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What happens during soil incubations? Exploring microbial biomass, carbon availability and temperature constraints on soil respiration




Birgé, Hannah E., author
Conant, Richard, advisor
Paul, Eldor, committee member
Wallenstein, Matthew, committee member
Stromberger, Mary, committee member

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Decomposition of soil organic matter (SOM) is one of earth's most important and dynamic biogeochemical cycles. Much research is devoted to separating and studying individual controls on SOM decomposition. A commonly-used approach is to incubate soils under controlled conditions to understand the drivers of SOM decomposition. In chapter 1, I explore the use of soil incubations to investigate SOM-temperature dynamics, and emphasize the importance of testing the assumptions of laboratory soil incubations. In chapter 2, I describe how I tested whether depletion of available SOM, soil microbial biomass, or extra-cellular enzyme pools drive the decline in soil respiration over the course of a long-term incubation in soils from two sites (a cultivated plot and a forested plot at Kellogg Biological Station, Hickory Corners, MI USA). I found that the availability of SOM was the key determinant of respiration, and the loss of microbial biomass and extra-cellular enzymes over the course of a long-term incubation did not limit the ability of the remaining microbial biomass to respire available SOM. I observed a sharp increase in respiration when the soils were mixed, which support availability as a key driver of soil respiration. My results support a paradigm in which physico-chemical drivers are the primary determinant of soil respiration over the course of a long-term incubation. In chapter 3, I describe how I investigated the validity of using constant temperatures - a departure from diurnal temperature oscillations soils experience in situ - in laboratory soil incubations. The effect of oscillating versus constant temperature in incubation experiments designed to measure soil organic matter (SOM) decomposition response to temperature is not well studied in the laboratory. I investigated the impact of oscillating versus constant temperature incubation regimes on soils from the two sites listed above with varying levels of available SOM, microbial biomass and extra-cellular enzymes. Over 42 days of incubation I measured changes in soil respiration, changes in the existing microbial biomass and extra-cellular enzyme pools, and shifts in the thermal optima of four common soil extra-cellular enzymes in response to oscillating (shifting between 25°C and 35°C every 12 hours) and constant (30°C) temperature treatments. I found that none of these soil pools were significantly affected by incubation temperature oscillations. My results justify the use of soils depleted of microbial biomass and a constant temperature regime to investigate SOM decomposition in laboratory soil incubations.


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