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The kinetics of particulate organic carbon (POC) mineralization in marine surface sediments is not well constrained. This creates considerable uncertainties when benthic processes are considered in global biogeochemical or Earth system circulation models to simulate climate-ocean interactions and biogeochemical tracers in the ocean. In an attempt to improve our understanding of the rate and depth distribution of organic carbon mineralization in bioturbated (0–10 cm) sediments, we parameterized a 1-D diagenetic model that simulates the reactivity of three discrete POC pools at global scale (a "multi-G" model). The rate constants of the three reactive classes (highly reactive, reactive, refractory) are fixed and determined to be 70 yr−1, 0.5 yr−1, and ~0.001 yr−1, respectively, based on the Martin curve model for pelagic POC degradation. In contrast to previous approaches, the reactivity of the organic material degraded in the seafloor is continuous with, and set by, the apparent reactivity of material sinking through the water column. The model is able to simulate a global database (185 stations) of benthic oxygen and nitrate fluxes across the sediment-water interface in addition to porewater oxygen and nitrate distributions and organic carbon burial efficiencies. It is further consistent with degradation experiments of fresh phytoplankton. We propose that an important yet mostly overlooked consideration in previous upscaling approaches is the proportion of the relative reactive POC classes reaching the seafloor in addition to their reactivity. The approach presented is applicable to both steady-state and non-steady state scenarios, and links POC degradation kinetics in sedimentary environments to water depth and the POC rain rate to the seafloor.

Natural porous media such as soils or aquifers are characterized by various pore-scale heterogeneities. This includes the temporal variability of substrate supply and the spatial heterogeneity of pore sizes. As a consequence, microbial growth conditions in natural porous medium environments may differ from typical laboratory setups used to study microbial behavior. Pore-size heterogeneities and the resulting transport regime can lead to highly complex distribution patterns of substrates and corresponding microbial growth conditions including the frequent occurrence of stress periods for the microbial population. Microorganisms can respond to such stress periods by switching from an active into an inactive or dormant state, and the corresponding microbial abundance and dynamics may exhibit rather complex temporal and spatial patterns. This study applies an extended modeling concept for the growth and degradation activity of microbial species able to switch between two different physiological states. The concept is implemented into a pore network model that allows simulating the changes in microbial growth conditions in heterogeneous porous media. The model is used to study the impact of pore-size heterogeneities on the distribution and activity of a bacterial community consisting of two competing species. Simulation results show that such pore-scale heterogeneity has a strong impact on bacterial abundance and activity, creating a complex structure of microbial niches. This leads to an increased coexistence of microbial species at both the microscale and the macroscale, although in the macroscale, the total biomass concentration is less affected by these heterogeneities.

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