The results presented here suggest that the application rate of the biochar needs to be increased to greater than 7 metric tonnes ha−1 a−1. Increases in crop productivity have been reported for biochar application rates ⩾10 metric tonnes ha−1 (Jha et al., 2010). Cassia biochar has a high C content (42% C); biochars with lower C content would need a higher application rate to achieve C-sequestration. Therefore, in high mineral soils without obvious soil aggregates, it could be considered good practice to apply >10 metric tonnes ha−1 a−1. This must be balanced with sustainable biochar feedstock availability (Woolf et al., 2010). Biochar addition is a sustainable agricultural practice, therefore, care must be taken to use available green waste and not infringe on native vegetation essential to the local ecosystem as this could result in soil degradation and erosion which would reverse the soil fertility benefits of biochar addition.
3. Conclusions
The biochars from different biomass feedstock contained >20% C and were high in macro- and micronutrients. In this study, biochar application of 6.6 metric tonnes ha−1 cassia biochar was enough to initiate C-accumulation, which is reflected in an increase in OM and a net reduction in soil bulk density. Biochar application rates >10 metric tonnes ha−1 a−1 are recommended for high mineral content tropical soils.