From soil to gas - high resolution insights into plant-soil interactions by integrating planar oxygen optodes, porewater chemistry, soil microbial analysis and trace soil gas flux using a rhizobox approach

Abstract

Ecosystem trace gas fluxes (CO2, CH4, N2O) are a critical component of the global greenhouse gas cycle, but uncertainty remains regarding the important mechanisms driving variability across the soil-plant-atmosphere interface. This is due in part to a lack of techniques that can integrate measurements across these interfaces at high spatial and temporal resolution under controllable experimental conditions. To improve upon these experimental techniques, we present a novel approach in which custom-made rhizoboxes, integrated with stateof-the-art planar oxygen (O2) optode sensors and outfitted with water, soil and gas samplers, allow for integration of porewater chemistry, soil microbiology, plant-soil trace gas flux, belowground root dynamics, along with a spatially and quantitatively resolved O2 profile (i.e., planar optode). We demonstrate our experimental design with a case study using three rhizoboxes at controlled water levels, one with soil only and two transplanted with Carex acutiformis, a wetland plant known to transport O2 belowground through the roots (i.e., radial oxygen loss). Our case study clearly illustrates that high spatially and temporally resolved data can be captured using planar chemical sensors and integrated with simultaneous measurements of soil, water, plant and gas variables. We find clear evidence for radial O2 loss, a mechanism occurring at the millimeter scale whereby roots emit O2 belowground. Additionally, we find that plant-soil gas fluxes are correlated to porewater chemistry (i.e., redox potential, pH, O2 concentration), soil microbial relative abundances and planar O2 optode profiles, underscoring the ability of this experimental design to simultaneously monitor a variety of measurement types with minimal disturbance. We find that CO2 uptake from the plants increases significantly (p = 0.003, R2 = 0.78) with belowground root radial O2 loss, indicating a tight coupling between above and belowground plant dynamics. Additionally, the bacterial genus Hydrogenophaga, often associated with denitrification, increased in abundance with a corresponding decrease in N2O flux over time. Finally, we find that conventional porewater O2 measurements provide an inaccurate characterization of soil O2 concentration when compared to planar optodes. Our rhizobox design is a promising strategy for solving fundamental knowledge gaps and mechanisms in biogeochemistry. Our hope is that this will be a useful tool for the community in generating data for improved ecosystem modeling since the setup can be modified to simulate variable environmental conditions and characterize a wide variety of plant-soil systems.