The Role of Microbial Dormancy in Soil Carbon-Climate Feedbacks

Alejandro Salazar, Purdue University

Abstract

Soil respiration, a process primarily driven by soil microbes, is the largest flux of carbon from terrestrial ecosystems to the atmosphere. As the Earth gets warmer, the annual amount of carbon that is respired from soil to the atmosphere is increasing. Even small increases in this flux could accelerate global warming. However, our ability to predict changes in soil respiration and its contributions to the Earth's climate is constrained by the limited mechanistic understanding that we have about the ways through which climate change is affecting soil microbial processes. In my dissertation research, I investigated different mechanisms through which microbes respond to the environment and influence soil carbon cycling. I focus much of the discussion on one microbial strategy that affects soil carbon cycling and that has been considered in large scale ecosystem models only recently: microbial dormancy. A vast majority of microbes in soil can switch their metabolic state between active and dormant in response to changes in environmental conditions. Only metabolically active microbes are capable of driving biogeochemical cycles. It is unclear whether changes in the Earth's climate system are affecting the metabolic state of microbes in soil, and if climate-driven activation of dormant microbes could contribute to a positive soil carbon-climate feedback. I conducted a series of laboratory, eld, and modeling experiments to asses the role of microbial dormancy on feedbacks between soil carbon and climate. First, I tested whether soil respiration rates are linked to the metabolic state of microbial communities in soil. To do this, I monitored changes in soil respiration and active and dormant microbial biomass under different moisture and temperature conditions. I found that pulses of soil respiration after wetting dry soils (a phenomenon known as the Birch effect) are partially caused by rapid activation of dormant microbes in soil (Chapter 2). The relationship between active microbial biomass and soil respiration rate is consistent across soils from different ecosystems and climatic regions (Chapter 3). I modied an existing microbe-focused soil carbon model to account for microbial dormancy and compared predictions of soil respiration with and without simulated dormancy under different warming and drying-wetting scenarios. Incorporation of dormancy into a soil carbon model made microbial biomass and respiration less susceptible to drying-wetting stress, especially under warming conditions (Chapter 3). Finally, I tested this hypothesis in a eld experiment at seasonal scale and found that seasonal soil respiration in an temperate old-eld is more closely linked to the abundance of active microbes in soil, than to total microbial biomass (active and dormant) or community composition (fungi:bacteria ratio; Chapter 4). I conclude that soil respiration rates are importantly linked to the abundance of metabolically active microbes in soil. This is true in soils from different ecosystems and climatic regions and across hours-to-months temporal scales. The influence of microbial dormancy on soil respiration rates has implications for soil carbon-climate feedbacks. Incorporation of microbial dormancy in a soil carbon model buffers the effect of drying-wetting stress on simulated biomass and respiration, especially under warming conditions. Overall, these results suggest that if the Earth keeps getting warmer and in many places the length of the dry periods between wetting events keeps getting longer, microbes in soil will emit more CO2 to the atmosphere than would be predicted by models that do not take dormancy into account.

Degree

Ph.D.

Advisors

Dukes, Purdue University.

Subject Area

Climate Change|Microbiology|Biogeochemistry|Soil sciences

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