Thesis

Effects of Nitrogen Addition on the Litter Decomposition Dynamics of the Deciduous Coastal Sage Shrub Artemisia californica Less.

This study focused on the in situ leaf decomposition of Artemisia californica Less. using the litterbag method. Litterbags (10 x 10 cm) were constructed using 2 mm mesh screening material for the top cover to allow microarthropods to pass freely and a 0.04 mm mesh bottom to prevent material loss. Litter was collected from two types of A. californica shrubs; ones exposed to elevated N levels (N+) for 8 years and others exposed to ambient N levels (control). Two grams of fresh litter were used to fill each bag, and 96 control litter and 96 N+ litter bags were deployed at the site for a total of 182 bags. Twenty-four bags (12 containing control litter and 12 containing N+ litter) were placed randomly within each of the eight (4 control and 4 N enriched) 10 m x 10 m plots. The bags were periodically brought back to the lab to assess mass loss and litter C and N concentration. Mass loss was determined by drying and weighing the litter remaining in the litter bag and litter N and C concentration was determined by elemental analysis. The hypotheses tested during this experiment were that nitrogen addition was going to cause an increase in litter decomposition rates because of an increase in litter quality; that nitrogen addition was going to cause an increase in mineralization because nitrogen enriched litter will provided more nitrogen for bacterial activity. Finally, that nitrogen addition was going to cause an increase in mineralization because there would be an extra amount of nitrogen in the soil which would decrease the potential for microbial nitrogen limitation. Experimental nitrogen addition altered the chemical composition of the litter by reducing the total carbon (C) and soluble C content of the litter. Litter collected from plots that had been previously enriched with nitrogen (N+) had higher decomposition rates (k-values) than the litter collected from control plots, which means that they lost mass quicker over time. The type of soil that the bags were incubating in did not have any effect on the litter decomposition rate regardless of | 2 the litter’s origin. N+ litter had higher levels of nitrogen concentration over time and litter decomposing in nitrogen enriched soils also had a higher concentration of nitrogen throughout the experiment. Because of the patterns observed throughout this experiment, we can conclude that experimental nitrogen deposition increased litter decomposition rates by improving litter quality (C:N ratio). Understanding how nitrogen deposition affects these communities will help understand how N and C cycling and soil C storage are altered by human-induced N inputs.

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