Pyrolytic biochars from sunflower seed shells, peanut shells and Spirulina algae: their potential as soil amendment and natural growth regulators

Abstract

Several studies have shown that pyrolysis conditions and feedstocks are the key factors infuencing biochar chemical and physical properties. The information on the nature of biochar is quite important, especially when this carbonaceous material is intended to be used as a potential soil amendment. In this study, we investigated the formation and characterisation of biochars produced from vacuum pyrolysis of sunfower seed shells (SSS), peanut shells (PS) and Spirulina algae (Sp) at 280 °C (for SSS, PS and Sp) and 350 °C (for PS). As a proxy to test the potential of each biochar assoil amendment, we assessed the germination and growth efects of the biochar water-extractable substances (BWES) at diferent concentrations (10; 7.5; 5; and 2.5% w/v) on Lactuca sativa. Results showed that the biochar from pyrolysis of PS at 280 °C would be the most suitable soil amendment, since its BWES did not afect germination and exhibited a remarkable growth-promoting efect (50?100%) on roots and stems of L. sativa. In contrast, BWES from SSS, Sp and certain concentrations of PS produced at 350 °C inhibited growth of Lactuca sativa, and particularly BWES of Spirulina dramatically reduced germination, posing a risk for direct application as soil amendment.The presence of carbonyl derivatives in the BWES from PS may be linked to the stimulatory efects of this extract. Aromatics could be responsible for the germination and growth inhibition in the BWES of SSS, while nitrogen organic compounds would enhance the inhibitory efect in BWES from Sp.