Carbon sources in seed carbonate of Lithospermum arvense

Plant species such as Lithospermum arvense accumulates CaCO3 in their seeds. Seed carbonate has been defined as a suitable material for radiocarbon dating because it is assumed that the total carbon in seed carbonate comes from the atmosphere and so 14C content in seed carbonate should be related to the atmospheric 14C concentration during seed development. However, plants can take up inorganic carbon from soil solution via roots. The 14C content of dissolved inorganic carbon is determined by respiration as well as carbonate minerals dissolution. While respired CO2 may show 14C content related to the 14C concentration in the atmosphere, the carbonate minerals are almost 14C depleted. Therefore, we hypothesized 14C ages based on seed carbonate will be overestimated if part of carbon in seed carbonate comes from the soil solution i.e. from the old carbon. To evaluate this, the allocation of photo-assimilated carbon and root uptake of HCO3- will be examined by 14C labeling and tracing. L. arvense will be grown in a carbonate-free or a carbonate-containing soil (Sand and Loess, respectively), where 14C will be provided as 1) 14CO2 in the atmosphere or 2) Na214CO3 in soil solution (by injecting the labeled solution into the soil) during seed development period.


Fig. 1: A ca. one-month-old Lithospermum arvense (family Boraginaceae) in a 250 mL growing pot (a); L. arvense flower (b); L. arvense seeds (c).


Fig. 2: Plants can take up HCO3- from soil solution. The source of HCO3- is dissolution of root- and rhizomicrobial-respired CO2 originally from the atmosphere (ACO2). In carbonate-containing soils, the dissolution of lithogenic carbonates (CaLCO3) is a second source.