Despite the old saying “dead as dirt,” Earth’s soil is an incredibly rich and dynamic environment. Invertebrates, microbes, and plants all interact beneath the surface in a complex dance that determines the health of ecosystems and plays a part in larger biogeochemical cycles. Plants, in particular, play a major role in mediating mass transfers among different compartments of the biosphere. Their root systems create twisting underground superhighways of gases and liquids, moving the molecules essential for life up into the plant to be broken apart or added together as needed and generating waste products to be exhaled and excreted into the air. The gas transporting capacity of wetland plants is due to porous, gas-filled root tissues called aerenchyma that transport oxygen to the anoxic root zone and also facilitate gas transfer in the opposite direction, from soils to atmosphere.
Scientists have spent considerable time investigating how key gases, such as oxygen and methane, are exchanged between earth and atmosphere by plants. The generalized kinetics of gas flow into root systems is still poorly understood, however. Here Reid et al. attempt to determine the speed at which wetland plants move various trace gas molecules through their root system.
The team relied on an experiment known as a “push-pull test,” in which a combination of dissolved gas and nonvolatile tracers are coinjected into a vegetated wetland soil (pushing) and their concentrations are measured over time by sampling (pulling) the gasses at the site of injection. The key to this experimental setup is to inject one or more volatile tracers, which will readily partition into gas-filled root tissues and be transported by the aerenchyma network, and a nonvolatile bromide tracer, which will only dissipate in pore waters by diffusion. By comparing the ratio of the tracers at the site of injection over time, scientists can determine how readily the dissolved gases are being taken up into the root network compared to their nonvolatile counterpart.
The authors analyzed sulfur hexafluoride and helium gases. These two gases were chosen because they are chemically inert and, because of their different properties, were expected to establish upper and lower limits on gas exchange rates. Measurements were taken at multiple depths and locations within a New Jersey tidal marsh in order to establish a rough estimate for how quickly gases can migrate from saturated soils into root systems in the wild. The density of the root system proved to be a particularly important variable, with more developed root systems significantly enhancing gas exchange.
Although sulfur hexafluoride and helium are not themselves relevant to ecological processes in wetlands, the rates inferred from those tracers should bookend the minimum and maximum rates exhibited by critical biogenic gases like carbon dioxide, methane, and nitrous oxide. This is an important step in understanding how physical-chemical transport systems interact with biological nutrient cycling in wetland soils, with implications for the greenhouse gas balances of wetland ecosystems. (Water Resources Research, doi:10.1002/2014WR016803, 2015)
—David Shultz, Freelance Writer
Citation: Shultz, D. (2016), Details of gas flow in wetland plant roots unearthed, Eos, 97, doi:10.1029/2016EO044981. Published on 4 February 2016.