"Role of Arbuscular Mycorrhizal Fungi in Nutrient Uptake and Growth of Durum Wheat"

Soil microbiome is involved at different levels in the food web, in bio-geochemical nutrient cycles and in several interactions with plants. Based on its key role in the agro-ecosystem processes, the soil microbiome has been identified as one of the principal factors in an agriculture addressed to the ecological intensification. Among the several relationships established between plants and soil microorganisms, arbuscular mycorrhizal (AM) symbiosis is the most widespread. Two out of three of all plant taxa (among others the main crops) are involved in the AM symbiosis which takes place between the plant root system and arbuscular mycorrhizal fungi (AMF), a monophyletic group of fungi belonging to the subphylum of Glomeromycotina. Although AM symbiosis can provide several positive services in the agroecosystem, the main benefit has always been highlighted in the increment of plant nutrition. However, the outcome of the AM symbiosis is context dependent. The greatest benefits ascribed to AMF has been observed on plant P acquisition under conditions of soil P-deficiency, whereas their contribution on plant N nutrition is still debate, since positive, neutral and negative effect has been observed. The reason of such contradictory results seems to rely on the soil N availability, since AMF have a notable N demand for their own metabolism and can even compete with the host plant for the soil N under soil N-deficiency. Additionally, although AMF can transfer N from organic source, no information is available on whether or not this ability change varying the organic source composition. Given the role of AMF in nutrient cycling, uptake and transfer to the host plant, increasing our knowledge about their role on plant nutrition is crucial in an agricultural addressed to the environment and economic sustainability. With the view to the agriculture sustainability, the reduction or the absence of tillage can provide several environmental and agronomical benefits. At the same time, different tillage system determines various pedo-climatic micro-environments which can profoundly influence the soil microbial community composition. Different pedo-climatic micro-environments are also observed along the soil profile with the consequence of drastic modifications in the community composition of bacteria and fungi (including AMF) along the soil depth. Modification in the community composition can differ in symbiotic efficiency and therefore drive the outcome of the interaction between crop and soil microorganisms. However, the potential effect of the different communities deriving from the above reported microenvironments on plant growth has not yet been investigated. In order to contribute to fill the above reported gap of knowledge a set of 4 experiments (described in chapter 2) was carried out. Two experiments (paragraph 2.1 and 2.2) aimed to evaluate the effect AM symbiosis outcome varying soil N and P availabilities, and the effect of AMF on plant growth, N uptake and N recovery from the applied fertilizer when N in soil was applied as mineral or organic source. The third experiment (paragraph 2.3) aimed to characterize the AMF community along the soil profile and to evaluate if the observed differences were able to affect the plant growth and nutrient uptake under adequate water availability or under drought stress. Finally, the fourth experiment (paragraph 2.4) focused on the effect of the soil microbial community deriving from different long-term tillage management and depth on plant growth an N uptake. In all the experiments durum wheat was used as focal plant. Results have shown that under soil N-deficiency AMF compete with the host plant determining a decrement of plant growth and N uptake. A negative effect of AMF on plant growth was also observed under very high soil N availability, in absence of other limiting factors. Whereas, a positive AMF effect was observed at intermediate soil N availability, when the host plant is still under N-limiting conditions and the fungal component has satisfied its own demand. In the latter case, AMF have shown the ability to transfer a substantial amount of N derived from mineral fertilizer and organic matter. However, the organic matter composition has strongly affected the effect of AMF on plant performance. In fact, while AMF increased the plant N recovery from the organic patch with a low C:N ratio, a detrimental effect of plant growth and N recovery was observed in presence of an organic source with a high C:N ratio. Results have also shown that the AM symbiosis outcome in presence of different soil N availability conditions may change in relation to the availability of other elements. In fact, while under conditions of high P availability, the mycorrhizal outcome shifted along the entire spectrum of the ecological relationships (mutualism, commensalism or parasitism) depending on the availability of N, under soil P-deficiency, AMF have always provided a benefit to the host plant, regardless the soil N availability. Results of the third experiment have highlighted a significant shift of AM fungal communities with depth and the existence of subsoil specific AM fungal phylotypes. The inoculation with living soil deriving from different depths resulted in variations in root colonization consistent with those detected by molecular analysis, but have had little or no effect on plant performance both with adequate water availability and in presence of drought. On the contrary, significant differences on root colonization, aboveground biomass production and N uptake were observed when plants were inoculated with living soil deriving from different tillage systems and soil depth (paragraph 2.4).