Journal Highlights

North American Wild Rice Faces Sulfide Toxicity

From Eos.org: Research Spotlights

Researchers have developed a model to inform the regulation of sulfate levels, in freshwater environments, that are threatening the iconic plant. 

Wild rice (Zizania palustris) is a native North American marsh grass, which grows in shallow aquatic environments such as wetlands, the margins of lakes, and slow-flowing streams, its roots favoring soft organic sediments. 

Native American tribes across the Great Lakes region have harvested wild rice for millennia. It not only is a significant part of their diet but also has sacred significance in their culture.

Commercial cultivation, largely in paddy fields, began in Minnesota in the 1950s, and the distinctive long, slender black grains are now a popular superfood. 

However, wild rice is under threat from sulfate pollution. As a result of human activities, including wastewater discharge and mining, sulfate enters the waterbodies where wild rice grows. In time, it penetrates into the saturated soils below where the plant’s roots grow. In these anaerobic conditions, bacteria transform (or “reduce”) the sulfate into sulfide. High concentrations of sulfide are toxic to roots and inhibit plant growth. 

The Minnesota Pollution Control Agency faces a decision as to whether to regulate sulfate levels in order to protect wild rice. To guide this decision, Pollman et al. developed a model to better understand what influences sulfide concentrations in “porewater,” the water contained within soil. 

The relationship between sulfate in surface water and sulfide in porewater is somewhat complex, and the researchers’ modeling effort aims to capture that complexity. They started by creating a conceptual model of the relationship between sulfate and sulfide based on certain assumptions about processes and relationships between components and variables. 

Next, they selected appropriate subsets of data collected from lake and stream sites in Minnesota as part of a larger study that spanned the state. After running statistical analyses on these data, they expanded their conceptual model to include more factors. The results showed that three variables were equally important in determining sulfide concentrations: sulfate levels, organic carbon in the soil, and iron in the soil. 

The strength of their modeling approach rests in not only how it allows for a range of variables but also how it provides a framework for modeling direct, indirect (i.e., interactions mediated through a third variable), and feedback relationships. 

While the model was shown to be successful in capturing the complexity of relationships in the system, the researchers cautioned against using it to determine an absolute level of sulfide that is toxic to wild rice. Environmental and geochemical conditions in the many different waterbodies where wild rice grows are varied; thus they recommend applying a multiple binary logistic regression model to calculate sulfate thresholds appropriate to each wetland location. 

Control of sulfate levels is seen as the best intervention to reduce sulfide toxicity in freshwater environments, but this study offers a model that can be used to tailor regulation for greater efficacy. 

-- Jenny Lunn, Contributing Writer,