In situ measurement of microbial activity and controls on microbial CO<sub>2</sub> production in the unsaturated zone

Wood, Brian D.; Keller, Catherine; Johnstone, Donald L.

doi:10.1029/92wr02315

Cited by 92 publications

(77 citation statements)

References 51 publications

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“…CO 2 from deeper layers migrates upwards to the depleted upper layers, where it stimulates carbonate dissolution and thus CO 2 consumption. Linan et al, 2008;Walvoord et al, 2005;Wood et al, 1993). Both biotic and abiotic processes have been demonstrated as sources of CO 2 in studies conducted in the proximity of the considered study site: (a) geotectonic mantle-derived CO 2 migrating upward from deeper parts of the crust through regional faults and seismic activity (Ceron et al, 1998), (b) microbial decomposition of dissolved and solid organic carbon near the groundwater table (typically hundreds of meters deep) (Benavente et al, 2010) and (c) CO 2 from local calcite precipitation near the groundwater table (Benavente et al, 2010).…”

Section: Discussionmentioning

confidence: 99%

Atmospheric turbulence triggers pronounced diel pattern in karst carbonate geochemistry

et al. 2013

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Abstract. CO2 exchange between terrestrial ecosystems and the atmosphere is key to understanding the feedbacks between climate change and the land surface. In regions with carbonaceous parent material, CO2 exchange patterns occur that cannot be explained by biological processes, such as disproportionate outgassing during the daytime or nighttime CO2 uptake during periods when all vegetation is senescent. Neither of these phenomena can be attributed to carbonate weathering reactions, since their CO2 exchange rates are too small. Soil ventilation induced by high atmospheric turbulence is found to explain atypical CO2 exchange between carbonaceous systems and the atmosphere. However, by strongly altering subsurface CO2 concentrations, ventilation can be expected to influence carbonate weathering rates. By imposing ventilation-driven CO2 outgassing in a carbonate weathering model, we show here that carbonate geochemistry is accelerated and does play a surprisingly large role in the observed CO2 exchange pattern of a semi-arid ecosystem. We found that by rapidly depleting soil CO2 during the daytime, ventilation disturbs soil carbonate equilibria and therefore strongly magnifies daytime carbonate precipitation and associated CO2 production. At night, ventilation ceases and the depleted CO2 concentrations increase steadily. Dissolution of carbonate is now enhanced, which consumes CO2 and largely compensates for the enhanced daytime carbonate precipitation. This is why only a relatively small effect on global carbonate weathering rates is to be expected. On the short term, however, ventilation has a drastic effect on synoptic carbonate weathering rates, resulting in a pronounced diel pattern that exacerbates the non-biological behavior of soil–atmosphere CO2 exchanges in dry regions \\mbox{with carbonate soils}.

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Section: Discussionmentioning

confidence: 99%

Atmospheric turbulence triggers pronounced diel pattern in karst carbonate geochemistry

et al. 2013

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“…Bacon and Keller, 1998;Murphy et al, 1992;Keller, 1991;Keller and Bacon, 1998;Wood and Petraitis, 1984;Wood et al, 1993;Hendry et al, 1993;Hendry and Wassenaar, 2005;and Lawrence et al, 2000), and isotopic evidence suggest that microbial respiration of organic material is the dominant source of subterranean CO 2 . Most relevant to karst settings, Benavente et al (2010) to À20.6‰ in the soil, and pCO 2 of >20,000 ppm in wells at depths of >10 m, with a d 13 C ranging from À19.7 to À22.1‰, suggesting the CO 2 was derived from an organic matter source.…”

Section: The Sources Of Speleothem Carbonmentioning

confidence: 99%

Radiocarbon evidence for decomposition of aged organic matter in the vadose zone as the main source of speleothem carbon

Noronha

Johnson

Southon

et al. 2015

Quaternary Science Reviews

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a b s t r a c tSeveral recent studies have used records of the radiocarbon ( 14 C) bomb peak in speleothems to inversely model the soil a 14 CO 2 and the age distribution of soil organic material (SOM) above caves, in part to investigate the potential of speleothems as sensitive records of past SOM dynamics. The results of these modeling studies have suggested that soil CO 2 at karst sites is derived primarily from the decomposition of SOM with turnover times on the order of decades to centuries. This result is in stark contrast with observations of soil a 14 CO 2 at non-karst sites, which indicate that soil CO 2 is derived primarily from root respiration and decomposition of SOM with much shorter turnover times. This discrepancy suggests that SOM in karst settings may have a very different age distribution than sites that have been studied previously and/or that soil CO 2 is not the main source of speleothem carbon. To help resolve this discrepancy, we present an improved inverse model which we use to estimate the age of CO 2 above several caves. We also present results from a detailed case study of soil carbon dynamics at Heshang Cave, China. This work demonstrates that SOM in karst sites may be much older than SOM in non-karst soils that have been studied previously, but that CO 2 produced in the shallow soil zone is unlikely to be the main source of speleothem carbon. A review of the literature suggests that the most likely explanation for the aforementioned discrepancy is that decomposition of down-washed SOM in the vadose zone is the dominant source of speleothem carbon.

