KBS062: CH4, N2O, CO2 Fluxes (round chamber method)


In use from 2008-03-01


Sampling Frequency: Every two weeks during the growing season for GLBRC; weekly during growing season for LTER. Starting in 2013, sample frequency is increased to weekly in June for GLBRC, and monthly winter samples added for GLBRC and LTER. Starting in 2015, only sample GLBRC treatments G1-G4, G5, and G10.

CO2, CH4, and N2O fluxes are estimated using in situ closed-cover flux chambers. Chambers are semi-permanently installed in replicates 1-4 of all treatments in the KBS GLBRC Intensive Sites, and in all replicates in the Arlington sites (GLBRC). Four buckets are semi-permanently installed in each of the KBS Marshall scale-up fields (GLBRC). Four buckets are semi-permanently installed in each plot of the Resource Gradient Experiment (LTER).

Gas fluxes are measured at 18 minute sampling intervals. Samples are taken from gas traps by inserting a 10 mL syringe into the rubber septum and drawing 10mL which is used to flush a vented 6 mL glass vial. Glass vials are sealed with a septum. Another 10 mL gas sample is taken from the trap and placed in the glass vial, giving the vial a gas overpressure. CH4 and N2O are determined by gas chromatography. CH4 is analyzed with a flame ionization detector (300 degrees C), while N2O is analyzed with a 63Ni electron capture detector (350 degrees C). CO2 is analyzed using an infrared gas analyzer. Gases are separated on a Poropak Q column (1.8 m, 80/100 mesh) at 80 degrees C. Carrier gas for CH4 is nitrogen, while carrier gas for N2O is argon/methane (90/10).


Sampling Procedure:

  1. Vials (Exetainer LABCO) should have fresh septa. Number the vials as appropriate (KBS LTER uses 4 sampling times (T0 – T3) per chamber). All vials are numbered with the set ID determined by the user.
  2. Check chambers for plant growth and damage (rodent etc) prior to sampling.
  3. Place chamber lids and sample vials next to relevant chambers.
  4. Check the septum on the chamber lids and replace if needed. Damage by repeated needle insertion occurs. Inspect the o-ring on each lid for correct placement and / or damage.
  5. Place a 25G needle into the chamber lid septum to relieve pressure inside chamber during lid placement. Snap the lid onto the installed bucket and IMMEDIATELY start the stopwatch. Mark the time the lid was snapped on the bucket. Remove the needle after the lid is firmly attached.
  6. Since multiple chamber measurements are made, the stopwatch should run continuously throughout the sampling regime. Do not reset the stopwatch at each chamber. Keep the clock running and write down the time each lid is deployed and each time a sample is taken.
  7. Insert one needle into the first sample vial (labeled Chamber ID -T 0) to act as a vent. Insert the syringe needle into the chamber lid septum and flush three times by pulling out and then pushing in the syringe (about 10 cm3 should be extracted then reintroduced to mix the chamber air prior to sampling). Repeat chamber air mixing X 3, then draw out a 10 ml sample and inject into the sample vial while the vent needle is in place. Listen for air escaping from the vent needle while vial is flushed. Remove the vent needle from the vial. Reinsert the syringe into the lid septa and mix chamber air 3 additional times before drawing the 10 ml sample. Remove the syringe from the lid septa and inject sample into the vial. RECORD THE SAMPLING TIME WHEN THE AIR SAMPLE IS DRAWN UP IN THE SYRINGE. This is sample T(0). Inject as much of the 10ml sample as possible into the 5.9 ml vial. The over pressure guards against sample contamination and is corrected during analysis.
  8. Record chamber / vial ID, along with sampling time on data sheet.
  9. If applicable, record soil temperature / moisture proximal to chamber during sampling period (ie during 1 round of sampling).
  10. Repeat sampling procedure for all chambers in a given plot / treatment / block etc, recording lid deployment and sampling times for all initial T 0 samples.
  11. Return to the first sampled chamber and repeat sampling at new time (T 1) for all chambers. KBS LTER takes four samples (T 0 – T 3) from each chamber over an approximately 1 hour sampling time (i.e., returning to the same chamber approximately every 20 minutes). Prior to actual treatment sampling, tests should be made to ensure fluxes of N2O are linear over the sampling time period. It is also a good idea to sample ambient air at the beginning and end of each campaign.

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. As a guideline, allow about 1 minute for each sampling procedure. Sample T 0 includes lid deployment, so may take a little longer.