Biogeochemistry Lab

Research

Nitrous oxide cycling in the aquatic environment

Nitrous oxide (N₂O), a powerful greenhouse gas (has ~300 times the global warming potential of CO₂) and the dominant ozone-destructing substance, is produced during the nitrogen transformation in the aquatic systems. The aquatic systems including ocean, estuaries and inland waters are large but poorly constrained sources of N₂O to the atmosphere. This uncertainty is partly due to the sparse observations and lack of mechanistic understanding of N₂O cycling processes. 

By applying both nitrogen stable isotope tracers and molecular techniques to directly measure nitrogen transformation rates and the microbes performing these processes, I provided the first comprehensive view of N₂O cycling in the seasonally anoxic Chesapeake Bay while I was a postdoc in the lab of Professor Bess Ward at Princeton University (Tang et al., 2022a and b). Nitrification and denitrification were identified as the dominant production pathway in oxygenated and anoxic waters respectively. A large potential of N₂O consumption in both oxygenated and anoxic estuarine waters were characterized for the first time. The ongoing work is assessing the response of N₂O cycling to environmental changes like warming, deoxygenation, acidification and nitrogen loading changes using both field observations and model simulations. 

Marine carbon cycle in a changing climate 

Nearly half of the net primary production on Earth occurs in the oceans, which plays an important role in supporting fishery and regulating the atmospheric CO₂ level. Marine primary production is affected by many environmental drivers including light, temperature, and nutrients. For example, iron is hypothesized to limit phytoplankton growth in the high-nutrient, low-chlorophyll (HNLC) regions including the North Pacific, Equatorial Pacific, and Southern Ocean. 

Using satellite observations, biogeochemical Argo floats and atmospheric modeling, we found iron in aerosols emitted from 2019-2020 Australian wildfires (from both biomass burning and dust) fueled widespread phytoplankton growth in the Southern Ocean, potentially leading to a substantial increase in carbon uptake by the ocean (Tang et al., 2021). This study raises public awareness of the far-reaching impact of wildfires and the complex role of wildfires in Earth’s climate system. Ongoing research in our group evaluates the controlling factors of marine productivity and carbon uptake in the global oceans.

Compilation and meta-analysis of nitrogen cycling processes in the global ocean 

The number of observations of different biogeochemical processes are accumulating globally. Compilation and meta-analysis of large datasets using statistical approaches and machine learning help us to better understand the distribution and controlling factors of biogeochemical processes and guide future observation efforts. 

A database of nitrification and nitrifiers in the global ocean has recently been constructed (Tang et al., 2023). We are developing a new model parameterization for nitrification based on the newly compiled database. The new parameterization will be incorporated in Earth System Models to assess changes in nitrification rates under climate change and its subsequent impacts on marine productivity. 

Machine learning algorithms have been applied to estimate N₂ fixation and diazotrophs in the global ocean using our compiled database of N₂ fixation rate and diazotrophs abundance (Tang et al., 2019 a and b). For example, machine learning-estimated global marine N₂ fixation ranges from 68 to 90 Tg N/year. Comparison to other model estimates shows substantial discrepancies in the global magnitude and spatial distribution of marine N₂ fixation, especially in the tropics and in high latitudes.

Method development for high resolution observations of biogeochemical processes 

Technological advances help to better observe and understand the Earth system. As a graduate student under the guidance of Professor Nicolas Cassar at Duke University, I developed and applied an unprecedented high-resolution underway method to measure N₂ fixation (a major external source of nitrogen to the ocean) (Cassar, Tang et al., 2018). Extremely high N₂ fixation rates and associated substantial contributions to primary production were discovered in the coastal waters of the eastern United States (Tang et al., 2019; 2020). These unexpected observations challenged the traditional view of the distribution of marine N₂ fixation in which N₂ fixation dominated in oligotrophic open waters. Development of other high-resolution observation methods and analytical techniques continues in our lab at USF.