Predicting the impact of spatial heterogeneity on microbially mediated nutrient cycling in the subsurface

Abstract

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 used 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 the presence of oxic niches in predominantly anoxic 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 in 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, for example, carbon and nitrogen cycling in groundwater over long periods at the catchment scale.

Spatial Control of Microbial Pesticide Degradation in Soil: A Model-Based Scenario Analysis

“…The spatial heterogeneity of degraders had less impact on MCPA dynamics than precipitation, but leaching of MCPA and leachate concentrations consistently increased with degrader heterogeneity within a given precipitation scenario. This is in line with simulation results of microbial nutrient cycling in spatially heterogeneous domains under fully saturated conditions . In our study, with extreme spatial heterogeneity of degraders, leaching from the topsoil was still ongoing at the end of the one year simulation period, however, only at low ng/L leachate concentrations (Figure A,B and SI1, Figure S6).…”

“…This is in line with simulation results of microbial nutrient cycling in spatially heterogeneous domains under fully saturated conditions. 55 In our study, with extreme spatial heterogeneity of degraders, leaching from the topsoil was still ongoing at the end of the one year simulation period, however, only at low ng/L leachate concentrations ( Figure 2 A,B and SI1, Figure S6 ). Despite these low concentrations, MCPA persistence until the next cultivation cycle might pose the risk of accumulation in the subsoil following several MCPA applications.…”

“…However, empirical data on the magnitude of COV is limited, and its determination outside of such idealized simulation studies appears largely impractical. While studies carried out in fully saturated conditions have indicated that using travel time of solutes may close the gap on estimating bulk reaction rates, ,, the estimation of the same in unsaturated settings remains elusive.…”

Characterizing Natural Organic Matter Transformations by Microbial Communities in Terrestrial Subsurface Ecosystems: A Critical Review of Analytical Techniques and Challenges

“…Thus, micro-scale heterogeneity ultimately impacts microbial activity, including the capacity to transform organic matter. Studies at centimeter scales indicate that microbial distribution can be highly variable in subsurface sediments over centimeter distances ( Brockman et al, 1997 ), and that microbial diversity changes with spatio-temporal variation of groundwater and sediment chemistry ( Smith et al, 2018a ; Khurana et al, 2022 ). However, methods to measure pore scale processes and scale those observations to field-relevant are still under development.…”

A perspective on applied geochemistry in porous media: Reactive transport modeling of geochemical dynamics and the interplay with flow phenomena and physical alteration

“…These authors explored the evolution of hydrodynamics as a result of the development of a biofilm growth in the pores for different biofilm properties, and some also introduced a porosity-permeability relationships for bioactive media as an alternative to the Kozeny-Carman equation (Thullner, Zeyer, and Kinzelbach 2002;Lopez-Peña, Meulenbroek, and Vermolen 2018). There has been recently a significant increase in the number of biogeochemical reaction models describing the intrinsic microbial behavior in more details (e.g., (Mellage et al 2015;Gharasoo, Thullner, and Elsner 2017;Ehrl, Gharasoo, and Elsner 2018;Khurana et al 2022a)) that can be implemented into pore-scale models in order to realistically simulate natural attenuation of compounds in porous environments. The same concept of coupling pore-scale models with more detailed and advanced reaction kinetics can be extended to other fields such as, CO 2 -methane exchange in gas-hydrate sediments for gas recovery (Gharasoo, Babaei, and Haeckel 2019), aerobic and anaerobic CO 2 respiration in soil (Davidson et al 2012;Pagel et al 2014), inhibitory effects of electron acceptors and donors in soil bioremediation (S. Zhang, Gedalanga, and Mahendra 2016), and the depletion of light non-aqueous phase liquids (LNAPL) in natural zones (Sookhak Lari et al 2019).…”