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“…The total amount of carbon in the dissolved phase was calculated according to Wood et al (1993) as the sum of H 2 CO 3 (aq) (which summarises CO 2 (aq) and H 2 CO 3 , as is commonly used) and HCO − 3 (bicarbonate). Thus,…”

Section: Soil Co 2 Transport Modelmentioning

confidence: 99%

Measuring and modelling the isotopic composition of soil respiration: insights from a grassland tracer experiment

Gamnitzer¹,

Moyes

Bowling

et al. 2011

Biogeosciences

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Abstract. The carbon isotopic composition (δ 13 C) of CO 2 efflux (δ 13 C efflux ) from soil is generally interpreted to represent the actual isotopic composition of the respiratory source (δ 13 C Rs ). However, soils contain a large CO 2 pool in airfilled pores. This pool receives CO 2 from belowground respiration and exchanges CO 2 with the atmosphere (via diffusion and advection) and the soil liquid phase (via dissolution). Natural or artificial modification of δ 13 C of atmospheric CO 2 (δ 13 C atm ) or δ 13 C Rs causes isotopic disequilibria in the soilatmosphere system. Such disequilibria generate divergence of δ 13 C efflux from δ 13 C Rs (termed "disequilibrium effect").Here, we use a soil CO 2 transport model and data from a 13 CO 2 / 12 CO 2 tracer experiment to quantify the disequilibrium between δ 13 C efflux and δ 13 C Rs in ecosystem respiration. The model accounted for diffusion of CO 2 in soil air, advection of soil air, dissolution of CO 2 in soil water, and belowground and aboveground respiration of both 12 CO 2 and 13 CO 2 isotopologues. The tracer data were obtained in a grassland ecosystem exposed to a δ 13 C atm of −46.9 ‰ during daytime for 2 weeks. Nighttime δ 13 C efflux from the ecosystem was estimated with three independent methods: a laboratory-based cuvette system, in-situ steady-state open chambers, and in-situ closed chambers.Earlier work has shown that the δ 13 C efflux measurements of the laboratory-based and steady-state systems were consistent, and likely reflected δ 13 C Rs . Conversely, the δ 13 C efflux measured using the closed chamber technique differed from these by −11.2 ‰. Most of this disequilibrium effect (9.5 ‰) was predicted by the CO 2 transport model. Isotopic disequilibria in the soil-chamber system were introduced by changing δ 13 C atm in the chamber headspace at the onset of the Correspondence to: U. Gamnitzer (ulrike.gamnitzer@wzw.tum.de) measurements. When dissolution was excluded, the simulated disequilibrium effect was only 3.6 ‰. Dissolution delayed the isotopic equilibration between soil CO 2 and the atmosphere, as the storage capacity for labelled CO 2 in waterfilled soil pores was 18 times that of soil air.These mechanisms are potentially relevant for many studies of δ 13 C Rs in soils and ecosystems, including FACE experiments and chamber studies in natural conditions. Isotopic disequilibria in the soil-atmosphere system may result from temporal variation in δ 13 C Rs or diurnal changes in the mole fraction and δ 13 C of atmospheric CO 2 . Dissolution effects are most important under alkaline conditions.

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In situ measurement of microbial activity and controls on microbial CO₂ production in the unsaturated zone

Cited by 92 publications

References 51 publications

Atmospheric turbulence triggers pronounced diel pattern in karst carbonate geochemistry

Atmospheric turbulence triggers pronounced diel pattern in karst carbonate geochemistry

Radiocarbon evidence for decomposition of aged organic matter in the vadose zone as the main source of speleothem carbon

Measuring and modelling the isotopic composition of soil respiration: insights from a grassland tracer experiment

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In situ measurement of microbial activity and controls on microbial CO2 production in the unsaturated zone

Cited by 92 publications

References 51 publications

Atmospheric turbulence triggers pronounced diel pattern in karst carbonate geochemistry

Atmospheric turbulence triggers pronounced diel pattern in karst carbonate geochemistry

Radiocarbon evidence for decomposition of aged organic matter in the vadose zone as the main source of speleothem carbon

Measuring and modelling the isotopic composition of soil respiration: insights from a grassland tracer experiment

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In situ measurement of microbial activity and controls on microbial CO₂ production in the unsaturated zone