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Enzyme activities and nitrogen transformations following fertilization of a Douglas-fir forest in coastal British Columbia Adams, Seth Daniel


Forest fertilization has long been used to enhance tree growth and timber yield. More recently, fertilization has become the focus of increased interest as a potential method for increasing soil C uptake, vis-à-vis suppression of microbial enzymes that decompose recalcitrant soil organic matter (SOM). However, fertilization is also associated with increased emission of the potent greenhouse gas N₂O. In this study I evaluate the effect of N amendment on activities of soil enzymes and on N ammonification and nitrification rates in a chronosequence of Douglas-fir stands on Vancouver Island, British Columbia. Ammonification and nitrification rates were also compared between two different fertilizers, urea and slow-release urea, which is urea with a polymer coating to decrease the rate of N release. Fertilization significantly decreased the activity of the ligninases, phenol oxidase and peroxidase in the mineral soil layer of all stand ages. However, the effect appeared to be temporary, as there was no difference between treatment and control 63 days after fertilization. Activity rates of three enzymes that degrade labile SOM - cellobiohydrolase, betaglucosidase and NAGase - were also measured, and there was a small decrease in activity rates of these enzymes in the forest floor. Urea amendment increased net rates of ammonification and nitrification. Addition of slow-release urea resulted in lower rates of ammonification and nitrification compared to urea, but the rates remained elevated longer. Nitrification rates were increased the most in the clearcut stand (7 years old), and less so in the two older stands (18 and 57 years old). Ammonification and nitrification rates were approximately an order of magnitude higher in the forest floor than in the mineral soil, and the response to fertilization was also greatest in the forest floor. Nitrification was increased by fertilization, but this effect was only significant in the clearcut stand. Nitrification at all sites increased more with urea addition than with slow-release urea. Further research is recommended to determine if there is a minimum threshold of available N necessary to cause the suppression of microbial enzyme activities, and if it could be reached through operational applications of slow-release urea fertilizer.

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