Carbon change in agricultural soils

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 measurements from long-term field experiments at Iowa State in conjunction with mechanistic models to understand where, when, and why soil carbon gains (or losses) can occur in conventional and diversified Corn Belt agricultural systems. In particular, we use measurements of the natural variation in stable isotopes of carbon in plants and soils to gain a richer understanding of how agricultural and conservation practices alter soil carbon cycling. A related study examines rates of soil carbon accrual in riparian buffers, using isotope measurements to understand the sources of and fate of carbon inputs. This work has been funded by USDA NIFA AFRI and the Iowa Nutrient Research Center. Representative publications: Ye and Hall (2020), Hall et al. (2019).

Wetland soils in agricultural landscapes

Are cropped depressions hotspots of nutrient losses and greenhouse gas emissions?

Much of the Midwestern landscape is comprised of poorly drained soils that supported wetlands prior to European settlement and have now been drained to allow row crop production. Many of these former wetland soils continue to pond intermittently following precipitation events, often causing crop failure, and potentially exacerbating leaching losses of nitrogen and phosphorus to downstream waters and increasing the emissions of the greenhouse gases nitrous oxide and methane. Our collaborative work with several other ISU colleagues has focused on quantifying the environmental impacts of depressions cropped with corn and soybeans, and on the capacity of perennial plants to mitigate these impacts. This work has been funded by USDA NIFA AFRI, the Iowa Nutrient Research Center, and the Leopold Center for Sustainable Agriculture. Representative publications: Lawrence et al. (2021), Yu et al. (2021), Lawrence and Hall (2020), Hall et al. (2018)

Fundamentals of soil organic matter (SOM)

How do biological and geochemical processes influence the composition and persistence of SOM at continental scale?

Despite more than a century of research, the factors that control the molecular composition and abundance of soil organic matter remain debated in the scientific community. We have used samples from the National Ecological Observatory Network 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 are interested in resolving the contentious role of lignin, a complex plant polymer, in influencing the decomposition of dead plant material and its ultimate contributions to SOM. This work has been funded by the NSF Macrosystems Biology Program. Representative publications: Yu et al. (2021, 2022), Hall, Ye, et al. (2020), Hall et al. (2020)

A new approach to measure soil nitrogen losses

Can we detect effects of agricultural practice changes on nitrate leaching and nitrous oxide emissions?

Mitigating the environmental impacts of nutrient losses requires the ability to measure whether a given agricultural management strategy has a significant impact relative to business-as-usual practices. Unfortunately, detecting small changes in nitrate leaching and nitrous oxide emissions remains challenging in typical field experiments, due to difficulties in quantitatively measuring drainage water and accounting for spatial variability in soil properties. With ISU colleagues and the Iowa Nutrient Research and Education Council (INREC), we have developed a unique infrastructure where replicate intact soil blocks (5’ x 5’ x 4’) are enclosed on the sides and bottom in a welded steel box, each with a single drain that allows us to capture the complete drainage volume for chemical analysis. Corn is grown in these soil blocks under representative field conditions. We are using this unique facility to test the potential of a novel microbial source of nitrogen to reduce nitrate leaching and nitrous oxide emissions relative to traditional nitrogen fertilizer. This work has been funded by INREC.

Oxygen variability in soil

How does redox cycling affect SOM persistence?

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. This work has been funded by the NSF Ecosystems program. Representative publications: Chen et al. (2020), Huang et al. (2021, 2020), Huang et al. (2017).

New biogeochemical measurements

Precise, robust, and low-cost measurement methods for biogeochemical analyses

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 recently began measuring soil nitric oxide emissions (aka NOx) using a custom dynamic chamber system plumbed to an optical NOx analyzer, as part of a collaborative USDA NIFA AFRI project 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.