Bomb-radiocarbon signal suggests that soil carbon contributes to chlorophyll a in archival oak leaves

Abstract. Carbon exchange between biosphere and rhizosphere is an important component of the global carbon cycle. Photosynthetic products being sequestered into soils have been intensively studied, yet the reverse pathway from rhizosphere to biosphere is poorly known. In the present study, we determined the radiocarbon content (Δ¹⁴C) of the bulk leaves of the deciduous Quercus oak and of chlorophyll a extracted from the same leaves collected in Switzerland during the 1950s and 2000s. Our results demonstrate that old soil-derived carbon significantly contributes to the synthesis of chlorophyll a, an essential molecule for photoautotrophs. The Δ¹⁴C values of chlorophyll a were consistently lower than those of bulk leaves which closely tracked bomb-derived Δ¹⁴C signals in the atmosphere. The results cannot be explained without invoking an additional carbon source with a turnover time exceeding 100 years. A two-pool mixing model assuming atmosphere and rhizosphere as two endmembers indicates that contributions of the soil carbon to chlorophyll a are 19 ± 5 % (n = 4), and turnover time of such soil carbon is no shorter than 1,300 years. We suggest that hydrophilic compounds such as amino acids or phytol are transferred into plant roots from soils through mycorrhizal symbionts, and chlorophyll a is one of the destinations of such ¹⁴C-depleted carbon in vascular plants.