Spatio-temporal patterns of rhizosphere CO₂ concentration are linked to root type and soil moisture dynamics in white lupine

Respiration by plant roots can seasonally account for the majority of soil CO2 production. At the plant scale level, the spatial distribution of soil CO2 concentration is linked to root metabolic activity, which may vary with root physiological function and development stage of the root system, but also to soil moisture, a key variable affecting gas transport in soils. We performed rhizotron experiments with white lupine (Lupinus albus) to measure spatiotemporal dynamics of pCO(2) (CO2 partial pressure) in the root zone and its dependence on root type and soil moisture. Via planar optode imaging, we measured pCO(2) daily at constant soil moisture (0.3 cm(3) cm(-3)), and hourly in a consecutive drying-rewetting experiment, where soil moisture was increased from 0.1 to 0.3 cm(3) cm(-)(3). During the first 16 days of plant growth, regions of high CO2 concentration formed around cluster roots, where pCO(2) locally increased to >20 %, while pCO(2) remained lower around lateral roots. Rewetting of dry soil had a pronounced effect on pCO(2): One hour after rewetting, pCO(2) at the root surface had increased significantly. Around cluster roots, the CO2 accumulation zone strongly increased from 0.3 mm (dry soil) to 10 mm (wet soil), but remained narrower around lateral roots (1.5-1.8 mm in dry vs. wet soil). Our study demonstrates the applicability of planar optodes to monitor pCO(2) in the rhizosphere at different soil moisture levels, and highlights that local CO2 concentration in the root zone is a function of root type and soil moisture.