Direct uptake of organically derived carbon by grass roots and allocation in leaves and phytoliths: 13C labeling evidence

In the rhizosphere, the uptake of low-molecular-weight carbon (C) and nitrogen (N) by plant roots has been well documented. While organic N uptake relative to total uptake is important, organic C uptake is supposed to be low relative to the plant's C budget. Recently, radiocarbon analyses demonstrated that a fraction of C from the soil was occluded in amorphous silica micrometric particles that precipitate in plant cells (phytoliths). Here, we investigated whether and to what extent organically derived C absorbed by grass roots can feed the C occluded in phytoliths. For this purpose we added C-13- and N-15-labeled amino acids (AAs) to the silicon-rich hydroponic solution of the grass Festuca arundinacea. The experiment was designed to prevent C leakage from the labeled nutritive solution to the chamber atmosphere. After 14 days of growth, the C-13 and N-15 enrichments (C-13 excess and N-15 excess) in the roots, stems and leaves as well as phytoliths were measured relative to a control experiment in which no labeled AAs were added. Additionally, the C-13 excess was measured at the molecular level, in AAs extracted from roots and stems and leaves. The net uptake of labeled AA-derived C-13 reached 4.5 % of the total AA C-13 supply. The amount of AA-derived C-13 fixed in the plant was minor but not nil (0.28 and 0.10 % of total C in roots and stems/leaves, respectively). Phenylalanine and methionine that were supplied in high amounts to the nutritive solution were more C-13-enriched than other AAs in the plant. This strongly suggested that part of AA-derived C-13 was absorbed and translocated into the plant in its original AA form. In phytoliths, AA-derived C-13 was detected. Its concentration was on the same order of magnitude as in bulk stems and leaves (0.15 % of the phytolith C). This finding strengthens the body of evidences showing that part of organic compounds occluded in phytoliths can be fed by C entering the plant through the roots. Although this experiment was done in nutrient solution and its relevance for soil C uptake assessment is therefore limited, we discuss plausible forms of AA-derived C-13 absorbed and translocated in the plant and eventually fixed in phytoliths, and implications of our results for our understanding of the C cycle at the soil-plant-atmosphere interface