TRANSCRIPT. Thanks Ashlee, my name is Rod, and we re going to open up with a poll question. [Pause for poll]

Transcription

1 TRANSCRIPT SLIDE 1 [00:00]: Speaker: Thanks Ashlee, my name is Rod, and we re going to open up with a poll question. SLIDE 2 [00:07]: The question should have popped up on your screen. How large is the soil carbon pool? Please answer by choosing the radio button next to your answer; we will give you a few moments. And just to explain the three options; it s 2 times the atmospheric, is it 3 times land plants or is it 1/3 rd of all fossil fuel reserves? Those are your 3 options. [Pause for poll] SLIDE 3 [01:06]: Thanks everyone for your answer, the correct answer is all three of them. So the soil carbon pool is obviously very large, and it SLIDE 4 [01:16]: basically equates to roughly 2 times the atmosphere or 3 times the land plants or 1/3 rd of all fossil fuel reserves. Continuing on with the webinar, why is soil CO 2 flux important? SLIDE 5 [01:33]: Well we just touched base on that a second ago with the question, it s because the amount of carbon in the soil is large and thus the amount of carbon released is very large. Page 1 of 9

2 SLIDE 6 [01:46]: Also soil flux is important for a number of reasons, they re a number of reasons people out there study soil CO 2 flux. Remediation, so reclamation of mine lands, of obviously fossil fuel reserves, it s important to understand the carbon dynamics when you re trying to remediate areas that have been depleted of obviously the natural vegetation, and so that s a group that s very interested in understanding carbon dynamics, and obviously carbon release from the soil. Also the people who work on both terrestrial and geologic sequestration, they are interested in understanding background rates of soil CO 2 flux as well. Also some of the people that are looking at just sequestration based off natural vegetation, just how effective is sequestration in these natural lands, when it comes to looking at climate change. People that study Ecology, so the people out there looking at the different natural ecosystems of the world, are very interested in understanding the amount of CO 2 that potentially is coming out of these systems, and ultimately by learning this information they can better manage those ecosystems. And finally Agricultural producers and researchers are interested in understanding the amount of carbon that s in their soil and is being released from their soil. The carbon in the soil helps with productivity, helps tie nutrients to the soil, and so they re very interested in understanding the affects in this of the CO 2 and carbon in the soil, and ultimately how much is being released. And again, better understanding the management practices that go from that. So those are just a few groups that are interested in understanding soil CO 2 flux. SLIDE 7 [03:54]: Where does soil CO 2 come from? Autotrophic respiration, so that s the root and micro-organism metabolism of substrate from plants. It comes from Heterotrophic respiration which is the breakdown of soil organic matter in plant litter by microorganisms. Just the respiration of soil fauna, so everything from the microbes to the small Collembola, to earth worms all obviously breath and respire as living organisms, so that is a function also of the CO 2 flux. And also the action of water on inorganic carbonates in the soil can also be a source of CO 2 flux from the soil. SLIDE 8 [04:39]: What affects soil CO 2 production? Well, obviously the biology of the soil, and so what s in the soil itself, the productivity of the nutrient transport in the soil. Metabolic activity, the plants, microorganisms, how many microorganisms are there, also temperature. Everyone talks about the Q10 relationship, where you increase the temperature 10 you double the enzymatic reactions, so temperature has a very, very Page 2 of 9

3 large effect on soil CO 2 flux. Moisture as well, if you increase moisture, increase temperature you have obviously more microorganisms that thrive in those environments, and thus you can increase both your autotrophic and heterotrophic respiration. So those are some of the biological effects of CO 2 production. And also just the chemistry of the soil, and what type of nutrients and elements and stuff are in the soil and how they interact with the soil can have a huge effect on soil CO 2 production. SLIDE 9 [05:51]: What affects soil CO 2 movement? The majority soil CO 2 movement is directly driven by concentration gradients, and so you basically have a very high concentration in the soil and a very low concentration in the atmosphere, and so it s a diffusion based process, is what we re trying to say by this slide. Is the soil flux, the natural soil flux is driven by diffusion, by the most part. SLIDE 10 [06:20]: The effects of soil porosity on different types of soil. If you look at this graph you can see how a very heavy clay soil responds, verses a light sandy loamy soil. Again, this is based off a gradient, a concentration gradient, so in this case you have a heavy soil where the concentration gradient from inside the soil to the atmosphere is very large, so you have a very large difference in concentration, verses a light sandy soil where you can have a very large underestimate in the actual rates of the soil flux, when you re closed down and trying to take a measurement, basically because that delta, that change in concentration is much smaller. And that s what this slide is trying to show when it comes to actually using a measurement technique, and looking at the way we analyze the equation. What effects soil CO 2 movement? SLIDE 11 [07:23]: Pressure has a very large impact on soil CO 2 movement. Whether it s a negative pressure or positive pressure on the soil can affect the soil CO 2 flux measurement. If it s a negative pressure, you have what s termed bulk flow of CO 2 out of the soil, and so you have this negative pressure in the atmosphere and it can actually push CO 2 out of the soil and have a much greater impact on the amount of CO 2 being released in the diffusion rate itself. A positive pressure will do just the opposite. If you have a positive pulse in the atmosphere, ---you re going to have a positive amount of CO 2 coming out of the soil through diffusion, but that rate will be significantly lower due to the positive pulse and ultimately bulk flow that occurs the opposite way. And so, pressure has a very, very large impact and a lot of the Page 3 of 9

