Composted biochar versus compost with biochar: effects on soil properties and plant growth

Abstract Biochar is widely recognized as an amendment that enhances soil properties and sequesters carbon, particularly in degraded soils. However, biochar applied solely to soil may also hinder plant development due to toxic by-products generated during pyrolysis or nutrient retention. To mitigate these adverse effects, it has been suggested to either mix biochar with compost or to process it by composting with fresh organic materials. To date, there is a lack of comparative studies evaluating the performance of these two approaches. In this study, three types of biochar, differing in their initial feedstocks (beech wood, hornbeam/beech/oak mixture, and digestate/cereal straw mixture), were investigated. These biochars were applied solely, mixed with green waste compost, or processed as composted biochar in two soils of different fertility (a Luvisol and a gleyic Fluvisol). A pot experiment was conducted under controlled conditions where lettuce was grown for three months. After harvesting, plant biomass, and soil microbial and physicochemical properties were measured. Composted biochar and compost additives maintained a neutral soil pH, contrary to biochar applied solely or mixed with compost. The dissolved organic carbon and total nitrogen were higher in composted biochar treatments, leading to a higher proportion of humified material with a high degree of condensed aromatic groups compared to other treatments. Microbial activities were higher in the composted biochar treatments compared to those in the compost with biochar, and more specifically in the less fertile Luvisol. Finally, composted biochar increased plant growth by almost six times compared to the control without amendments, whereas the mix of biochar and compost increased it by only three times. Solely applied biochars did not affect lettuce growth. This study demonstrates that biochar composting is more beneficial than mixing biochar with compost in terms of improving soil fertility and mitigating the negative effects associated with pure biochar application. Graphical Abstract