In recent decades, previously unobserved trace compounds have become more widely detected in wastewater treatment effluents and freshwater ecosystems. Emanating from various sources and presenting potential human health and ecological risks at much lesser concentrations than traditional contaminants, detection of “emerging contaminants” has increased with improvements in analytical techniques. The behavior of emerging contaminants in wetlands is a topic of increasing interest, as natural wetlands are known to transform and sequester pollutants and constructed or treatment wetlands are widely utilized to address elevated concentrations of constituents of concern. Both natural and constructed wetlands are complex biogeochemical systems with interrelated abiotic and biotic mechanisms leading to the removal of emerging contaminants. A literature review was performed to assess the current state of knowledge of various wetland mechanisms involved in removing these contaminants from surface waters and effluents. The primary mechanisms discussed in the literature are sorption, photodegradation, microbial biodegradation and phytoremediation. The most influential mechanisms are dependent on the properties of the contaminants and wetland systems studied. Common trends exist for different constructed wetland designs to leverage various mechanisms based on hydrology, substrate and vegetation plantings. Much remains to be understood about the various processes occurring in wetlands as they relate to emerging contaminant removal. Improving the understanding of the potential role of wetland mechanisms can help manage this environmental challenge more effectively.

Nitrous oxide (N₂O) is a potent greenhouse gas that contributes to stratospheric ozone depletion and global climate change. Soil denitrification has two potential end-products, N₂O and dinitrogen (N₂), and the ratio of these end-products (N₂O:(N₂O+N₂) or the N₂O ratio) is controlled by various factors. This study aims to quantify the influence of soil pH on the ratio of denitrification end-products in Oklahoma soils with different soil textures. Six natural grassland soils encompassing three distinct soil textures were incubated in the laboratory under natural and modified pH with an overall tested pH ranging from 2 to 10. Denitrification end-products were measured in the laboratory using the acetylene inhibition technique and further estimated using a process-based biogeochemical model. Both the laboratory and model results showed that soil pH and texture influenced the ratio of the denitrification end-products. Generally, as soil pH increased the N₂O ratio decreased, although both lab and model results indicated that this relationship was not linear. Soil texture may have an indirect effect on the N₂O ratio, as two soils of the same texture could have different N₂O ratios. However, clay percentage of the soil did show a linear positive correlation with the N₂O ratio, suggesting components of soil texture may be more influential than others. Overall, soil pH was a controlling factor in the ratio of denitrification end-products and the newly observed nonlinear relationship warrants further study, particularly when considering its effects in different soil textures.

Nitrous oxide (N₂O), which contributes to global climate change and stratospheric ozone destruction, can be produced during denitrification. Although the N₂O ratio, a measure of denitrification completion, is influenced by various properties, studies have largely been limited to site- or treatment-specific conclusions. The primary objective of this study was to identify important factors driving N₂O ratios and their relationships in soils by systematically reviewing and quantitatively evaluating results from published laboratory denitrification studies. A database with 60 studies (657 observations) was compiled, including studies meeting the minimum criteria: (i) laboratory experiments on soils, (ii) nutrient (carbon and/or nitrogen) addition, and (iii) N₂O and dinitrogen gas measurements. Of these, 14 studies (100 observations) had sufficient data for inclusion in the meta-analysis to assess the effect of added nutrients on the N₂O ratio. Furthermore, we modeled the effect of moderators on treatment effect by fitting a meta-regression model with both quantitative and categorical variables. Close review of studies in the database identified soil pH, carbon addition, and nitrogen addition as important variables for the N₂O ratio, but trends varied across studies. Correlation analysis of all studies clarified that soil pH was significantly correlated with the N₂O ratio, where soils with higher pH had lower N₂O ratios. The meta-analysis further revealed that nutrient addition had an overall significant, positive treatment effect (0.30 ± 0.03, P<.0001), indicating that experimentally adding nutrients increased the N₂O ratio. The model was most significantly improved when soil texture was used as a moderator. The significance of soil texture for the N₂O ratio was a major finding of this study, especially since the assays were usually conducted with soil slurries. Overall, this study highlights the importance of field soil properties (i.e., pH, texture) and laboratory conditions (i.e., nutrient addition) in driving the N₂O ratio and N₂O production from denitrification in soils.