I am currently a research scientist at the National Center for Water Quality Research at Heidelberg University. Prior to this, I was a postdoctoral research associate in Dr. Todd Royer's laboratory at Indiana University in Bloomington. I received my PhD from the Department of Biological Sciences at University of Notre Dame in Dr. Jennifer Tank's laboratory. I received a B.S. in Biology from Virginia Tech in 2002 where I worked at the Stream Team for 2 years. I was inspired to pursue research because of the support from Dr's. Jack Webster, Fred Benfield, and Maury Valett. Throughout my career so far, I have had the opportunity to pursue research in variety of different places and topics with numerous collaborators, which has served to stimulate my excitement about research particularly in stream ecosystems.
Current and former projects
The extent of soil phosphorus stratification in the Sandusky River Watershed
Increased dissolved reactive phosphorus (DRP) loads from agricultural watersheds over the past decade have stimulated the return of algal blooms to Lake Erie. Previous research suggests that the accumulation of phosphorus (P) on the surface of agricultural soils is linked to elevated DRP runoff. In this collaborative project among the NCWQR, the Sandusky River Watershed Coalition, and Seneca County Soil and Water Conservation District, we have measured the stratification of P in over 1400 agricultural fields in the Sandusky River Watershed using standard soil testing procedures (Mehlich 3 P, or M3P extractions). Preliminary results indicate that P stratification is prevalent in the Sandusky River Watershed. In 75% of all fields, surficial P (0–2 inches) was 7–176 ppm higher than mean M3P from 0–8 inches. Although the absolute level of surficial P was correlated with mean M3P, the highest levels of stratification were at soil test levels <100ppm. Our results indicate a surficial P soil test should be included in the P-index, a metric used to identify fields with high potential for DRP runoff to undergo targeted management. Click here for more information.
The Heidelberg Tributary Loading Program
The tributary loading program at the NCWQR is a one-of-a-kind long-term monitoring program that measures water quality at 15 stations in 9 different river watersheds whose area covers over 50% of Ohio and parts of Michigan and Indiana. This program has been widely influential and has provided essential information for those trying to understand long-term changes in nutrient and sediment loading from large river watersheds. I am lucky to now be involved in this program and look forward to helping analyze and present results from this large database. Click here for more information.
The influence of human land use on nutrient cycling
Agriculture and urbanization can lead to profound changes to stream ecosystems including nutrient enrichment, altered hydrology, and diminished riparian zones. To date, most investigations of human impacts on streams have focused on the role of inorganic nutrients (i.e. nitrate, ammonium, phosphate). For my dissertation research, I focused on the influence of human activities on organic nutrients and found that dissolved organic carbon (DOC) and nitrogen (DON) can cycle as rapidly as inorganic nutrients thereby potentially contributing to local and downstream eutrophication (Johnson et al. 2009a, Johnson and Tank 2009). In most cases, the potential for nutrients to cause eutrophication is associated with the autotrophic (algae and macrophytes) responses. Yet I found that heterotrophic components of stream biofilms were particularly sensitive to nutrient enrichment from agriculture and urbanization indicating both autotrophic and heterotrophic metabolic pathways influence eutrophication (Johnson et al. 2009b). Much of my dissertation research was conducted in conjunction with the Lotic Intersite Nitrogen eXperiment II (LINX II), a large collaborative project examining the fate of nitrate (NO3) in streams using 15NO3 stable isotope additions (Mulholland et al. 2008). I am leading two synthesis papers from LINX II examining the production of DON within streams and the ultimate fate for nitrate after removal from the water. Thus far, results suggest that DON production can be prevalent in stream ecosystems and challenges the paradigm that stream DON is only terrestrially-derived. Also, a majority of removed N was transformed into other solutes within days to weeks. Thus, it appears that streams are not simple conduits for N, but rather are highways with multiple rest stops— i.e., when NO3 enters a stream it is rapidly cycled within days to weeks into a variety of forms while spiraling downstream.
The interaction between carbon and nitrogen cycles in agricultural streams
Denitrification, the reduction of NO3 to N2 and N2O, is most commonly performed by facultative anaerobic bacteria in anoxic environments. Dissolved organic carbon is the electron donor in this reaction and is thought to play an important role in controlling denitrification, but many questions remain regarding the mechanisms behind DOC-denitrification interactions. For my postdoctoral research, I have been investigating the interactions among DOC, denitrification rates, and microbial community structure in an agricultural stream network. Using a combination of field studies, laboratory assays, and whole-stream manipulations, we have found that water-column dynamics are often separated from processes occurring in shallow sediments. More specifically, in these agricultural streams the diversity of denitrifying bacteria can influence denitrification rates (Baxter et al., 2012a), yet the availability of DOC in the water column has little influence on denitrification rates (Johnson et al., 2012). Further, preliminary results from a DOC enrichment conducted in southern Indiana indicated that DOC in the water column did not reach sediment porewater as little as 5 cm deep. Since denitrification appears to rely on sediment organic matter rather than water-column DOM as a source of carbon, these results have implications for efforts to influence biogeochemical processes in streams through management of DOC. The use of ecosystem experimentation in conjunction with isotopic tracer methods and molecular biology has proven to be a powerful approach for mechanistically addressing complex biogeochemical interactions – this is an approach I plan to use and expand upon in my future research.
