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Integrating biochar and compost into soils can enhance both carbon (C) sequestration and soil fertility. However, their combined effects on C mineralisation and priming in C-poor soils are not well understood. To address this gap, we conducted a 120-day laboratory incubation study. We applied biochar (2%) and compost (2%) individually and in combination (1% each) to a soil with low organic C content (0.12%). The biochars included poultry biochar, rice husk biochar, and Eucalyptus biochar, all produced at a temperature of 550 °C. Our findings indicate that co-application of compost and biochar resulted in non-additive effects on decomposition. Co-application increased decomposition (by 18%, poultry biochar), decreased decomposition (by 10%, rice husk biochar), or was similar (Eucalyptus biochar) compared to decomposition predicted from their separate applications. The difference between actual and predicted decomposition (i.e., priming effect) was positively related to microbial biomass C and pH, indicating that biochar properties can modulate the priming effect by changing microbial activity and pH, but possibly also by affecting C adsorption. Future research should also consider their combined effects on C stabilisation in different soils. Overall, our study underscores that co-application of compost and biochar can have non-additive effects on overall decomposition and that biochar properties such as pH should be considered when applied with compost for their C sequestration potential in soil.
Syaranamual et al. (Mon,) studied this question.