| Human activity has increased the flux of nitrogen from land to the oceans by twofold globally over the past 40 years. The increasing loads of nitrogen have caused serious anthropogenic eutrophication which would contributes to series negative effects such as decreased water transparency, increased oxygen demand and even a threat to both human health and economy. As a critical link in nitrogen cycle, nitrification, the oxidation of ammonia to nitrate via nitrite, is thought to connect biological N fixation and anaerobic N losses. Moreover DO in the water column and the sediments can be depleted by intense nitrification, leading to the formation of hypoxic zone at continental shelves. Denitrification, the conversion of NO3﹣or NO2﹣into gaseous forms either N2O or N2, is of both fundamental and practical contribution to calculate nitrogen cycle flux, control nitrogen eutrophication and study global climate change. Thus there is considerable concern on the nitrification and denitrification process influenced by pollution, for example, global warming, contamination with recalcitrant organic compounds, and nitrogen overloading.Several environmental variables have been demonstrated to influence nitrification and denitrification including temperature, oxygen, ammonium, salinity and so on. Nitrification is always rate limited in most ecosystems by activities of autotrophic ammonia-oxidizing bacteria (AOB) that are responsible for the oxidation of ammonia to nitrite by the enzyme ammonia monooxygenase (amo). Recent advances in DNA-based techniques for direct whole microbial community analysis have made it possible to study AOB communities without culturing by using probes or PCR primers target amoA, a functional gene coding for the active subunit of amo. Although it's difficult to use culcure-independent methodologies for a wide range of taxonomic groups, denitrifier quantity could be estimated by the application of MPN. However, as yet few special and systematical studies have reported on relationships ?among environmental variables, characteristic bacteria community and nitrification or denitrification process.The present study was explored the processes of nitrification and denitrification occurring in surface sediment at the Changjiang River, Yellow River and Colne River Estuaries. The aim is to investigate: (i) the rates of nitrification and denitrification; (ii) distributions of nitrifier and dinitrifier; (iii) the role of bacteria in and the influence of environmental factors such as temperature and salinity on the nitrification and denitrification processes; (iv) the environmental effects of nitrification and denitrification through calculating the nitrogen and oxygen flux.For Changjiang River Estuary, the nitrification rates ranged from 100.3μmol·m-2·h-1 to 514.3μmol·m-2·h-1 which decreased from nearshore to offshore. The denitrification rates were in the range between 101.3μmol·m-2·h-1 and 731.9μmol·m-2·h-1 and had a decreased gradient from nearshore to offshore sediment with two high value regions located at the mouth of Yangtze River and Hangzhou Bay respectively. The nitrifying bacteria were counted as (1.87-3.53)×105 cells·(gram wet weight) -1 and exhibited salt tolerance to some extent. The nitrifier quantity was a main factor to effect nitrification rates at high-salinity sea area, with an influence ratio of 87.7%, at a level of 60% biodiversity similarity. The quantities of denitrifying bacteria ranged from 3.9×105 cells·(gww)-1 to 110.0×105 cells·(gww)-1, which were significant correlated with the denitrification rate, indicated that denitrifying bacteria was the determining factor. The denitrification rate was also primarily affected by the salinity and the concentrations of DO and ammonia. Fluxes of transformed nitrogen and consumed oxygen by nitrification process were 4.68×105 kg·N and 6.07×104mol per day respectively, meanwhile the nitrogen flux by the denitrification process was calculated to be 8.19×105 kg·N·d-1, suggesting that nitrification and denitrification would be important factors for the distribution of DIN species and DO at bottom water in early summer.For Yellow Riever Estuary, the range of the nitrification rates was between 30.3μmol·m-2·h-1 and 76.5μmol·m-2·h-1, with higher value at the mouth of Yellow River and decreasing gradually to adjacent area of Bohai Sea and Shandong Peninsula. In addition, denitrification rates ranged from 3.49μmol·m-2·h-1 to 19.09μmol·m-2·h-1. The distribution of denitrification rates showed radial decrease from two higher-value areas at the mouth of Yellow River and adjacent sea area of the Laizhou Bay to the lower-value area at the Bohai Sea extended area. Nitrification and denitrification processes have no significant correlation. The numbers of the nitrifier ranged from (1.87±0.19)×104 cells·(gww)-1 to (3.53±0.34)×104 cells·(gww)-1, meanwhile the numbers of denitrifier were counted as (1.2-11.0)×103 cells·(gww)-1, which had no significant differences among sampling sites. Nitrification was influences by many environment factors, and the most impact factor is nitrifier quantities with influence ratio of 72.9%. The remarkable correlation coefficient indicated that denitrifier quantity was the most impact factor for denitrification. The fluxes of Nitrogen and Oxygen generated by nitrification process were 1.65×105 kg·N·d-1 and 1.37×104 mol·d-1, respectively. And the flux removed by denitrification process was 4.10×104 kg·N·d-1.For Colne River Estuary in UK, denitrification rate were measured between (23.99±8.85)μmol·m-2·h-1 and (154.74±46.45)μmol·m-2·h-1 in winter, with the values of upriver stations much higher than that of the river mouth. The percentages of coupled nitrification-denitrification in the total denitrification were from 52.98% to 96.10%. The range of potential nitrification rate was between 3.33μmol·m-2·h-1 and 8.89μmol·m-2·h-1 which has no significant correlation with denitrification rate or coupled nitrification-denitrification. The nitrifier quantities ranged from 1.24×106 cells·(gww)-1 to 16.92×106 cells·(gww)-1, with increasing monthly at Hythe and Alresford sites and varying unobviously at Brightlingsea site. Nitrifier populations were dominated byβ- andγ-Proteobacterium, while members of theβ-Proteobacterium were mainly belonging to the Nitrosomonas and Nitrosospira. Regression analysis showed that there was some correlation between nitrifier quantity and potential nitrificant rate at a level of 60% biodiversity similarity.
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