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Format:
Print
Author:
Fredriksen, Guinevere
Dept./Program:
Plant and Soil Science
Year:
2005
Degree:
MS
Abstract:
Undisturbed northern forest ecosystems are generally nitrogen limited, nitrogen is conservatively cycled between the plants, biota, and soil of these ecosystems without large inputs or outputs of N. Over the last century humans have significantly increased the amount of inorganic nitrogen cycling through northern forest ecosystems from atmospheric deposition. The accumulation of inorganic nitrogen in these ecosystems has the potential to alter the way nitrogen is cycled within them, progressively moving the system from a state of nitrogen limitation to a state of nitrogen excess. Nitrogen excess can cause the acidification of soil and water and the depletion of base cations from the soil profile, which decreases soil fertility and water quality within an ecosystem, which may result in the alteration of natural communities. This study investigates the temporal variability of net nitrification and mineralization rates in two deciduous forest stands over two growing seasons from 2003 and 2004, and the relationship between soil properties and net rates. Gross rates of ammonification and nitrification were also measured in composite and intact core samples twice over the course of this study at one site and compared to net rates.
Temporal patterns were not consistent between the years of this study. Both study sites exhibited greater temporal variability in net nitrification and mineralization rates in the first year of the study than in the second year. This indicates that while temporal patterns may be inherent in N transformation rates in these northern deciduous forests, the patterns are not consistent from year to year. Of the soil properties measured, which included pH, temperature, soil water, NH₄⁺ substrate availability, and the C:N ratio, NH₄⁺ substrate availability explained the greatest amount of variability in net nitrification rates. Soil water content predicted the greatest amount of variability in net mineralization rates, although the relationship was weak. Complex interactions were shown to exist between the soil properties themselves at these watersheds, questioning the reliability of linear regression models at explaining soil N transformation rates.
The isotope pool dilution method was used to assess gross and net rate production of NH₄⁺ and NO₃⁻ in intact core and composite samples from each year at one location. Gross ammonification rates were not different-between the different sample types but gross nitrification was much greater in the composite samples than in the intact core samples. Gross ammonification was significantly greater than net ammonification, while there was no significant difference between gross and net nitrification rates, indicating that NO₃⁻ consumption is low in these soil samples. A comparison of different incubation lengths on gross and net rates in composite samples found that rates of ammonification and nitrification decreased with increased incubation length, although the greatest variability in rates was also measured at the shortest incubation length. Net rates were generally good predictors of gross rates, and gross nitrification rates explained over half of the variability in gross NH₄⁺ consumption rates. These findings indicate that at this site the disturbance caused by mixing soil samples stimulates both net and gross nitrification rates and may be caused by an increase in the spatial availability of NH₄⁺, combined with a decrease in competition for NH₄⁺, caused by the removal of plants.