Consequences of elevated atmospheric carbon dioxide on soil microbial processes of an oak forest

Abstract

The atmospheric concentration of carbon dioxide (CO2) continues to rise due to anthropogenic activity. Globally, forests cover some 43% of the earth’s surface and account for 70% of terrestrial net primary productivity. Above ground processes have been well evaluated in relation to elevated CO2 concentrations, and the consensus is that plants will respond positively to the CO2 fertilisation effects of elevated CO2. This increase in photosynthesis and primary production is expected to mediate a larger flux of carbon below ground. On the one hand, extra carbon allocated below ground may induce positive effects on microbes, where soil microorganisms are stimulated to break down soil organic matter releasing extra CO2 to the atmosphere. On the other hand, greater below ground inputs of carbon may promote carbon storage leading to sequestration of CO2 from the atmosphere. These contrasting hypotheses require additional experiments to disentangle the response of soil microorganisms to elevated CO2 concentrations. This thesis reports results from two sets of experiments designed to test how elevated atmospheric CO2 concentrations may affect the activity and biomass of free-living and symbiotic soil microorganisms. The first experiments used a simplified rhizosphere experiment to assess how root exudates under elevated CO2 may affect soil nutrient availability, enzyme activity and carbon cycling in an oak woodland. Data from root exudation studies under elevated CO2 were used to create artificial exudate mixtures mimicking scenarios at atmospheric CO2 concentrations of 350ppm, 525ppm, and 700ppm. In a separate experiment, 13C labelled exudate mixtures were applied to the soil to investigate how the fate of carbon added to the soil changes with increased exudate concentration. It was found increased exudation did not linearly affect most parameters tested. The treatments significantly reduced soil extractable nitrogen with a non-significant trends towards an increase in extractable phosphorus. Microbial activity was significantly stimulated at exudation equivalent of 525ppm compared to ambient concentrations. Overall, the most sensitive transition in CO2 concentration was found between 350ppm and 525ppm. This indicates the sensitivity of the system to small changes and suggests that as the current carbon dioxide concentration is 410ppm, we soon may experience a shift in how microorganisms process extra inputted carbon to the soil. The final experiment used the Birmingham Institute of Forest Research free air carbon dioxide enrichment facility (BIFoR FACE) to test if elevated CO2 affects the production of mycorrhizal fungal mycelium in a mature oak forest. Carbon dioxide enrichment had been applied for two years, and consequently no difference in production was expected. Mesh bags filled with acid washed sand were incubated in the upper soil for 111 days over the growing period in elevated and ambient treatment rings. No significant differences were found but a trend towards increased production in plots receiving elevated atmospheric CO2 was observed. This thesis highlights the sensitivity of mature oak forest soils to changes in atmospheric CO2 concentrations. It also highlighted the non-linear response to incremental increases in carbon addition to the soil. The results presented emphasise the complex nature of mature oak forest soil and the sensitivity of the 350ppm to 525ppm carbon dioxide concentration transition.

Date of Award	1 Jan 1824
Original language	English
Awarding Institution	The University of Manchester