Comparative Meta-Analysis of Physical and Chemical Properties of Food Waste and Conventional Biochar

Conventional biochar from woody biomass (woodchips, sawdust, bamboo etc.) and crop residues (rice husk, wheat straw, coconut shell etc.) enhances long-term soil carbon (C) storage with slow-release soil fertiliser input. However, the soil response to food waste biochar application is yet to be established. In order to optimise food waste biochar as a soil amendment product, a comparative meta-analysis of food waste biochar to other established conventional biochar types is needed. This study compares the effect of feedstock type and pyrolysis temperature on biochar physical-chemical properties, chemical stability and aromaticity to infer how food waste biochar may alter soil properties.

Data was synthesized from 33 peer-reviewed articles on food waste and 50 on conventional biochar. Feedstocks were categorised into cooked food waste, fruit-vegetable peels/seeds, woody biomass and crop residues. Pyrolysis temperatures were classified as slow (300 to 500^oC), fast (550 to 650^oC) and flash (700 to 1000^oC). Pairwise comparison of feedstock types was achieved by Welch’s t-test and Cohen’s d effect size to assess impacts of temperature on biochar properties.

Pyrolysis temperature predominantly governs biochar properties, with minimal impact from feedstock choice. Lack of significant differences (p > 0.05) and low-to-high effect sizes were observed in moisture content (d = 0.10 – 1.08), surface area (d = 0.12 – 0.35), ash content (d = 0.19 – 0.74) and pH (d = 0.02 – 0.75) at fast temperature regime in most food waste and conventional biochar pairwise comparisons. Food waste biochar was similar to conventional biochar in total potassium (p > 0.05; d = 0.06 – 0.25) at slow temperature regime but a significant difference and large effect sizes (p < 0.05; d > 0.5) in total nitrogen was observed between food waste and conventional biochar at the three temperature regimes. Food waste biochar had lower total C content across all temperature regimes compared to conventional biochar (p < 0.05; d > 0.5) but showed similarity in fixed C at flash temperature. In addition, most food waste biochar clustered in the coal and anthracite regions of the van Krevelen diagram at flash pyrolysis temperature, indicating lower H/C (0.0 – 1.2), O/C (0.0 – 0.76) ratios that signify enhanced aromatic C and potential for long-term stable C. Overall, these findings show that food waste feedstocks differ in nitrogen content compared to conventional feedstocks – which are high in total C –, with resulting biochar capable of enhancing N availability in soil. Despite limited field and laboratory data on the potentials of food waste biochar on enhancing soil C stocks and associated soil co-benefits, evidence suggests that pyrolysis temperature can be used to optimise food waste biochar for total C, fixed C and porosity to match the soil carbon stock potential of conventional biochar. The N and K content variations amongst food waste and conventional feedstocks emphasises the potential to engineer biochar production from co-blend of food waste and crop residues to produce balanced nutrient content for targeted soil fertility management.