Does biochar enhance soil organic matter formation in tropical soils?

Tropical soils are often deeply weathered and vulnerable to degradation. Biochar is a promising means to improve physico-chemical characteristics such as pH or bulk density. Sustainable soil amelioration is best achieved by improving biological activity, resulting in enhanced soil organic matter (SOM) stocks. In a meta-analysis, we investigated, if biochar amendment to tropical soils led to native SOM stock increases. We found a mean additional C accumulation (MAC) of 0.29% soil dry weight (% dw). MAC was independent of study duration, climate, and biochar addition rate, but strongly linked to soil type and nutrient status prior to the experiment: In Nitisols, MAC was highest (0.99% dw) and initial C and N contents were higher in these soils. MAC was negative in Ferralsols and Oxisols (-0.01% dw and -0.2% dw respectively). MAC as a percentage of initial C content was <50% for most soil types, but -50% in Ferralsols, Oxisols and Ultisols. We conclude that while biochar can enhance SOM stocks, attention has to be paid to the soil environment it is amended to. In low-activity clay soils, biochar amendment can lead to C mining and should therefore be co-amended with nutrient-rich organic amendments.