Effluxed CO2-13C from sterilized and unsterilized treatments of a calcareous soil

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Abstract

Soil inorganic carbon (C) represents a substantial C pool in arid ecosystems, yet little data exist on the contribution of this pool to ecosystem C fluxes. A closed jar incubation study was carried out to test the hypothesis that CO2-13C production and response to sterilization would differ in a calcareous (Mojave Desert) soil and a non-calcareous (Oklahoma Prairie) soil due to contributions of carbonate-derived CO2. In addition to non-sterilized controls, soils were subjected to sterilization treatments (unbuffered HgCl2 addition for Oklahoma soil and unbuffered HgCl2 addition, buffered HgCl2 addition, and autoclaving for Mojave Desert soil) to decrease biotic respiration and more readily measure abiotic CO2 flux. Temperature and moisture treatments were also included with sterilization treatments in a factorial design.

The rate of CO2 production in both soils was significantly decreased (36–87%) by sterilization, but sterilization treatments differed in effectiveness. Sterilization had no significant effect on effluxed CO2-13C values in the non-calcareous Oklahoma Prairie soil and autoclaved Mojave Desert soil as compared to their respective non-sterilized controls. However, sterilization significantly altered CO2-13C values in Mojave Desert soil HgCl2 sterilization treatments (both buffered and non-buffered). Plots of 1/CO2 versus CO213C (similar to Keeling plots) indicated that the source CO213C value of the Oklahoma Prairie soil treatments was similar to the δ13C value of soil organic matter [(SOM); −17.76‰ VPDB] whereas the source for the (acidic) unbuffered-HgCl2 sterilized Mojave Desert soil was similar to the δ13C value of carbonates (−0.93‰ VPDB). The source CO213C value of non-sterilized and autoclaved (−18.4‰ VPDB) Mojave Desert soil treatments was intermediate between SOM (−21.43‰ VPDB) and carbonates and indicates up to 13% of total C efflux may be from abiotic sources in calcareous soils.

Introduction

Ecosystem carbon (C) fluxes have been studied intensively in recent years to understand how increasing CO2 levels and atmospheric temperature may affect storage of ecosystem C. To date, however, most of these studies have focused on processes involving organic forms of C, since this is the dominant form of soil C in many temperate ecosystems. By contrast, the amount of pedogenic inorganic C (PIC) can be up to 10 times greater than that of organic C in arid and semiarid regions (Schlesinger, 1982), yet few studies have addressed the potential contribution of this C pool to ecosystem C fluxes.

Pedogenic carbonates occur mainly in arid and semiarid regions receiving annual precipitation of less than 500 mm. Net PIC accumulation rates in arid and semiarid soils are estimated to range from 0.5 to 5.1 g C m−2 yr−1, similar in magnitude to long-term, nonsteady-state accumulation of organic C in grassland soils of 1 g C m−2 yr−1 (Bachman and Machette, 1977; O’Brien and Stout, 1978; Schlesinger, 1982; Monger and Gallegos, 2000). The turnover time of the global PIC pool is estimated to be 85,000 yr, but recent studies in New Mexico indicate that turnover times can be as short as 120 yr (Monger and Gallegos, 2000). Therefore, PIC may play an important role in regulating C fluxes on a decadal timescale.

Formation and dissolution of CaCO3 in soils is driven primarily by the partial pressure of CO2 in the soil (PCO2) and soil moisture (Eq. (1), Birkeland, 1984):CaCO3 (s)+CO2 (g)+H2O (l) Ca2+ (aq)+2HCO3 (aq).

Vegetation is one of the main drivers regulating both soil PCO2 and soil water content. Belowground CO2 is produced by root and heterotrophic respiration and lost through soil respiration, while soil water content is a function of precipitation, water uptake by plants, evapotranspiration, and leaching. Increased levels of atmospheric CO2 may result in increased belowground net primary production and increased water-use efficiency (e.g. Rogers et al., 1994). These increases may result in a persistent increase in soil CO2 concentrations and perhaps soil moisture, thereby favoring CaCO3 dissolution [Eq. (1)].

To date, few studies have addressed the potential contributions of carbonates to ecosystem C fluxes. In a discussion of methods to quantify soil respiration, Alef (1995) stated that measurements using closed-jar incubations can be influenced significantly by the abiotic CO2 production of alkaline soils containing CaCO3, but processes that could interfere with these measurements were not mentioned. Recent field studies conducted in southeast Arizona involving Bowen ratio measurements suggest C fluxes in arid regions may be seasonally altered by the dissolution and precipitation of inorganic C in soils (Emmerich, 2003).

Isotopic measurement of effluxed CO2-13C is potentially a useful method to quantify abiotic CO2 production from PIC, as the δ13C values associated with pedogenic carbonates and organic matter are distinctly different (Cerling, 1984). In the field, however, the mixing of atmospheric CO2 with soil CO2 would complicate separating soil CO2 into its biotic and abiotic components. As an initial step in determining if PIC contributes to total soil C efflux, we conducted a laboratory incubation study and hypothesized that effluxed CO213C would differ in a calcareous (Mojave Desert) soil versus a non-calcareous (Oklahoma Prairie) soil and that the response of the soils would differ to sterilization treatments (designed to inhibit biotic respiration) due to contributions of carbonate-derived CO2.

Section snippets

Laboratory procedures

Soil samples (depth of 0–5 cm) were collected from either the Mojave Global Change Facility on the Nevada Test Site (mean annual temperature, 16.1 °C; mean annual precipitation, 74 mm), which is located approximately 65 miles northwest of Las Vegas, Nevada, and from an Oklahoma Prairie site (mean annual temperature; 16 °C; mean annual precipitation, 1050 mm) near Purcell, Oklahoma. Ten grams of air-dried soil were measured into Corning™ 250-ml Pyrex™ bottles with septa-fitted, polypropylene-plug

Results

Sterilization significantly reduced CO2 production in all treatments of both soils, but sterilization techniques were not equally effective (Table 2, Table 3). In the Oklahoma Prairie soil, addition of HgCl2 decreased the mean respiration rate by 85% averaged over all temperature and moisture treatments (Fig. 1). In the Mojave Desert soil, the respiration rate was reduced by 87% in the buffered HgCl2 treatment, 74% in the autoclaved treatment, and 36% in the unbuffered HgCl2 treatment. As

Discussion

The isotopic composition of respired CO2 indicated that the efficacy of sterilization was not related strictly to the ability of a particular method to suppress microbial activity in a calcareous soil. For instance, the isotopic data showed that the relatively low effectiveness of unbuffered HgCl2 in suppressing respiration in the Mojave Desert soil was caused by dissolution of carbonates. Though there was no difference in effluxed CO213C between autoclaved and non-sterilized treatments in

Acknowledgments

The authors wish to thank Dawn Girard for laboratory assistance, and Graham Sparling, Giles Marion, and Jennifer Lease for providing helpful comments on the manuscript. The work was funded by NSF Grant #DEB 0318646 to Paul Verburg.

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