Riparian plant biodiversity reduces stream channel migration rates in three rivers in Michigan, U.S.A.

Recent work has shown that the biodiversity of organisms can influence geophysical processes such as the transport of streambed sediments and the erosion of soils. Yet most of this work has been conducted in smallâ scale fluvial system mimics, demonstrating a clear need to investigate the relationship between biodiversity and erosion in natural systems. We conducted an observational field study across 3 rivers in forested watersheds in northern Michigan, U.S.A., quantifying streambank retreat rates using aerial photos and measuring riparian plant community biodiversity and abundance. We hypothesized that more diverse riparian plant communities would produce greater woody plant stem density and basal area, which in turn would reduce erosion rates of streambanks due to increased root production. We used structural equation modelling to compare causal networks using plant biodiversity metrics to predict streambank migration rate indirectly through effects on plant abundance, as well as models that used migration rate to predict plant abundance indirectly through effects on plant biodiversity. Although structural equation models explained both causal pathways successfully, models using biodiversity to predict migration rate were a better fit to data than models that used migration rate to predict plant biodiversity and abundance. The best performing models suggested plant biodiversity was indirectly negatively correlated with erosion rate (average standardized path coefficient = â 0.22), after accounting for environmental differences between sites. This work adds to a growing body of evidence indicating that biodiversity can modify geophysical processes, demonstrating the need to explicitly account for biological variation when considering ecogeomorphic feedbacks.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/144301/1/eco1972_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/144301/2/eco1972.pd

Water Temperature, pH, and Road Salt Impacts on the Fluvial Erosion of Cohesive Streambanks

Increasing human populations and global climate change will severely stress our water resources. One potential unforeseen consequence of these stressors is accelerated stream channel erosion due to increased stream temperatures and changes in stream chemistry, which affect the surface potential and hence the stability of soil colloids. The objectives of this study were to determine the effect of water temperature, pH, and salinity on streambank erosion rates; determine how erosion rates vary with clay mineralogy; and, explore the relationship between zeta potential and erosion rate. Remolded samples of natural montmorillonite- and vermiculite-dominated soils were eroded in a recirculating hydraulic flume under multiple shear stresses (0.1–20 Pa) with different combinations of water temperature (10, 20, and 30 °C), pH (6 and 8), and deicing salt (0 and 5000 mg/L). The results show that erosion rates significantly increased with increasing water temperature: a 10 °C increase in water temperature increased median erosion rates by as much as a factor of eight. Significant interactions between water pH and salinity also affected erosion rates. In freshwater, erosion rates were inversely related to pH; however, at high salt concentrations, the influence of pH on erosion rates was reduced. Results of this study clearly indicate water chemistry plays a critical role in the fluvial erosion of cohesive streambanks and suggest that channel protection efforts should include controls for stream temperature, in addition to peak flow rates, to maintain channel stability

Influence of Sample Holding Time on the Fluvial Erosion of Remolded Cohesive Soils

Despite extensive research on bridge scour and channel erosion, predicting the occurrence and rate of cohesive soil erosion remains problematic. The lack of standard procedures for sample preparation and testing has resulted in wide variations in testing conditions, devices, and soil properties across erosion studies, ultimately preventing the synthesis of cohesive erosion studies and progress in understanding the fundamental processes of cohesive soil erosion. Therefore, the objective of this study was to evaluate the effects of sample holding time on the fluvial erosion of remolded cohesive soils to inform the development of standard testing procedures. Three different soils (fat clay, lean clay, and silty sand) were tested in a flume following multiple sample holding times. Results show that erosion rate can decrease 85-95% within 72 h of soil wetting, depending on clay mineralogy. These results highlight the importance of maintaining a consistent soil preparation protocol in cohesive soil erosion experiments and reporting soil sample holding durations when conducting cohesive erosion research using remolded samples

Fluvial Erosion Rate of Cohesive Streambanks is Directly Related to the Difference in Soil and Water Temperatures

Despite decades of research in the field of cohesive soil scour, a major challenge in water resource engineering is an understanding of the fundamental processes governing the erosion of cohesive streambank soils. Given that cohesive soil erodibility is affected by many factors simultaneously, it is necessary to study these factors independently to obtain insights into the erosion process. Three natural soils with different mineralogies were chosen for this study: montmorillonite-dominated fat clay, vermiculitedominated lean clay, and kaolinite- and illite-dominated silty sand. The soils were remolded at maximum dry densities and optimum moisture contents and subjected to 15-min erosion tests in a laboratory flume. Erosion tests were performed at water temperatures of 15 and 25°C and corresponding soil temperatures of 0, 15, and 25°C, and 15, 25, and 40°C. Test results show that, irrespective of soil type, erosion rate increased with an increase in water temperature but decreased with an increase in soil temperature. When soil and water temperatures were equal, there was no significant change in erosion rate (α = 0.05). Further analyses showed that, irrespective of soil type, erosion rate was a function of the difference in soil and water temperatures and not either temperature alone, indicating that the important thermal factor in the erosion process was the difference in soil and water temperatures. These results show the importance of accounting for soil and water temperatures in erosion studies and suggest that the use of stormwater control measures to control runoff temperatures may be necessary to combat streambank degradation resulting from urbanization

Engineered and Environmental Controls of Microbial Denitrification in Established Bioretention Cells

Bioretention cells (BRCs) are effective tools for treating urban stormwater, but nitrogen removal by these systems is highly variable. Improvements in nitrogen removal are hampered by a lack of data directly quantifying the abundance or activity of denitrifying microorganisms in BRCs and how they are controlled by original BRC design characteristics. We analyzed denitrifiers in twenty-three BRCs of different designs across three mid-Atlantic states (MD, VA, and NC) by quantifying two bacterial denitrification genes (<i>nirK</i> and <i>nosZ</i>) and potential enzymatic denitrification rates within the soil medium. Overall, we found that BRC design factors, rather than local environmental variables, had the greatest effects on variation in denitrifier abundance and activity. Specifically, denitrifying populations and denitrification potential increased with organic carbon and inorganic nitrogen concentrations in the soil media and decreased in BRCs planted with grass compared to other types of vegetation. Furthermore, the top layers of BRCs consistently contained greater concentrations and activity of denitrifying bacteria than bottom layers, despite longer periods of saturation and the presence of permanently saturated zones designed to promote denitrification at lower depths. These findings suggest that there is still considerable potential for design improvements when constructing BRCs that could increase denitrification and mitigate nitrogen export to receiving waters

Plant biodiversity effects in reducing fluvial erosion are limited to low species richness

It has been proposed that plant biodiversity may increase the erosion resistance of soils, yet direct evidence for any such relationship is lacking. We conducted a mesocosm experiment with eight species of riparian herbaceous plants, and found evidence that plant biodiversity significantly reduced fluvial erosion rates, with the eight‐species polyculture decreasing erosion by 23% relative to monocultures. Species richness effects were largest at low levels of species richness, with little increase between four and eight species. Our results suggest that plant biodiversity reduced erosion rates indirectly through positive effects on root length and number of root tips, and that interactions between legumes and non‐legumes were particularly important in producing biodiversity effects. Presumably, legumes increased root production of non‐legumes by increasing soil nitrogen availability due to their ability to fix atmospheric nitrogen. Our data suggest that a restoration project using species from different functional groups might provide the best insurance to maintain long‐term erosion resistance.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/117138/1/ecy1222_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/117138/2/ecy1222.pd