Measurement of Greenhouse Gas Flux from Agricultural Soils Using Static Chambers

The overall goal of this procedure is to measure trace gas flux from soils under different vegetation types or management practices. This is accomplished by capturing gases emitted from the soil surface within a chamber and measuring change in concentration over time to parasite. A cylindrical or rectangular anchor is first placed into the ground to prevent lateral flow of gases.

When ready to sample, a lid is sealed on top of the anchor trapping emissions from the soil system. Samples are then collected from the chamber headspace with several samples collected over a predetermined time course. The samples are subsequently analyzed by gas chromatography to determine the concentration of the gases of interest.

Gas concentration is then plotted against time, and the dataset is subjected to linear or curve linear regression using a predetermined flux model appropriate to the system in order to determine the rate of flux gas fluxes may be compared between treatments or across seasons. To explore system dynamics, these methods can help answer questions about how emissions of greenhouse gases, such as carbon dioxide, nitrous oxide, or methane from the soil are affected by management practices and weather. The main advantage of this technique over other approaches for measuring greenhouse gases is that it provides the capacity to monitor many sites at once with relatively inexpensive equipment.

This is not a novel method, but is one that's been used by researchers over a number of years. In fact, the sampling protocol that we'll be describing has been documented in publications done by researchers associated with Grace net A-U-S-D-A program, or by the Global Research Alliance and International Group. The purpose of our publication then is to demonstrate this technique visually.

For those who haven't had experience with it, A variety of chamber designs exist. Each chamber contains two parts, an anchor that is inserted into the soil and a lid that is placed on top of the anchor. During flux measurement, chambers should be constructed of sturdy, non-reactive material, such as stainless steel or PVC, and should be insulated and covered with reflective material to prevent heat buildup during measurement, the system should include a mechanism for sealing the lid onto the anchor, a septum to allow sample collection and vent tube to prevent pressure changes during chamber deployment and sample removal.

In designing chamber shape and size, consider spatial factors such as crop row spacing, fertilizer banding, and plant height. Note that trade offs exist between chamber height and detection sensitivity. Here we'll use simple cylindrical chambers constructed from utility pans at least one day prior to sampling.

Install chamber anchors in the soil at desired locations, apply even pressure across all points so that the anchor does not warp sink anchors into the ground to a depth of at least eight centimeters with no more than five centimeters remaining above the soil surface. Anchors should sit as close to the soil surface as possible to minimize microclimate effects and water ponding in order to calculate chamber headspace volume. Anchor height above the soil surface must be measured at the time of installation and any time a significant change in soil surface occurs prior to beginning the experiment, it is necessary to determine a suitable sampling time course to follow.

Optimal timing will depend on the system under study and the chambers being used. Time course calibration can be performed by sampling intensively over the space of an hour. Under conditions expected to produce a relatively high flux of trace gases of interest.

Concentration can then be plotted against time in order to visualize the system's flex dynamics and determine the most appropriate duration of chamber deployment, time point spacing, and linear or non-linear flex model to be used in data analysis. Here we show carbon dioxide and nitrous oxide concentrations measured several hours after a heavy rainfall and a suggested sampling time course that would allow for linear regression in this experimental system. At a minimum, a time course should include three, preferably four sampling time points based on the optimal timing divides a sampling scheme that covers all sites, treatments or replicates, and allows personnel to move through the chamber sites efficiently.

This may require dividing the chamber sites into rounds that can be sampled one after the other, construct rounds out of blocks of replicates rather than treatment by treatment. In order to avoid bias and plan for sampling time, walking time between sites as well as time for the collection of any ancillary measures. Keep in mind that if measurements are to be used as representative of a whole day, samples should be collected at a time that reflects daily average temperatures, typically mid to late morning on each sampling date, follow the predetermined sampling plan and timing because concentrations will ultimately be plotted against time.

