Biochar from sugarcane residues: An overview of its sequestration potential in Sao Paulo, Brazil

Harvesting sugarcane (Saccharum officinarum) produces large quantities of biomass residues. We investigated the potential for converting these residues into biochar (recalcitrant carbon rich material) for soil carbon (C) sequestration. We modified a version of the RothC soil carbon model to follow changes in soil C stocks considering different amounts of fresh sugarcane residues and biochar (including recalcitrant and labile biochar fractions). We used Sao Paulo State (Brazil) as a case study due to its large sugarcane production and associated soil C sequestration potential.

Mechanical harvesting of sugarcane fields leaves behind > 10 t dry matter of trash (leaves) ha^-1 year^-1. Although trash blanketing increases soil fertility, an excessive amount is detrimental and reduces the subsequent crop yield. After the optimal trash blanketing amount, sugarcane cultivation still produces 5.9 t C ha^-1 year^-1 of excess trash and bagasse (processing residues) which are available for subsequent use.

The available residues could produce 2.5 t of slow-pyrolysis (550°C) biochar C ha^-1 year^-1. The model predicts this could increase sugarcane field soil C stock on average by 2.4 ± 0.4 t C ha^‑1 year^‑1, after accounting for the climate and soil type variability across the State. Comparing different scenarios, we found that applying fresh residues into the field results in a smaller increase in soil C stock compared to the biochar because the soil C approaches a new equilibrium. For instance, adding 1.2 t of biochar C ha^‑1 year^‑1 along with 3.2 t of fresh residue C ha^‑1 year^‑1 increased the soil C stock by 1.8 t C ha^‑1 year^‑1 after 10 years of repeated applications. In contrast, adding 0.62 t of biochar C ha^‑1 year^‑1 with 4.5 t of fresh sugarcane residues C ha^‑1 year^‑1 increased the soil carbon soil stock by 1.4 t C ha^‑1 year^‑1 after 10 years of application. These are reductions 25% and 40% of the potential soil C accumulation rates compared with applying available residues as biochar.   

We also tested the sensitivity of the model to biochar-induced positive priming (i.e. increased mineralization of soil organic C) using published values. This showed that the C sequestration balance remains positive over the long term, even considering an extremely high positive-priming factor. Upscaling our results to the total 5 Mha of sugarcane in Sao Paulo State, biochar application could sequester up to 50 Mt of CO₂ equivalent per year, representing 31% of the emissions attributed to the State in 2016.

This study provides first insights into the sequestration potential of biochar application on sugarcane fields. Measurements of changes in soil C stocks in sugarcane field experiments are needed to further validate the model, and the emissions to implement the practice at large scale need to be taken into account. As the climate crisis grows, the need for greenhouse gas removal technologies becomes crucial. Assessing the net effectiveness of readily available technologies is essential to guide policy makers.