Plants control soil gas exchanges possibly via mucilage

Abstract

Background: Gaseous matter exchanges in soil are determined by the connectivity of the pore system which is easily clogged by fresh root exudates. However, it remains unclear how a hydrogel (e.g., mucilage) affects soil pore tortuosity and gas diffusion properties when drying. Aims: The aim of this viewpoint study is to extend the understanding of gas exchange processes in the rhizosphere by (a) relating it to the patterns formed by drying mucilage within pore space and (b) to give a concept of the effect of drying mucilage on soil gas diffusivity using the combination of experimental evidence and simulations. Methods: To describe the effect of mucilage on soil gas exchanges, we performed gas diffusion experiments on dry soil–mucilage samples and took images of glass beads mixed with mucilage to visualize the formation of mucilage after drying, using Environmental Scanning Electron Microscopy. Finally, we set up simulations to characterize the geometric distribution of mucilage within soil during the drying process. Results: Experiments of gas diffusion show that mucilage decreases gas diffusion coefficient in dry soil without significantly altering bulk density and porosity. Electron microscopy indicates that during drying mucilage forms filaments and interconnected structures throughout the pore space reducing gas phase connectivity. The evolution of these geometric structures is explained via pore scale modelling based on identifying the elastic strength of rhizodeposition during soil drying. Conclusion: Our results suggest that releasing mucilage may be a plant adaption strategy to actively alter gas diffusion in soil.

Extracellular polymeric substances from soil-grown bacteria delay evaporative drying

“…This difference could have been caused by dry xanthan structures deposited in the pores above the evaporation plane limiting water vapor diffusion. Reduced gas diffusion was reported for coarse sand amended with dried seed mucilage (Haupenthal et al, 2021).…”

Coupling non‐invasive imaging and reactive transport modeling to investigate water and oxygen dynamics in the root zone

“…While the mucilage layer around roots facilitates water uptake (Ahmed et al., 2014) and nutrient diffusion (Zarebanadkouki et al., 2019) in drying soils, gas transport to the roots’ surface is impeded as the O 2 diffusion coefficient within this layer is estimated to be as low as 2 × 10 −10 m 2 s −1 and O 2 solubility is limited (Ben‐Noah & Friedman, 2018). Drying mucilage may also disconnect gas diffusion paths even at low soil water content (Haupenthal et al., 2021). At the micro‐scale, several models of O 2 root uptake and transport consider a ‘boundary layer’ of saturated soil surrounding the roots to account for limited diffusion in the rhizosphere soil (Bartholomeus et al., 2008; Cook & Knight, 2003).…”

Physics of Viscous Bridges in Soil Biological Hotspots

“…After its deposition, the polymer network can further enhance water retention by acting as a new solid matrix preventing or delaying the air entry. This new matrix maintains the connectivity of the liquid phase, thus enhancing the unsaturated hydraulic conductivity (Benard et al., 2019) and the diffusion coefficient in soils (Zarebanadkouki et al., 2019), and it reduces gas diffusion (Haupenthal et al., 2021), thus reducing evaporative fluxes. Additionally, the formation of extensive 2D structures corresponds to a sudden increase in soil water repellency, which reduces the rewetting kinetics (Benard, Zarebanadkouki, & Carminati, 2018).…”