Soil is a composite of minerals, organic matter, water, and air. The structural arrangement of these components into pore networks define where different biogeochemical processes can take place. The distribution of such processes across a volume of soil is therefore inherently heterogeneous. Recent literature has shown that soils that are nominally oxic can contain smaller anoxic regions (so-called anoxic microsites) in which biogeochemistry markedly differs from the surrounding oxic soil volume. These regions form where the supply of oxygen by diffusion outpaces oxygen demand by respiring microbial communities. The resulting shift in respiration pathway from aerobic to anaerobic causes a slowdown in organic carbon mineralization and, depending on the respiration pathway, a buildup of reduced metal species.
Here, we used time-integrative measures of past anoxia (i.e., electrochemical measurements and quantification of anaerobic functional genes) to determine how the spatial distribution of anoxic microsites varies between aggregate interiors and bulk soils in soils of two distinct textures across multiple depths in a California grassland. We found greater evidence of anoxia in topsoils vs. subsoils and finer vs. coarse-textured soils. Counter to many traditional depictions of soil aggregates, we observed that aggregate interiors showed equal or less evidence of anoxic microsites than bulk soils. Across the entire dataset, our combined proxies for anoxic microsite prevalence were strongly and positively correlated with organic C concentration (R2 = 0.80), highlighting the importance of soil organic C availability and microbial oxygen demand in creating anoxic microsites. Our results contribute to a growing body of evidence that soil oxygen demand (i.e., microbial respiration) can play a more prominent role in anoxic microsite formation than soil oxygen supply, provoking questions about the suitability of using aggregate size and moisture as lone proxies for soil oxygen availability.
People: Meret Aeppli
Collaborators: Emily Lacroix
Publication: Emily M. Lacroix, Anna Gomes, Alexander S. Honeyman, Katie R. Huy, Scott Fendorf, Vincent Noël, Meret Aeppli. Soil carbon concentration drives anoxic microsites across horizons, textures, and aggregate position in a California grassland. Geoderma, Volume 454, 2025, 117165, ISSN 0016-7061, doi: 10.1016/j.geoderma.2025.117165.