Porewater Constituents Serve As A Control And Sensitive Indicator Of The Belowground Carbon Cycle Response To Climate Change Drivers
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Song, Tianze
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Abstract
Although comprising only ~3% of the terrestrial surface area, peatland ecosystems store an estimated one-third or more of global soil carbon. In addition to this role as a net carbon sink, freshwater wetlands, such as peatlands, account for approximately one-third of global methane (CH4) emissions to the atmosphere. While CH4 emissions from peatlands are expected to disproportionately increase due to warming, the environmental controls remain poorly constrained. Any increase in CH4 emission is of great concern due to the fact that the sustained-flux global warming potential of CH4 is estimated to be 34-times greater than that of carbon dioxide (CO2) on a 100 y timescale.
The majority of heterotrophic organic matter decomposition and the production of greenhouse gases in peatlands occurs in the soils, which are composed largely of solid degraded peat and interstitial waters or porewaters. Microorganisms in peatlands soils reside attached to the solid peat particles or are free-living in the planktonic porewater environment. Dissolved organic matter (DOM) in porewaters represents a primary driver of heterotrophic respiration and methanogenesis in surficial peat soils across many peatland types. Evidence also indicates that DOM can inhibit anaerobic decomposition and CH4 production by serving as a source of alternative electron acceptors, acidifying the environment, or releasing anti-microbial polyphenolic compounds, which calls for further mechanistic understanding of porewater-peat interactions. To this end, this dissertation employed laboratory and field studies to investigate the response of peat and porewater microbial communities to climate change factors (warming, elevated atmospheric CO2).
In Chapter 2, the response of soil organic matter decomposition and greenhouse gas (GHG) production to climate drivers was investigated in a laboratory study of peat soils collected from a temperate bog dominated by peat mosses (S1 bog) at the Marcell Experiment Forest in northern Minnesota. Amendment of peat soil microcosms with porewater inhibited GHG production and contributed to acetate pooling at close to in situ temperature (4°C) but stimulated GHG production and acetate consumption rates by up to a factor of 2 at warmer temperatures (14 and 25°C). Elevated temperature (25°C) led to a slight decrease in microbial diversity and stimulated the growth of methanogens and specific syntrophic taxa. These results confirm that DOM is a primary driver of decomposition in peatland soils, contains compounds that inhibit microbial metabolism, and show that inhibition is alleviated by warming.
In Chapter 3, porewater microbial communities were characterized in peatlands for the first time and compared to their counterparts attached to solid phase peat soil. In addition, given that soil core sampling can be tedious and destructive, especially in the context of whole ecosystem climate manipulation experiments, the feasibility of using porewater microbial communities as an indicator of ecosystem response to climate drivers (warming and elevated atmospheric CO2) was explored. This portion of my research leveraged the whole ecosystem warming (WEW) experiment entitled, Spruce and Peatland Responses Under Changing Environments (SPRUCE) located in a northern peatland. Results revealed that planktonic microbial communities in porewaters are distinct from attached communities associated with the solid phase of peat soil. Microbial abundance was two to three orders of magnitude lower in porewater compared to peat, whereas porewater communities contained a substantially higher alpha diversity compared to peat. Pronounced shifts in the abundance, diversity, and composition of porewater microbial communities were observed with warming. In contrast, attached peat microbial communities showed a relatively muted response to increasing temperature, although microbial abundance did increase with warming at the surface. Specific functional guilds of microorganisms that mediate the methane cycle (methanotrophs) were shown to increase in relative abundance with increasing temperature. While porewater microbial communities show promise as a sensitive indicator of climate change, further investigation is warranted to elucidate the metabolic potential of predominant taxa in porewaters and whether the response of porewater community dynamics to climate drivers is representative of overall peat soil community dynamics.
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2022-12-14
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Dissertation