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Reconciling Negative Soil CO2 Fluxes: Insights from a Large-Scale Experimental Hillslope

1
Biosphere 2, University of Arizona, Tucson, AZ 85623, USA
2
Honors College, University of Arizona, Tucson, AZ 85721, USA
3
Department of Hydrology and Atmospheric Sciences, University of Arizona, Tucson, AZ 85721, USA
4
School of Natural Resources and the Environment, University of Arizona, Tucson, AZ 85721, USA
*
Author to whom correspondence should be addressed.
Soil Syst. 2019, 3(1), 10; https://doi.org/10.3390/soilsystems3010010
Received: 30 October 2018 / Revised: 24 December 2018 / Accepted: 3 January 2019 / Published: 13 January 2019
(This article belongs to the Special Issue Formation and Fluxes of Soil Trace Gases)
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Abstract

Soil fluxes of CO2 (Fs) have long been considered unidirectional, reflecting the predominant roles of metabolic activity by microbes and roots in ecosystem carbon cycling. Nonetheless, there is a growing body of evidence that non-biological processes in soils can outcompete biological ones, pivoting soils from a net source to sink of CO2, as evident mainly in hot and cold deserts with alkaline soils. Widespread reporting of unidirectional fluxes may lead to misrepresentation of Fs in process-based models and lead to errors in estimates of local to global carbon balances. In this study, we investigate the variability and environmental controls of Fs in a large-scale, vegetation-free, and highly instrumented hillslope located within the Biosphere 2 facility, where the main carbon sink is driven by carbonate weathering. We found that the hillslope soils were persistent sinks of CO2 comparable to natural desert shrublands, with an average rate of −0.15 ± 0.06 µmol CO2 m2 s−1 and annual sink of −56.8 ± 22.7 g C m−2 y−1. Furthermore, higher uptake rates (more negative Fs) were observed at night, coinciding with strong soil–air temperature gradients and [CO2] inversions in the soil profile, consistent with carbonate weathering. Our results confirm previous studies that reported negative values of Fs in hot and cold deserts around the globe and suggest that negative Fs are more common than previously assumed. This is particularly important as negative Fs may occur widely in arid and semiarid ecosystems, which play a dominant role in the interannual variability of the terrestrial carbon cycle. This study contributes to the growing recognition of the prevalence of negative Fs as an important yet, often overlooked component of ecosystem C cycling. View Full-Text
Keywords: net soil exchange; biosphere 2; carbonate weathering; negative emission technology; microbial induced carbonate precipitation net soil exchange; biosphere 2; carbonate weathering; negative emission technology; microbial induced carbonate precipitation
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Cueva, A.; Volkmann, T.H.M.; van Haren, J.; Troch, P.A.; Meredith, L.K. Reconciling Negative Soil CO2 Fluxes: Insights from a Large-Scale Experimental Hillslope. Soil Syst. 2019, 3, 10.

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