海岸带地区的固氮、氨化、硝化与反硝化特征

The characteristics of nitrogen fixation, ammonification, nitrification and denitrification in coastal zones

海岸带是海洋环境中受人类活动影响最大、生物地球化学循环最为活跃的地区。这一地区氮的生物地球化学循环包括 :生物固氮、有机氮的氨化、氮的硝化、反硝化等 4个主要过程。概括性地介绍了有关这四个过程的发生机制、环境影响因素及研究方法等方面的研究动态、进展、存在的科学问题与今后的研究方向。过去十几年来 ,固氮主要集中在对束毛藻属的研究上 ,其间有两个重要发现 ,一是生物固氮在海洋氮循环中的作用远比人们以前的想象要重要得多 ;二是蓝细菌已经在海洋中存在了 2 0亿年 ,它们有可能调节大气中的 CO2 ,进而影响全球气候。由于有机物的结构千差万别 ,含氮有机物的氨化过程可能是一个简单的矿化反应 ,也有可能是一系列复杂的代谢过程 ,在水解酶的作用下含氮有机物降解为下一级化合物。硝化过程分两步进行 ,氨的硝化为反硝化细菌提供了重要的硝酸盐来源 ,通常采用同位素方法来研究硝化过程。发生在沉积物中的反硝化过程是氮循环的关键步骤 ,反硝化过程一方面减少了海水中初级生产者可利用的氮 ,另一方面产生了终结产物 N2 和 N2 O,而 N2 O是一种温室气体 。

The biogeochemical cycling of nitrogen is a significant factor influencing global climatic change. It is a complex process involving interactions between the atmosphere, seawater, sediments, and microorganisms. Nitrogen cycling is particularly active in estuarine and coastal zones, regions where human activities can impact on the natural process. There are four major processes in biogeochemical cycling: nitrification fixation;organic nitrogen ammonification;nitrification, and denitrification. The mechanisms and physico-chemical factors regulating the four major processes will be reviewed, along with an outline of the active research directions in this field. Recent research on nitrogen fixation has highlighted the role of Trichodesmium. Biological nitrogen fixation is a more important process than previously understood and may have an influence on the capacity of the oceans to sequester carbon. The nitrogen fixation rates of cyanobacterial mats can range from 0.8 to 76 g N /(m·a). Usually, the contribution of cyanobacterial nitrogen fixation in estuaries is relatively small, but in tropical coastal marine lagoons, it may account for a significant proportion of the total annual nitrogen inputs. This research suggests that available organic carbon is probably the main factor limiting the nitrogen fixing potential in oligotrophic marine environments. The addition of carbon compounds such as glucose, polysaccharides, xylan or alginate can stimulate nitrogen fixation activity. Depending on the structural complexity of the organic matter, ammonification can be either a simple mineralization reaction or a complex series of metabolic steps involving a number of hydrolytic enzymes during which N-containing polymers are broken down to their soluble sub-units. The ammonification rates for coastal sediments reported is ranged from 7 to 644 (mg N/(m^2·d),) and is controlled by temperature, oxygen penetration into the sediment, the nature and concentration of organic matter and the physiological characteristics of microbial communities. It is now recognised that the oxidation of ammonium plays a pivotal role in generating a source of nitrate for denitrifying bacteria. Ammonia oxidation to nitrate is a two-stage process, and a number of different methods have been developed to estimate nitrification rates in marine sediments. One of the most widely used is the ^(15)N-NO^-_3 isotope dilution technique. Many physico-chemical and biological factors are important in regulating nitrifying activity in coastal marine sediments including temperature, ammonium concentration, oxygen tension, pH, CO_2 concentration, salinity, presence of inhibitory compounds, light, and macrofaunal activity.Denitrification is a key process in the sediment nitrogen cycle since it decreases the amount of nitrogen available to the primary producers, as the gaseous end-products N_2O and N_2 diffuse into the atmosphere. In coastal marine systems that receive large quantities of nitrogen from anthropogenic sources, it also provides a mechanism to remove excess nitrogen and helps to control the rate of eutrophication in these environments. On the other hand, N_2O is a greenhouse gas that plays a key role in both the stratospheric ozone and tropospheric heat budget. A number of methods have been employed to estimate denitrification rates in coastal marine sediment, such as mass balance, acetylene inhibition and the use of ^(15)N as an isotopic tracer. Denitrification rates are regulated by a number of physico-chemical and biological factors including nitrate concentration, available carbon, temperature, oxygen depth penetration, Eh, pH, concentration of inhibitory compounds, salinity, light, and macrofaunal activity.