A trend of increasing tillage intensity in U.S. corn and soybean production in recent years has led to increased greenhouse gas emissions from agricultural fields, a new study finds.
The study drew on years of survey data that asked thousands of U.S. farmers about their tillage practices. The researchers then fed the relevant data into sophisticated ecosystem models to see how tillage decisions affect soil greenhouse gas emissions, including carbon dioxide and nitrous oxide.
Survey data indicate that farmers relied less on tillage between 1998 and 2008, but this trend began to reverse around 2009 when tillage intensity began to increase.
Growing weed resistance to the common herbicide glyphosate has likely contributed to increased tillage, says Chaoqun Lu, associate professor of ecology, evolution, and organismal biology at Iowa State University, as well as lead author of the study in natural food.
Genetically engineered herbicide-tolerant crops first appeared on the agricultural scene in the late 1990s, and their adoption freed farmers from some of their dependence on tillage as a method of weed control.
But an increasing number of herbicide-resistant weed species have emerged over the decades, reducing herbicide effectiveness and making tillage a more attractive weed control option again. And as tillage intensity increases, more carbon and nitrogen stored in the soil are released into the atmosphere as greenhouse gases, Lu says.
“One of the interesting things we found in this study is that tillage intensity went from a decreasing trend to an increasing trend since 2008,” says Lu. “Our regression analysis suggests that this trend is correlated with the widespread adoption of herbicide-tolerant crops before 2008 and emerging weed resistance after 2008. We cannot assert a strict causal relationship, but regression analysis reveals a strong relationship between them .”
The survey asked about farmers’ decisions about seed varieties and the intensity of cultivation practices. Survey topics included no-till, conservation tillage (eg, ridge tillage, mulch tillage), and conventional tillage (eg, ., furrow plough, scissor plough, disc harrow).
Data shows that no-till increased by about 12 million acres for corn production and nearly 17 million acres for soybeans between 1998 and 2008. But no-till corn acres declined by nearly half a million acres between 2009 and 2016 and declined by nearly 6 million acres of soybeans during that time, according to the survey.
Corn acreage under conservation tillage and soybean acreage under conservation tillage and conventional tillage showed similar trends, first declining between 1998 and 2008 before recovering to earlier levels in 2016.
Integration of the data into terrestrial ecosystem models shows that the gains in tillage intensity since 2009 have offset the greenhouse gas mitigation benefits obtained from the decline in tillage from 1998 to 2008.
The study reveals a relationship between weed resistance, seed technology and greenhouse gas emissions that could lead to a better understanding of how agricultural practices can mitigate climate change, Lu says.
His team’s previous research has shown that nitrous oxide emissions from farmland in the U.S. Corn Belt have increased in recent years, largely due to the widespread application of nitrogen fertilizers to farmland. The added nitrogen is partially used by crops, but the rest stays in the soil or is lost to the environment. During this process, microorganisms living in soils consume nitrogen-containing compounds and release nitrous oxide as a byproduct.
During this time, the organic matter in the soil breaks down and partially converts into carbon dioxide. Both are potent greenhouse gases that have the potential to warm the climate. Intensive tillage practices disturb the soil, alter soil moisture and aeration, and mix heavy crop residues into soils, which together alter soil greenhouse gas production rates and allows more of them to escape, says Lu.
Lu points to the use of alternative herbicides to control glyphosate-resistant weeds, or the use of glyphosate in fewer consecutive years, as well as crop diversification beyond corn and soybeans as options to control weeds without increasing greenhouse gas emissions.
“Without an effective strategy to control weeds, tillage intensity could continue to increase in the future and could undermine the greenhouse gas mitigation achievements of other agricultural activities,” said said Lu.
Additional co-authors are from Nanjing University of Information Science and Technology, Auburn University, Tennessee State University, and Iowa State.
Source: Iowa State University