The cover crop workshop in Kansas last week reminded me to revisit the principle issue at hand in this blog: exploring the means by which atmospheric carbon dioxide (CO2) gets sequestered in soils for the benefit of all. Here’s a quick review: the rising content of heat-trapping trace gases in the atmosphere, CO2 especially, poses a dramatic threat to life on Earth. I won’t go into the details. Instead, here’s a graph that I found from the Scripps Institute at the University of California, San Diego, which sums it all up. It’s a scientific projection of CO2 under current emission trends:
Scripps estimates that levels of CO2 will rise to above 1500 ppm and not return to pre-industrial levels for thousands of years into the future. Dr. James Hansen, the Director of NASA’s Goddard Institute for Space Studies and the nation’s top climate scientist, put the situation this way in a paper published in 2008: “Business-as-usual greenhouse gas emissions, without any doubt, will commit the planet to global warming of a magnitude that will lead eventually to an ice-free planet.”
A safe target, wrote Dr. Hansen and his co-authors, is 350ppm and no higher. They argued that paleoclimate evidence plus ongoing global changes imply that the current level of CO2 (currently 394 ppm) is already too high to maintain climatic conditions to which civilization has adapted. We need to get back to 350ppm. But how?
Dr. Hansen provided an answer in an editorial published in 2009: “cut off the largest source of these emissions – coal – and allow CO2 to drop back down to 350 ppm through agricultural and forestry practices that increase carbon storage in trees and soil.”
Actually, a 50 ppm drawdown this way is quite plausible. That’s because planetary soils hold twice as much carbon as the atmosphere does and have the potential to hold a lot more – if we manage them properly. Through the miracle of photosynthesis, CO2 is transformed into a solid (i.e., glucose) inside a green, growing plant which separates the carbon from the oxygen and sends a goodly portion of the C into the soil via its roots to feed the microbial wildlife. More green plants = more CO2 sequestration = climate mitigation.
Cover cropping is an important part of this equation because it covers bare soil with green, growing plants (via no-till methods), as I learned in Kansas. But there’s more. Cover cropping is about the breadth of agriculture, i.e., the lateral extent of the green plants across the soil surface. Frequently lost in the discussion is the issue of depth – the vertical extent to which plant roots grow. Depth matters – and it matters a lot under a changing climate.
When farmers, ranchers and others consider the carbon benefits of green plants they rarely think ‘below the surface’ (because their focus is on food for people or forage for animals), and if they do they don’t think very deeply. That’s because the roots of many plants – annuals especially – don’t go very deep! Besides, the top two to three feet of soil is where the action is. The carbon cycle works quickest there, as plants make ‘fresh’ carbon available to hungry microbes, including protozoa, fungi, and nematodes, which in turn die and decompose, releasing CO2 back into the air, round and round.
The real potential for climate mitigation, however, lies deeper – four to ten feet below the soil surface, or more. That’s where the ‘steady-state’ carbon is found, so-called by scientists because it is rarely disturbed by human or geological activity. That means carbon can be stored deep in the soil profile for thousands of years or more. One scientist wrote recently that increasing soil carbon in the steady state by just 15% would lower atmospheric CO2 by 30%!
The key, of course, are roots – deep roots. Many trees have deep roots and scientists know that over the course of the Earth’s geological history trees played a vital role in scrubbing excess CO2 from the atmosphere and storing carbon deep in the soil. Trees are trying to perform the same role today. Unfortunately, we are mismanaging our forests badly and therefore decreasing their potential as mitigating agents. That’s why scientists are looking now at deep-rooted grass and bush plants as a more viable strategy to ‘soak up’ the excess CO2 in the air.
Deep-rooted plants have other important qualities as well. They are very drought-tolerant, for example, because they can tap water at much deeper levels than shallow-rooted plants can. Deep-rooted plants also resist erosion, since they are harder to dislodge from the soil. They also have access to essential nutrients deeper in the soil profile than shallow-rooted plants – nutrients which can improve their health and the health of whatever eventually eats them. Recent research also indicates that deep roots stimulate higher rates of photosynthesis, which might, in turn, store more carbon in the soil.
Who has deep roots? Generally, it’s the perennials, such as grasses and trees. Annuals, such as corn and wheat, have much shallower roots. But that begs a question: what’s stopping us from breeding plants to have deeper roots? Nothing except our attitude. Fortunately, this is changing. Instead of breeding plants for maximum yield and profits, as the leaders of the (now) infamous Green Revolution did, researchers, farmers, ranchers, and agronomists are looking at ways to create plants that maximize carbon sequestration. That includes deeper roots. There is certainly good reason to try – our survival comes to mind, for one.
All of this is easier said than done, of course. Breeding plants is a complicated job, involving sophisticated knowledge of plant genetics, carbon and nutrient cycling, and various microbial relationships. Fortunately, this work is underway in labs around the world as researchers begin to appreciate the implications of success. It all points in a new and hopeful direction.
Let’s wish them god speed.