Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Biology

Supervisor

Hugh Henry

Abstract

Climate change is expected to alter the intensity and dynamics of soil freezing as a result of increased air temperatures and reduced snow cover. Soil freezing can influence ecosystem nitrogen (N) cycling by damaging plants and soil microorganisms, but little is known about how soil freezing effects on ecosystem N cycling may combine or interact with increased atmospheric N deposition, which is also expected to exert a strong influence on terrestrial ecosystems in the coming decades. The objective of my thesis was to examine the combined and possibly the interactive effects of climate induced changes in soil freezing and N addition on plant productivity, soil microorganisms, and soil nutrient cycling in a grass-dominated temperate old field ecosystem. First, using 15N tracer, I investigated N retention by different nitrogen pools (plant, litter, roots, soil and simulated N deposition) in response to soil freezing under current and projected future atmospheric N deposition rates. My results indicated that soil freezing can increase N losses from soil over the winter and from atmospheric N deposition during the growing season, with the latter occurring due to decreased plant productivity. Second, I combined increased freezing (both in controlled environment chambers and in response to snow removal in the field) with N addition to explore whether soil freezing effects are mostly transient (i.e. over winter and spring melt), or whether there are legacy effects of freezing that continue over multiple years. My results indicated that the legacy effect of soil freezing reduced plant productivity over multiple years, but that N addition counteracted these declines in plant productivity. With respect to soil responses, freezing only caused short term (over winter) increases in extractable nitrogen pools, although there were also declines in fungal biomass during the second growing season as a legacy effect of freezing. Overall, my results indicate that intense soil freezing and increased atmospheric N deposition can both alter plant productivity and ecosystem N retention, although there were few significant interactions between these two factors.


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