Soil invertebrates could withstand biochar geoengineering

But new study advises wetting of either the soil (after mixing) or the char (just before) with large-scale biochar application, in order to keep Eisenia fetida happy.

E. Fetida is a very successful earthworm, native to Europe and (un)intentionally introduced to any other continent, except Antarctica. It helps break down dead organic matter and thus increases nutrient-availability and overall soil fertility and plant biomass production.

Heaving soil nutrients readily available could be important for plant’s CO2 absorption we recently learned, so perhaps earthworms are ‘good for the climate’.

This led 4 scientists, connected to Earth Science and Environmental Engineering departments of Rice University Houston and Baylor University Waco, to wonder if there could be a downside to soil CO2 management, by mixing agricultural soils with biochar, an oxidation-resistant form of carbon, and a product of pyrolysis of crop residues.

‘Innocent geoengineering’

Although an ancient Amazonian technique to increase soil productivity, large-scale application of biochar receives modern attention mostly as a (rather innocent) option* for CDR, carbon dioxide removal, a geoengineering route that abates climate change directly at its source of elevated atmospheric CO2 concentrations (instead of trying to influence the Earth’s albedo, so called Solar Radiation Management, or SRM).

[*) Biochar is by no means the silver bullet to our climate. It could well be a nice extra though – but requires -like all effective climate policies- international cooperation around a significantly higher carbon price. A 2007 study in Nature, by Johannes Lehmann of Cornell University, states biochar will be cost-effective from a price of 37 dollars per tonne of CO2 onwards. Current EU price is 23 dollars, trading among 10 US States goes at 1 dollar 90 per tonne of CO2. The same Johannes Lehmann links ‘full agricultural implementation’ of biochar to a mitigation potential of around 1 gigatonne of sequestered carbon (3.7 Gt CO2) per year, some 10 percent of global emissions, land use included. Even James Hansen (who opposes SRM geoengineering) includes biochar as a CDR option in his FFPO/350 ppm Scenario.]

Initial worm biochar response

Altering soil composition and chemistry could well influence the biology of such soils, the new study, published in Soil Biology and Biochemistry suggests. “Earthworms avoided soils containing 100 and 200 g/kg dry biochar at statistically significant levels (p < 0.05),” the researchers write. Biochar application also had a negative impact on the worms’ body masses – but did not influence their reproduction.

However in their analysis of what caused the negative effects, possible intrinsic characteristics to biochar soils (nutrition deficiency, desiccation, presence of toxic hydrocarbons) were excluded.

Biochar application did also lead the soils to dry out [we expect this is due to biochar absorbing a lot of water at first, as it is still a dry sponge after the pyrolysis]. And we all know earthworms like a little moisture – so we get to reach a solvable problem, the researchers think:

“Wetting the biochar to its field capacity resulted in statistically undetectable avoidance relative to control soil, indicating that insufficient moisture could be a key factor affecting earthworm behavior in soil amended with dry biochar.”

“To avoid desiccation of invertebrates and enable their beneficial ecosystem services, we recommend wetting biochar either before or immediately after soil application.”

© Rolf Schuttenhelm |

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