Abstract
Composting of biowaste to organic fertiliser promotes resource recycling with various environmental co-benefits, including mitigation of nutrient loss, greenhouse gas emissions, and soil enrichment. As produced from the pyrolysis of organic waste, biochar could be added to compost for enhanced performances. The use of either compost or biochar or both has shown a positive effect on the overall soil quality, such as increasing soil pH and electrical conductivity, increasing soil organic matter, promoting soil carbon storage, and reducing the bioavailability of heavy metals. However, studies have reported contradictory observations and varying degree on the positive effect of such amendments on the aspects mentioned above. This review aims to evaluate the effect of biochar on composting towards a greener and cleaner process. The interacting mechanisms among biochar, compost and biochar-compost amendment upon soil application are discussed. The addition of biochar to compost effectively reduces nutrient loss and gaseous emission and promotes humification. The presence of biochar enrich specific groups of microbes that encourage nitrogen immobilisation. Biochar is more effective in improving the soil carbon pool, whereas compost has a more direct and persistent impact on the soil pH and cation exchange capacity. Upon applying mixed compost with biochar in soil, the organic amendments reduced heavy metals' bioavailability through the respective mechanisms. Different effects of compost and biochar on the soil properties and microbial community were observed, depending on the amendment type, soil condition and length of the application period.
