Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Science

Program

Biology

Supervisor

Dr. Hugh Henry

Abstract

Increased atmospheric nitrogen (N) deposition and climate warming are both anticipated to influence the ecosystem N dynamics of northern temperate ecosystems substantially over the next century. Nevertheless, in field experiments with N addition and warming treatments, temporal scale can play an important role in determining the extent of treatment effects on N dynamics, and it is unclear to what extent the results of short-term studies can be extrapolated to responses over longer time scales. I compared the short-term versus long-term effects of N addition and warming on net N mineralization, N leaching, and N retention in a grass-dominated old field. I examined new N addition and warming plots (3 years old) added to an existing field experiment (16 years old) to compare the treatment effects on the N responses over the two time scales while controlling for variation in background environmental conditions. I used in situ soil cores to quantify net N mineralization and leaching, and 15N tracer to estimate ecosystem N retention. Nitrogen addition significantly increased total N mineralization over the growing season and there was a significant interaction between plot age and N addition over this period. Similarly, N leaching significantly increased under N addition over winter. However, there were no significant interactions between either of the treatments and plot age for 15N retention. The latter result might be attributed to the lack of plant community composition shifts over the 16 years of the field experiment in response to the treatments, and it also indicates the new N addition plots may have already reached the point of N saturation.

Summary for Lay Audience

Nitrogen (N) is a key element in many ecosystems, given that the availability of N in forms that can be taken up by plants often limits plant growth and influences species composition. Since the industrial revolution, the quantity of biologically available N resulting from human actions has increased by an order of a magnitude. A large majority of this N is applied directly to agricultural fields in excess amounts. The excess N that does not become taken up by plants or soil microorganisms can pollute the atmosphere. Likewise, the burning of fossil fuels can contribute to atmospheric N pollution. Atmospheric N pollution can be carried long distances, and subsequently be deposited back onto the landscape in precipitation or via dust, potentially disrupting natural ecosystems. Climate warming is another important factor that can alter N in ecosystems. In field experiments with N addition and warming treatments, time scale can play an important role in determining the extent of treatment effects on N dynamics. I compared the short-term (3 year) versus long-term effects (16 year) of N addition and warming on N availability and N retention in a grass-dominated field. I examined the response of N mineralization, whereby soil microorganisms break down decaying organic matter to obtain energy and, in the process, can release N back to the soil. I also examined N losses via leaching (losses of N in the water draining through the soil), and I added N labelled with 15N, the heavy isotope of N, to trace the retention of N added to the soil. For N mineralization, plot age influenced the warming and N addition effects to some extent, but plot age did not influence their effects on 15N retention. The lack of treatment effects on the plant composition of the plots after 16 years may explain why plot age was not an important factor in influencing the effects of the treatments on 15N retention.

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