Contrasting impact of soil amendments on bioavailability, mobility and speciation of zinc in an acidic sandy loam soil

Fifty percent of Indian soil is deficient in zinc (Zn), on the contrary to it, anthropogenic activities are causing Zn contamination in soil. Metal speciation in soil reflects its fate of mobility, bioavailability and chemical behavior. Soil amendments play crucial role on metal bioavailability. A pot experiment was conducted to investigate the effects of hardwood biochar (BC), vermicompost (VC) and farmyard manure (FYM) (2.77 g kg−1) on Zn speciation and phytoavailability in a Zn contaminated (150, 250, 300, and 350 mg kg−1) acidic sandy loam soil (pH 6.41) using Xanthium strumarium L. as an indicator plant. The amendments significantly enhanced the photosynthetic rate (PN), dry biomass and tolerance index (TI) of the plant. The efficiency was in the order VC > FYM > BC. Reduced antioxidants (SOD, CAT, POD, and flavonoids) activities under application of the amendments signify their efficacy to diminish the metal induced oxidative stress in the plant. Biochar reduced the phytoavailability of Zn up to 29%. However, VC and FYM increased it up to 13.5% and 7.6%, respectively. Biochar significantly immobilized soil Zn (7.3 - 25.5%) by shifting its exchangeable form to carbonate and organic matter bound forms, more particularly to humic acid fraction. Conversely, VC and FYM elevated Zn mobility (38.5 - 81.8% and 38.4 - 74.5%) by encouraging Zn binding to highly soluble fulvic acid fraction of organic matter. In the tested acidic sandy loam soil biochar markedly reduced Zn bioavailability while VC and FYM increased its bioavailability. Hence application of BC in Zn deficient soil can aggravate the Zn deficiency. Thus, screening of soil amendment on the basis of soil Zn content is vital to alleviate Zn toxicity or deficiency to crops.