Imaging Rhizosphere CO2 and O2 Concentration to Localize Respiration Hotspots Linked to Root Type and Soil Moisture Dynamics

Abstract PurposeRhizosphere respiration strongly affects CO2 concentration within vegetated soils and resulting fluxes to the atmosphere. Respiration in the rhizosphere exhibits high spatiotemporal variability that may be linked to root type, but also to small-scale variation of soil water content altering gas transport dynamics in the soil. We address spatiotemporal dynamics of CO2 and O2 concentration in the rhizosphere via non-invasive in-situ imaging.MethodsOptodes sensitive to CO2 and O2 were applied to non-invasively measure in-situ rhizosphere CO2 and O2 concentration of white lupine (Lupinus albus) grown in slab-shaped glass rhizotrons. We monitored CO2 concentration over the course of 16 days at constant water content and also performed a drying-rewetting experiment to explore sensitivity of CO2 and O2 concentration to soil moisture changes. ResultsHotspots of respiration formed around cluster roots and CO2 concentration locally increased to > 20 % pCO2 (CO2 partial pressure). After rewetting the soil, cluster roots consumed available O2 significantly faster compared to non-cluster lateral roots. In wet soil, CO2 accumulation zones extended up to 9.5 mm from the root surface compared to 0.3-1 mm in dry soil.ConclusionResults from this imaging experiment indicate that respiratory activity differs substantially within the root system of a plant individual and that cluster roots are hotspots of respiration. As rhizosphere CO2 and O2 concentration was strongly sensitive to soil water content and its variation, we recommend monitoring the soil water content prior and during the measurement of rhizosphere respiration.