Effects of harvesting and burning on forest N dynamics
采伐和火烧对森林氮动态的影响

Forests are important terrestrial ecosystems, with particular nutrient cycling mechanisms to maintain structure and functions. Nitrogen is essential for forest growth and development, and commonly limiting for the forest productivity. N dynamics in forest ecosystems are frequently disturbed by intensive human activities. Based on a variety of research results, some potentially important human disturbances such as harvesting and burning are discussed and their effects on forest N dynamics are reviewed. It deserves more attention that the change of environment after harvesting will affect N dynamics in forest ecosystems. One of the major concerns with forest harvesting is loss of N due to increased N leaching, N removal in biomass, and loss of forest floor and soil N due to increased erosion and run-off. These N losses may lead to N limitations to the growth and productivity of regenerating forests. Also, nitrogen uptake rates generally decline after harvest, but rapid vegetation regrowth can lead to increases in nitrogen uptake rates. Ammonification and nitrification processes are stimulated after harvesting, by which N is becoming more moveable. Unfortunately in the situation of no assimilation after harvesting, much of N will be lost out of the ecosystems. Fire affects N dynamics via multiple mechanisms. First, fire volatilizes N during combustion, leading to a net loss of N from the system. Second, a pulse of increased N availability is created by fire. This pulse could be the combined effect of ash residue, root and microbe death, and increased N mineralization from organic matter. As time passes, N availability drops, likely as a result of lost organic matter from combustion, increased plant uptake, leaching or erosion loss. However, long-term ecological effects of fire, including linkages between aboveground restoration and belowground biogeochemical processes, are still poorly understood. Future experimental work should be focused on understanding how the short-term responses of N to harvest and fire influence the structure and function of forest ecosystems in the long term. Also, the effects of harvest and fire on ecosystem N should be weighed for implementation of any forest management programs.