The potential fertilizer value of Namibian beach-cast seaweed (Laminaria pallida and Gracilariopsis funicularis) biochar as a nutrient source in organic agriculture

Abstract

Biochar has been reported as an efficient tool for carbon sequestration and soil quality improvement, though most research has evaluated the quality of biochar from terrestrial based materials. The study evaluated the macro; micro nutrients and heavy metal concentrations of biochar from seaweeds from Namibian coast. Seaweed species Laminaria pallida and Gracilariopsis funicularis were pyrolysed at different temperatures i.e. control (uncharred); 200; 400; 600 and 800 °C in order to determine the chemical and physical properties of the biochar produced. There were significant interactions (P < 0.05) between temperature and seaweed species on most parameters measured. A pyrolysis temperature of 400 °C resulted in an almost 50% reduction in biochar yield, with higher temperatures resulting in even more yield loss whilst producing biochar with higher alkaline pH above 7 and high EC above 20 mS/cm. G. funicularis biochar resulted in the highest macro element content with total C 38.3%; N 4.3% and P 6.3 g/kg, whilst L. pallida biochar had the highest contents of cations of 16.2 g/kg Ca; 6.4 g/kg Mg; 151 g/kg K and 45 g/kg Na. Higher pyrolysis temperatures did not result in consistent changes in total heavy metal content, with only Cd content being above the maximum permissible limits for biochar of 3.9 mg/kg. Generally, a pyrolysis temperature of 400°C was observed to give biochar with the best quality in terms of macro elements, pH and total C. G. funicularis biochar showed significantly higher nutrient concentrations, therefore has great potential in soil quality improvement.