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

Alejandro Cueva,Peter A. Troch,Till H. M. Volkmann,Laura K. Meredith,Joost van Haren

doi:10.3390/soilsystems3010010

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.

Highlights

Arid and semiarid ecosystems across the globe play a fundamental role in the interannual variability of the terrestrial carbon cycle [1,2] and might be considered model systems to understand a future world that is becoming drier and warmer [3]
We demonstrated that LEO hillslopes are a consistent carbon sink, with comparable CO2 flux rates to natural deserts across the globe
We showed that negative soil CO2 fluxes are possible and probable, and should not be discarded in future studies, as they can influence local carbon balances that, in turn, propagate into global estimates

Summary

Introduction

Arid and semiarid ecosystems across the globe play a fundamental role in the interannual variability of the terrestrial carbon cycle [1,2] and might be considered model systems to understand a future world that is becoming drier and warmer [3]. Soil fluxes of CO2 (Fs ) drive significant carbon exchange between the terrestrial surface and the atmosphere, second only to photosynthesis [14,15] Due to their temperature sensitivity, Fs are considered to be the main determinant of future feedbacks to ongoing global climate change and, on a global scale, contribute an order of magnitude more than does anthropogenic activity [16]. The physical and biogeochemical mechanisms that promote capture of CO2 by soils are still uncertain and spark debate, even regarding whether negative Fs represent carbon uptake by the ecosystem [19,20], especially over short temporal scales. This knowledge gap challenges confidence in the understanding and quantification of ecosystem carbon balance, from local to global scales

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Discussion

Conclusion