Browsing by Subject "dissolved organic carbon"

This study seeks to investigate the dynamics of dead plant carbon over fifty years of old-field forest development at the Calhoun Long Term Soil-Ecosystem Experiment (LTSE) in South Carolina, USA. Emphasis is on the transition phase of the forest, which is less well studied than the establishment and early thinning phase or the steady state phase. At the Calhoun LTSE, the biogeochemical and ecosystem changes associated with old field forest development have been documented through repeated tree measurements and deep soil sampling, and archiving of those soils, which now allow us to examine changes that have occurred over the course of forest development to date.

In this dissertation, I first quantify the accumulation of woody detritus on the surface of the soil as well as in the soil profile over fifty years, and estimate the mean residence times of that detrital carbon storage. Knowing that large accumulations of C-rich organic matter have piled onto the soil surface, the latter chapters of my dissertation investigate how that forest-derived organic carbon has been incorporated into mineral soils. I do this first by examining concentrations of dissolved organic carbon and other constituents in soil solutions throughout the ecosystem profile and then by quantifying changes in solid state soil carbon quantity and quality, both in bulk soils and in soil fractions that are thought to have different C sources, stabilities, and residence times. To conclude this dissertation, I present the 50-year C budget of the Calhoun LTSE, including live and dead plant carbon pools, to quantify the increasing importance of detrital C to the ecosystem over time.

This exceptional long term soil ecosystem study shows that 50 years of pine forest development on a former cotton field have not increased mineral soil carbon storage. Tree biomass accumulated rapidly from the time seedlings were planted through the establishment phase, followed by accumulations of leaf litter and woody detritus. Large quantities of dissolved organic carbon leached from the O-horizons into mineral soils. The response of mineral soil C stocks to this flood of C inputs varied by depth. The most surficial soil (0-7.5cm), saw a large, but lagged, increase in soil organic carbon (SOC) concentration over time, an accumulation almost entirely due to an increase of light fraction, particulate organic matter. Yet in the deepest soils sampled, soil carbon content declined over time, and in fact the loss of SOC in deep soils was sufficient to negate all of the C gains in shallower soils. This deep soil organic matter was apparently lost from a poorly understood, exchangeable pool of SOM. This loss of deep SOC, and lack of change in total SOC, flies in the face of the general understanding of field to forest conversions resulting in net increases in soil carbon. These long term observations provide evidence that the loss of soil carbon was due to priming of SOM decomposition by enhanced transpiration, C inputs, and N demand by the growing trees. These results suggest that large accumulations of carbon aboveground do not guarantee similar changes below.

A cost-saving strategy of large-scale algae cultivation that can lead to more economical production of algal food, feed, and fuels involves reusing the cultivation water after algae harvesting. Few studies have focused on predictors of algae growth success in reused water, or explained these results in terms of algal ecology. Factors such as dissolved organic matter accumulation and interactions with bacteria are also understudied in the context of water reuse, yet could inform cultivation decisions to maximize water reuse without losses in algal productivity. This dissertation investigated trends in previous studies and also used an experimental approach to determine how reusing cultivation water affects algae growth, dissolved organic carbon (DOC) accumulation, DOC release rates, and relative abundances of bacteria. Based on over 80 previous studies testing algae growth in reused water, algae taxon was the only factor significantly associated with algae growth response. A possible explanation for this result could be differences in the amount and composition of DOC excreted by different algae. I therefore experimented with three taxonomically and physiologically distinct algae, two diatoms and a green alga, to test their growth responses, DOC excretion rates, and the extent of DOC accumulation in reused water. Algae growth response in reused water varied by algae, yet was not correlated with DOC concentrations. Additionally, DOC concentrations steadily increased with each water reuse, suggesting a build-up of recalcitrant DOC after bacteria, or possibly algae, degraded more labile DOC. To further explore the extent of strain-specific growth responses, I tested the effect of reused water from a self-inhibitory algae strain on other algae strains. This reused water did not inhibit two other algae strains, suggesting that the inhibitory mechanism was strain-specific and was likely from build-up of a certain DOC compound. Across the three algae cultures, different bacteria taxa became enriched or depleted in reused water, despite all cultures being exposed to the same reused water source. DOC composition and concentration in reused water were therefore likely not driving observed differences in final bacteria communities, and DOC produced from the growing algae may be more influential. Overall, results from this dissertation support strain-specific features of algae growth responses, and suggest that algae screening processes should include tests in reused water. Identifying algae strains with uninhibited growth responses will be important for implementing water reuse for algae cultivation, to ultimately improve the economic feasibility of algae bioproducts.