Articles | Volume 13, issue 5
Biogeosciences, 13, 1693–1703, 2016
Biogeosciences, 13, 1693–1703, 2016

Research article 18 Mar 2016

Research article | 18 Mar 2016

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

Anne Alexandre1, Jérôme Balesdent1,4, Patrick Cazevieille3, Claire Chevassus-Rosset3, Patrick Signoret4, Jean-Charles Mazur1, Araks Harutyunyan2, Emmanuel Doelsch3, Isabelle Basile-Doelsch1, Hélène Miche1, and Guaciara M. Santos2 Anne Alexandre et al.
  • 1Aix-Marseille Université, CNRS, IRD, INRA, CEREGE UM34, 13545 Aix en Provence, France
  • 2Department of Earth System Science, University of California, B321 Croul Hall, Irvine, CA 92697-3100, USA
  • 3CIRAD, UPR Recyclage et risque, 34398 Montpellier, France
  • 4INRA UR 1119 GSE, 13100 Aix-en-Provence, France

Abstract. 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 13C- and 15N-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 13C and 15N enrichments (13C excess and 15N 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 13C excess was measured at the molecular level, in AAs extracted from roots and stems and leaves. The net uptake of labeled AA-derived 13C reached 4.5 % of the total AA 13C supply. The amount of AA-derived 13C 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 13C-enriched than other AAs in the plant. This strongly suggested that part of AA-derived 13C was absorbed and translocated into the plant in its original AA form. In phytoliths, AA-derived 13C 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 13C 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

Short summary
This 13C labeling experiment demonstrates that carbon can be absorbed by the roots, translocated in the plant, and ultimately fixed in organic compounds subject to occlusion in silica particles that form inside plant cells (phytoliths). Plausible forms of carbon absorbed, translocated, and fixed in phytoliths are assessed. Implications for our understanding of the C cycle at the plant-soil-atmosphere interface are discussed.
Final-revised paper