Crop geoengineering #3: doubling root depth would store 230 Gt carbon in agricultural soils – minus 118 ppm CO2

Breeding crops with deeper (and larger) root systems could help to lower atmospheric CO2 levels, while also making the crops better drought-resistant, Douglas Kell, a Professor of Bioanalytical Science at the University of Manchester says.

Crop CDR geoengineering: longer roots store more carbon in soils
But of course ever getting to such enormous numbers requires a lot of very unlikely things, like full implementation. Besides, the carbon part of the story needs to be fitted in the context of the Earth’s atmospheric carbon cycle, where everything is large-scale, important and inter-dependent. (So just to be safe one could also express the benefits of the idea the other way around: drought-resistant crops, with a carbon dioxide removal (CDR) extra.)

Carbon calculator shows climate potential

It is encompassing carbon sequestration in the deeper parts of the soils that would be key, as the carbon here would be relatively long-lived and protected from oxidation. But still we will reach another state where biomass growth will equal biomass break-down, and if not in CO2, then perhaps in other greenhouse gases. The value would be in [a compensating increase of] the ‘steady state carbon’:

“The carbon that can be sequestered in the steady state by increasing the rooting depths of crop plants and grasses from, say, 1 m to 2 m depends significantly on its lifetime(s) in different molecular forms in the soil, but calculations suggest that this breeding strategy could have a hugely beneficial effect in stabilizing atmospheric CO2.”

The study, published yesterday in Annals of Botany, comes with a special soil carbon calculator, which assumes the following values to get to 230,000 megatonnes of extra sequestered carbon and to lowering the atmospheric CO2 concentrations to a level that James Hansen would call a bit too radical:

The amount of carbon as CO2 currently in the atmosphere is approx 750 Pg, and that in the soil is approx 1500 Pg.

If the global land area for crops is 2300 Mha and that for grassland is 2300 Mha, and an extra 1.0 m depth of roots are grown, and they take up 1.0 % (by mass at an equivalent carbon density of 1000 kg/m3 ) of the relevant soil volume, then the extra amount of carbon that could be sequestered by the above land areas is 10 kg.m-2 = 100 t.ha-1.

This equates to 230 Pg (230,000  M tons) , if the C in the roots and other sequestered carbon are re-respired over a period of 2.0 years.

As 1 Pg is equivalent to 0.51 ppmv atmospheric CO2, this would decrease the CO2 in the atmosphere by 118 ppmv, the current value of 385 ppmv would decrease to 267 ppmv.

If however you play around with the settings the picture can change a lot. For instance halving the respiration time to 1.0 years would ‘only’ lower CO2 concentrations by 21 ppm.

Other (dreamed) crop climate improvements

If we continue the game we wonder what kind of climate-beneficial synergy can be achieved if you combine this approach with biochar (for the above ground crop residues – do remember to wet your char first) and albedo crop geoengineering – if you breed them longer roots, you may just as well breed your crops lighter-coloured leaves.

You´d have your adaptation (drought resistance, higher crop yields), CDR (carbon stored in biochar and in structurally higher root biomass) and SRM (more reflection of solar radiation) all in one. And you’d have to ask all the world’s farmers to change their entire practice – just once.

And even then there is another thing* that would help us even more: eating the crops ourselves, instead of using them as animal feed for billions of cows and pigs. Because you know which crops will easily more than double your average wheat plant’s rooting depth? Yes, trees.

[*) And indeed, of course, this ‘debate’ has some long-run importance for the world’s agricultural carbon balance too.]

© Rolf Schuttenhelm |

Comments are closed.