It's what we do.
It's what we do.
I am a biogeochemist and ecosystem ecologist with broad interests in understanding the functioning of ecosystems, how ecosystems respond to environmental change, and how processes in one ecosystem can affect surrounding systems. Most of the Wetland Biogeochemistry Lab's research takes place in tidal wetlands, an important component of the hydrological continuum that stretches from non-tidal rivers through tidal estuarine waters to the coastal ocean, although my intellectual interests span aquatic to terrestrial environments and freshwater to marine habitats. Depending on the research question, my work considers scales from microbial interactions to the effects of ecosystems on global climate.
The following sections describe the major focal areas in the lab right now. There is undeniably some overlap between the individual focal areas (e.g., saltwater intrusion is caused by sea level rise but both are treated separately below). I hope that the rationale for this (somewhat artificial) splitting of interrelated research topics becomes apparent. Without further ado, please explore the lab's current research projects:
For much of the last decade, I have been studying how tidal freshwater wetlands are affected by saltwater intrusion, the movement of saline water into regions that have historically been exposed to freshwater conditions. In the coastal zone, saltwater intrusion occurs due to sea level rise, low freshwater discharge, and following episodic events such as storm surges, all of which allow the influence of the ocean to be felt farther upstream. Our research in this area has involved long-term in situ wetland salinity manipulations, short-term laboratory experiments, and measurements in wetlands along existing estuarine salinity gradients. Sampling locations have been as far south as the South Carolina and as far north as New Jersey, with most current work taking place in local Virginia systems. Together with colleagues at VCU and elsewhere, we have approached saltwater intrusion from the scale of the composition and activity of soil microbial communities through ecosystem-scale exchanges of greenhouse gases between the wetland and atmosphere. You can read about some of our activities in general-audience articles written by VCU News that describe some of the science and our outreach efforts at a local elementary school. The following video shows us in action during a soil sampling event at our Cumberland marsh site on the Pamunkey River, Virginia:
The accumulation of mineral and organic matter in tidal marsh soils allows the marshes to build soil volume and accrete vertically; this is critical in allowing tidal marshes to keep pace with rising sea level. In tidal wetlands along the upper James River estuary, Virginia, we are installing a series of SETs (surface elevation tables) to answer the fundamental question of whether these wetlands are “keeping up” with sea level rise or not? Those measurements will provide a direct answer to that question but, in many ways, most of the research that I conduct is relevant to understanding how environmental changes affect wetland soil accretion. For example, I have addressed how the disturbances of saltwater intrusion and nutrient enrichment affect rates of decomposition that, along with organic matter production, play a key role in determining rates of organic matter accumulation and wetland accretion. The factors controlling the preservation of organic matter are especially important in tidal freshwater marshes, where organic matter accumulation accounts for more vertical accretion than does the accumulation of mineral sediments. However, that's not to say that mineral inputs are unimportant! With a new NSF grant, colleagues and I are exploring how historical changes in mineral sediment delivery to the coastal zone (e.g., due to afforestation and dam construction) have affected the ability of salt marshes to keep up with rising sea levels in marshes along the Atlantic Coast of the United States. We will use this information as part of a modeling effort to predict salt marsh vulnerability to future sea level rise.
Despite the brevity of this section, the question of "Can tidal wetlands keep up with rising sea levels?" is probably the most important question that coastal wetland scientists are studying. Quite simply, if a wetland falls behind in its race with sea level, the wetland will be flooded more and more, eventually reaching a point where the flooding frequency and/or depth of flooding are too great to support plant growth. When that happens, the tidal wetland will become a barren mud/sand flat (= no more wetland!) and the ecosystem functions provided by that wetland will be lost.
VCU’s Rice Rivers Center contains a 70 acre restored tidal freshwater wetland that is a fantastic resource for understanding how wetland processes change during ecosystem establishment. How closely does this recently restored wetland along Kimages Creek compare with the “pristine” tidal freshwater forested wetland in the adjacent Harris Creek system? To that end, we have installed an eddy covariance flux tower within the restored marsh so we can continuously determine rates of CO2, CH4, water vapor, and energy exchanges between the wetland and atmosphere. As a logical outgrowth of my interest in the effects of sea level rise, we are installing a series of SETs (surface elevation tables) in the tidal portion of the Rice/Kimages wetland, the nearby Harris Creek wetland, and other tidal wetlands in the upper James River. Are all of these wetlands are “keeping up” with sea level rise? Is the restored Rice wetland functioning similarly to natural wetlands in the region? Our work at the Rice Rivers Center wetland also relates to my interests in the effects of nutrient enrichment on wetland processes. I’m working with a graduate student who is fertilizing marsh plots with varying levels of nitrogen and phosphorus fertilizers to see if there are threshold responses and if the outcome of plant competition for nutrients differs between low and high nutrient loading rates.
