RESEARCH
Grassland ecosystem recovery from chronic nitrogen fertilization
For my PhD, I worked with the Konza Prairie Long Term Ecological Research (LTER) program to study the changes in plants, soils, and microbes after ceasing long-term fertilization. By reversing this treatment, I was able to assess the mechanisms of ecosystem recovery from long-term nitrogen (N) fertilization, and how long fertilization legacies persist in this ecosystem. In addition, the field experiment was designed to study the direct and indirect effects of fire as a driver of N dynamics.
What we learned is that chronic N fertilization (a total of 300 g of N per square meter added!) changes above- and belowground dynamics. Plant production remained high in the annually burned prairie plots that never dissipated during my time at Kansas State University, and this was driven by the replacement of big bluestem (Andropogon gerardii) with switchgrass (Panicum virgatum). Additionally, plant and microbe available N was elevated due to more N-enriched soils which contributed to higher rates of nitrification and denitrification - though these legacies weakened over time. Microbial community composition is more sensitive to fertilization in the unburned prairies related to the lack of fire supporting greater background N availability. Surprisingly, fire didn't accelerate ecosystem recovery.
Nieland & Zeglin. Plant and microbial feedbacks maintain soil nitrogen legacies in burned and unburned grasslands. Journal of Ecology (2024). 10.1111/1365-2745.14386
Nieland, Moley, Hanschu, & Zeglin. Differential Resilience of Soil Microbes and Ecosystem Functions Following Cessation of Long-Term Fertilization. Ecosystems (2021). 10.1007/s10021-021-00633-9
Microbial functional legacies in restored grassland systems
Most of the prairies in the US Midwest has been transformed to support row-crop agriculture, with only a few pockets of native prairies left on the landscape. There is growing interest among farmers to integrate strips of prairie on their farms to mitigate against nutrient runoff and support biodiversity. How effective this management strategy is, and how long it takes belowground functions to recover, remains an outstanding question.
We know that previous land management can alter how soil microbes function. But why this happens, and how persistent these effects last, is not known. In a project funded through a USDA postdoctoral fellowship, I'm leveraging a sequentially restored prairie chronosequence at the Konza Prairie Biological Station. We are asking which ecosystem C and N pools and processes are less or more resilient to previous agricultural practices, what mechanisms prevent or promote recovery, and if soil microbial functions recover fully as if they were never subjected to agriculture.
Nieland & Keiser. Using ecological restoration to disentangle the mechanisms and longevity of soil functional legacies. Functional Ecology (2025). 10.1111/1365-2435.70123
Coupled carbon-nitrogen dynamics
Just like us, soil microbes are made of a constrained ratio of carbon and nutrients. Many microbial heterotrophs release enzymes into their extracellular environment to breakdown organic matter so they can acquire nutrients to maintain this ratio. Other microorganisms, though, harvest energy through other metabolic pathways, and thus don't require energy from organic sources. This differential demand for N, in particular, can result in intense microbe-microbe competition that controls the availability and form of N. Ambient environmental conditions are likely to alter this competition, and it's starting to become clear that human activities can also alter competition.
Much of my research looks into the investment of soil microbial communities to break down organic material, and how human activities affects this process. Soil microbial communities can adjust their relative investment to synthesis enzymes based on nutrient availability. But, atmospheric deposition of N can shift competition - under higher N deposition rates, nitrifiers can use this N source to carry out their chemolithoautotrophic lifestyle, suggesting weaker competition.
In addition to 'natural' ecosystems, I'm also looking at the decomposition dynamics of cover crops in managed ecosystems. We are using this framework of C-N coupling to study how we can better manage agricultural landscapes to reduce N runoff and increase crop yield.
