Effect Of Flooding On Soil Nitrogen Forms Distribution And Related Enzyme, Bacteria In The Water Level Fluctuating Zone Of Three Gorges Reservoir

After the achievement of the Three Gorges Reservoir(TGR) engineering, a 30 meters high water level fluctuating zone(WLFZ) appear. Being the alternation zone between land and water area, WLFZ also belongs to the ecological sensitive area. Frequently wetting and drying cycle will impact on nitrogen forms distribution characteristics in the soil of WLFZ. Therefore, the study of nitrogen forms transformation has great significance in illuminating the effects of nitrogen transfer and transformation between water and soil. The purple soil is typical in the WLFZ of TGR. Nitrogen forms distribution of the soil, relationship between nitrogen transformation and soil enzyme, bacteria activities were investigated at four water depths(0, 2, 5 and 15 m) and four flooded time(30, 60, 120 and 180 d) in situ experiment. The results are stated as follows:(1) A revised sequential extraction technique of sediment nitrogen classification is used in the study to discuss the flooding influence on nitrogen forms distribution characteristics in the soil. With an increase of water depth, the content of total nitrogen has decreased in period of flooding as a whole. During drying period, the content of total nitrogen is less than flooded period. The content of four nitrogen forms of TF-N could be ordered as OSF-N >IMOF-N >IEF-N >CF-N. During flooding period, the content of IEF-N and IMOF-N are decreased with an increase of water depth. Compared to non-flooded(0 m), the content of CF-N in flooded soil increased, but the content of OSF-N is stable. On the whole, the content of TF-N in flooded soil are all significantly higher than non-flooded(0 m)(p< 0.05). With an increase of flooding time, the content of TF-N in flooded soils showed a increasing tendency. A part of NTF-N change into TF-N make the content of NTF-N and TN are all decreased in flooded period.(2) The relationship between nitrogen forms of flooded soils and water environment such as water temperature, electrical conductivity, p H, dissolve oxygen are analyzed. The contents of IEF-N and IMOF-N have a positive correlation. All the indexes of water environment are significantly correlated to TN. Positive correlation exists among water temperature and IMOF-N, between illumination and IEF-N, IMOF-N. Dissolve oxygen is positively correlated to IEF-N, but negatively correlated to CF-N. In conclusion, water environment have an effect on nitrogen forms distribution in the soil.(3) The content of inorganic nitrogen in different water depth changed. The ammonium and nitrate of soils were influenced by water depth significantly(p< 0.05). The contents of ammonium and nitrate decreased gradually with increasing water depth after 180 days flooded; compared to non-flooded(0 m), the contents of ammonium increased significantly at depth 2 m and nitrate decreased significantly at depth 15 m(p< 0.05). Soil inorganic nitrogen content(ammonium and nitrate) at depth 15 m were lower than those of non-flooded soil. The contents of ammonium and nitrate in flooded soils are all significantly lower than non-flooded(0 m) in second flooding(p< 0.05). Nitrogen releasing will aggravate eutrophication problem of TGR in second flooding.(4) Water environment affected nitrogen transformation in the soil according to the study. The relationship between water environment and four enzymes related nitrogen transformation in flooded soils are analyzed. Results showed that: positive correlation exists among four enzymes activities and water temperature, turbidity; except for nitrate reductase, other enzymes activities are positively correlated to electrical conductivity. Meanwhile, there is no significant correlation between dissolve oxygen and four enzymes activities; p H is negatively correlated to urease and nitrite reductase activities; except for nitrite reductase, other enzymes activities are positively correlated to illumination. Moreover, the quantity of ammonifying, nitrite, denitrifying bacteria are decreased with an increase of water depth in period of flooding. It is the same as the change of soil inorganic nitrogen content. So the water depth has an important influence on nitrogen transformation and soil enzyme activities and microbe biomass.