We know that green plants are important for keeping our planet cool: Around the globe, they consume billions of tons of the greenhouse gas carbon dioxide (CO2), converting it, through photosynthesis, to sugars and ultimately to wood. But CO2 is also released back into the atmosphere in a process called respiration, both from living plants and from the decomposition of detritus. Thus the total amount of CO2 plants take out of the atmosphere equals the quantity they absorb for photosynthesis, minus the amount they return to the atmosphere through respiration. It sounds like a simple equation, but until now, measuring its components has been anything but simple; scientists have been stumped for years trying to come up with an accurate means to tease out the two components of the “simple equation” – an analysis that is critical to predicting the future impact of plants on our climate.
Recently, a research team in the lab of Prof. Dan Yakir
of the Environmental Sciences and Energy Research Department demonstrated a new method for just measuring how much CO2
is taken up, independent of the amounts released back in respiration. This method, based on another chemical compound in the atmosphere – one found in extremely tiny amounts – should help scientists around the world clarify the picture of the ongoing exchange of carbon between plants and the atmosphere, hopefully leading to better climate models and predictions. Measuring the levels of this compound, COS, involved developing new, ultra-precise tools to reveal its presence.
In the commonly used methods for figuring carbon exchange, only the net difference between photosynthesis and respiration can be directly measured in the air above vegetation. But that doesn’t indicate specifically how much CO2 is lost or gained – information that is critical when trying to understand how patterns change daily or seasonally, across regions or in response to climate change. Researchers trying to tease apart CO2 fluxes have used various “tricks” – for example, measuring at night, when photosynthesis shuts down; but the results have still been only approximations.
Yakir and his team decided to act on an idea that had come up several years earlier but was considered too impractical to be of use: Measure the flux of another substance, carbonyl sulfide (COS) into the plant biosphere. COS is similar to CO2, except that instead of two oxygen atoms attached to the carbon, the molecule contains an oxygen and a sulfur atom. Plants absorb COS along with the CO2; but they don’t release it again in respiration, so measurements could serve as a good proxy for CO2 uptake. The only problem was that there is about a million times less COS in the atmosphere than CO2: If CO2 is measured in parts per million, COS must be measured in parts per trillion (that is one molecule of COS for every trillion molecules of gas in the atmosphere).
Though these figures were daunting, the researchers convinced a scientific instrument company in the US to build them a tool – called a quantum cascade laser (QCL) – capable of accurately measuring minute amounts of COS. Research student Keren Stimler then spent around five years testing the QCL with plants in the lab, “developing the ‘language’ of COS exchange, the instrument, and the analysis of its results,” says Yakir.
In the team’s recent work
, which appeared in Nature Geoscience
, Yakir and postdoctoral fellow Dr. David Asaf decided it was time to take the QCL on the road. They loaded it into the Biosphere-Atmosphere Mobile Research Lab (see below) and took it to various sites to see how well it works in real conditions – in pine forests in a variety of climate regions and in agricultural fields. Their results demonstrate that COS measurements based on the new QCL technology are able to provide the numbers on CO2
uptake that scientists have been seeking for years. Indeed, the team can now confirm their previous finding that pine forests in Israel are as effective in carbon storage (carbon sequestration) as European pine forests: While the semi-arid local forests take up much less CO2
in photosynthesis than European ones, they also lose much less CO2
in respiration, so the net carbon storage is the same in both forests.
Going mobile pays off
Two years ago, when Prof. Dan Yakir together with staff scientist Dr. Eyal Rotenberg and their team finally (after two years of development) started up the four-by-four, 12-ton truck that houses the Biosphere-Atmosphere Mobile Research Lab, they knew they were taking a bit of a risk. Although they had visions of the exciting contributions the one-of-a-kind transportable lab could be making to global research on the exchange of materials between forests and the atmosphere, there were no guarantees that the new approach would be widely endorsed or that other researchers in this field would be lining up on their doorstep.
This summer, any fears they may have had were laid to rest when research groups from the US, Spain, Austria and Germany all showed up to work together out of the Mobile Lab. The teams brought along their research tools – including large pieces of equipment that had to be carefully shipped halfway around the world. The idea was to combine the mobile platform, with its fully operational (and air-conditioned) laboratory, on the one hand, and, on the other hand, to make use of the mobile lab’s 28-meter-high telescopic mast, which, according to Yakir, reaches the critical layer “between the plant canopies and the airspace below the mixed atmosphere.” Each group planned to investigate one slice of the larger question: How do the volatile compounds released by trees (think of the terpenes that give pines their characteristic scent) link to CO2 exchange, and how does this affect atmospheric chemistry and ultimately cloud formation and climate?
One group, for example, took detailed measurements of the trees at ground level; another measured levels of the volatile compounds above the forest canopy; a third monitored the size of particles, which, above a certain diameter, can serve as nuclei for the condensation of water drops in clouds; and so on. Combining the data from all of the groups, says Yakir, “is beginning to give us an overall picture of the evolution of climate – from the trees to clouds and rain.”
“We were very excited by this campaign,” he says. “It was a great example of scientific collaboration in which different groups worked together out of one small traveling lab. It was a further test of our own instruments, including the new mobile platform and the QCL system, and it looks like the results will be really interesting. Most of all, it showed us that the investment in the Biosphere-Atmosphere Mobile Research Lab was worth the risk – it is clearly paying off.”
Prof. Dan Yakir's research is supported by the Cathy Wills and Robert Lewis Program in Environmental Science; the estate of Sanford Kaplan; and the