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

Abstract

Under arid and semiarid conditions, pedogenic (secondary) carbonates are formed in soil by precipitation of Ca 2? from soil parent material with dissolved CO 2 originating from root and rhizomicrobial respiration. d 13 C values of secondary CaCO 3 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 14 C isotopic exchange method. 14 CO 2 was assimilated by plants, respired into the rhizosphere and subsequently incorporated into secondary carbonate by recrystallization of primary loess carbonate. With ascending number of 14 CO 2 pulses, the amount of rhizosphere 14 C recovered in loess CaCO 3 increased linearly, leading to recrystallization rates of 3.2 9 10 -5 and 2.8 9 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 CaCO 3 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.

Pedogenic carbonates: Forms and formation processes

“…Carbonate solubility increases near roots because of (1) up to 100 times higher CO 2 concentration in the rhizosphere versus atmosphere and (2) up to two units lower local pH because of H + and carboxylic acid release by roots (Andrews and Schlesinger, 2001;Berthelin, 1988;Gocke et al, 2011b). The higher ions concentration leads to two-orders-ofmagnitude-faster PC accumulation close to the roots compared to root-free soil (Gocke et al, 2011b;Kuzyakov et al, 2006), e.g.…”

“…The higher ions concentration leads to two-orders-ofmagnitude-faster PC accumulation close to the roots compared to root-free soil (Gocke et al, 2011b;Kuzyakov et al, 2006), e.g. to rhizolith formation (Fig.…”

Differentiation of plant derived organic matter in soil, loess and rhizoliths based on n-alkane molecular proxies

“…This carbonatic encrustation of roots, probably generated during short periods of few years to decades (Gocke et al 2010b), might entail conservation of the former root tissue as well as preservation of rhizodeposits released during the root's lifetime. Therefore, rhizoliths as well as surrounding loess are suitable to estimate amounts of root derived C incorporated in loess.…”

Organic matter stabilization and ecosystem functions: proceedings of the fourth conference on the mechanisms of organic matter stabilization and destabilization (SOM-2010, Presqu’île de Giens, France)

“…To improve understanding of OM in loess profile and to determine whether the loess OM reflects original or postsedimentary OM, Gocke et al (2012) assessed the sources of loess and rhizolith OM via molecular proxies.…”

“…The 14 C tracer can be used for estimating C ages, but 14 C-enriched organic substrates are also useful for tracing biochemical and geochemical C pathways (e.g. Rovira et al 1998;Kuzyakov and Jones 2006;Bruun et al 2008;Philip and Simard 2008;Gocke et al 2011). In this issue, Lopez-Sangil and Rovira (2012) propose a simple new experimental design which eliminates the risk of cross-contamination during sample preparation for 14 C measurement.…”