The crop of the future: harvesting carbon
Whether you call it global warming or climate change, the emotionally charged topic generally associated with greenhouse gases brings an array of reactions – from genuine concern to belief in a conspiracy. Granted, natural shifts in global temperatures have occurred throughout human history. However, the fact remains that Earth’s average surface temperature has increased 1.3ºF. over the past century and is projected to increase by an additional 3.2ºF. to 7.2ºF. over the 21st century. It is happening at a faster rate than ever before.
Fortunately, U.S. farmers and ranchers are poised to make a difference. In fact, they have already adopted technologies in many instances that are helping to slow greenhouse gas emissions – even if it has been inadvertent – in the quest for reduced soil erosion, lower input costs, or improved water conservation.
TEMPERATURE CHANGE IS BOTH GOOD AND BAD
While the global temperature change may seem slight and insignificant, it does pose implications – both good and bad – for farmers and ranchers. According to the EPA, it can lead to a longer growing season in some regions, yet have an adverse effect on crops where summer heat already limits production.
Global warming can also lead to an increase in soil evaporation rates, as well as the chances of severe drought. It’s believed that climate change may encourage a northern migration of weeds and greater disease pressure in crops and livestock, due to warmer winters and earlier springs.
It’s been well publicized that the temperature rise is attributed to an increase in carbon dioxide and other greenhouse gases that collect in the atmosphere. Although carbon dioxide (CO2) and certain other gases are always present and critical in the atmosphere, an unnatural increase leads to a warming effect that is similar to that found inside a greenhouse. Hence, the term greenhouse effect.
According to Dan Kane, a doctorate student at Yale University’s School of Forestry and Environmental Studies, who is researching carbon cycling and the role of soil carbon in agriculture, the primary greenhouse gases associated with agriculture are carbon dioxide, methane (CH4), and nitrous oxide (N2O). Although carbon dioxide is the most prevalent greenhouse gas in the atmosphere, methane and nitrous oxide have much longer durations and absorb more long-wave radiation. Fact is, the comparative impact of methane is 25 times greater than that of carbon dioxide, while nitrous oxide has a global warming potential that is around 300 times that of carbon dioxide.
“There are numerous management strategies for drawing carbon out of the atmosphere and holding it in the soil,” says Kane. “In general, they include reducing tillage in order to decrease the level of soil disturbance, practicing crop rotation and rotational grazing, incorporating nitrogen-fixing crops, and using cover crops. They all sort of focus on getting more plant biomass back into the soil and disturbing the soil less. However, going forward, I think we’re going to have to take more of an all-of-the-above approach.”
Globally, soils are estimated to contain approximately 1,500 gigatons of organic carbon to 1 meter in depth – more than the amount in vegetation and the atmosphere combined.
Carbon sequestration, meanwhile, is the process involved in carbon capture and the long-term storage of atmospheric carbon in the soil or other carbon sinks. In effect, modification of agricultural practices is one of the most recognized methods of carbon sequestration, as soil can act as an effective carbon sink offsetting as much as 20% of carbon dioxide emissions annually. Reducing tillage reduces soil disturbance and helps mitigate the release of carbon to the atmosphere. Carbon dioxide is just one of the three gases attributed to agriculture. Some of the methane and much of the nitrous oxide in the atmosphere also come from agriculture. While methane emissions are often associated with livestock and the decay of organic waste in landfills, they’re also emitted from wet soil, particularly rice fields, which are typically flooded.
“One positive impact farmers can make is to manage irrigation applications in relationship to crop needs and install or maintain soil drainage systems,” Kane notes.
Still, methane accounted for only about 10% of all U.S. greenhouse gas emissions from human activities in 2015. Even though CO2 accounted for about 82.2% of all U.S. greenhouse gas emissions from human activities, only about 8% of it came from agricultural activities. That’s not the case with nitrous oxide emissions.
Although some of it is attributed to fuel combustion and industry, up to 75% of N2O emissions in the U.S. are attributed to the addition of nitrogen to the soil through the use of ammonia and urea-based fertilizer and inefficient soil management, which results in nitrogen volatilization. Fortunately, precision farming and advanced technology have the potential to significantly reduce that figure, as well. A Georgia study, for example, estimates that a cover crop of rye captured from 69% to 100% of the residual nitrogen left after a corn crop.
“If you look at it globally, nitrogen-use efficiency is still less than 50%,” says Raj Khosla, professor of precision agriculture at Colorado State University. “Fortunately, we have much of the technology and knowledge we need to change that number.”
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