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ESS Oral Defense: Emily Lacroix - October 13, 9am, Green 365 / Zoom

 Stanford University
*** Ph.D. Thesis/ Oral Defense ***

Revealing the unforeseen role and sensitivity of anoxic protection in soil carbon cycling 

Emily Lacroix

Thursday, October 13, 9:00am

Green 365

Department of Earth System Science

Advisor: Dr. Scott Fendorf

Increasing soil carbon (C) content promises to mitigate climate change and enhance soil fertility. Soil C content is determined, in part, by microbial respiration, which converts soil C into carbon dioxide. Carbon protection mechanisms represent processes and conditions that limit microbial respiration of soil C. Anoxic microsites, zones of oxygen depletion in otherwise oxic soils, are a recently recognized and under-studied soil C protection mechanism. In this dissertation, I use field and laboratory methods to determine the extent and contribution of anoxic microsites to soil C protection within natural and agricultural systems. In Chapter 2, I measure the dissolved oxygen content of soil porewater from California agricultural soils. I show that physical disturbance destroys anoxic microsites through enhancing oxygen supply to the   smallest soil pores. In Chapter 3, I show that oxygen limitations constrain OM turnover in a Hawaiian rainforest soil, an environment where mineral content is presumed to be the dominant soil C protection mechanism. In Chapter 4, I examine a soil textural gradient at the Stanford Dish and demonstrate that anoxic microsites are particularly important for protecting C in coarsely textured soils. Finally, in Chapter 5, I use droplet digital PCR to quantify anaerobe DNA (a proxy for anoxic microsites) in soils from four long-term agricultural experiments across the continental United States. I show that anoxic microsites vary with soil properties, respond to management, and uniquely contribute to soil C stabilization within cropland soils. In sum, this dissertation reveals that anoxic microsites represent a vulnerability in the soil C stock but also an opportunity to enhance soil C storage.

Zoom:  Available per request.