4 pressure we talk about with soil comes from the wind and obviously the atmospheric pressure changes that occur. And those ---typically occur very fast. SLIDE 12 [08:31]: What else affects soil CO 2 movement? Precipitation can have a large, very fast impact on soil CO 2 diffusion. Basically what happens, two things can happen, the first being displacement, and so as the water precipitation falls the water fills up the pores spaces and you have this instant pulse of CO 2 being released. And you can see that on the graph here, where we have the label 3.3mm rain event, you have this instant pulse of CO 2 due to the displacement. What else can happen is if you have a dry sort of soil and you put some water on it, the microbial activity starts to increase and you have respiration of microbes, and so ultimately what happens is you have this long term, longer term effect on soil CO 2 flux due to just moisture and increase microbial activity. SLIDE 13 [09:33]: The opposite can also occur with moisture, if you have too much moisture, you have that initial pulse from the first part of the rain fall event, and then as the pores fill up and sort of flood the soil, you can have diffusive resistance, and so you can actually have lower soil CO 2 flux rates over time with too much moisture as the CO 2 has no ability to escape, and that s due to the diffusive resistance going on with the moisture. Now I m going to turn it over to Jason Hupp, an application analyst here at LI-COR and he will talk about some of the challenges with soil CO 2 flux measurements. Moderator: Thanks Rod, while Jason takes his place and gets comfortable in front of the computer, I want to take a moment and thank everyone for submitting questions via the questions panel. We will read a few questions live at the conclusion of the webinar. But whether or not your question is answered verbally, please keep submitting them. Also, please feel free to follow the conversation on soil flux off line, via the Tweeter tag #soilflux. If you have questions following the webinar, you can ask them on Twitter using the tag or contact our speakers for more information. Remember, the webinar will be available off line in a few days, and watch your for a handout with in-depth information on the content of the webinar. Jason. Page 4 of 9

5 Speaker 2: SLIDE 14 [11:01]: Thanks Ashlee, and by all means please keep the questions coming. Well, Rods been talking about sort of the mechanisms and sources of CO 2 flux, I want to take a bit and talk a bit about its measurement and some of the challenges that we face there. The first thing to say about soil CO 2 flux measurements is that it s highly sensitive, and it s highly variable. And so Rod talked about how the fluxes primarily driven by diffusion, SLIDE 15 & 16 [11:30]: across a concentration gradient between the soil atmosphere and the atmosphere above the soil. And so anything that alters that gradient will alter the flux. He also talked about how pressure differentials between the atmosphere and the soil atmosphere can induce bulk flows. And so anything in the measurement that causes a pressure differential between the measurement device and the soil atmosphere can influence the flux that you re trying to measure. SLIDE 17 [12:04]: When we talk about soil CO 2 flux being highly variable, the first thing we want to talk about is it being variable in space. One of my favorite sayings about soil is; the only thing homogenous about soil is its heterogeneity. And this is as true for soil texture and composition and microbial community as it is for the soil CO 2 flux. SLIDE 18 [12:25]: It s also very variable in time, there s plenty of evidence that shows fluxes that show diurnal patterns, seasonal patterns or even patterns over longer time scales. So how do we measure soil CO 2 flux? SLIDE 19 [12:43]: One of the most common approaches is to take a closed gas exchange system approach. Or use something called a static chamber. In this approach you take a seal volume, your chamber, and you use it to cover over the area of soil you re interested in measuring the flux from. And in this chamber you measure the CO 2 concentration as it accumulates and if you plot that concentration over time and take Page 5 of 9

6 the slope of this curve, this dc dt term, the change in CO 2 concentration with time, and multiply that by the ideal gas law, you get a flux; units carbon released from your soil per unit time. This raises an interesting point here, because we ve said a couple times here that soil CO2 flux is diffusive in nature and it follows concentration gradients. And when you close over the head space above a volume of soil and let CO 2 build up in there, you re altering the concentration gradient that s driving the flux that you re trying to measure. Typically over short time scales this isn t an issue. But if you leave the chamber closed long enough eventually that chamber head space comes into equilibrium with the soil atmosphere below it and you re flux goes to zero. SLIDE 20 [14:04]: So the other issue that can occur when you leave a chamber on too long occurs when you re trying to make repeated measurements. And so not only does the CO 2 concentration build up in the chamber if it s left on too long, but it builds up in the upper soil strata as well. And so the figure that we re showing here is repeated flux measurements, and throughout this period we leave a chamber closed for the amount of time marked with the arrows, and so in the beginning we make measurements as we normally would, a minute or so, and then we leave the chamber closed for 5 minutes and we open it up, and it vents back to atmospheric, we close it down and we make a flux measurement, and that flux is significantly higher than the flux we measured before we closed the chamber for 5 minutes. You do the same thing for 10 minutes and the flux is even more is even higher, and it takes it longer to recover to pre-capping levels. And so what s happening here, is as the CO 2 concentration builds up in the upper soil strata, even though the CO 2 concentration in the chamber goes to ambient, because there s resistance to diffusion in the soil, it takes it longer to move back to those pre-disturbed conditions. SLIDE 21 [15:24]: So wind can also affect our soil CO 2 flux measurements when using chambers. And so what we re showing here is a plot of CO 2 concentration of the chamber over time, plotted on the primary y-axis, and wind speed over the measurement period plotted on the secondary x-axis. And when I was talking about how we make close system measurements, I showed a curve of CO 2 concentration in the chamber over a period of time, and it was a nice straight line. A few slides back, Rod showed a figure that had a couple of curves with some different measurements, one was a nice straight line and one was a nice clean exponential curve. This one for lack of a better term is wonky, and it s wonky when the wind speed goes high. And so that raises the question of, well, how does wind affect our soil CO 2 flux measurement? Page 6 of 9

