CAREER: Introducing Dynamic Sulfur Chemistry into Hydrogels to Promote Water Retention and Healthy Microbe Growth in Soil

NON-TECHNICAL SUMMARY

The world’s population is growing rapidly, requiring the dramatic expansion of food production. Combined with increased urbanization, climate change, and competition for water, this poses an extreme challenge for the agricultural industry. These changes alone are difficult to manage, much less achieve sustainably, with limited impacts on economics, public health, and surrounding ecosystems.

Agriculture is the top consumer of water in the United States. With much of the western U.S. experiencing extreme drought in recent years, the water we do have must be used more efficiently. Although it is established that sulfur provides many benefits to plants, until recently, there has been ample sulfur deposition into soils from burning fossil fuels.

As society moves away from petroleum and air quality standards improve, sulfur deposited in soil will continue to decrease, increasing the pressure to add sulfur-based fertilizers to crops. Despite the necessity of sulfur for successful crop production, it also poses a pollution risk. Since commonly used fertilizers have less than 50% uptake by crops, sulfur-based fertilizers are also unlikely to be fully assimilated and could leach into surface waters, negatively impacting the environment.

This work aims to create absorbent, sulfur-based materials to aid in water retention and provide controlled delivery of key nutrients, including sulfur, to the soil. More effective nutrient delivery to plants could enhance crop production while limiting the impact on surrounding ecosystems. Exposing these materials to common bacteria found in soil will help determine the impacts of these materials on microbes in the soil.

Additionally, this research provides undergraduates with the training and mentoring necessary to join the STEM workforce after graduation. It also establishes a summer research experience (SRE) focused on scientific communication for all research students in the Chemistry Department. The SRE will enhance students’ ability to effectively learn from and share data with other scientists as well as better share their results with the general population.

Over time, the SRE will include underserved and low-income high school students in the region creating connections between them and higher education in STEM. TECHNICAL SUMMARY

Sulfur has been used for centuries as a pesticide in agriculture. However, its application as a fertilizer has not been considered until recently. Burning fossil fuels has deposited sulfur into the environment over many decades.

As society moves away from petroleum and air quality continues to improve, sulfur deposited into the soil will decrease, increasing the pressure to add sulfur-based fertilizers to crops. Inverse vulcanization (IV) has enabled the expansion of sulfur-based materials over the past decade. IV uses the heat labile bonds in S8 to act as the initiator, solvent, and monomer, leading to polysulfides with sulfur-contents ranging from 20-90%.

However, the hydrophobicity of S8 makes polymerizing polar monomers challenging, yielding very few materials that can interact with water. Garlic essential oil (GEO), rich in diallyl disulfides, offers a strategy to combine sulfur with polar monomers. By introducing dynamic sulfur-chemistry into a hydrophilic gel platform, this work aims to develop a new class of sulfur-based polymers and provide insight into the role of sulfur-loops, GEO, and the 3D polymer structure on the material properties.

These materials will be exposed to microbes, known to perform redox chemistry with sulfur, to determine if they can utilize the sulfur within these gels. Further analysis will connect our understanding of the polymer structure and oxidation state of sulfur to the bacterial response and provide insight into bacterial modification of the polysulfides.

Additional testing with soil bacteria will be performed to begin to understand how sulfogels impact the soil microbiome. Further analyses will determine if microbes are able to access the sulfur within the polymer and convert it to a form accessible to plants such as sulfates. These analyses would aid in the development of materials that can provide the controlled delivery of water, nutrients, and beneficial bacteria to crops.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.