Greenhouse gas emissions from agricultural watersheds and the role of artificial subsurface drainage
As a part of the LINX II project, we found that streams were almost always a source of N2O, a potent greenhouse gas, to the atmosphere and the highest N2O emissions were in streams influenced by human land use (Beaulieu et al. 2010). In addition, recent research suggests a substantial amount of greenhouse gases (GHG) in agricultural watersheds are emitted indirectly through the artificial subsurface drainage systems (tile drains) used extensively throughout the poorly-drained soils of the Midwestern U.S. I am currently involved in a project to determine the importance of indirect emissions of GHG’s from tile drains and the stream in an agricultural watershed. Tile drains are porous pipes installed ~1m below the soil surface that convey excess water to a nearby ditch in order to maintain soil conditions suitable for agriculture. Biological activity in the soil generates GHG’s that can be directly emitted to the atmosphere or can become dissolved in soil water, transported through tile drains, and emitted to the atmosphere upon discharge of the drainage water to a ditch or stream. Although greenhouse gas fluxes from agro-ecosystems are a substantial contributor to the atmosphere, indirect emissions from tile drains are not currently incorporated into estimates of greenhouse gas emissions from agricultural areas. Preliminary results indicate that the tiles and stream are consistent sources of GHG’s to the atmosphere, which could have implications for regional GHG inventories, land management, and our basic understanding of biogeochemistry in agro-ecosystems. The results are currently in preparation for publication and I anticipate these results will be the foundation for a proposal to examine indirect GHG emissions across the Midwest region.
The importance of riparian zones in stream nutrient retention
In agricultural streams, small riparian zones within the incised channel act as mini-floodplains when inundated during high flows. These mini-floodplains, or benches, help reduce water velocity and increase interactions between water column NO3 and surfaces colonized with denitrifying bacteria that can permanently remove NO3. As a part of a Nature Conservancy (TNC) funded project conducted at the University of Notre Dame, we found that expanding the area of riparian benches enhances NO3 removal, but the proportion removed is minimal when NO3- concentrations are exceptionally high (Roley et al., 2012). I also helped mentor a master’s student at Indiana University who examined denitrification in naturally-formed riparian benches in an agricultural ditch. His research showed that denitrification was stimulated during, and up to two days following, high flow events (Weatherwax et al., 2010 presentation). This project has been expanded by another master’s student who is examining how these patterns are mediated by microbial community structure. As it appears that riparian zones in agricultural streams can be important areas for denitrification and other biogeochemical process, I intend to pursue this research in the future.
Restoration of stream ecosystem function
Stream ecosystems provide a number of important services to society including clean water for drinking, recreation, and aesthetics; however, these services are often diminished as a result of human activities. Stream restoration is a common practice to help improve stream ecosystem functions, yet the effectiveness of restoring streams is not well documented. Currently, there is a large-scale restoration of the Fawn River in northern Indiana. In 1998, an enormous volume of organic sediment was released from a fish hatchery reservoir into the Fawn River, which covered the gravel sediments with up to 2 meters of mud in some locations. My collaborators and I are monitoring how the removal of organic sediment using a water jet/suction system may restore the interaction of the water column with the hyporheic zone, which is the zone in the sediments beneath the stream where shallow groundwater and surface water mix. In previous research on an agricultural ditch restoration, we found that increased interactions between the water-column and the bottom of the stream (i.e. the benthos) enhanced NO3- removal via denitrification (Roley et al., 2012). We predict increased interactions with the hyporheic zone will lead to a similar result and enhance ecosystem services such as nutrient retention. Further, this is one of few restorations aiming to restore interactions between surface and subsurface water, which could inform future efforts at restoring streams with limited hyporheic zones as found in many agricultural and urban ecosystems. This project began recently and is an ongoing research topic I plan to continue pursuing in the future.
The Lotic Intersite Nitrogen eXperiment II (LINX II)
I was lucky to be involved in the LINX II project during my PhD- a project that examined the influence of human land use on the fate of nitrate in streams. For more on the overall project, check out the project's website: http://www.faculty.biol.vt.edu/webster/linx/.