Geology topics

Olivia Johnson

Publications and source records attributed to Olivia Johnson.

3 recordsLinked to original sources

USGS2024

Common use herbicides increase wetland greenhouse gas emissions

Wetlands play a disproportionate role in the global climate as major sources and sinks of greenhouse gases. Herbicides are the most heavily used agrochemicals and are frequently detected in aquatic ecosystems, with glyphosate and 2,4-Dichlorophenoxyacetic acid (2,4-D), representing the two most commonly used worldwide. In recent years, these herbicides are being used in mixtures to combat herbicide-tolerant noxious weeds. While it is well documented that herbicide use for agriculture is expected to increase, their indirect effects on wetland greenhouse gas dynamics are virtually unknown. To fill this knowledge gap, we conducted a factorial microcosm experiment using low, medium, and high concentrations of glyphosate or 2,4-D, individually and in combination to investigate their effects on wetland methane, carbon dioxide, and nitrous oxide fluxes. We predicted that mixed herbicide treatments would have a synergistic effect on greenhouse gases compared to individual herbicides. Our results showed that carbon dioxide flux rates and cumulative emissions significantly increased from both individual and mixed herbicide treatments, whereas methane and nitrous oxide dynamics were less affected. This study suggests that extensive use of glyphosate and 2,4-D may increase carbon dioxide emissions from wetlands, which could have implications for climate change.

Science of the Total Environment↗

USGS2023

Practical guide to measuring wetland carbon pools and fluxes

Wetlands cover a small portion of the world, but have disproportionate influence on global carbon (C) sequestration, carbon dioxide and methane emissions, and aquatic C fluxes. However, the underlying biogeochemical processes that affect wetland C pools and fluxes are complex and dynamic, making measurements of wetland C challenging. Over decades of research, many observational, experimental, and analytical approaches have been developed to understand and quantify pools and fluxes of wetland C. Sampling approaches range in their representation of wetland C from short to long timeframes and local to landscape spatial scales. This review summarizes common and cutting-edge methodological approaches for quantifying wetland C pools and fluxes. We first define each of the major C pools and fluxes and provide rationale for their importance to wetland C dynamics. For each approach, we clarify what component of wetland C is measured and its spatial and temporal representativeness and constraints. We describe practical considerations for each approach, such as where and when an approach is typically used, who can conduct the measurements (expertise, training requirements), and how approaches are conducted, including considerations on equipment complexity and costs. Finally, we review key covariates and ancillary measurements that enhance the interpretation of findings and facilitate model development. The protocols that we describe to measure soil, water, vegetation, and gases are also relevant for related disciplines such as ecology. Improved quality and consistency of data collection and reporting across studies will help reduce global uncertainties and develop management strategies to use wetlands as nature-based climate solutions.

Wetlands↗

USGS2020

Vegetation affects timing and location of wetland methane emissions

Common assumptions about how vegetation affects wetland methane (CH) flux include acting as conduits for CH release, providing carbon substrates for growth and activity of methanogenic organisms, and supplying oxygen to support CH oxidation. However, these effects may change through time, especially in seasonal wetlands that experience drying and re-wetting, or change across space, dependent on proximity to vegetation. In a mesocosm study, we assessed the impacts of on CH flux using clear flux-chamber measurements directly over plants (‘whole-plant’), adjacent to plants (where roots were present but no stems; ‘plant-adjacent’), and plant-free soils (‘control’). During the establishment phase of the study (first 30-days), the whole-plant treatment had ~5-times higher CH flux rates (51.78±8.16 mg-C md) than plant-adjacent or control treatments, which was primarily due to plant-mediated transport, with little contribution from diffusive-only flux. However, high fluxes from whole-plants depleted porewater CH concentrations both directly below whole-plant and in neighboring plant-adjacent treatments, while controls accumulated a highly concentrated reservoir of porewater CH. When the water table was drawn down to simulate seasonal drying, reserve porewater CH from control soil was released as a pulse, equaling the earlier higher CH emissions from whole-plants. Plant-adjacent treatments, which had neither plant-mediated CH transport nor a concentrated reservoir of porewater CH, had low CH flux throughout the study. Our findings indicate that in seasonal wetlands, vegetation affects the timing and location of CH emissions. These results have important mechanistic and methodological implications for understanding the role of vegetation on wetland CH flux.

Journal of Geophysical Research: Biogeosciences↗