Reducing the effects of soil heavy metal pollution in Vicia villosa L. by using biochar, trichoderma and phosphorus fertilizer management

IntroductionHeavy metal contamination in soil refers to the excessive accumulation of elements called heavy metals, which are capable of causing significant levels of biological toxicity. These metals, in concentrations higher than of tolerable threshold of plants, often cause metabolic disorders, inhibition of growth and physiological process and eventually lead to death of plants. Lead (Pb) and zinc (Zn), as the two common coexisting mineral heavy metals around the world, accumulate in the soil due to mining activity and can adversely affect the crop productivity, the agro-ecosystem components and ultimately the human and environmental health. Biochar, as a potent soil amendment agent, is often obtained by pyrolysis of agricultural organic wastes such as crop residues under low oxygen pressure condition. Trichoderma harzianum, as a plant growth promoter, has capability in stimulating plant growth under varying environmental stresses and therefore, inoculation of plant roots with this fungus could be a conceivable approach to cope with the heavy metal stress. Hairy vetch (Vicia villosa Roth) is a winter annual forage species that shows better growth performance than the other forage legumes such as alfalfa or clover under the severe environmental conditions, especially winter frost and drought.phosphorus supplementation and their interactions on the growth, physiology and P, Pb and Zn uptake by hairy vetch in heavy metal-polluted soil.Materials and methodsThe experiment was carried out in the greenhouse of Faculty of Agriculture and Natural Resources of Mohaghegh Ardabili University as a factorial experiment based on a completely randomized design (CRD) in three replications. Heavy metal contaminated soil was collected from wheat fields around Angouran village (Angouran Rural District, Angouran District, Mahneshan County, Zanjan Province, Iran), downstream of the Zanjan's lead and zinc processing plant (Calcimin® Co.; 36°34'13.2"N 47°37'21.4"E), from a depth of about 0 to 25 cm. Two levels of biochar application (non-biochar control, 5% peanut hull-derived biochar), two levels of Trichoderma inoculation (non-inoculation control and inoculation with conidial suspension) and three levels of P supplementation (0P, non-supplementation; 11P, 11 mg P.kg-1 and 22P, 22 mg P.kg-1) were applied as the three experimental factors. The feedstock of peanut hull (PH) used in the present experiment were purchased from the local market. After thorough washing, PHs were air-dried and then well ground with a lab-scale mill and sieved using a 100 mesh sieve size. T. harzianum Rifai - T22 isolate acquired from the Laboratory of Plant Pathology, Faculty of Agriculture and Natural Resources, University of Mohaghegh Ardabili was revived on PDA (potato dextrose agar) medium and further incubated for at least one week at 28 °C. This experiment was conducted in an open-air vegetation yard with mean day/night temperatures of 25/17 °C, respectively. The experiment lasted for 50 days, in the period from May 16, 2020 (sowing the seeds) until July 5, 2020 (final sampling). In each plastic pot (25 cm in diameter × 20 cm in height), pre-filled with the pot mixture according to the treatments, 20 surface-sterilized hairy vetch (Vicia villosa Roth., Local landrace from Ardabil, Iran) seeds were sown and irrigated with tap water (EC = 0.21 dS m-1 and pH = 7.2) to 100% field capacity. Throughout the experiment, all the pots were re-irrigated when the soil water content dropped to 70% of the field capacity.Results and discussionApplication of biochar and Trichoderma fungus at not using phosphorus (P0), resulted in higher aerial biomass. Trichoderma fungus alone or with P11 level had a better result in increasing plant height. The concentration of chlorophyll a was in highest level by biochar combined Trichoderma, and P0 phosphorus applied, as well as the application of Trichoderma caused the highest concentration of chlorophyll b. The changes in relative leaf water content ranged from 85.5 to 90.5% among the treatment combinations. Phosphorus consumption at the maximum level (P22) showed the same effect in improving leaf protein compared to the P0 level. Moderate phosphorus consumption (P11) combined with biochar and Trichoderma caused the highest soluble sugar content. The highest proline content of leaves was obtained by using Trichoderma without applying biochar and phosphorus. The use of biochar alone or with Trichoderma could improve the amount of phosphorus absorption as much as the use of phosphorus fertilizer alone. Combined application of biochar and Trichoderma fungus with P11 and P22 fertilizer levels increased phosphorus absorption by 41.6% and 22.3% respectively compared to P0 level. Trichoderma fungus had no significant effect on the amount of Pb and Zn absorption in the aerial parts, but biochar significantly reduced their absorption in the aerial parts of Hairy vetch plants.ConclusionBased on the results, the application of biochar alone or in combination with Trichoderma fungus improved the vegetative traits and physiological characteristics, including increasing the concentration of photosynthetic pigments, the relative content of leaf water and soluble sugar, the content of protein and proline in the cluster flower vetch. Phosphorus absorption increased significantly when using biochar and Trichoderma. It seems that biochar helps to increase photosynthesis by creating suitable conditions in soil porosity, humidity, increasing cation exchange capacity and reducing the concentration of heavy elements, with better absorption of elements and thus improving plant growth and development. Trichoderma fungus with biological control increases the availability of elements including phosphorus by increasing the transfer of sugar and amino acids and creating induction resistance and stimulating the growth of beneficial microorganisms, and by making changes in the physiological characteristics, it increases the resistance of the plant against the stress of heavy elements.