Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article


Master of Science




Henry, Hugh A. L.


Climate warming and atmospheric nitrogen deposition, two elements of global change, are expected to exert strong effects on northern temperate ecosystems over the next century. I added new nitrogen addition and warming plots to a pre-existing nitrogen and warming field experiment in London, Ontario to compare the short-term (1-2 year; new plots) versus long-term (14-15 year; old plots) treatment effects on soil carbon and microbial activity. I used soil density fractionation and size fractionation to separate soil carbon fractions and analyzed carbon quality using Fourier-transform infrared spectroscopy (FTIR). I used extracellular enzyme assays to assess microbial activities. The soil organic matter free light fraction recovery increased with nitrogen addition in the old plots but decreased in the new plots. Interactions between warming and plot age were significant for some hydrolase enzymes. These results confirm short-term responses of soil carbon and microbial activity differed from long-term responses in this field experiment.

Summary for Lay Audience

Since the industrial revolution, the burning of fossil fuels and agricultural intensification have increased globally. These activities have accelerated global change through the release of greenhouse gases, such as carbon dioxide (CO2) and methane (CH4), and increased nitrogen pollution. Greenhouse gases trap radiation from the sun in the form of heat, ultimately increasing the Earth’s global surface temperatures. In the coming decades, average global surface temperatures are expected to increase around 2 ℃, with the greatest warming effect towards the poles. In addition to warming, nitrogen pollution threatens the surrounding environment when excess nitrogen enters a system. Nitrogen-based fertilizers are often applied excessively to agricultural soils as nitrate, ammonia, urea, and/or ammonium, and can have non-specific effects, such as runoff contaminating aquatic environments. Nitrogen released to the atmosphere also can be deposited back into ecosystems through atmospheric nitrogen deposition. I was interested in understanding how soils would respond to warming and nitrogen addition over time. To study this, I conducted a long-term field experiment in a temperate old field to examine the short-term (1-2 year) versus the long-term (14-15 year) responses of soil organic matter (primarily decomposing plant material) and microbial activities to warming and nitrogen treatments. Rather than comparing short-term responses from previous studies with long-term responses, new treatment and control plots were established to control for weather variation over time. Overall, long-term plots exhibited stronger treatment responses than the short-term plots. Nitrogen addition enhanced the accumulation of organic matter in the long-term plots for the free light fraction of organic matter in soil, which is comprised mostly of root fragments. Warming alone had no effect on organic matter accumulation but, when combined with nitrogen addition, organic matter in soil aggregates (i.e. small soil clumps protected from microbial decomposition) was greater. Lastly, both warming, and nitrogen addition treatments affected microbial enzyme activities, but carbon-acquiring enzymes, targeting easily decomposable organic matter, responded more to treatments than the enzymes produced to target more difficult substrates. Results from my thesis suggest that short-term treatment responses cannot be extrapolated to the long-term, and that cumulative responses can occur in the long-term.