Preprints
https://doi.org/10.5194/bg-2021-72
https://doi.org/10.5194/bg-2021-72

  18 May 2021

18 May 2021

Review status: this preprint is currently under review for the journal BG.

Predicting the impact of spatial heterogeneity on microbial redox dynamics and nutrient cycling in the subsurface

Swamini Khurana1, Falk Heße2,3, Anke Hildebrandt2,4,5, and Martin Thullner1 Swamini Khurana et al.
  • 1Department of Environmental Microbiology, Helmholtz Centre for Environmental Research – UFZ, Leipzig, 04318, Germany
  • 2Department of Computational Hydrosystems, Helmholtz Centre for Environmental Research – UFZ, Leipzig, 04318, Germany
  • 3Institute of Earth and Environmental Sciences, University Potsdam, Potsdam, Germany
  • 4Institute of Geoscience, Friedrich-Schiller-University Jena, Jena, Germany
  • 5German Centre for Integrative Biodiversity Research, Leipzig, Germany

Abstract. The subsurface is a temporally dynamic and spatially heterogeneous compartment of the Earth's Critical Zone, and biogeochemical transformations taking place in this compartment are crucial for the cycling of nutrients. The impact of spatial heterogeneity on such microbially mediated nutrient cycling is not well known which imposes a severe challenge in the prediction of in situ biogeochemical transformation rates and further of nutrient loading contributed by the groundwater to the surface water bodies. Therefore, we undertake a numerical modelling approach to evaluate the sensitivity of groundwater microbial biomass distribution and nutrient cycling to spatial heterogeneity in different scenarios accounting for various residence times. The model results gave us an insight into domain characteristics with respect to presence of oxic niches in predominantly anaerobic zones and vice versa depending on the extent of spatial heterogeneity and the flow regime. The obtained results show that microbial abundance, distribution, and activity are sensitive to the applied flow regime and that the mobile (i.e., observable by groundwater sampling) fraction of microbial biomass is a varying, yet only a small, fraction of the total biomass in a domain. Furthermore, spatial heterogeneity resulted in anaerobic niches in the domain and shifts of microbial biomass between active and inactive states. The lack of consideration of spatial heterogeneity, thus, can result in inaccurate estimation of microbial activity. In most cases this leads to an overestimation of nutrient removal (up to twice the actual amount) along a flow path. We conclude that the governing factors for evaluating this are the residence time of solutes and the Damköhler number (Da) of the biogeochemical reactions in the domain. We propose a relationship to scale the impact of spatial heterogeneity on nutrient removal governed by the log10Da. This relationship may be applied in upscaled descriptions of microbially mediated nutrient cycling dynamics in the subsurface thereby resulting in more accurate predictions of e.g., carbon and nitrogen cycling in groundwater over long periods at the catchment scale.

Swamini Khurana et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-72', Anonymous Referee #1, 09 Jun 2021
    • AC1: 'Reply on RC1', Swamini Khurana, 13 Jul 2021
  • RC2: 'Comment on bg-2021-72', Anonymous Referee #2, 06 Sep 2021
    • AC2: 'Reply on RC2', Swamini Khurana, 15 Sep 2021
  • RC3: 'Comment on bg-2021-72', Anonymous Referee #3, 15 Sep 2021

Swamini Khurana et al.

Swamini Khurana et al.

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Short summary
In this study, we concluded that the residence time of solutes and the Damköhler number (Da) of the biogeochemical reactions in the domain are governing factors for evaluating the impact of spatial heterogeneity of the domain on nutrient removal. We, thus, proposed a relationship to scale this impact governed by Da. This relationship may be applied in larger domains thereby resulting in more accurate predictions of nutrient removal in groundwater.
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