Effects of eucalypt and black wattle biochars from vacuum pyrolysis on sandy soil quality and cauliflower yield

Woody invasive alien plants (IAPs), including black wattle (Acacia mearnsii De Wild.) and eucalypt species (Eucalyptus camaldulensis Dehnh.), present significant environmental challenges in South Africa. Biochar from these plants can enhance soil carbon sequestration and fertility. This study examines the effect of biochars from different IAP feedstocks and pyrolysis temperatures on soil quality and crop production. Five biochars were produced at a range of temperatures (500–800 °C) from eucalypt and a single temperature (600 °C) from black wattle biomass using low-vacuum pyrolysis. Biochars were applied at 1.5% (wt.) to acidic sandy soil in which cauliflower was cultivated in a fertilised greenhouse experiment. Soil chemical, physical and microbiological properties were determined at planting and harvest. The 600 °C eucalypt biochar increased cauliflower yields by 53%, attributed to higher soil basic cation availability and N retention. In contrast, higher temperature eucalypt biochars and the black wattle biochar decreased curd yields by 26–79%, attributed to phytotoxicity, alkalinity, and reduced boron availability. Additionally, biochars reduced plant available water due to high porosity. These findings highlight the critical role of pyrolysis conditions and feedstock selection in determining biochar performance in soil and crop systems.