Bioavailability of phosphorus, other nutrients and potentially toxic elements from marginal biomass-derived biochar assessed in barley (Hordeum vulgare) growth experiments

Biochars produced from marginal biomass feedstocks are a potential source of recycled nutrients for agriculture, but may also contain potentially toxic elements (PTEs) which can cause phytotoxicity. We assessed the potential for nutrient recycling from such materials against potential environmental risks in 17 biochars containing high concentrations of various PTEs and nutrients. Methods for investigating the risk of biochar-derived PTEs were developed and assessed. Short-term (21 days) growth experiments with barley (Hordeum vulgare) in 5% biochar/sand mixtures were used to present the ‘worst-case scenario’ of high dose and low pH buffering. We compared plant nutrient and PTE concentrations with amounts extracted from the same biochars using 1 M NH₄NO₃ or 0.01 M CaCl₂ (buffered and unbuffered, respectively) and Mehlich 3 to analyse whether such extractions could be used to predict bioavailability. The yields of barley grown with biochars “EPOCAD550”, and “WLB550” were significantly higher than the control (p < 0.05). Total phosphorus (P) concentration in above-ground biomass was higher than the control for the EPOCAD550 treatment (p < 0.01). Both buffered and unbuffered 0.01 M CaCl₂ biochar extractions were significantly positively correlated with plant leaf concentration for six of the 18 elements investigated, more than any of the other extractions. CaCl₂ extractions provided the most representative assessment of element bioavailability from marginal biochars compared to more resource-intensive growth experiments. Our results provide new insights into the bioavailability of elements in biochar and the standardisation of methods which accurately assess this attribute, which is necessary for promoting use of biochars from marginal biomass for recycling nutrients from wastewater and to agricultural production.