CH₄, N₂O, CO₂ Fluxes (square chamber)

Field Gas Sampling Protocol

Field Sampling: Usually March through November

Materials

• Tins or plastic bags: for soil samples (1 per chamber)
• Small Level: for leveling chamber base
• Rubber Mallet: for hammering base
• Wood Block (2″ × 4″ × 18″) or Plywood square (16 × 16″): for hammering base
• Soil Probe
• Syringes (at least 2 per person)
• Needles (at least 10 per person)
• Septa (from 3 ml venoject vials): for lids
• Bottle of water
• Labeled vials, 4 per chamber
• Stop watch
• Soil thermometer (Taylor small dial thermometers with 13 cm stems)
• Ruler: to measure chamber height
• Field apron or equivalent: for carrying supplies
• Pencils
• Data sheets
• Clippers
• Gas box frames for square chambers:
  • Gas box base (aluminum, 27.3cm x 27.3cm x 10cm) – installed in plot
  • Gas box lid (29cm x 29cm x 14cm)
• Gas box frames for round chambers – NO LONGER IN USE
  • Chamber base (PVC, 25cm dia.) – installed in plot
  • Chamber cap
  • Elastic band

Preparation

1. Place chamber base in ground, bottom edge approximately 1 inch below surface. The frames must be level. On badly sloping sites, this can be achieved by carefully hammering a wooden board placed over the top of the base; use a level to check angle. Frames should be set into ground at least 1 day before sampling; they remain in the soil until winter, and are not removed between sampling times except for agronomic treatments.
2. Plant growth within the box frames needs to be clipped to base height (do not clip to soil surface) without disturbing the soil prior to sampling. Litter is left in place. The area inside the frame should be as representative as possible to the area outside the frame.
3. Vials need to be recapped prior to each sampling date. Number of chambers *4 sampling times (T0 – T3) + 4 ambient vials = number of vials needed. Each vial is labeled with a unique number for gas sample identification.
4. Check the chambers (frames and lids) for plant growth and damage. Level the frames and clean the lids the day before sampling.
5. The chamber lids need to be kept clean; clean the inside with water (only) before each use. The epoxy seals lining the edges of the box lid should be checked annually by filling the boxes with water – look for leaks. We’ve also been sealing the bottom channel on the gas box frames to prevent leaking.

Sampling

1. Stay on trails – do not trample plants or compress soil near chamber.
2. Record T0 soil temperature on data sheet; temperature probe should be inserted 10 cm.
3. Record gas box lid type – it will be marked with a letter on the top.
4. Check the frames for obvious cracks. Fill the frame troughs with water (square chambers) without spilling water onto the soil (added water may stimulate biological activity).
5. Check septa on the box lids and replace if needed; they get dirty and gooey as they weather.
6. Sample chambers
   1. Square Chambers
      1. Place PVC lid on frame and check to see if a proper seal has formed; if lid has been laying flat on the ground, wave it around to make sure you’re not just moving the air that has been trapped inside for weeks directly onto the frame. To ensure the seal is good, gently pull lid up and feel for suction or check water level.
      2. Flush the sample vial with air from the chamber using a vent needle. Remove the vent needle and inject sample. For 5 ml vials, we usually flush with 10mL and inject with as much of an additional 10mL as possible. (The over pressure guards against sample contamination, and provides an easy means for removing known volumes from the vial during gas analysis). For large (10mL) vials, flush with 30 mL and add an additional 10 mL.
   2. Round Chambers
      1. Place the PVC lid on frame and seal by putting an elastic band around latex skirt.
      2. Flush the sample vial using the 3-way stopcock and tubing (this keeps the chamber from moving when removing samples). For large (10 ml) vials, flush with 30 mL of chamber air and add an additional 10 mL.
7. Start timer after the first sample is injected into the vial; record vial number and time on data sheet.
8. Use soil probe to take soil sample to 25 cm for moisture (and nitrate if desired) at least 0.5 m away from chamber. Put sample in tin or plastic bag.
9. Measure and record the height of each chamber in four places (cm).
10. Fill 4 vials with ambient air taken above your head; overfill vials as for chamber samples. These samples are to check for vial effects.
    

Helpful hints: It is easiest to keep track of where you are in the vial sequence by placing the vent needle in the next vial to be used and turning the “used” vials upside down in the tray. Some of the septa/vials may leak – this should be fairly obvious, listen for the leak. If rain is expected, data sheets can be photocopied onto rainproof paper.

Treatments sampled

One chamber is sampled per replicate unless otherwise specified.

• T7 fertilized microplot sampled 2008 – present
• T8 sampled 1992 – 1998, 2008 – present
• CF sample 1993 – 1997, 2008 – present (including N deposition microplots)
• DF sampled 2000 – 2007 (2 chambers/plot), 2008 – present (1 chamber/plot, including N deposition microplots)
• SF sampled 1993 – 1997, 2007 – present (including N deposition microplots)

Vial, syringe and sampling interval

The initial vials used for sampling gas were 3mL glass vials crimped with butyl septa seals through 2002-11-01. A 10mL syringe was used with the 3mL vials. Following the 3mL vials, 12mL glass vials with screw top caps and pierceable rubber septa were used through 2004-05-05. With the switch to larger vials came a switch to sampling with a 20mL syringe. Next the 12mL vials were replaced with smaller, 6mL vials with the same closure type and sampling done with a 10mL syringe again. These 6mL vials have been used to the present.

A sampling interval, or the time between individual samples from one chamber, was approximately 40 minutes through 2008. Beginning in 2009, a 20 sampling interval was used.

Analysis

In the lab, analyze vials for CH₄ (using the GC-FID), N₂ O (GC-ECD), and CO₂ (IRGA). Analyze CH₄ prior to N₂ O, because the ECD carrier gas contains 10% CH₄ you will contaminate CH₄ samples with carryover from the ECD injection port. Samples should be analyzed within a day or two of collection, or else you may start having some problems due to the sample vial septa. Among the problems you might encounter are septa absorbing the compound you are interested in, the septa giving off the compound, or the septa giving off a different compound that interferes with analysis of your compound. (If prompt analysis is not possible, store standards in sample vials the same day you collect your samples. That way what ever happens to your samples will also be happening to your standards, and your standard curves should generate reasonable sample ppms).

Run a full set of standards at the beginning and end of a run; r ² should be >0.99. Run a check standard every 10 – 15 samples and if off more than 10%, run full standards.

Calculation

To convert ppm/min to grams of compound per hectare per day, use to following formula:
Equation to calculate flux

where a is the slope in ppm/min, b is the headspace volume of the container in Liters and c is the molecular weight of the compound.