Laboratory for Observing Anoxic Microsites in Soils (LOAMS)

Redox reactions essentially underlie several biogeochemical processes and are typically spatiotemporally heterogeneous in soils and sediments. However, redox heterogeneity has yet to be incorporated into mainstream conceptualizations and modeling of soil biogeochemistry. Anoxic microsites, a defining feature of soil redox heterogeneity, are non-majority oxygen depleted zones in otherwise oxic environments. Neglecting to account for anoxic microsites can generate major uncertainties in quantitative assessments of greenhouse gas emissions, C sequestration, as well as nutrient and contaminant cycling at the ecosystem to global scales. However, only a few studies have observed/characterized anoxic microsites in undisturbed soils, primarily, because soil is opaque and microsites require µm-cm scale resolution over cm-m scales. Consequently, our current understanding of microsite characteristics does not support model parameterization.

To resolve this knowledge gap, we simultaneously (i) study impact from anoxic microsites on biogeochemical cycles at the soil scale and (ii) detect, quantify, and characterize anoxic microsites directly from natural cores.

We have examined the influence of anoxic microsites on biogeochemical cycles of nutrients (C, S, and Fe) and contaminants (Zn, Ni, As, U), combining results from experimental columns and natural cm-scale anoxic microsites of floodplain sediments at the upper Colorado River Basin scale. In parallel, we have demonstrated through a proof-of-concept study that X-ray fluorescence (XRF) 2D mapping can reliably detect, quantify, and provide basic redox characterization of anoxic microsites using solid phase “forensic” evidence. Rapid screening of large cores at high spatial and energy resolution, i.e. 1-100 µm resolution over cm-m areas, followed by systematic algorithm-driven data processing, allows for relatively quick identification, quantification, and characterization of actual anoxic microsites. To date, these investigations have revealed direct evidence of anoxic microsites in predominantly oxic soils such as from an oak savanna and toeslope soil of a mountainous watershed, where anaerobicity would typically not be expected. We also revealed preferential spatial distribution of redox microsites inside aggregates from oak savanna soils. We anticipate that this approach will advance our understanding of soil biogeochemistry and help resolve “anomalous” occurrences of reduced products in nominally oxic soils.