BayCEER Workshop 2008

14C pulse labeling of plants applied for quantification of pedogenic carbonate recrystallization rate

CaCO₃ constitutes up to several mass percents in soil. These pedogenic (secondary) carbonates originate by dissolution and recrystallization of lithogenic (primary) carbonates with CO₂ from soil air. Plants are an important factor for CO₂ concentration and distribution in soil. Pedogenic carbonates are used for the reconstruction of paleoclimatic conditions (δ¹³C and δ¹⁸O) and for radiocarbon dating of paleosoils. For precise results, the recrystallization rate of pedogenic carbonates has to be known. To ascertain this, we use a ¹⁴C tracer method which reveals a relation between rhizosphere CO₂ of plants and amount of recrystallized CaCO₃. Under controlled conditions, plants (e.g. wheat, maize) were grown on loess. Contrary to soil this loess contains only primary CaCO₃, has a high CaCO₃ content (30%) and no organic carbon. The plants were exposed to an artificially labeled ¹⁴CO₂ atmosphere during discrete pulses in 5 day intervals. Prior to the next labeling, the amount of incorporated ¹⁴C in shoots, roots, dissolved organic and inorganic carbon, respired CO₂ and CaCO₃ of loess were measured by liquid scintillation counting. With the CaCO₃ activity and the specific activity of respired CO₂, the amount of recrystallized CaCO₃ and thus the recrystallization rate were calculated. As shown by the expamle of pedogenic carbonate recrystallization, ¹⁴C labeling offers a unique low-cost and very sensitive approach for estimation of slow rates of steady state processes in soils.