Joshua J. LeMonte and Dr. Bryan G. Hopkins, Plant and Wildlife Sciences
Farmers typically apply relatively high rates of nitrogen (N) fertilizers, often in excess. This is because N is the mineral nutrient most commonly deficient in agricultural soils. Soil-plant system inefficiencies prevent complete utilization of the N, leaving residual N in the soil, wasting natural resources and causing environmental concern. To compensate for inefficiencies and improve crop yield, supplementing crops with N fertilizers has long been understood to be integral. Nitrogen fertilizer accounts for 40% of the per capita increase in food production in the last half-century, and is the source of 40% of the world’s dietary protein.
Increasing nitrogen use efficiency (NUE) has been a focus in the agricultural industry for years. Increased efficiency is beneficial for the grower (more profits and less labor), the fertilizer industry (new technology and resource conservation), and society (decreased negative environmental impacts). Recent innovations have been targeted to meet the growing demands for fertilizer sources and management which can decrease environmental impacts such as ground and surface water contamination, increased erosion, and air pollution. Polymer coated urea (PCU) is one fertilizer-N innovation which has shown promising results. Substantial research has been conducted regarding ground and surface water pollution and erosion. Air quality impacts of agriculture have also been investigated. However, this research has been somewhat limited by technology and the labor-intensive nature of the sampling and analysis. Recent innovations in technology and more attention from the media and funding agencies mandate more comprehensive research be conducted to give a more accurate assessment of the air quality impacts of conventional fertilizers (especially nitrogen) and high efficiency fertilizers.
The effect of air quality on climate change has been hotly debated. Many scientists agree that increases in greenhouse gas (GHG) concentrations in the atmosphere may lead to climate change and less weather predictability. Carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) are the major anthropogenic GHGs which come from agriculture. While agriculture is not the leading contributor of GHG emissions in the U.S., it must not be overlooked as a possible source for ameliorating a significant amount of emissions annually. Carbon dioxide is emitted in the largest concentrations nationally and internationally each year and consequently dominates the press and scientific research. However, N2O is 296 times as potent as CO2, although in much smaller atmospheric concentrations. Agriculture produces nearly 80% of the nation’s N2O. Of that, 35% of the total is estimated to be fertilizer-induced. In addition to N2O emissions, agriculture also volatilizes ammonia (NH3). Excess NH3 in the atmosphere leads to catalytic ozone depletion, haze, and offsets the sensitive N cycle. For the purposes of this project, N2O and NH3 were the gases of focus.
Nearly 30 years ago, a method was developed to measure N2O in agricultural soils using gas chromatography (GC). This method involved short-term sealing off of a specific site within a trial to allow gas build up. Multiple samples were taken at given intervals. Using this method, it has been common for samples to be taken weekly to try to determine the N2O emission rate. This is inadequate however, when dealing with gas fluxes. As mentioned above, many factors contribute to the amount of gas being lost to the atmosphere at any given point. Therefore, it is not sufficient to give a snapshot view of the N2O emissions from a study site or growing field once a week. But this method is not without merit. A good deal of information has been obtained by the GC method. An alternative method to GC is photoacoustic infrared detection (PID). This method has been employed in indoor air-quality studies and for occupational health and safety measurements. Although former versions of these analyzers have been unreliable, there is reason to believe that recently developed and updated versions of the analyzers may be accurate, reliable, and stable as gas monitors. The objective of this study was to determine if relative differences in N2O and NH3 emissions could reliably be gathered by using PID.
A glasshouse study was conducted in winter of 2008-2009. A randomized block design was used, with three replications of four fertilizer-N treatments (control, urea, and two PCU rates) applied to three soil types: sandy soil, loam soil, and a sandy loam. The treatment was mixed with the top 5 cm of soil. Twelve chambers were constructed out of standard 19L PVC buckets. The chambers were designed to maximize accumulation of soil evolved gases, yet also allow gas exchange with the atmosphere. Each chamber contained approximately 13.5 kg of soil during each trial. The soil came to within 8-12 cm of the top of the chamber. Corn (Pioneer 35F38) was hydroponically germinated and four corn individuals with shoots 8-12 cm high were transplanted into each chamber. It was necessary for the individuals to be approximately this height to allow light to reach plant tissue and foster plant growth. Nitrogen treatments were then applied at varying rates.
Nitrous oxide emission and ammonia volatilization samples were collected using an Innova 1309 multi-port sampling unit. This unit was controlled by an Innova 1412 Photoacoustic Field Gas Analyzer, which also performed gas analysis on-site using PID. The 1412 unit was connected to a computer which controlled which gases were sampled (N2O and NH3), time intervals of sampling and analysis, and data input for further statistical analysis. A complete sample set (12) was taken with the multiplexer (Innova 1309) in thirty minute intervals, as directed by the user. Immediately following sample collection, the air sample was pumped into the 1412 where it was then analyzed and recorded. Each sample took approximately 20 seconds to be sampled, analyzed and recorded. A total of approximately 800 samples were taken and analyzed for each chamber.
Significant results in NH3 and N2O emissions were obtained using the Innova 1412 field gas analyzer using PID method of analysis. This method shows potential to help better understand the real-time flux of gases in agricultural systems. Before generalizations can be made more research must be conducted in longer intervals than in this experiment. Comparing urea and PCU with this method allows for qualitative comparisons to be made, and with more funding in the future, could also show great quantitative results as total N fluxes and losses are computed. Nitrogen loss to the atmosphere is of growing concern in today’s world, especially in agriculture. Steps must be taken to accurately account for these N losses and ways to increase NUE must be explored.