Since the start of the industrial era human activities have been impacting climate with increasing emissions of greenhouse gases (GHGs), altering the Earth's energy balance. Nitrous oxide (N2O) and methane (CH4), together with carbon dioxide (CO2) are the principal GHGs with increasing concentration in the atmosphere. The global warming potential (GWP) of N2O is 298 times and CH4 is 34 times higher relative to that of CO2 on a 100 year time horizon. Global changes, including climate change, are known to impact GHG fluxes but a mechanistic understanding of what drives feedback responses is not fully known. For example, major pathways of GHG production and consumption in terrestrial ecosystems are microbial processes. However, the response of functional microbial groups associated with GHG fluxes to global change conditions and its consequences for GHG feedback responses remains largely unknown. In fact, most field studies addressing GHG emissions fail to include all three GHGs and microbial communities present in the soil. In this study, I aimed to improve projections of GHG emissions under current management practices and future climatic conditions and to further improve the mechanistic and regulatory understanding of soil GHG fluxes to the atmosphere. Firstly, I investigated the long-term effect (six years) of management practices, routinely implemented in forestry plantation to maximize yield production, as well as land-use change, particularly afforestation. This study was conducted in an Australian forest dryland ecosystem, characterized by low water and nutrient availability resulting from low precipitation and high evaporation. Secondly, due to the high vulnerability of nutrient poor soils, a controlled environment laboratory experiment was established with soil monoliths from three nutrient poor sites from New South Wales, Australia. Soil monoliths were exposed to future projected concentrations of CO2 (600 ppm) and elevated temperature (+3°C) across a nine-month period. Finally, the long-term effect of climate change was addressed in a five-year manipulative field study in a boreal forest warming (+3.4°C) experiment (B4Warmed) together with reduced precipitation (≈45% reduction) during summer at two sites in northern Minnesota, USA. In all experimental studies, the fluxes of the three GHGs N2O, CH4 and CO2 were measured in order to quantify GHG flux feedback response rates (in CO2 equivalents) under the various treatments. The balance among net exchanges of N2O, CH4 and CO2 emissions was further assessed by quantifying the net soil GWP-induced by each treatment. The mechanistic and regulatory understanding of GHG feedback responses was explored by explicitly considering shifts in functional microbial communities' abundance and soil physicochemical properties. Specifically, ammonia-oxidizing archaea and bacteria and N2O-reducing bacteria for N2O, methanotrophs for CH4 and total bacteria for CO2. Soil abiotic properties such as available NH4+, NO3-, PO43-, pH, moisture, temperature, texture and total C and N were taken into account. In a dryland ecosystem N2O and CO2 fluxes were limited by water whereas N2O and CH4 were constrained by N availability. From all the climate change treatments considered, increasing temperatures showed the strongest effects on GHG fluxes. Elevated CO2 and elevated temperature treatment effects, when combined, were less than additive, over a nine-month incubation period. In a boreal forest, CO2 fluxes were strongly reduced, but N2O and CH4 were unaffected, under combined reduced precipitation and warming treatments. Overall, GHG emissions demonstrated a positive feedback to global change treatments. Combined climate effects also tended to offset, to some extent, the single temperature effect. Such response was reflected in the net GWP-induced treatment, with CO2 being responsible for the direction due the larger amount being produced compared to N2O and CH4. Overall, the research presented in this thesis demonstrates the importance of functional microbes, moisture, temperature and nutrient availability in GHG emissions under different global change treatments. Particularly, nitrification-mediated pathways showed a strong relationship with N2O emissions in all experimental studies conducted. Methane uptake, although strongly linked to methanotrophs abundance, was also strongly dependent on soil abiotic characteristics capable of affecting gas diffusion. Carbon dioxide emissions were characterized by higher complexity with multiple drivers responsible for net emissions, particularly bacterial abundance and temperature. My study provides a strong framework for future studies on feedback responses of GHG fluxes under current and future climate conditions and proposes biological (functional microbes) variables should be explicitly considered when developing mechanistic understanding regulating feedback responses.
| Date of Award | 2016 |
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| Original language | English |
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