Does biochar contribute to soil organic matter accumulation? – A tropical perspective

Tropical soils are often deeply weathered and vulnerable to degradation. Biochar appears a promising means to improve soil quality while sequestering carbon into the soil. Yet, sustainable soil amelioration depends on stable soil organic matter (SOM) stocks for nutrient retention, water uptake and as habitat for soil life. In a literature meta-analysis, we investigated, if biochar amendment to tropical soils led to SOM increases additional to biochar C. 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 slightly 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. Changes to soil microbiomes were more conclusive and included elevated enzyme activities and shifts from bacterial to fungi dominated microbiomes. We conclude that soil nutrient status prior to amendment, which is often linked to microbial activity, determines if the alteration of soil conditions caused by the biochar can be buffered ecologically, so that fresh organic residues are transformed into SOM. Additionally, we remarked that research on biochar – SOM interactions in tropical soils largely depends on cooperations with institutions from North America and Europe for funding and analytical infrastructure. Researchers, institutions, and funding bodies need to be creative and cautious to realise equitable participation of all partners in international research projects designed to render added value for societies around the world.