Research Areas
Aquatic fate of millennia-aged soil carbon following disturbance
Tremendous amounts of carbon have been stored for hundreds to thousands of years in frozen permafrost soil and in water-logged peatland soils on Earth. As warming thaws permafrost soils in the Arctic and land use change disturbs the hydrology of peatlands soils in the tropics, the millennia-aged carbon stored in these soils is becoming vulnerable to oxidation and release to the atmosphere. Current estimates suggest that the annual carbon emissions from thawing permafrost and disturbed peatlands are on the same order of magnitude and, together, contribute ~1.5 Pg (or 1.5 billion metric tons) of carbon per year to the atmosphere. However, neither of the estimates from these ecosystems include how much additional carbon may be emitted to the atmosphere following the export of carbon from permafrost or peatland soils to freshwaters.
My research has addressed this gap by showing that the degradation of permafrost organic carbon in arctic lakes and rivers may contribute an additional ~14% of carbon loss on top of current estimates (Bowen et al. 2020 GRL) and that the degradation of peat organic carbon in tropical canals may contribute an additional ~10% on top of soil emissions (Bowen et al. 2024 Nature Geoscience; Bowen et al. 2024 GCB). This work has been highlighted by Nature News & Views, EOS, and Grist. Our ongoing research focuses on constraining the aquatic fate of previously-stored soil carbon following wetland disturbance and the impacts on downstream coastal ecosystems.
Deforestation & land management impacts on greenhouse gases
Forested streams and swamps are globally important emitters of greenhouse gases to the atmosphere. As these forested areas become cleared during land use change, more of the carbon in freshwaters and soils is getting exposed to sunlight. Greater sunlight exposure has implications for the temperatures that drive microbial activity and the sunlight-driven photochemical reactions that can add to greenhouse gas production. However, the impact of this exposure on the greenhouse gas emissions from streams and swamps is highly uncertain.
My research has shown that the sunlight exposure of semi-forested streams can increase degradation rates for dissolved organic carbon by ~12% through photochemical degradation pathways (Bowen et al. 2020 L&O). Sunlight exposure can also double the rates for organic carbon degradation in the canals and ditches dug throughout deforested and drained peatlands (Bowen et al. 2024 Nature Geoscience). Our ongoing research focuses on the role of sunlight in the transfer of heat to freshwater bodies and the impact of deforestation and sunlight wavelengths on greenhouse gas emissions.
Degradation products that enhance microbial activity
Microbes play a major role in the turnover of carbon in soil and aquatic ecosystems. Our ability to predict how this might change with ecosystem disturbance is limited, however, by our understanding of the molecules that fuel metabolism and/or how the availability of these molecules might change in the future.
My research has addressed this gap by identifying types of molecules that are produced during abiotic degradation pathways, such as sunlight-driven photochemical reactions in sunlit waters, that support microbial metabolism. For example, my work has shown that sunlight exposure can break down larger aromatic-rich organic molecules into smaller pieces that are more readily respired by stream microbes (Bowen et al. 2020 L&O) and that the photodegradation of organic nitrogen can be fast enough to supply ammonium to primary producers in oligotrophic lakes (Bowen 2021). I have used combinations of various qualitative and quantitative organic characterization methods, including high-resolution mass spectrometry (FTICR-MS), tandem mass spectrometry (LC-MS/MS), and fluorescence spectroscopy (EEM-PARAFAC), to tackle this question. Our ongoing research focuses on the timescales over which products are formed during abiotic degradation relative to microbial degradation and how these change along environmental gradients (e.g., redox, light).
Transferring lab-based experimental results to the environment
Quantifying how much microbes versus abiotic pathways contribute to the total degradation of dissolved organic matter to greenhouse gases in aquatic ecosystems requires an understanding of how their rates compare and how they change under different environmental conditions (redox state, hydrology, light availability). Because it is often a challenge to estimate degradation rates in situ from field measurements alone, there is a need to better translate lab-based experimental results to degradation rates in the environment.
In my research, I aim to address this gap by designing experiments and field monitoring campaigns to better transfer lab-based experimental results to the environment using the water chemistry, water residence time, and meteorology of the ecosystem. I have used LED-based reactors to quantify the wavelength dependence of products formed during photochemical pathways to better scale the rates of their formation in sunlit waters (Ward et al. 2021 ES&T). I have also used plug-flow biofilm reactors to quantify the rates that dissolved organic matter is respired by microbes in the streambed (Bowen et al. 2020 L&O). Ongoing research involves the use of field sensors to monitor meteorology and water chemistry at high-frequency and computing to estimate rates in the environment.
Collaboration
Understanding how the carbon cycle will change in the future or how to mitigate these changes requires collaboration across disciplines.
Thus, I work with others who study controls on methane oxidation (a more potent greenhouse gas than carbon dioxide) and soil respiration, and the effectiveness of nature-based solutions.
Methane Oxidation
By measuring the rates and isotope fractionation factors for methane oxidation, Perryman et al. (2024) estimated that the oxidation of methane between anoxic peat porewater and oxic canal waters mitigates three-fourths of the methane emissions from canals draining disturbed tropical peatlands.
Restoration & Conservation
By measuring the greenhouse gas emissions from drained and restored tropical peatland soils, Novita et al. (2024) estimated that restoration can reduce respiration rates by one-third. Asyhari et al. (2024) showed that conservation of one of the last remaining unprotected peat swamp forests on Borneo has a strong climate mitigation potential because carbon losses are half of those in disturbed peatlands.
Soil Respiration
By measuring landscape-level changes in water table and soil temperature across a secondary forest and an area managed for palm oil production, Ritonga et al. (prepared for submission) estimated that the spatial heterogeneity in vegetation cover caused by land management practices strongly impact landscape-level emissions estimates.