森林土壤氧化亚氮排放对大气氮沉降增加的响应研究进展

STUDY ON THE RESPONSES OF NITROUS OXIDE EMISSION TO INCREASED NITROGEN DEPOSITION IN FOREST SOILS: A REVIEW

森林土壤N2O来源于土壤氮素的氧化还原反应,硝化、反硝化、硝化细菌反硝化以及化学反硝化是其产生的四个关键过程。当前,氮素富集条件下森林土壤N2O排放存在硝化和反硝化主导作用之争,对大气氮沉降增加的响应模式以及微生物驱动机制尚不清楚。综述了森林土壤N2O来源的稳定性同位素拆分,森林土壤总氮转化和N2O排放对增氮的响应规律,增氮对N2O产生菌群落活性和组成的影响,并指出研究的薄弱环节与未来的研究重点。总体而言,森林土壤N2O排放对大气氮沉降增加的响应呈现非线性,包括初期无明显响应、中期缓慢增加和后期急剧增加三个阶段,取决于森林生态系统"氮饱和"程度。施氮会引起森林土壤有效氮由贫氮向富氮的转变,相应地改变了土壤硝化细菌和反硝化细菌群落丰度与组成,进而影响土壤N2O排放。由于森林土壤N2O排放监测、土壤总氮转化和N2O产生菌群落动态研究多为独立进行的,难以阐明微生物功能群与N2O排放之间的耦合关系。未来研究应该有机结合15N-18O标记和分子生物学技术,准确量化森林土壤N2O的来源,揭示森林土壤N2O排放对增氮的非线性响应机理。

Continuous accumulation of nitrous oxide （N2O） in the atmosphere leads to global warming and ozone depletion. Forest ecosystems act as source and sink of atmospheric N2O, posing a great uncertainty in budgeting of atmospheric N2O. Exogenous nitrogen inputs into terrestrial ecosystems are an alternative explanation for this uncertainty. Therefore, exploring mechanisms involved in responses of N2O emission from forest soils to increased atmospheric nitrogen deposition is of some important theoretical and practical significance. However, due to complexity of soil nitrogen cycling and high spatial heterogeneity of forest ecosystems, progress of the research on soil N2O flux response to N addition has been quite slow. N2O emission in forest soils is mediated by microbial communities, and nitrification, denitrifieation, nitrifler denitrification and chemical denitrification are the four main processes of soil N2O production. Presently, which one of nitrification and denitrificatiopn is the leading contributor to soil N2O emission under nitrogen enrichment is still controversial; and how N2O emission responds to increased N deposition and what mechanism is involved in soil microbes driving the porcess are not well known. In this paper, a review is presented of the progresses of the study on identification ofsources of N2O in forest soils using the stable isotope labelling technique, laws of the responses of total N transformation in and N2O emission from forest soils to nitrogen addition, as well as effects of increased N depostion on activity and composition of soil microbial community. Also, the paper points out weak links in the present studies and possible research priorities in the future. Generally, soil N2O flux is influenced by many environmental factors including soil temperature, soil moisture, pH, Eh, and N availability. Increased nitrogen deposition may increase, decrease or have little effect on forest soil N2O emission, depending on forest types, initial nitrogen content in soils, and dose and duration of nitrogen application. Overall, the response of N2O emission from forest soils to increased atmospheric N deposition exibits a nonlinear pattern, including no significant response at the early stage, linear increase at the medium stage, and exponential increase at the late stage. The three-stage pattern depends on degree of ＂ N saturation＂ of the forest ecosystems. Besides, significant relationships were observed between soil NO3^- content and abundance of denitrobacterial genes, between soil NH4^＋ content and abundance of nitrobacterial genes, and between soil N2O flUX and abundance of denitrobacterial genes. Nitrogen application leads to change in status of soil available N from N deficiency to N sufficiency, and hence changes in abundance and composition of nitrobacteria and dennitrobacteria, thus affectng soil N2O emission. Moreover, as the monitoring of N2O emission from forest soils and the researches on transformation of soil TN and dynanucs of N2O producing bacterium communities are often carried out independently, making it hard to elaborate on the coupling relationship between soil microbial functional groups and soil N2O emission. It is, therefore, suggested that future researches should focus on the following three aspects 1 ） to lay out and conduct some multi-dosage multi-form N fertilizer application experiments to work out equations for response of soil N2O to N addition and threshold of N deposition that may cause significant change in N2O flux response curve; 2） to define relationships between soil N transformatioon and soil N2O emission and explore relative contributions of soil nitrification and denitrification to soil N2O generation by means of the 15^N-18^O labeling technique in combination of molecular biology, of which the findings may explain the difference between the forests in North China and in South China in source of soil N2O ; and 3） to quantify the coupling relationships of soil N2O flux with major soil microbial functional groups, such as nitrifiers, denitrifiers, and ammonia-oxidizing bacteria, and use molecular biological and matagenomic methods and techniques to determine effects of N addition on abundance and composition of N2O producing bacterial community. By so doing, it is expected that the mechanism of the non-linear response of N2O emission to increased N depostion in forest soils could be fully understood.