The environmental roles of cryptic and restored wetlands in agricultural landscapes

Decreasing nutrient losses and greenhouse gas emissions by targeting hydric soils, in-field and edge-of-field wetlands, and riparian zones

In agricultural landscapes, interfaces between land and water are critical leverage points for decreasing water pollution and greenhouse emissions. Hydric soils and partially drained wetlands within crop fields can be hotspots of nutrient leaching and greenhouse gas emissions, and impacts may be addressed with targeted sub-field management and conservation. At the field edge, we are studying the role of wetlands and riparian buffers in mitigating direct and indirect greenhouse gas emissions and nutrient export to downstream waters. This work is a large collaborative effort involving numerous colleagues at ISU and other institutions.

Funding sources: USDA NIFA (2018, 2023), US EPA (2017), Iowa Nutrient Research Center (2016, 2018), Leopold Center for Sustainable Agriculture (2016)

Representative publications: Hall et al. (2023), Lawrence et al. (2021), Yu et al. (2021)

Carbon change in agricultural soils (or, is it really about the nitrogen?)

Where and why do perennials and diversified cropping systems increase soil carbon storage?

It is often assumed that agricultural systems which incorporate perennial plants, cover crops, reduced tillage, and complex crop rotations can increase the storage of carbon in soil organic matter (aka, “carbon sequestration”). However, detecting soil carbon change remains challenging, and increases in carbon storage following management change are not guaranteed. We are using long-term field experiments at Iowa State and mechanistic models to understand where, when, and why soil carbon gains can occur. In particular, we use measurements of the natural variation in stable isotopes of carbon in plants and soils to gain a richer understanding of mechanisms by which how agricultural and conservation practices alter soil carbon and nutrient cycling.

Take-home point: By altering soil metabolism and decreasing external nutrient requirements, diversified cropping systems may have tremendous benefits for water quality and climate, even when there is no change in soil carbon.

Funding sources: USDA NIFA (2020), Iowa Nutrient Research Center (2020), Center for Global and Regional Environmental Research (2017)

Representative publications: Ye and Hall (2020), De et al. (2020), Huang et al. (2023)

Fundamentals of soil organic matter (SOM)

How do biological and geochemical processes influence the composition and persistence of SOM at site to continental scales?

Despite more than a century of research, the factors that control the molecular composition and abundance of soil organic matter remain hotly debated in the scientific community. We have used samples from the National Ecological Observatory Network, and from our local agricultural ecosystems, to ask basic questions about what soil organic matter is, at a molecular level, as well as how biological, geochemical, and physical factors interact to control its distribution. In particular, we have focused on where, when, and why mineral phases can protect organic matter vs. stimulating its decomposition. We have also addressed the contentious roles of lignin, a complex plant polymer, in influencing plant litter and SOM decomposition. In short, controversies about the sources and persistence mechanisms of SOM can be partially reconciled by accounting for variation in climate, plant/microbial, and geochemical factors at site to continental scales.

Funding sources: NSF Macrosystems Biology (2018)

Representative publications: Hall et al., (2022), Yu et al. (2021), Hall et al. (2020)

Evaluating practices to mitigate nutrient leaching through improved measurements

Evaluating impacts of microbial amendments and nitrogen management on nutrient leaching and greenhouse gas emissions at plot scale

Numerous practices have been proposed to decrease N and P leaching from row crop agriculture, yet evaluating their effectiveness is challenged by the difficulties in achieving accurate and precise measurements of nutrient leaching at the scale of research plots. With ISU colleagues, we have developed a new alternative to traditional drainage plot studies that enables quantitative collection of drainage water from intact soil monoliths planted to corn. We are using this system to evaluate the environmental benefits that might be achieved by reducing synthetic N fertilizer and partially replacing it by N fixed through microbial sources.

Funding sources: Iowa Nutrient Research and Education Council (2018)

Oxygen variability in soil

How does redox cycling affect SOM persistence and greenhouse gas emissions?

Many soils experience zones of oxygen depletion that vary over space and time. Limited oxygen availability is thought to decrease the microbial decomposition of organic matter, and this phenomenon is well known to contribute to organic matter persistence in some wetland and sediment environments. However, the impact of oxygen variability in terrestrial soils is less well understood. Oxygen limitation affects interactions between SOM and minerals, particularly iron oxides, which may increase or decrease rates of organic matter decomposition and greenhouse gas production in soil. Resolving these mechanisms is key for understanding the potential climate feedbacks of soils from humid environments.

Funding sources: NSF DEB, Ecosystems program

Representative publications: Chen et al. (2020), Huang et al. (2021, 2020), Huang et al. (2017)

Advancing biogeochemical measurements to address new questions in agroecosystems

Advances in biogeochemical measurement approaches

Our group likes to tinker. We have refined or developed methods for trace gas analyses to support the research themes described above, and to explore new questions. We developed a low-cost and robust automated chamber method for measuring N2O emissions in very wet soils, where commercially available automated chambers are prone to failure. We recently began measuring soil nitric oxide emissions (aka NOx) using a custom dynamic chamber system as part of a collaboration with colleagues at NYU, Cornell, and NCAR. This project seeks to model the contributions of Corn Belt soils to tropospheric ozone and the resulting impacts on crop productivity. We also developed a fully automated incubation apparatus to measure CO2 fluxes and carbon isotope composition from hundreds of cores, which revealed the importance of cover crops for the metabolism of deep agricultural soils.

Funding sources: USDA NIFA (2018 and 2021), Iowa Nutrient Research Center (2020)

Representative publications: Lawrence and Hall (2020), Ye and Hall (2020)