Pedogenic carbonate formation: Recrystallization versus migration-Process rates and periods assessed by C-14 labeling

Abstract

Under arid to semihumid climatic conditions, dissolution of primary carbonate and recrystallization with carbon (C) from soil CO2 leads to accumulation of significant amounts of pedogenic (secondary) carbonate. Most soils of arid and semiarid regions contain a carbonate accumulation horizon, the depth of which is related to climatic conditions and properties of parent material. It remains unclear whether this carbonate migrates from the upper horizons before or after recrystallization with soil CO2. The aim of this study was to determine recrystallization rates during initial pedogenesis and to estimate the accumulation depth of secondary carbonate based on C isotopic exchange during secondary carbonate formation in an experiment with alternating moisture conditions. Maize grown on 1 m high loess-filled columns was pulse labeled in (CO2)-C-14 atmosphere every 3 weeks. After 6 months, portions of secondary (recrystallized) CaCO3 were determined in 5 cm segments, based on C-14 respired in the rhizosphere and subsequently incorporated into newly formed secondary carbonate. More than 80% of recrystallized carbonate ((CaCO3)-C-14) was leached from the uppermost 15 cm of the loess column, and more than 70% of total secondary carbonate were accumulated in a depth between 15 and 50 cm. Based on the recrystallization rate calculated for the uppermost 15 cm of the loess column (1.77 +/- 0.26 . 10(-5) day(-1)), between 300 and more than 1,700 years are necessary for complete decalcification of the upper 15 cm. Our modeled data are consistent with formation of calcic horizons under relatively humid conditions.