Dissertations, Theses, and Capstone Projects

Date of Degree

9-2021

Document Type

Dissertation

Degree Name

Ph.D.

Program

Earth & Environmental Sciences

Advisor

Zhongqi Cheng

Committee Members

Peter M. Groffman

Karen Vaughan

Chester B. Zarnoch

Subject Categories

Biogeochemistry | Climate | Soil Science

Keywords

Climate Change, Wetland, Soil, Carbon, Nitrogen, Halophyte

Abstract

Wetlands are complex environments that play a critical role in regulating the global biogeochemical cycle of carbon (C) and nitrogen (N). Wetlands are critical contributors to global climate change and atmospheric chemistry since they store as much as 33% of the world’s soil organic carbon (SOC), release more than 20% of the atmospheric methane (CH4), and produce nitrous oxide (N2O), an extremely potent greenhouse gas (GHG). Despite the enormous radiative forcing potential of carbon dioxide (CO2), CH4 and N2O derived from wetlands, uncertainties over the rates of C sequestration and GHG production persist. Here we present data on the carbon accretion rates (CAR), carbon stocks, GHG production and N cycling for a lagoon salt marsh system in California. I quantified C and N stocks, CAR, and CO2, CH4, and N2O efflux during a 32-day incubation experiment, in Scott’s Creek Marsh, a lagoon salt marsh system in California, with a focus on variation with the halophytic plant community within the marsh.

Scott’s Creek Marsh’s C pool of 305.77 Mg C ha-1 is high relative to other studied salt marshes, with significant variation in halophytic zones. Zones dominated by Typha latifolia, an exotic, invasive plant had the highest %C (4.68%), C density (0.069 g cm-3), total C (347.3 Mg C hectare-1) and CAR (195.8 g C m-2 yr-1). The average CAR for SCM, 157.81 ± 9.23 g C m-2 yr -1 was low compared to other studied salt marshes along the Pacific coast of the United States (173.6 g C m-2 yr -1) and globally (244.7). However, it is important to note that our study did not include the O horizon of the soils and the soils are not saturated year-round. There is a clear need for further study of the dynamics of Typha invasion and its effects on salt marsh C and N dynamics.

Hydrology, which controls oxidized versus reduced soil conditions is a clear driver of C and N dynamics in these salt marshes. The 32-day incubation revealed a significant difference in the amount of CO2 respired under oxidized and reduced soil conditions; however, the type of halophytic zones did not significantly influence CO2 production. Flooding the soils reduced the rate of respired C-CO2 by 30.51% (Distichlis), 53.76% (Juncus), and 50.47% (Typha), respectively. Our data revealed no significant differences (P > 0.05) in methane efflux between treatments (oxidized vs. reduced) and all three halophytic zones. Based on our annual methane efflux data, it appears that C-CH4 does not constitute a large portion of the soil carbon. The C-CH4 efflux comprised less than 0.01% of the initial carbon.

The N2O concentrations did not significantly differed between zones but did so based on treatment, with oxidized samples producing significantly more N2O than reduced. The nitrification variation between reduced and oxidized samples was significant based on our statistical analyses (P < 0.001). Nitrification among the oxidized samples based on our calculations would constitute a total per annual basis of 5.71% of the N stock. Regression analyses indicated there was a direct relationship between mineralization and nitrification for the oxidized samples during the incubation (R2 = 0.99, P < 0.001).

This study provided novel and important information on an important ecosystem type that is affected by multiple components of global environmental change that influences plant community composition and hydrologic conditions. The information will increase our ability to understand and manage these ecosystems to achieve environmental goals related to climate change, water and air quality, and biodiversity.

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