Carbon dioxide fluxes from cyanobacteria crusted soils in the Kalahari

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Type Article
Original languageEnglish
Pages (from-to)254-263
Number of pages9
JournalApplied Soil Ecology
Volume39
Issue number3
Early online date04 Mar 2008
DOI
Publication statusPublished - 01 Jul 2008
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Abstract

The surface of dryland soils is frequently characterised by a biological crust comprising of various combinations of cyanobacteria, algae, moss and lichens. In the Kalahari of Botswana, soil crusts are predominantly made up of cyanobacteria, which when moist, are capable of fixing N and C. Many cyanobacteria also produce extracellular polymeric substances (EPS) which bind soil particles together and decrease erodibility. The physical integrity and metabolic activity of soil crusts is thus critical to ecological productivity,
erodibility and CO2 fluxes in dryland regions. There are, however, few studies of the magnitude and controlling factors of soil CO2 flux within these systems.
Our aim was to quantify in situ soil CO2 flux during contrasting antecedent moisture conditions in the south west Kalahari of Botswana. We have designed a gas exchange chamber for field deployment coupled to a portable gas chromatograph, control and data logging instrumentation. The optical and active thermal control specifications of the chamber have been designed to permit photosynthesis and cope with the temperature extremes of the Kalahari whilst minimizing disturbance to the cyanobacteria soil crust. This approach has enabled CO2 fluxes to be monitored in situ with a high degree of precision for
extended periods. In August 2005, when the surface and subsoils were dry, the ambient CO2 efflux was negative and low during the daytime (6.15 mg C m2 h1). When 8 mmrainfall equivalent of water was added to the surface there was an immediate uptake of CO2 during the daytime at rates up to 75 mg Cm2 h1 demonstrating that rates of net photosynthesis are greatly enhanced by available moisture. In contrast, in May 2006 following a prolonged wet period when the subsoil was moist, there was a net positive efflux of CO2 from the soil at rates of up to 60 mg C m2 h1 irrespective of whether the surface soil was moist or not. This is consistent with subsoil heterotrophic bacterial respiration becoming an important contributor to soil efflux.

Keywords

  • carbon dioxide, soil efflus, cyanobacteria soil crusts, Karahari