7 Well we know soil is porous, and one of the key assumptions of a closed gas exchange system is that, the pressure is constant during the measurement. And the way that we achieve a constant pressure in a closed system measurement like this is, we pretty much put a hole in the side of the chamber. SLIDE 22 [16:40]: Traditionally these holes have been a short tube, and when wind blows across the tube it creates a Venturi effect, and this affects the dynamic pressure inside the chamber. SLIDE 23 [16:51]: And so, dynamic pressure is a function of wind speed. The higher the wind speed, the higher the dynamic pressure. But we know that total pressure isn t changing, atmospheric pressure, soil surface and atmospheric pressure at the top of our chamber isn t significantly different and so this has to be balanced by a static pressure. And so ultimately the static pressure in the chamber has to be equal in magnitude to the dynamic pressure. With the Venturi event it does that by pumping air in or out of the chamber. Well there s a different solution here, and that s to apply the Bernoulli Effect SLIDE 24 [17:32]: to remove the Venturi effect on the pressure vent. And with the vent design that LI-COR uses, we actually slow down the wind speed as it moves across the pressure vent, and this reduces that affect. And so we can see that if we compare fluxes obtained SLIDE 25 [17:52]: with an improper vent design, and one measured with a chamber using the LI-COR vent, and so these measurements were made on the same soil collars, the same soil area, one right after the other, and measurements made with the improper vent we see are no different than the LI-COR vent at low wind speeds, but as we d expect as wind speed increases, the improper vent, the one subject to the Venturi effect, causes an overestimation in the flux that we re measuring. SLIDE 26 [18:28]: Page 7 of 9

8 So back to the high variability in soil CO 2 flux, what we re showing here are measurements made in a corn field in Nebraska in the fall. And the dark circles are measurements made within the row, so the chambers setting between corn plants, and the open circles were made between rows. And what we see is that there s a distinct difference in the soil CO 2 flux, even for chambers that are only separated by a few feet. The other thing to note, is that early on in this dataset we see high diurnal variability. Then a frost event occurs, we re in a seasonal shift with this dataset, and the CO 2 flux has become much flatter. And that s all I have to say. Ashlee. Moderator: Thanks Jason, and thanks Rod for your presentation. I have a couple questions here from the audience that we ll take first before we talk about the upcoming webinars in the series. The first question I have is; what type of soil typically have the largest and smallest CO 2 flux rates? Speaker: Thanks Ashlee. Well typically highly organic moist soils in warm temperatures have the highest flux rates. Whereas the smaller flux rates will occur on very dry desert soils. And again that s not always the case, but typically that s what happens in both, both those environments. Moderator: Thanks Rod. The second question I have is; you discussed the chamber pressure vent, where can I purchase such a vent? Speaker 2: Thanks Ashlee. Well, the chamber pressure vent is something that came about after much research, it s not an apparently obvious problem or it s not an apparently obvious solution to an obvious problem. The pressure differentials are small and they re difficult to measure and it took quite a bit of work to develop that vent. It s actually a patented design; we don t sell it separately from the LI-COR CO 2 flux chambers. But if you want to learn more about it, we ve published some research that went into its development in the Journal of Geophysical Research, and Ashlee can we include a copy a reference to that in the content pack? Page 8 of 9

9 Moderator: Yea, no problem Jason, we ll make sure we get that out to everyone. Alright, that s all I have for questions. Before we end the webinar, I d like to let everyone know that SLIDE 27 [21:08]: the webinars that are coming up in the soil flux series, are on your screen right now. Next up is Soil CO 2 flux measurement techniques, following that is; Advantages of the LI-8100A system, and finally we ll have Collecting and quality control of soil flux data. So you guys should be getting announcements when these are available so just watch your , and you ll be able to register for those once they are ready to go. SLIDE 28 [21:31]: Thank you everyone for joining us, our panelist will stick around until everyone has had their questions answered. And if you have another question that you haven t posed yet you can contact Rod or Jason at the information on the screen. Thanks for joining us. Page 9 of 9