Agronomic role of arbuscolar mycorrhizal fungi in agro-ecosystems managed with organic wastes and low quality water

In the conventional or intensive agriculture systems, to support the ever-increasing demand for agricultural products, the widespread use of mineral fertilizers and agro-chemicals (fungicides and insecticides), has led to the loss of soil fertility and soil biodiversity with a negative impact on the sustainability of the agro-ecosystem. In this context, a viable alternative to conventional or intensive agriculture systems, can be considered the low-input and/or organic ones, based on the use of organic fertilizer and/or amendment, and on the promotion of the role played by native soil microorganism or by bio-fertilizers introduced ones, to maintain the crop productivity. Arbuscular mycorrhizal fungi (AMF) are an important group of soil microorganism that establish a symbiotic relationship with about 80-90% of plants. AMF play an essential role to support a sustainable environment, improving the nutrient availability (particularly P) to the plant, enhancing tolerance and/or resistance to abiotic and biotic stress, thus maintaining the crop productivity. Their beneficial effects can be inhibited or reduced by conventional or intensive agriculture practices. However, these negative effects can be mitigated by the use of organic wastes as fertilizer/amendment produced by agro-industrial activities (olive mill wastewater) and biogas process (digestate). Digestate ((liquid (DLF) and solid fraction (DSF)) is a by-product of anaerobic digestion process for biogas production, characterized by a different physical-chemical composition mainly due to degraded feedstock. Generally, DLF is characterized by a high NH4-H:TN ratio, lower organic matter contents and N:P ratio shifted to nitrogen (N), while opposite characteristics are found in DSF. The physical-chemical characteristics of digestate determine its agronomic value, which can be spreaded (DLF) or applied (DSF) directly in soil as organic fertilizer/amendment. Although several studies reported a digestate positive effect on soil microbial communities, to our knowledge the effect of DLF and DSF distribution on AMF inoculated plants has not yet been investigated. In Mediterranean basin, from olive oil process, large amounts of olive mill wastewater (OMW) are produced in a short-period of time (2-4 months during year). Also the OMWs, in relation to their physical-chemical composition, can be used in agriculture as fertilizer/amendment in soil, especially providing organic matter and potassium (K). Nevertheless, OMW has antimicrobial and phytotoxic effects due to the high contents of phenolic compounds. So far, the OMW spreading effect on AMF symbiosis is poorly investigated. However, some authors reported a negative OMW effect on AMF symbiosis with a decrease of root colonization. In semi-arid and arid Mediterranean regions with limited freshwater resources, especially during spring-summer period, the use of poor quality water for irrigation is becoming a viable alternative to maintain crop yield and agriculture sustainability. Although this wastewaters contain dissolved salts which can negatively affect the soil microbial communities and crop production, it is widely demonstrated that AMF symbiosis improves nutrient and water uptake under saline conditions, even if the physiological and biochemical mechanisms involved are still unclear. Considering the above mentioned, the aim of this Ph. D. research was to study and evaluation, in two different Italian bioclimatic contexts (Sicily and Veneto), AMF inoculation effects on biomass production on energy crops fertilized with digestate (liquid and solid fraction) and forage crops fertilized with olive mill wastewater or irrigated with poor quality water. The research activities conducted in Veneto has been focused on the agronomic role of arbuscular mycorrhizal fungi inoculation, considering also the environmental impacts in terms of soil CO2 emissions and nitrogen leaching, on: i) energy crop biomass production using DLF as fertilizer; ii) triticale biomass production under different fertilization rates (mineral fertilizer, DLF, DSF and not fertilizer) using seed-coated fungicides. The most relevant findings are the following: - on energy crops: AMF inoculation did not increase the biomass production, except in Jerusalem artichoke managed with double biomass cutting. In the un-inoculated plots, a root colonization by indigenous mycorrhizal community was found. AMF inoculation contributed to NH4-N leaching reduction but increased NO3-N leaching and soil CO2 emission. Among the studied species, giant reed was the most productive energy crop in terms of biomass production, followed by miscanthus, sorghum, maize, Jerusalem artichoke, and lolium; however it showed a lower nitrogen use efficiency and higher cumulative soil CO2-C emissions. Miscanthus and sorghum showed the best N and P use efficiency, with a moderate NO3-N leaching and lower cumulative soil CO2-C emissions. - In triticale: AMF root colonization was inhibited by the presence of seed-coated fungicide. Nevertheless, in DSF treatment, the highest AMF colonization observed was probably due to the low available P form and the high organic matter content which probably contributed to mitigate fungicide negative effect. Mineral fertilization determined the highest triticale dry biomass production compared to organic fertilizer (DLF and DSF) even if the environmental (e.g. higher CO2(eq) emission) and economical (e.g. fertilizer costs) aspects should be considered. The researches conducted in Sicily have been focused on AMF inoculation effects on: i) a three years legume-based succession producing forage (Triticum durum Desf.-Medicago scutellata L. intercropping), broad bean (Vicia faba L. minor) and chickpea (Cicer arietinum L.) grain, using different OMW volumes as the only fertilization source; ii) a yearly evaluation of millet genotypes for forage production using irrigation water at two salinity levels and two water restitution regimes. The most relevant findings are the following: - on crop succession: in the first year, AMF inoculation increased the durum wheat biomass yield, but not affected the medicago scutellata biomass production. AMF root colonization was not influenced by OMW volumes, with a significant higher N and P uptake observed in inoculated durum wheat; while, no statistical effect of AMF was found on N and P uptake in scutellata. In the second year, AMF inoculation did not affect broad bean grain yield and determined a higher P uptake in broad bean grain. Moreover, AMF inoculation promoted root nodule number and weight in broad bean in absence of OMW volumes. Considering the two years, OMW spreading promoted the fabaceae productions with a higher biomass in scutellata and grain yield in broad bean, whereas it did not show any effect on durum wheat biomass production. OMW spreading reduced the weed presence in the first year and broad bean nodulation in the second one. - On millet: salt stress conditions negatively influenced AMF inoculum effects on forage production, without any difference between the two studied genotypes (Unikum and Kinelskoje). In absence of salinity treatment, AMF inoculation increased the forage production only in Unikum. Under water stress (25% ETm restitution) AMF inoculation promoted the forage production, whereas no significant AMF effect was observed in well-watered condition. Unikum proved to be the best millet genotype in experimental field conditions with more than double forage production compared to Kinelskoje