Effects of mucilage and extracellular polymeric substances on soil gas diffusion

Abstract

Gas exchange in the soil is determined by the size and connectivity of air-filled pores. Thereby, water saturation, soil compaction, and organic matter fraction are the main barriers for gas movement in soil. For optimal growth, life in soil requires a balance between water and oxygen content. However, fluctuations in moisture conditions challenge this balance. By exuding mucilage and extracellular polymeric substances (EPS), plants and bacteria can alter the physical properties of the soil in their vicinity. It is considered that by releasing these hydrogel-like substances, plants and bacteria increase their resilience against drought stress. However, we still lack knowledge on how these substances affect the soil gas diffusion. An improved understanding of the complex interactions between plants, bacteria, and soil, which have great implications for root water and nutrient uptake, and biogeochemical turnover and respiration processes in the soil, could provide valuable insights for optimizing crop performance and improving water and nutrient use efficiency.<br /> The focus of this thesis was to investigate the effect of mucilage and EPS on soil gas diffusion, aiming to improve understanding of gas diffusion processes in soil by explaining the geometric alterations of the pore space induced by mucilage and EPS. Laboratory measurements were conducted to determine soil gas diffusion coefficient (D_p) supported by advanced imaging techniques such as X-ray Computed Tomography (X-ray CT) and Environmental Scanning Electron Microscopy (ESEM) to quantify and visualize mucilage-induced alterations of the pore space and simulations to characterize the geometric distribution of mucilage within soil during the drying process.<br /> Initially, a conceptual model was developed to describe alterations in the pore space geometry induced by mucilage under dry soil conditions. Laboratory measurements indicated that mucilage decreases the gas diffusion coefficient under dry conditions without affecting bulk density or porosity. Depending on its content in the soil, mucilage forms various structures within the pore space. The evolution of these structures was explained via pore scale modeling based on identifying the elastic strength of rhizodeposition during soil drying. Next, the influence of mucilage on soil gas diffusion at different water contents during a drying-rewetting cycle was investigated. In soils without mucilage, a hysteresis in the gas diffusion coefficient was observed. The extent of the hysteresis depended on particle size. Furthermore, X-ray CT imaging indicated a hysteresis in gas-phase connectivity for samples without mucilage. The effect diminished with increasing mucilage content. In addition, ESEM imaging of sandy soil samples mixed with mucilage confirmed the formation of liquid structures in the pore space. However, these structures showed slightly different shapes in comparison to those in glass bead samples, likely due to the higher surface roughness of soil particles. Finally, diffusion measurements conducted on soil samples containing EPS demonstrated a similar effect of EPS and mucilage on gas diffusivity.<br /> In conclusion, the findings of this thesis suggest that plants and bacteria balance oxygen availability and water content by releasing polymeric substances, even under fluctuating moisture conditions. Through these exudates, they employ similar strategies to engineer their surroundings, modifying the physical properties of their local environment in ways that enhance their survival and resilience.