Investigation of small-scale processes in the rhizosphere of Lupinus albus using micro push-pull tests

Abstract

Rhizosphere processes affect the mobility, phytoavailability and toxicity of solutes in soil. To study reactions in the rhizosphere under quasi in situ conditions, we recently developed the “micro push-pull test” (μPPT) method, combining micro-suction cups with the principle of the “push-pull test” method known from groundwater applications. Here we report the application of μPPT to investigate rhizosphere reactions in situ, i.e. degradation of deuterated citrate (citrate-d4) in the rhizosphere of Lupinus albus grown in sand-filled rhizoboxes. In a μPPT, a solution containing reactive (citrate-d4) and non-reactive solutes (bromide) is injected into a porous medium and shortly thereafter, the pore water solution is re-extracted from the same location. Concentration (“breakthrough”) curves of extracted reactants can be compared to those of the non-reactive solute, allowing the determination of reaction rates. We applied the μPPT in rhizoboxes with Lupinus albus and sampled different types of micro-habitats: bulk soil, rhizosphere of normal roots and rhizosphere of cluster roots of different ages. Breakthrough curves of citrate-d4 varied considerably between tests adjacent to cluster roots and normal roots, and in bulk soil. Degradation of citrate-d4 in bulk soil and adjacent to normal roots was below detection, while we found strong degradation of citrate-d4 adjacent to 4 to 5-days old cluster roots. In situ degradation rate constants for citrate-d4 around cluster roots were found to be in the range from 0.38 to 0.71 h−1. We successfully applied the μPPT to the rhizosphere. The μPPT is useful to investigate local processes in microcosms and to monitor processes also over time (e.g., during cluster-root development) due to its non-destructive nature.