Abstract Co-application of biochar with other amendments is generating interest as a means to increase biochar effectiveness for improving soil health. Yet, the extent to which such co-application improves soil health is unclear. This paper (i) synthesized the impact of biochar applied with or without organic and inorganic amendments on soil health indicators including soil physical, chemical, and biological properties using field studies, (ii) discussed potential factors that may affect the performance of the co-application, and (iii) summarized research needs. Based on 28 peer-reviewed publications up to September 30, 2024, biochar co-application improved 9 of 16 soil properties compared to biochar alone. It enhanced wet aggregate stability in 5 of 9 comparisons by 45%, saturated hydraulic conductivity in 5 of 6 by 17%, field water content in 8 of 14 by 20%, cation exchange capacity in 9 of 17 by 58%, and organic matter concentration in 5 of 9 by 37%. Also, co-application of biochar increased soil microbial biomass C, phosphatase activity, and N and P concentrations by 33% to 76% in most comparisons. However, it had mixed effects on bulk density, pH, electrical conductivity, C and K concentrations, as well as urease and dehydrogenase activities. Biochar co-application with organic amendments (compost/manure) improved soil physico-chemical properties (bulk density, C, N, P, K) more consistently than with inorganic amendments (NPK). The benefits of biochar co-application increased with higher application rates. These findings suggest that biochar co-application can improve selected soil properties more than biochar alone, with benefits for soil structure, water retention, nutrient availability, and microbial activity, though results for some properties remain inconsistent. Long-term studies (>5 years) across diverse soils and climates are needed to further elucidate these effects and optimize biochar co-application strategies for sustainable soil management. Highlights Biochar co-applied with amendments (compost/manure/NPK) improves some soil properties over biochar alone Soil benefits increase with an increase in biochar co-application rate Biochar + organic amendments are more effective than biochar + inorganic amendments Graphical Abstract
Adetunji et al. (Fri,) studied this question.