Blue Dragon Energy & Environmental Blog 2.0: The Science Behind Soil and Water Conservation: Measuring Trace Gases and Greenhouse Gases CO2, Methane, and N2O. Webinar Summary and Review. University of Arkansas, Division of Agriculture

This was a very informative webinar. It was specifically focused on the state of Arkansas and its agricultural issues related to soil and water conservation. It focused heavily on measuring trace gases, specifically the greenhouse gases CO2, methane, and nitrous oxide (N2O). The gases were measured under different agricultural parameters to ascertain the effects of specific management practices on emissions of these gases.

Soil and water resources are required for agriculture. Productive soil can be degraded quickly, but it takes hundreds of years to create it. Soil erosion is a problem. It erodes into waterways, increasing turbidity. Turbid water is hazardous for aquatic organisms. Poultry production waste is applied to the landscape for nutrient value in part of the state, but there is excess. In other places in the state, nutrients are deficient. Keeping sediments and nutrients out of waterways, particularly nitrogen and phosphorus, is important since this can lead to eutrophication, which can kill fish and other aquatic organisms. In Eastern Arkansas, water comes from groundwater aquifers and is lowering the water tables.

Air quality is also an issue in the form of greenhouse gases, CO2, methane, and N2O. CO2 is emitted fairly uniformly through soil respiration. Methane and N2O are emitted in pockets where they form. Methane requires saturation or ponding and happens in wetlands. N2O requires a source of nitrate or nitrogen, typically organic or inorganic fertilizers. Denitrification yields N2, which is no problem, but when denitrification is short-circuited, it yields nitrous oxides. CH4 is roughly 30 times the global warming potential (GWP) of CO2 by weight. N2O = 300 times the GWP of CO2. Flood-irrigated rice releases methane. Arkansas is the leading US state for rice production. Flood-irrigated rice loses oxygen in a few weeks, making the system revert from aerobic decomposition to anaerobic decomposition, which enhances CH4 production. Soil temperatures are a factor as well. Silt loam produces more methane than clay loam. Organic fertilizers like poultry waste increase methane emissions more than inorganic fertilizers. Flooding methods can affect methane production. Continuous flood irrigated rice produces the most methane. Interrupted flooding results in fewer methane emissions. Rotation of crops can influence methane emissions. The timing of nutrient applications can be a factor. Giving a little at a time can result in fewer N2O emissions. There are tillage effects on N2O. No till = lower N2O. Regarding methane and N2O, when one is increased, the other is decreased, so that is challenging. Rice is now being tested on raised beds in uplands with furrow irrigation. This results in a slight yield loss but less labor and less water usage. More area stays aerobic, but it is still challenging. Corn, cotton, and soybean crops are being studied with different combinations of paired management practices. Conventional tillage with no cover crop vs. less tillage with cover crops is being studied. Biochar has many soil benefits. It is being studied. Furrow irrigated cotton and corn show very little effect on CH4 and CO2, but maybe some reduction in N2O. In-field analyzers are being used more to get faster and more data.

Measuring Trace Gases, Specifically the Greenhouse Gases CO2, Methane, and N2O

Trace gas measurement in row crops utilizes in-field technologies. New instrumentation relies on new calculations. Tech is improving quickly. Reliable, fast field results are now possible. Analytic techniques assess agronomic and environmental parameters. There are specific sampling and analysis techniques. The use of a vented chamber, a portable weather station, laptop. etc. is one setup. The chamber vents are plugged to allow the collection of soil gas samples. A thermometer measures chamber temperature. The cap is placed on the chamber, so it is sealed. Wires are connected, and a fan turns on. Gas samples are collected using a 20ml syringe, slowly pulled out. 20 ml of gas is injected into a 10 ml vial, so it is slightly pressurized. Vials are stored upside down in a special suitcase. The chamber can be used in wet or dry conditions. We can now take real-time assessments in the field to assess data quality. Analysis with this method is via gas chromatography. The gas chromatograph (GC) uses carrier gases, helium, argon, and compressed air. The GC has three detectors: 1) flame ionization detector, 2) electron capture detector, and 3) thermal conductivity (TC) detector. We used the first and third of these detectors with a GC to analyze oil & gas well gases. It takes up to 18 minutes per vial to analyze. It could take days to analyze data, but the accuracy is very good.

There are also portable in-field instruments, three of which are shown and demonstrated. One involves enhanced cavity absorption with optimal feedback using infrared light and lasers. The chamber creates a sealed environment allowing for real-time measurements. This system has 8-hour batteries. Water vapor in the system is removed. The system can be controlled remotely with a phone or laptop. One can record trace gas concentrations every second. Typically, about 5 minutes is enough time to get the desired sample. Software called Soil Flux Pro is used to analyze the samples. Real-time data is an advantage as it can determine if something goes wrong instantly. It can analyze other gases too, such as ammonia. However, it cannot work in flooded environments.

The next in-field instrument is another trace gas analyzer made by Li-Cor. It uses the same chamber as the previous system and also uses infrared laser technology. It uses a Fourier Transformation to analyze, converting an interferogram to an absorbance spectrum where concentration can be derived automatically via software. The chamber has sensors that measure soil conductivity, soil temperature, and other parameters. It is measured against background air by flushing out air before analyses, so there are no trace gases before samples are taken. This system is very sensitive to water vapor and temperature. A cover outfitted with highly reflective tape is used to limit high field temperatures. It can measure up to 50 trace gases. There is a need to bring a heavy pressurized gas tank into the field so that is one disadvantage.

The third in-field sampler/analyzer is the smart chamber, which can be powered with solar panels and attached to a central computer to be operated remotely. The chambers can open and close on a timer. It can operate continuously and can be controlled remotely. This system allows for more data with less time in the field.

Analysis of biogeochemical cycles in the field can be improved and better understood with these methods.

The Q&A portion of the presentations included a question about how herbicide and insecticide applications may affect measurements. This is generally not an issue, especially as more pre-emergent herbicides are being used. I assume he means herbicides built into seeds and seed genetics. Another question asked if a crop rotation from rice to soybeans, then back to rice, results in soil loss. They answered that it depends on tillage and flatness. Less topography and less tillage always result in less soil loss. Biochar question -can it filter out some water impurities? Biochar could possibly absorb nutrients and perhaps release them slowly.

Trace gases in tillage with no crop rotation vs. furrow/raised bed with crop rotation is one ongoing study. A two-year study is in progress to determine the effect on trace gases. It takes several seasons to confirm. Other management practices can be compared in different combinations. There was a funding question. They noted that instrumentation is expensive and needs to be very well maintained, and that manufacturers provide good support. Below is the Li-Cor analyzer and a video about how it works.