陆地生态系统氮状态对碳循环的限制作用研究进展

The Effects of Nitrogen Limitation on Terrestrial Ecosystem Carbon Cycle: a Review

陆地生态系统碳循环和氮循环密切相关,碳贮量与碳通量在很大程度上受氮循环的影响和限制。由于氮循环的复杂性,在以往的大多数碳循环研究中,更多考虑水分、温度和大气CO2浓度等因子的影响,考虑碳氮相互作用的研究较少。氮素可限制植物光合、有机质分解、同化产物的分配以及生态系统对大气CO2浓度升高的响应。根据目前有关碳氮模型的发展状况可将碳氮耦合循环模型分为三大类:一是静态模型,它的土壤养分水平或者叶氮含量不变,是常数,这类模型适合于在站点或氮素浓度变化不大的区域应用;二是土壤氮限制模型,能够保持稳定的生态系统氮收支,在NPP(Net Primary Productivity,净初级生产力)的模拟中考虑土壤氮有效性的动态变化的影响,使模拟结果更为合理;三是叶氮限制模型,在NPP的模拟中考虑叶片氮浓度的动态变化的影响。这三类模型虽然都考虑了氮对碳循环的限制作用,但在氮碳循环机理方面尚有不少欠缺,所以在研究中可能会带来很大的不确定性。在以后的研究中,应通过加强碳氮相互作用的实验研究,增进对碳氮过程的深入了解,进而建立综合动态的碳氮耦合模型,以减少目前碳循环研究中的不确定性。

Terrestrial carbon cycle and nitrogen cycle are closely coupled. Some factors, such as temperature, water and CO2 concentration, were more considered in the previous carbon researches than the nitrogen because of the complexity of nitrogen cycle, but this situation has changed in recent years. Nitrogen has great effects on the plant photosynthesis, organic matter decomposition, carbon allocation and the responses of ecosystem when the atmospheric CO： concentration increases. There are generally three types of carbon models to consider nitrogen limitation on carbon assimilation： （1） static models： these type models usually use a constant soil fertility index or leaf nitrogen concentration and are fit for a site or district where the nitrogen variability can be neglected; （2） soil nitrogen limitation models： these models can maintain stable nitrogen budgets and dynamic soil nitrogen can turn the potential NPP to actual NPP; （3） leaf nitrogen limitation models： such models are similar to soil nitrogen limitation but feature a further dynamic leaf-level nitrogen regulation of NPP. The common approach uses relative leaf nitrogen concentration to scale down proportionally either the maximum Rubisco or the NPP. Although the three type models all consider the nitrogen effects on carbon cycle, they may produce great uncertainty in carbon research because of the partly understanding of the interaction between carbon and nitrogen. Future studies should focus on both the experiment and observation about the nitrogen feedbacks on carbon cycle and the development of integrated dynamic ecosystem models that can describe the interaction of carbon and nitrogen cycle, contributing to decreasing the uncertainty in the carbon research.