Radon as a natural tracer of gas transport through trees

Abstract

Trees are sources, sinks, and conduits for gas exchange between the atmosphere and soil, and effectively link these terrestrial realms in a soil-plant-atmosphere continuum.We demonstrated that naturally produced radon-222 ( 222 Rn) gas has the potential to disentangle the biotic and physical processes that regulate gas transfer between soils or plants and the atmosphere in field settings where exogenous tracer applications are challenging.Patterns in stem radon emissions across tree species, seasons, and diurnal periods suggest that plant transport of soil gases is controlled by plant hydraulics, whether by diffusion or mass flow via transpiration.We establish for the first time that trees emit soil gases during the night when transpiration rates are negligible, suggesting that axial diffusion is an important and understudied mechanism of plant and soil gas transmission.

Isotopic evidence for axial tree stem methane oxidation within subtropical lowland forests

“…Megonigal et al . (2020) also showed that inert sedimentary gases and tree stems were connected using the natural soil gas tracer radon ( 222 Rn), which positively correlated with temporal changes in stem CH 4 fluxes. Our stable isotope data provide a further line of evidence indicating that, in lowland flooded forest stands such as Melaleuca and Casuarina , trees act as soil gas transport conduits to the atmosphere, and can allow for some degree of bypassing of soil/water column CH 4 oxidation processes.…”

“…Consequentially, the average afternoon oxidation rates between lower and upper measurements (51.9 ± 5.3%) were twice that of the average dawn oxidation rates (26.0 ± 5.1%). We speculate that this temporal oxidation hysteresis phenomena may be caused by one of more of the following: (1) diurnal temperature driven differences driving microbial metabolism; (2) mixing of CH 4 isotopic gas signatures by upwards diffusive transport (dawn and afternoon) vs active transpiration transport (afternoon only); (3) changes in sap‐flux and oxygen gradients assisting aerobic MOB during photosynthesis periods (Barba et al ., 2019b); (4) shifts in soil source values due to rhizosphere oxidation during photosynthesis (Chanton & Dacey, 1991; Cho et al ., 2012); and (5) reduction of tree‐stem pore water content during transpiration allowing both oxidation and more rapid upwards diffusion gas transport (Teskey et al ., 2008; Steppe et al ., 2015; Megonigal et al ., 2020). Diel patterns of tree stem CH 4 oxidation and transportation processes clearly require further research to ascertain the precise processes responsible.…”

Impact of forest plantation on methane emissions from tropical peatland

“…Our study did not aim to conduct direct measurements to establish the relative importance of these different processes. Quantifying the pathway‐specific emissions and improving our understanding on the impact of root distribution by depth and dissolved CH 4 concentration profile are important future study (Barba et al, ; Megonigal, Brewer, & Knee, ).…”

Multiple processes contribute to methane emission in a riparian cottonwood forest ecosystem

“…Other mechanisms that can contribute to temporal and spatial variation within tree stems are: the density of wood, water content and presence of wet heartwood, abundance of lenticels, tree diameter, and differences in diffusivity of stem tissue types (heartwood, sapwood, bark) (Zeikus & Ward, 1974; Pangala et al ., 2013; Wang et al ., 2017; Barba et al ., 2019; Covey & Megonigal, 2019). In addition, a recent study based on radon measurements has indicated that transport of gases within tree stems can be affected on a diurnal basis by changes in plant hydraulics and transpiration rate (Megonigal et al ., 2020).…”