Linda Benedict, Liu, Shuai, Harrison, Stephen A., Tian, Zhou, Kongchum, Manoch, Dodla, Syam, Wang, Jim Jian, Viator, Sonny
Jim J. Wang, Syam K. Dodla, Sonny Viator, Manoch Kongchum, Stephen Harrison, Sanku D. Mudi, Shuai Liu and Zhou Tian
The increasing concentrations of greenhouse gases in the atmosphere have been projected to cause an average global temperature rise of 3.6-10.8 degrees Fahrenheit by the end of 21st century. To combat this trend, efforts have been made to reduce the amount of greenhouse gas emissions from all contributing sectors. Agricultural activities account for about 14 percent of overall greenhouse gas emissions globally. A recent report from the U.S. Environmental Protection Agency indicates that the agricultural sector contributes to 6.9 percent of all U.S. greenhouse gas emissions. Understanding the factors that control greenhouse gas emissions and developing appropriate strategies to mitigate greenhouse gas emissions from agricultural sources are important tasks faced by today’s agriculture.
Carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) are the three major greenhouse gases.Carbon dioxide has been the largest emission because of the combustion of fossil fuel. However, methane and nitrous oxide have a global warming potential of 25 and 298 times greater than carbon dioxide because of their longer atmospheric lifetime and greater potential to absorb infrared radiation. Therefore, even though methane and nitrous oxide emissions are generally much lower than carbon dioxide, they are major contributors to climate change, especially nitrous oxide. Soil emissions from agricultural fields contribute all three greenhouse gases. The magnitudes of these emissions are influenced by soil properties and crop production as well as associated management practices. The carbon dioxide is primarily from soil respiration associated with decomposition of soil organic matter, and thus it is directly related to soil organic matter stability. Major methane sources from agriculture include digestive processes in ruminant animals and manure management, which accounted for a combined 30 percent of overall emissions from human activities. While upland crop production systems generally have low methane emissions, flooded soils such as those under rice cultivation as well as natural wetlands produce a significant amount of methane.
Methane production from flooded soils is primarily from microbial activities under anaerobic conditions, although recent research has indicated that potential methane production from aerobic soil conditions is also possible. Nitrous oxide generation highly depends on the availability of nitrogen for nitrification (biological conversion of ammonium to nitrate) or denitrification (biological conversion of nitrate to nitrogen gas). Therefore, factors that affect the availability of nitrogen are expected to significantly influence nitrous oxide emission. Nitrous oxide is produced from incomplete reduction of nitrate during the denitrification process under anaerobic condition. About 69 percent of all U.S. nitrous oxide emissions have been attributed to agricultural soil management activities. Globally, soil emissions of nitrous oxide could account for as much as 90 percent of overall nitrous oxide sources. Therefore, evaluation of different agricultural management practices for developing effective mitigation strategies to reduce these greenhouse emissions is critical and necessary.
Louisiana has a unique combination of land resources, from the northeastern upland to the noncultivated coastal marshland. Major agricultural crops include soybean, corn, rice, cotton and sugarcane. In addition, Louisiana has a significant portion of forestland as well as rich coastal wetlands.
LSU AgCenter scientists have been actively characterizing soil emissions of greenhouse gases from various crops as well as from natural wetlands. In a U.S. Department of Agriculture-funded project, AgCenter scientists examined effects of different harvesting and residue management practices on greenhouse gas emissions from soils under sugarcane production. In sugarcane production, harvest trash residue is either burned or retained; both of which are common practices in Louisiana. Subsequent to the imposition of residue management treatments, the emission of nitrous oxide on a Commerce silt loam soil was considerably higher where the residue was retained compared to burning (Figure 1). The former also yields slightly higher carbon dioxide emissions than the latter (Figure 2). Liquid urea ammonium nitrate (UAN) fertilization has much lower soil emission of nitrous oxide than urea (Figure 1).
In a different study, AgCenter scientists compared effects of polymer-coated granular urea and uncoated regular urea on greenhouse gas emissions from wheat in a Thibaut silty clay soil. The polymer-coated urea, in general, reduces both nitrous oxide and carbon dioxide emissions as compared to uncoated urea (Figure 3).
These studies have clearly shown the benefit of UAN or coated urea application in reducing the emissions of greenhouse gases, especially nitrous oxide, in crop production.
As for rice production in which methane is of primary interest, the average methane emission from Louisiana rice cropping is small during the first growing stage, peaks during the midseason (at about 50-70 days after planting) and then decreases again (Figure 4). For a ratoon crop, methane emission is significantly increased after re-flooding the rice field for which the peak can be up to 31-40 pounds per acre per day. The methane emission over the entire cropping season ranges from 133-312 pounds per acre for the main crop and from 446-1,072 pounds per acre for the ratoon crop. Rice straw left in the field after harvesting the main crop has been attributed as the cause of the higher emissions in the ratoon crop. In addition, application of urea fertilizer has resulted in greater methane emissions than ammonium sulfate and potassium nitrate. Longer flooding periods also increase the amount of methane emissions. Nitrous oxide emission is not considered as an important source of greenhouse gas from the Louisiana rice ecosystem.
Jim J. Wang is a professor, Syam K. Dodla is a post-doctoral research scientist, Manoch Kongchum is an instructor, Stephen Harrison is Walker Nolin Professor in Agronomy and Sunku D. Mudi, Shuai Liu and Zhou Tian are students in the School of Plant, Environmental & Soil Sciences. Sonny Viator is a professor at the Iberia Research Station in Jeanerette.
(This article was published in the spring 2013 issue of Louisiana Agriculture magazine.)