Effect of biochar doses under various levels of salt stress on soil nutrient availability, soil enzyme activities and plant growth in a marigold crop

Context Biochar application not only remediates soils contaminated by heavy metals but also improves soil fertility and plant growth in salt-affected soils. Aims The aim of the study was to investigate the effects of water source salinity and biochar on soil properties and growth of a marigold (Tagetes erecta) crop. Methods This pot study used a factorial completely randomised design with four levels of salinity of the water source (0.04, 2.01, 3.32 and 5.16 dS m(-1)) and five biochar doses (0, 2.5, 5.0, 7.5 and 10 t ha(-1)). Salinity treatments contained different proportions of treated tannery effluent, and biochar was derived from water hyacinth (Eichhornia crassipes). Soil physico-chemical properties, nutrient contents and enzyme activities, and plant biomass, were measured during crop growth and post-harvest. Key results Water salinity level and biochar dose both had significant effects on soil properties. Higher salinity of water and biochar dose increased soil pH, whereas electrical conductivity decreased with biochar application in soils receiving higher salinity water. Soil organic carbon and available nutrients were significantly affected by salinity level and biochar dose. Irrespective of salinity level, biochar application at 10 t ha(-1) increased soil organic carbon and available nitrogen, phosphorus and potassium by 40.5%, 23.1%, 15.2% and 30.5% in post-harvest soils. Enzyme activities and plant growth were significantly reduced at higher salt levels, whereas application of biochar improved these parameters. Conclusions The results indicate that application of biochar at 10 t ha(-1) significantly promotes nutrient availability and enzyme activities in soils with various levels of salinity. Biochar application enables plant growth primarily by trapping the soluble salts on the pore space available in its surface. Implications Biochar application may help to mitigate nutrient deficiency and crop failure under salt-stress conditions.