Carbonate recrystallization in root-free soil and rhizosphere of Triticum aestivum and Lolium perenne estimated by 14C labeling

Under arid and semiarid conditions, pedogenic (secondary) carbonates are formed in soil by precipitation of Ca2+ from soil parent material with dissolved CO2 originating from root and rhizomicrobial respiration. delta C-13 values of secondary CaCO3 record the photosynthetic pathway of former vegetation and is therefore used as a tool for paleoenvironmental studies. The time scale of pedogenic carbonate formation as well as the influence of several environmental factors are crucial, yet poorly known. We estimated the recrystallization rate of pedogenic carbonate by the C-14 isotopic exchange method. (CO2)-C-14 was assimilated by plants, respired into the rhizosphere and subsequently incorporated into secondary carbonate by recrystallization of primary loess carbonate. With ascending number of (CO2)-C-14 pulses, the amount of rhizosphere C-14 recovered in loess CaCO3 increased linearly, leading to recrystallization rates of 3.2 x 10(-5) and 2.8 x 10(-5) day(-1) for wheat and ryegrass, respectively. In loess close to roots, recrystallization rates more than twice as high were obtained. Extrapolating these rates we showed that several hundred years are necessary for complete recrystallization of primary loess CaCO3 in root-free substrate, assuming that both primary and secondary carbonate is recrystallized several times. In contrast, the process probably takes only decades in rhizosphere loess if carbonaceous encrustations form around the root, impeding repeated recrystallization. This indicates the importance of rhizosphere processes (e.g. respiration of roots and microorganisms, exudation) for secondary carbonate formation.