Cover crops, CO2 and carbon credits?
April 11, 2018
Cover crops are grown for several of their benefits, ranging from protecting and enhancing soil to pest suppression. What about using them to sequester atmospheric carbon to counter climate change, which could serve as an income source from carbon credits? While carbon credits aren’t a current reality, once a cap and trade system is in place, farmers may be able to cash in from a practice many are already doing, and this financial incentive could lead to greater cover crop use. When the Chicago Climate Exchange was in existence a decade ago, contracts traded for a much as $7.40 per ton.
To be prepared for a carbon trading system, we need data on how much carbon we can realistically expect to capture with cover crops. With USDA funding, we analyzed our archive of cover crop biomass samples for carbon content, then, based on measured dry matter yield, were able to calculate the atmospheric CO2 equivalent using atomic weight. This is possible because all the carbon in plant tissue comes from the atmosphere through photosynthesis.
The CO2 equivalent of several cover crops we work with is shown in figure 1. In addition to significant levels of carbon capture, what’s interesting is several of the species and mixes do it in a relatively short timeframe. The sorghum-sudan, Sunn hemp and combination of the two is planted around July 1 and sampled in mid-October after its killed by frost. These cover crops would be used after short- season crops, in situations where planting is prevented or as part of an organic soil improvement program. In addition to carbon capture, Sunn hemp is a legume which fixes atmospheric nitrogen which becomes available for the next crop reducing the need for fertility inputs. Winter rye, a commonly used cover is planted in October and terminated in early June before planting of the cash crop. The radish-rye combination is intended for use after short season crops like vegetables or wheat. Radish is planted in early August, mechanically terminated in mid-October by roller crimping and rye then no-till planted into the residue. This combination is part of study looking at several cover crop species planted in August and their effect on the rye cover crop. It turns out many of them produce synergistic or antagonistic effects on rye, and radish produces a positive effect. In other words, rye produces more biomass following radish than the other covers or rye grown without a previous cover. Finally, red clover is frost seeded into wheat in March and terminated in October. This system provides season long soil protection, captures nearly one-half of the season-long solar radiation and heat unit accumulation, all while fixing nitrogen for the next crop.
This carbon capture is not permanent but rather a strategy to take CO2 “out of play” in the short term. Some of it will be released rapidly as CO2 as residues decompose while the more stable forms will stay in soil for years improving soil health/ quality. For this to be an effective carbon sink, cover crops need to be planted every year and the greater the planted acreage, the greater the effect.
Figure 1 Carbon dioxide equivalent of the tissue carbon found in the above ground biomass of
several cover crop species used in the Institutes research, 18 site-years, 87 observations.
Sunn hemp + sorghum-sudan cover crop mix