Modeling microbial mediated carbon decomposition in the spatio-temporally heterogeneous subsurface

The Critical Zone is a highly dynamic natural system. Temporal dynamics in surficial processes such as weather events lead to mobilization and transport of organic matter from the surface to spatially heterogeneous subsurface. The inherent spatial heterogeneity of the subsurface, in turn, influences access to water flux and nutrients there in. This variable access to nutrients influences the formation of microbial diversity as well as of hot spots of microbial activity in the subsurface. As a consequence, the dynamics of biogeochemical cycles is variable in space and time. A detailed assessment of these dynamics and of its consquences on groundwater quality is challenged experimentally by the limited observational opportunities of subsurface compartments, and numerically by reactive transport model approaches typically making the use of numerous simplifying assumptions. These assumptions may include steady state conditions, uniform flow conditions, or homogeneous flow field conditions in the subsurface. There is, thus, a need to evaluate relative importance of spatial heterogeneity and temporal dynamics on subsurficial biogeochemical processes such as the decomposition of organic carbon compounds.

In this study, we undertake a numerical modeling approach to study the impact of spatio-temporal heterogeneities on organic carbon transformations in aquifers. We used an already established biogeochemical process network, and an established numerical tool (Centler et al., 2010) to simulate microbially driven transformations of different organic carbon compounds in temporally dynamic and spatially heterogeneous subsurface settings, driven by data obtained from a subject site (AquaDiva Critical Zone Observatory, Hainich National Park, Thuringia, Germany (Küsel et al., 2016)).

The obtained results reveal the feedback between spatial heterogeneity effects and temporal dynamic effects on microbial activity, and subsequent impact on nutrient discharge in groundwater. This feedback may be either positive or dampening (negative) depending on the biogeochemical potential of the domain (easily estimated in terms of the Damköhler number). In turn, the results provide the basis for the consideration of heterogeneity effects in effective rate descriptions of biogeochemical processes.