Afterlife of Leaves: Professor Allison Gill on Litter Decomposition and Soil Formation for Log Lunch

Pictured: Log Lunch lecturer Allison Gill smiles at the camera.

March 11 marked biology professor Allison Gill’s first Log Lunch lecture to a packed room of students and faculty, who turned out en masse for her talk “Afterlife of Leaves: Effects of Anthropogenic Nitrogen Fertilization on Plant Litter Decomposition and Soil Formation.” Gill discussed her most recent research, which employed a previously unused empirical model to reveal how nitrogen fertilization impacts plant litter decomposition, an important component of the global carbon cycle.

Pictured: Lecturer Allison Gill presents to the audience.

Gill opened her presentation with an overview of how carbon is stored in soils across the planet, made possible by the decomposition of leaf litter through microbes that degrade the physical matter and uptake carbon while they are alive, but release carbon dioxide once dead. Carbon is preserved in soils through chemical interactions, physical entrapment, and biochemical processes, a complex system that manages the chemical uptake and loss in terrestrial ecosystems.

In leaf litter decomposition, these mechanisms are altered by the presence or absence of fertilizers. Nitrogen is one powerful fertilizer with a dramatically increased presence in global systems, as a result of its widespread use for anthropocentric farming. Gill explained that a hypothesis based on a basic biochemical understanding of the process suggested that adding nitrogen would increase microbial activity and speed up litter decomposition. However, research in 2005, 2010, and 2018 asserted that nitrogen did not influence litter decay at all.

Pictured: The Log Lunch menu is displayed on flip paper.

For Gill, these conclusions were at odds with how such a process should play out in reality. She found that all the researchers used the same exponential model, which wasn’t capable of allowing for “flexibility in decomposition rates through time.” By utilizing a more complex alternative that could show change temporally and applying it to over 500 distinct decomposition data sets globally, she found that nitrogen-induced litter decomposition accelerated the process early on but slowed significantly in the later stages.

“This was global evidence that nitrogen fertilization changed the dynamics of litter composition… not just on tree leaves, but any type of litter,” she said.

Pictured: A plate of salad, lasagna, artichoke dip, and baguette.

Further research that Gill conducted along with other scientists across 20 different sites across the world found that microbial biomass had increased as a result of nitrogen influence, as well as the formation of minerals associated with soil organic matter. At the Hopkins Memorial Forest in Williamstown, Gill continues researching how nitrogen influences the storage or release of carbon along with undergraduate students at Williams. This research holds important implications for the relationships between litter decomposition, carbon sequestration, and soil organic matter in an era of climate change and fertilizer utilization.

The lecture was accompanied by a hearty meal of vegetable lasagna, artichoke dip and freshly-baked baguettes, and a fresh winter citrus salad. Spinach, artichokes, and onions were sourced from a local farm. Sweet brown butter toffee chocolate chip cookies followed the lunch.


Log Lunch is a CES program hosted every Friday at noon. During Log Lunch, a vegetarian meal prepared by Williams students is served, followed by a talk on an environmental topic. Speakers are drawn from both the student body and faculty of Williams, as well as from local, national, and international organizations. Learn more here.