The eddy flux tower was purchased with funds from the Higher Education Equipment Trust Fund (HEETF), with additional support for our other work at the Rice Rivers Center provided by the Center itself through awards to faculty and students.
Wetlands respond to climate change and also play an important role in affecting the rate of ongoing climate changes. Through the process of photosynthesis, wetland plants remove the greenhouse gas carbon dioxide (CO2) from the atmosphere and store that carbon in wood and wetland soils. However, wetlands are the largest single source of the greenhouse gas methane (CH4, also known as marsh gas) and can also emit the greenhouse gas nitrous oxide (N2O, also known as laughing gas). What is the overall role of wetlands with respect to global climate? Do they have a net warming or cooling effect and is this likely to change in the face of a changing environment?
Many of the environmental changes that our lab is studying have the potential to affect the uptake and/or release of greenhouse gases from wetlands and are therefore relevant to understanding the climatic role of wetlands. For example, some of our research is showing that saltwater intrusion generally decreases CH4 emissions from tidal freshwater marshes but also decreases carbon storage during some years (but not others). We are also working to understand the balance between greenhouse gas uptake and release following wetland restoration at the VCU Rice Rivers Center using an eddy covariance flux tower (see photo at the top of this section).
Ultimately, the balance between the amounts of greenhouse gases added to and removed from the atmosphere will determine the net climatic role played by wetlands (or other ecosystems) but the issue is complicated because each greenhouse gas differs in its ability to trap heat and also in the length of time the gas persists in the atmosphere. In a paper published in 2015 in the journal Ecosystems, we showed that the commonly-used global warming potential is flawed when applied for this purpose in studies on the climatic role of ecosystems. In our paper, we proposed two new metrics – the sustained-flux global warming potential (for use when ecosystems emit greenhouse gases) and the sustained-flux global cooling potential (when there is greenhouse gas uptake) – that may be more appropriate in the context of understanding ecosystem greenhouse gas fluxes. This article was recommended by a Faculty of 1000 reviewer and was summarized in a VCU News article.
The discovery of the Haber-Bosch process and the subsequent application of nitrogen-rich fertilizers to agricultural fields around the globe has significantly increased global food production; this is a good thing. However, inefficient and excessive fertilizer use has led to eutrophication of coastal waters and wetlands, with impacts on productivity, species composition, and ecosystem functioning. I am working with a graduate student who is studying at VCU’s Rice Rivers Center to determine how increased nutrient loading affects wetland plant productivity and interspecies competition. By fertilizing marsh plots in the restored wetland with varying levels of nitrogen and phosphorus fertilizers, we are going to see if there are threshold responses to fertilization and determine if the outcome of plant competition for nutrients differs between low and high nutrient loading rates (in other words, do the same species “win,” regardless of the level of nutrient availability?).
This builds on some of our earlier work that examined whether wetland plants and soil microbes are limited by the same or different nutrients. Working in wetlands from Rhode Island to Georgia that varied in salinity (two salt marshes, two freshwater wetlands), soils (two mineral soils, two organic soils), vegetation, and hydrology, we set up an array of field fertilization plots to selectively relieve nutrient limitation by adding nitrogen, phosphorus, or nitrogen+phosphorus. During regular sampling campaigns, my collaborators and I measured ecosystem carbon dioxide and methane exchanges, soil and porewater enzyme activities, soil mineralization, leaf-level photosynthesis, plant/soil nutrient contents, and porewater chemistry. At two of our tidal marsh sites, plant growth was limited by nitrogen, with one site showing secondary limitation by phosphorus availability. (We suspect that plant growth at a non-tidal freshwater site was limited by phosphorus, although the shrubby vegetation at that site grew so slowly that we did not see a response during the study period). In contrast, multiple lines of evidence suggested that soil microbes across the sites were limited by phosphorus, regardless of which nutrient limited aboveground primary production. Despite large differences in ecosystem-scale primary production and respiration among the different fertilization treatments in a single site, the ratio of gross primary production:respiration did not change as a function of fertilization, suggesting that selectively relieving the nutrient limitation of the primary producers or soil microbial heterotrophs does not affect the proportion of primary production that is sequestered in wetland soils.
The ongoing work at the Rice Rivers Center is funded by a student award from the Center. The differential nutrient limitation research was funded by the National Science Foundation.