Calhoun, GRAD STUDENT
Knowledge about how temperature will influence microbial decomposition of soil organic matter and resultant CO2 efflux from soils is of importance to understand the global C balance and more accurately project future climate. Yet, it is difficult to quantify microbial activities in their natural habitats due to inherent heterogeneity in soils. Here I investigate microbial activities in controlled experiments ranging in complexity, such that I can parse the temperature responses of multiple processes operating simultaneously in soils. Soil microbes exude exo-enzymes into the soil matrix, where they break down organic molecules into smaller substrates. Microbes assimilate these smaller substrates to grow, while respiring CO2. In chapter 1, I demonstrate that pH can differentially influence temperature responses of distinct exo-enzyme activities relevant to C and N acquisition from soil organic matter. Expanding this, in chapter 2 I find that temperature and substrate availability can interactively influence biomass-specific respiration rates, C use efficiency (CUE), and stable isotope C discrimination from an isolated population of microbes. Enhancing the complexity again, in chapter 3 I show that temperature responses of exo-enzyme activities and respiration of natural microbial communities in soils are conserved at biomass-specific rates across diverse timescales and in spite of distinct community structures, while those at soil mass-specific rates are not. This body of work has ecological implications about microbial adaptation in a warmer world. Different temperature responses of exo-enzymes and CUE with varying environmental conditions can lead to microbial communities with distinct strategies of resource allocation to production of exo-enzymes and biomass for survival and reproduction. These inferences about microbial adaptation obtained from simplified systems can help understand driving mechanisms of apparent temperature responses and guide parameterizations in microbially-explicit Earth-system models to better project microbial feedbacks to climate.
Min, Kyungjin (2017): Temperature responses of microbial soil organic matter decomposition and associated respiration at various scales, ranging from exo-enzymes to populations and communities. PhD Dissertation, University of Kansas.