Quantifying anoxic protection of soil carbon

Emily Lacroix (Earth System Science, Stanford)

Emily Lacroix sampling soil and sediment in and near Searsville Lake

Summary

Anoxic microsites are small zones in soil where oxygen is depleted. Within these microsites, soil carbon is protected from microbial respiration, but the contribution to overall soil carbon protection (anoxic protection) remains poorly understood. Doctoral student Emily Lacroix, working with professor Scott Fendorf, is using a novel method for quantifying anoxic protection. Emily takes intact cores of soil, and then enhances aeration in the core by disaggregating and distributing the soil across a broad area incubator. The increase in carbon dioxide efflux after disaggregation, normalized to the total amount of carbon in the core, serves as a relative measure of carbon protection within anoxic microsites and their susceptibility to physical disturbance. Fendorf and Lacroix term this measure the “Disturbance Index.” Iron and manganese reduction in the soils both before and after disturbance provide an index of the prevalence of anaerobic metabolism in the intact and disaggregated cores, connecting increased carbon dioxide efflux after disturbance to the aeration of anoxic microsites. They account for potential increases in efflux from freed carbon by studying a parallel set of incubations in an anaerobic chamber. The goal is to understand how moisture, texture, organic matter content, and mineral surface area can influence anoxic protection in soil. Lacroix and Fendorf are studying carbon dynamics on the margin of Searsville Reservoir, in lakebed sediments that were exposed in fall of 2020, and on other Stanford lands.