Predicting the impact of anaerobic microsites on soil organic matter mineralization rates in upland soils

Abstract:

Soils are a crucial component of the global carbon (C) cycle, representing a highly dynamic and large reservoir of C stored as soil organic matter (SOM). An important control on SOM residence time is microbial mineralization. While the impact of climactic and site-specific constraints on SOM mineralization rates are recognized, the role of oxygen limitations remains elusive. If oxygen consumption (via heterotrophic respiration) outpaces supply (via diffusion), anaerobic microsites can occur even within seemingly well-aerated upland soils. Under anaerobic conditions, SOM mineralization rates are expected to be slower due to metabolic constraints on microbial C oxidation. Process-based C cycling models have begun to incorporate the inhibiting effect of oxygen limitations by estimating anaerobic pore volume. However, such model predictions still lack experimental validation and research on environmental controls thus far has largely been focused on soil moisture.

Here we aimed to determine the extent of anaerobic microsites within seemingly well-aerated upland soils experimentally and identify whether texture, SOM content, and microbial biomass can act as useful predictors in modeling frameworks. To this end, we monitored oxygen dynamics in soils spanning natural and artificial gradients in texture, SOM content and microbial biomass. Anaerobic microsites was visualized using a planar optode imaging system. Oxygen consumption rates were determined using gas chromatography, while oxygen diffusion rates were estimated based on porosity and pore-size distribution quantified by x-ray microtomography.

Our results show that bulk oxygen concentrations ranged from 70% to as low as 20% saturation. However, all soils showed substantial micro-scale variability in oxygen concentrations, leading to the formation of anaerobic microsites even at modest moisture content. The extent of anaerobic microsites correlated with an overall reduction in SOM mineralization rates, and depended on the interactive effect of texture-imposed diffusion limitations and the amount of bioavailable SOM driving oxygen consumption. Based on these results, a numerical model is presented to predict the extent of anaerobic microsites in a specific soil system using texture, SOM content and microbial biomass information.