采伐和火烧对森林氮动态的影响

森林是重要的陆地生态系统类型,它通过特有的养分循环机制维持其结构和功能.其中氮素对林木生长和发育十分重要,而且常是森林生产力的限制因素.另一方面,森林氮动态又常受到人类活动干扰的影响.根据国内外研究结果综述了采伐和火烧对森林氮动态的影响.结果表明采伐后环境因素的变化将影响森林N动态,其中最为关注的是采伐后一系列因素引起的N损失,如:N淋溶增加、伴随生物量的N迁移以及因径流或侵蚀增加造成的枯枝落叶层和土壤层N流失.这些N损失又将影响更新林分的生长和生产力.此外,采伐后N吸收速率一般下降,但随着植被快速生长N吸收速率将不断增加.采伐后氨化和硝化过程增强,但因短期内同化作用较弱,生态系统中大部分N将发生损失.火烧对森林N动态的短期影响主要包括:第一,火烧时N直接挥发损失;第二,火烧后N有效性增加,这主要由灰分沉积、根和微生物死亡及有机质N矿化增强等综合造成.随着时间延长,N有效性逐渐降低,这可能与火烧引起的有机质损失、植物N吸收增加、淋溶或侵蚀损失有关.然而,目前关于火烧造成的长期生态影响,如火烧后地上植被恢复与地下生物地球化学过程变化有何联系仍不太清楚.未来研究应着重于探讨氮素对森林采伐和火烧作出的短期响应将如何长期影响森林的结构和功能.此外,建议在实施营林方案时需考虑采伐和火烧对生态系统氮的影响.

Effects of harvesting and burning on forest N dynamics

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.