Â鶹ÒùÔº

Compost and biochar could boost carbon sequestration by crushed rock

Enhanced rock weathering (ERW) is a proposed method of carbon dioxide sequestration that involves spreading crushed silicate minerals on soils to drive chemical reactions that form carbonate minerals: Essentially, the idea is to boost the natural process of rock weathering, in which carbon is transferred from the atmosphere into rocks. But few large-scale field studies of ERW exist, making it difficult to determine the technique's practical feasibility and what factors might limit or enhance its success.

To address this knowledge gap, Tyler Anthony and colleagues conducted a 3-year, ecosystem-scale study to assess ERW in a California grassland environment, as well as the benefits of enhancing applications of crushed rock with organic additives. Their findings are in the journal AGU Advances.

The researchers spread finely crushed metabasaltic rocks across test plots in Browns Valley, California, in each of the three years. Along with the crushed rock, some of the applications included compost or a combination of compost and biochar (in this case, burned pine and fir left over from local logging). Other plots were treated with only compost, and a group of control plots received no treatment. Throughout the year, the team monitored each plot for levels of soil organic and inorganic carbon, pore water dissolved carbon, aboveground biomass, and greenhouse gas emissions.

The results showed that the rock-only plots sequestered only small amounts of carbon, though they helped reduce organic carbon losses compared with the control plots. Combining crushed rocks, compost, and biochar yielded the best results; in addition to sequestering carbon, the mixture both reduced nitrous oxide emissions and increased methane conversion, resulting in increased greenhouse gas mitigation overall.

The researchers estimate that if the combination of all three materials were expanded to cover 8% of California's total rangelands, it could sequester up to 51.7 million metric tons of carbon dioxide equivalent per year. However, that amount is about a quarter of the theoretical maximum for carbon sequestration from ERW in the area, according to the authors, indicating that achieving theoretical yields may be difficult in practice.

The authors note that their study took place during a drought, which may have decreased sequestration. They also point out that other "life cycle" emissions associated with ERW, such as those generated by producing, transporting, and applying treatments, must be factored into full assessments of the method's impacts.