it is easily precipitated into insoluble forms (CaHPO4, Ca3(PO4)2, FePO4, and AlPO4) and becomes unavailable for plant uptake. Some soil microorganisms are known to be involved in the solubilization of insoluble phosphate by excreting organic acids, phenolic compounds, protons, and siderophores. Among phosphate-solubilizing microorganisms, it has been reported that mycorrhizal fungi have the ability to actively mobilize and translocate nutrients from minerals and soil organic matter, directly to their host plant. Mycorrhizal fungi constitute a key functional group of soil biota that greatly contribute to productivity and sustainability of terrestrial ecosystems. These are ubiquitous components of most of the ecosystems throughout the world and considered key ecological factors in governing the cycles of major plant nutrients and in sustaining the vegetation cover. It has been suggested that the integration of key processes from natural ecosystems (plant-plant facilitation, positive plant soil feedback) in agricultural management systems could resolve increasing agricultural problems. Since these natural processes are frequently connected with arbuscular mycorrhizas, it is necessary to apply mycorrhizal inoculation technologies or to manage native arbuscular mycorrhizal fungus communities to replace or reinforce the mycorrhizal potential in these degraded areas. This chapter aims to describe the influence of some cultural practices (rotation, intercropping, mycorrhizal inoculation) that mimic these natural processes in agrosystems, on soil microbiota (i.e.: soil mycorrhizal infectivity) leading to a sustainable microbial complex with high efficiency against phosphorus mobilization and transferring phosphorus from the soil organic matter or from soil minerals to the host plant.

Increasing the Role of Mycorrhizal Symbiosis in Plant-Plant Facilitation Process to Improve the Productivity and Sustainability of Mediterranean Agrosystems

Plant-plant facilitation is an ecological process occurring in most terrestrial ecosystems. Plant-plant facilitation is considered as a positive interaction between both plant partners in which one plant species promotes the growth, survival or reproduction of the neighbouring plant. Recent studies have underlined the role of mycorrhizal fungi, i.e. arbuscular mycorrhizal fungi (AMF) and particularly extraradical hyphae of AMF, in interconnecting plants and consequently their importance in plant-plant facilitation process. Networks of AMF impact soils both physically and biologically and are considered as an important pathway for the transference of nutrients such as nitrogen and phosphorus. In parallel, AMF increase plant growth and nutrient uptake and decrease the deleterious effects of pathogens and drought. The aims of this chapter are to describe how the AMF are involved in the plant facilitation process and to assess the main mycorrhizal effects on the plant nutrition (P and N) and plant health. Also the contribution of AMF in cropping systems as well as agricultural strategies which improve AM associations in arid agrosystems will be reviewed and illustrated by experimental results from field studies in Mediterranean environment.