硝化和反硝化对湖泊有机质沉积成岩前降解作用的研究

Organic degradation by nitrification and denitrification before diagenesis in the Hongfeng Lake, SW China

贵州红枫湖10月叶绿素a(chla)和NO-3含量均比7月明显降低。利用氮同位素等数据对此进行了研究,结果表明,含量的降低是由不同的生物地球化学作用引起的。chla含量的降低主要是水体中有机质降解(硝化)所致,而NO-3含量的降低则是缺氧季节湖泊沉积物表层反硝化作用的结果。缺氧季节表层水体仍然能发生较强烈的硝化作用。硝化作用和反硝化作用分别发生在热分层湖泊的上层和沉积物表层。反硝化作用不仅消耗大量的NO-3,而且还造成了一定量的有机质降解(有机碳作为电子受体)。据估算,在红枫湖后五测点和大坝测点,总有机碳在沉积成岩前分别降解了78%和68%。其中由硝化作用引起的总有机质降解量分别为35.8%和25.9%,而反硝化作用则分别为13.4%和9.2%。

Nitrification and denitrification are the two most important nitrogen biogeochemical processes occurred in the Hongfeng Lake in oxygen depleted season. In order to study organic degradation by nitrification and denitrification before diagenesis in the Hongfeng Lake, Concentrations of chlorophyll a (Chl a) and nitrate, δ 15N values have been analyzed in July and October. Concentrations of Chl a and nitrate fell from July to October in the lake, which was caused by different biogeochemical processes from the nitrogen isotope data. The decrease of Chl a concentrations was mainly caused by organic degradation (nitrification), while the fall of nitrate concentrations was contributed by denitrification occurred at the surface layer of sediments. The interior nitrogen sources of nitrification have not been thought more of than exterior nitrogen inputs before in the lake. In addition to the decrease of Chl a concentrations, nitrification also produces much nitrate, which has strengthened denitrification. Not only has denitrification consumed substantial nitrate, but decomposed a certain mass of organic matter. It was calculated that 78% and 68% of organic matter has been decomposed before diagenesis in Houwu and Daba sites, respectively. The decomposition ratios caused by nitrification and denitrification in the two sites were 35.8% and 25.9% , and 13.4% and 9.2% , respectively. Nitrification and denitrification will not occur to the same site or at the same time. The good relationships between the net concentration changes of Chl a and nitrate at the upper layers of water columns and bad at the lower layers suggest that nitrification only occurred at the upper layers, which is consistent with comparatively higher DO concentrations there. In oxygen depleted season, denitrification layers in sediments go up and thus more nitrate will enter the layer and be utilized by denitrification bacteria. This has been confirmed by the decrease of nitrate concentrations and the increase of δ 15N values from July to October.