|Strategy||Frequency||Detailed Instructions||Accessory Sampling||Other Information||Shipping|
This is a new project and likely be changed and adapted to needs along the way. We will greatly appreciate any feedback and suggestions.
Participants in this campaign receive boxes of air sample flasks approximately every month during the growing season.
Sampling is easy but requires some attention. It is important to remember that we are likely to measure signals that are near the detection limit, and consistency and care over all steps of the operation (although ridiculously simple) are critical.
Air samples MUST be dry. We provide drying tubes that must be connected before the flask. Flushing of the flask with about 3 litres of sample air is a minimum, over pumping will quickly exhaust the drying tubes.
At this first stage we propose to limit sampling to nighttime (with, say, 2 h after dark and before dawn as the limits) and use 10 flasks per night (two nights per one box of 20 flasks). Nighttime is simpler because variable photosynthetic discrimination and complex boundary layer behaviour do not complicate the respired isotopic signal. Within this framework, the important thing is to obtain, with the 10 air samples, a large range of CO2 (and 13C) concentrations. A 30 ppm range in CO2 concentration among the flask is already difficult to get any meaningful results. To maximise the CO2 concentration range, one can sample over time during the night and/or over a height gradient. We suggest using both approaches to maximize the CO2 range and therefore the quality of the results. The first application of the results will be in constructing Keeling plots from which the 13C of respired CO2, and possibly of recently fixed C, and variations in them, will be obtained. (Other ideas to get at this product will of course be welcome).
We suggest looking at the pattern of the nighttime CO2 changes at your tower and identify the period that will give the max change in CO2 (without getting any daytime contamination). Concentrate the sampling over that period (e.g. 7 PM to midnight in our site).
Samples can be taken from the sonic height, by connecting the flask on-line between the Licor and the air pump (as done in our site, we have a union on the tube connecting the Licor and pump that can be replaced with the flask system during sampling periods). If you get enough of a range in CO2, this sampling location may be sufficient. Otherwise, we suggest to sample also inside the canopy, but note that the lower you get the smaller the "footprint" of the sample (best would be near the canopy). Samples can be taken alternatively at two locations (sonic and canopy) or in any other pattern that you think will maximise the concentration range. Ideally, one should be able to watch the Licor during and between sampling to insure good range of CO2 is obtained.
The target is to sample at least once a month over the season. We would like to trace changes in ecosystem function as reflected in the 13C signal over the seasonal cycle and over a wide geographical/climatic range. The latter is taken care of by including a range of sites distributed mainly throughout Europe. To cover seasonal changes we should sample every few weeks. This will depend on the efficiency of the shipping/mailing system.
Initially, to get familiar with the sampling process, we propose that two samplings (two sets of 10 flasks each) be taken within about a week so they can serve as semi-replicates. In addition, throughout the season, if there is a major weather event such as a major storm, freeze event etc. it would be interesting to compare sample (sets of 10) before the event with another taken about 5 days after the event.
The sampling unit should consist of: an inlet tube (from the sampling location such as sonic, canopy, below canopy etc.), dryer, cajon-type T connecter, small tubing to flush flask, flask (200ml), pump (e.g. conventional air pump used in most sites). The unit should be set-up as shown in the diagram below. At flux tower sites one ideal option is to insert the sampling kit between the outlet of the Licor and the inlet of the air pump (see small inset diagram below).
*Cajon fitting is an O-ring seal connector that needs only hand tightening to give a very good seal. Please make sure that the tubing is inserted past the O-ring before tightening. In case you need to open the fitting, note that an O-ring and a compressor ring must be inside, in the right order and direction (see diagram).
**The drying trap (magnesium perchlorate) should last for more than the 20 flasks you get. But of course if uncontrolled pumping (rate and duration) is applied, the dryer will be exhausted and the samples contaminated by water vapour. Please use with caution and if 'drierite' indicator at the outlet of the drier shows any indication of color change (from light blue to pink) samples will be too wet for analysis. The trap must be replaced.
To extend our abilities to interpret data, we ask you to sample and send with the return shipment samples of 1) leaves, 2) stems and 3) soil taken at the day preceding or following the nighttime sampling (that is, two sets of three samples per box you receive). Vacuum tight test tubes (Exetainers) are included in the Kit for this purpose.
Please note that we rely on tightly sealed tubes that don't loose water vapor, and we need to extract at least 1 ml of water from each sample (for 18O analysis).
Please provide some background information on your site:
Site name and location:
Use our address on back of mail label on box. Please "seal" on strap the box with one plastic tie provided with the kit before mailing back (in the same way you should have received the box). Also, we added a short note to customs officials who are usually a major cause of breakage. If you think a translation to local language is necessary, please add it and we will use it for future boxes.