Plant uptake of soil carbon: an overlooked flux in carbon cycling?

Plants are an essential part of the terrestrial C cycle, and it is commonly assumed that they acquire C solely from the atmosphere. However, evidence from plant physiology research indicates that plants can also take up dissolved C via their roots. This concept, however, is not commonly acknowledged within the soil science community. If such an uptake mechanism existed, it could significantly impact C cycling studies involving plants. Here, we aimed at quantifying plant C uptake from solution and soil in two pot experiments. In a first experiment, seedlings of Plantago lanceolata and Picea abies were grown in hydroculture enriched with ¹³C-labelled glucose or L-lysine. After four weeks, 2.0-5.5% of the applied ¹³C was recovered in the plants, along with a shift in the δ¹³C signal of the plant tissues from -31‰ up to +78 ‰. Here, our findings suggest a larger uptake of lysine as compared to glucose. In a second experiment, plants were grown for eight weeks in both sandy and silty soil amended with ¹³C-labelled soil organic carbon (SOC). After harvesting, we found up to 0.74% of the available tracer substance to be recovered in the plant tissues. Our findings show that the pathway of root C uptake from soil exists, but this mechanism has minor relevance in soil, likely due to competition with microbes for available C. However, it indicates the need for adjustments in our understanding of plant-soil interactions with respect to SOC dynamics, especially in experimental setups where ¹³C is added to soil: A decrease in soil ¹³C cannot be attributed exclusively to mineralization processes.