| 其他摘要 | Nitrogen cycle is an important nutrient phenomenon in aquatic environment, which can provide a theoretical basis for evaluating natural waters as well as solving the problems related to nitrogen controlling in the restoration of eutrophic lakes. The organic substances in water can be divided into two major parts: particulate organic matter (POM) and dissolved organic matter (DOM). They interact and transform with inorganic nitrogen. These three species together control the nitrogen cycle in aquatic ecosystem. Stable nitrogen isotope measurements of the inorganic or organic materials have been proven to be a powerful tool to assess its sources and identify the biogeochemical process in aquatic ecosystems. Therefore variations in the isotopic composition of organic matter is helpful to understand the role of nitrogen thereby proving the direct proof for the existence of special organic matter-related biogeochemical process in aquatic ecosystems. Such a study may assist to further understanding of the nitrogen cycle in aquatic environments.
At first, the present study solved some problems in the measuring technique which restricted the application of the nitrogen isotopic composition of DOM. Secondly, we measured the seasonal and profile variations of nitrogen isotopic ratios of particulate organic nitrogen (δ15NPON) values in surface water of Lake Hongfeng and Lake Baihua in Guizhou province from 2003 to 2004. It is also investigated the causes of the difference in seasonal variations between two lakes and the factors which control the changes of δ15NPON values in vertical profiles. Finally, several typical months were emphasized to represent the different thermal stratified period from 2006 to 2007 based on the former studies about Hongfeng Lake. The content of three nitrogen species in vertical profile waters combined with its δ15N values and other hydrochemistry parameters were used to demonstrate the existence of special biogeochemical processes in nitrogen cycle. There is a great difference on the content and the δ15N values of three nitrogen species between riverine inputs and the inner lake water, which were used to trace the sources as well as to understand the effect of allochthonous river input. The results obtained in this study can be summarized as follows:
1. The seasonal variations of δ15NPON values can be used to act as an indicator of the trophic state, reflecting the effect of allochthonous industrial wastewater and sewage. The δ15N values of surface PON were seasonally varied to a range from +3.7 to +14.9‰ in Lake Hongfeng and +1.3 to +8.7‰ in Lake Baihua during the study period from 2003 to 2004. It indicated that POM is greatly varied in these two lakes during the study period. In Lake Hongfeng, the higher δ15N PON values appeared in winter (February, 2004) and summer (September, 2003). The highest δ15N PON values in winter were attributed to the input of high contents and 15N-enriched inorganic nitrogen or POM released from various industries situated near the lakes. In Lake Baihua, the higher values of δ15NPON were appeared in summer (September, 2003) whilst the values were low during the winter season. These high and low values were possibly aroused by the growth of primary production and large 15N-depleted organic particles input from sewage respectively.
2. We gained the variation ranges of δ15NDON values in lacustrine environments at first time. The δ15N values of DON varied from +1.0 to +12.3‰, which were similar to the variation ranges of δ15NPON values (+2.8 to +16.8‰) and δ15NNO3- values (+5.9 to +22‰). During the growth of primary production, the δ15N values of these three nitrogen species followed the same results, showing the δ15NNO3- values were the maximum and the δ15NDON values were the minimum. Because the phytoplankton is the main component of POM and its δ15NPON values are controlled by the degree of nitrate utilization. Moreover, phytoplankton preferentially release 14N-enriched DON, so these three nitrogen species have the inheritance relationships with each other.
3. The seasonal variations of three nitrogen species will reflect the changes of its sources. In the different vertical profiles, the maximum values emerged in January 2007, which was influenced by the input of high contents and 15N-enriched nitrate. The smaller values appeared in April 2006 and March 2007 attributed to the strong nitrification which generated massive 14N-enriched nitrate at the inner water body. Except that the δ15NDON values were abnormally large in March 2007, the δ15NDON values in growth seasons were generally larger than that of other seasons in common. There were no obvious shifts in waters of the total vertical profile. The maximum δ15NPON values appeared in January 2007 which may affected by the input of 15N-enriched POM.
4. Different degradation pattern may lead to its products with different δ15NDON signals. Under the aerobic conditions, PON preferentially released 14N-enriched DON components during the processes of mineralization and nitrification, for example: the δ15NDON values decreased in the water above 10 m in April and July 2006. On the contrary, PON generated 14N-depleted DON components preferentially at anaerobic conditions, an increasing trend was found in the water below 18 m in April 2006 and the water below 10 m in July 2006.
5. The components of PON showed a markable variations in δ15NPON values. While the bacteria including the nitrobacteria and the denitrifying bacteria became the main component of PON, its δ15N values will decrease dramatically. For example, the δ15NPON values changed abruptly in the middle-bottom water in July 2006 and March 2007.
6. The combination of the δ15N compositions of three nitrogen species will effectively use to trace the sources of organic matter at further step. The nitrogen content vertically only changed a little on January 2007, but the δ15N values showed distinct variations. The δ15N compositions of three nitrogen species varied abruptly at 20 m which was the boundary of allochthonous inputs and inner water body. In the transverse sections, the concentration of nitrate increased unexpectedly but its δ15N values remained constant at one sample (the forth sampling site), meanwhile the δ15N values of DON decreased markedly. All these phenomena showed that there is possibly new sources of nitrogen at this site. It can be speculated that this new high content of nitrate and the low δ15N values of DON originated from the accumulation of fish farming.
Therefore all of these results can conclude that the nitrogen isotopic composition of organic matter might be a direct tool for tracing the organic matter sources and identifying the organic matter-related biogeochemical processes in lacustrine environments. |
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