Root-mediated carbon uptake: a limited but detectable pathway in plant–soil systems

Plants are important drivers of C cycling in vegetated terrestrial ecosystems and are commonly assumed to acquire C solely from the atmosphere. However, plant-physiological studies indicate that plants can also take up C from the soil via their roots. We used two complementary 13 C-tracer experiments to determine under which conditions root-mediated C uptake occurs and how net tracer incorporation varies with C substrate, growth medium, soil texture, and plant functional type. First, seedlings of Plantago lanceolata and Picea abies were grown in nutrient solutions containing 13 C-labelled glucose or L-lysine. After four weeks, 2.0–5.5% of the applied 13 C was recovered in plant tissues, and δ 13 C values shifted from approximately −31% to as high as +78%. Tracer recovery tended to be higher for lysine than glucose. In a second experiment, both species were grown for eight weeks in two contrasting soils pre-incubated with 13 C-labelled glucose. Up to 0.74% of the tracer was recovered in plant tissues, providing evidence for root-mediated incorporation of tracer-derived C from soil, although recovery was markedly lower than in nutrient solution. In soil, tracer recovery was higher in Picea abies than in Plantago lanceolata , whereas soil texture had no significant effect, despite a tendency toward higher recovery in the sandy soil. Our findings show that root-mediated C uptake occurs in the investigated woody and herbaceous species but is strongly constrained under soil conditions. Although quantitatively small, this pathway should be considered when interpreting tracer budgets and C fluxes in planted soil systems.