Non-invasive imaging of CO2 and O2 concentration reveals hotspots of root respiration

Root respiration constitutes a major contribution to the CO₂ efflux from vegetated soils. Amongst temperature, soil moisture is a key environmental variable determining respiration in soils, because it affects the amount of oxygen available for respiration as well as the CO₂ gas transport within the soil pore space.

Non-invasive imaging techniques facilitate the in situ observation of the complex respiration patterns in the rhizosphere. We applied planar optodes (80x100 mm²) to map the CO₂ and O₂ concentration in the rhizosphere of white lupine plants (Lupinus albus) grown in slab-shaped glass rhizotrons (150x150x15 mm³) in sandy soil under P-deficient conditions. Respiration was measured daily for 19 days after planting at constant soil moisture content as well as during a drying-rewetting experiment, during which soil volumetric water content varied between 0.1 and 0.3 cm³ cm^-3.

During their development, the plants exhibited a heterogeneous spatial pattern of root respiration; the highest CO₂-concentrations were measured at the root tips and along younger parts of the root system. Heterogeneity in CO₂ and O₂ patterns was most pronounced in the drying-rewetting experiment: Distinct hotspots of CO₂-release and oxygen consumption emerged 30 to 60 minutes after watering. The hotspot-regions correlated with the location of cluster roots growing close to the optodes, where up to three times increased CO₂ concentrations occurred. Overall CO₂ concentrations in the bulk soil increased as CO₂ accumulated over time as gas diffusion in the wet soil was limited.

Our results highlight the strong spatial and temporal variability of root respiration throughout the growth and development of the root system, and particularly in response to an increase in soil moisture. Further experiments aim to combine CO₂ and O₂ optode measurements with neutron computed laminography, a tomographic imaging method suited to capture the 3D root system architecture of plants grown in laterally extended rhizotrons in order to link root respiration to root branching order, diameter and functional type.