It is critical to plan out and or record sampling times precisely prior to sampling pre label and arrange collection vials for maximum efficiency and to reduce the likelihood of confusion during sampling. One vial is required per time point per chamber, collect additional equipment including a syringe, stop, cock, needles, and a stopwatch. Equipment and sample volume can vary depending on the collection and transfer methods being employed.

At the first site to be sampled, attach and seal a chamber lid to the pre-installed anchor. Immediately after sealing the lid, start a stopwatch. This is time zero or T zero with an empty 30 milliliter syringe fitted with a needle and stop cock in the open position.

Draw a 30 milliliter sample of air from a location adjacent to the chamber at the approximate height of the chamber top and close the stop cock. This will be the two zero sample, which must now be transferred to a collection vial with the syringe needle. Pierce the septum of a collection vial that already has another needle inserted near the edge of the septum.

Open the syringe, stop cock and inject approximately 20 milliliters of the sample into the vial. Then in a smooth motion, remove the extra needle while continuing to inject as much of the remaining sample as possible. Slightly over pressurizing the vial.

Close the stop cock and withdraw the syringe needle from the septum. Insert the extra needle into the next vial to be used, and turn the filled vial upside down to distinguish it from unfilled vials. In the flushing method of sample transfer shown here, initial sample injection results in the vials previous contents being cleared through the extra needle.

Then when the extra needle is removed, the vial is filled to a positive pressure with sample as an alternate to the flushing method. Samples may instead be loaded into pre evacuated or non evacuated vials. After completing collection of one sample, proceed to the next chamber site and repeat this process.

Sealing the lid on the correct predetermined time point. When attaching lids, take care to ensure that a good seal is achieved with no gaps or debris allowing leakage continue until all chambers have been sealed and T zero samples have been collected. Then return to the first chamber to collect a T one sample.

As the time approaches, 10 seconds until T one pierce the chamber lid septum with a syringe needle within a ten second range of T one, withdraw a 30 milliliter sample of air from inside the chamber and close the stop cock. Remove the needle from the chamber septum. Transfer the sample to a collection vial l.

Continue to move through chambers until all samples have been collected. Air temperature at the time of sampling must be recorded. Measurement of soil temperature and water content are also recommended and other ancillary measures such as soil bulk density and soil nitrate and ammonium may be required periodically depending on research goals.

Ambient air samples may also be collected for approximation of T zero or for use in non-linear flux models. Determine the concentration of gases of interest for each sample by gas chromatography. Using equipment fitted with an electron capture detector for nitrous oxide, an infrared gas analyzer or thermal conductivity detector for carbon dioxide and a flame ionization detector.

For methane, convert trace gas concentration from volumetric units to mass using the ideal gas law for each time series plot. Time by concentration follow a consistent quality control procedure for the identification and removal of suspect time points or time series, which could be caused by chamber or vial leakage or instrument malfunction. For example, here we show carbon dioxide and nitrous oxide time series from the same set of samples.

The drop in concentration observed for T one compared to other time points suggesting a trend of increase in concentration is likely due to vial leakage. Visual inspection of time series plots allows such problems to be identified. If using a linear flux model evaluate the graph for linearity, either using goodness of fit or by visual inspection later time points showing signs of plateau may be removed as long as a minimum of three time points, including T zero are retained for flux calculation.

Once data quality has been confirmed, perform linear regression and use the slope of the regression to calculate flux. Keep in mind that in some cases a non-linear flux model may be more appropriate than a linear model. Rates of flux may be tracked between treatments or over time.

To explore system dynamics while using this method, it's important to keep in mind that greenhouse gas flues will vary in response to weather, soil disturbance, and agricultural management practices, and that sampling dates should be arranged to capture this variability. Weekly sampling is typical with more frequent sampling around events such as rainfall, tillage, and manure or fertilizer application. After watching this video, you should have a good understanding of how to design and conduct a program of chamber based measurement of greenhouse gas fluxes and soil systems.