| With the rapid development of economy and urbanization process in the coastal area of China over the past three decades, the water bodis in the urban area were contaminated seriously due to lack and incomplete of environmental facilities. Riparian zone has drawn widespread concern for its special geographic location and environmental function. To study the removal capability of riparian zone for nitrogen in scattered point sources and non-point sources, and to discuss nitrogen transformation in soil of riparian zone as well as to construct engineering application to abate terrigenous nitrogen into urban water bodies at the most extent in relatively limited space of urban riparian zone has become not only an important direction but also a hot area of research.In this study, four parts of the work were carried out. The first one, a micro catchment area was constructed at Changfeng green belt of Suzhou River riparian in Shanghai city to study nitrogen removal efficiencies in vertical infiltration by using field investigation of rain and simulated rainfall-runoff. The second one, nitrogen transformable rates were systematically measured and studied by buried bags method in situ and acetylene inhibition method in lab at six typical urban riparian zones of Shanghai city. The third one, greenhouse gas (GHG) fluxes, which was an important negative environmental effect after different concentrations of nitrate were retained were measured using static closed chamber/GC technique in micro catchment area. The last part, one pilot-scale apparatus of engineering application was structured by coal cinder, sawdust, zeolite, medical stone and local soil, as well as Cynodon dactylon cultivated on the surface. Capability of the multiple-composing apparatus to removal the nitrogen in simulated urban rainfall runoff was studied. The main conclusions in this paper were as follows:(1) Nitrogen in vertical infiltration, which formed either by rain or by stimulative rainfall-runoff, was partly removed by artificial green riparian belt. NH3-N was mainly removed at0-60cm layer, but it was at60-90cm layer for NO3--N. The removal rate was significantly influenced by rainfall intensity. Removal rates for NH3-N and NO3--N were respectively over12.9%and0.64%at0-90cm layer in heavy rainfall event. But the removal rates were over43.8%and49%in two light rains. As for the stimulative rainfall-runoff containing relatively high nitrogen concentration, the removal of nitrogen mainly occurred in0-30cm layer. Removal rates of TN, NH3-N and NO3--N+NO2--N by soil at0-30cm layer were over39.9%,39.8%and10.0%respectively. However nitrogen removal rate decreased with the waterlogging time increasing. There was conspicuous spatial variation of nitrogen concentrations in vertical infiltration at different sites in catchment area. With the increase of flow distance of surface runoff, concentrations of nitrogen in vertical infiltration first increased and then decreased. Nitrogen concentration partly influenced treatment load of green belt on stimulative rainfall-runoff, but it was not the main influencing factor.(2) The net ammoniation rates, net nitrification rates and net mineralization rates were generally low and showed significant spatial-temporal difference at0-35cm layer in urban riparian zone of Shanghai city. The net ammoniation rates were from-0.115to0.154mg·kg-1·d-1with an average of0.004mg·kg-1·d-1.The net nitrification rates varied between-0.093and0.100mg·kg-1·d-1with an average of0.010mg·kg-1·d-1, and the net mineralization rates ranged from-0.127to0.233mg·kg-1·d-1with an average of0.019mg·kg-1·d-1. The soil in riparian zone of Shanghai city tended to accumulate available nitrogen. The denitrification rates were from0.002to33.915ng·g-1·h-1with an average of3.08ng·g-1·h-1at0-35cm layer in urban riparian zone of Shanghai city.(3) Physico-chemical properties of soil (SOC, NO3--N and NH4+-N) were main influencing factors in regional difference of net ammoniation rates, net nitrification rates and net mineralization rates. High concentrations of NO3--N and NH4+-N in soil could increase net ammoniation rates and net nitrification rates, but it turned out to be the opposite for SOC.The denitrification rates were not significantly influenced by physico-chemical properties of soil. But in small areas where the soil properties were homogeneous, the denitrification rates increased with the concentrations of SOC, NO3--N and NH4+-N in soil. Temperature was the main influencing factor in seasonal change for nitrogen transformable rates. Generally, nitrogen transformable rates increased with temperature.(4) Moisture content and vegetation type were also main influencing factors for nitrogen transformable rates. When moisture content was lower than20%, there was significant difference for net ammoniation rate, net nitrification rate and net mineralization rate in soil under different vegetation type. It was on the contrary when moisture content was above20%. Generally, net ammoniation rates, net nitrification rates and net mineralization rates in soil under different vegetation type increased with moisture content.(5) Net ammoniation rate, net nitrification rate, net mineralization rate and denitrification rate in soil with the treatment of wetting and drying in different time interval all significantly increased compared with the initial values. But with the increase of the time interval of wetting and drying, the rates reduced. Especially, under the long-term drought condition denitrification rates were lower than the initial values of fresh soil, but it is the opposite for net ammoniation rate, net nitrification rate and net mineralization rate.(6) Due to the input of NO3--N, GHG fluxes in short time were changed, which showed significant seasonal difference. N2O fluxes in spring, summer and autumn increased significantly, while the GHG fluxes reduced accordingly in other seasons suggesting that the input of NO3--N promoted the soil to absorb GHG. After the input and retention of NO3--N, daily fluxes of N2O, CH4and CO2significantly reduced in a short time, ie.3days in summer,4-5days in spring and autumn, which might be the reason that the GHG fluxes in soils treated with input of different NO3--N concentrations presented no significant difference.(7) Temperature and light intensity accounted for the seasonal change of GHG fluxes. There were significant correlations between GHG fluxes and temperature and light intensity. Specifically, CO2discharge fluxes significantly reduced with the increase of light intensity, while N2O, CO2and CH4fluxes correlated significantly with the soil temperature at different depths, and the fluxes all increased with the temperature. (8) The constructed pilot scale apparatus could partly purify NO2--N, NH3-N, TN, TP and CODCr, in which the purification effect of NH3-N and TP was best. The concentrations of NH3-N and TP in effluent flowing through the apparatus were generally lower than the type V standard values of GB3838-2002National Environmental Standards for Surface Water and the average removal rates were65.6%and54.9%for NH3-N and TP respectively. The purification effect for NO2--N, TN and CODCr was not as good as that for NH3-N and TP, and the removal rates were38.8%for NO2--N,21.1%for TN and32.9%for CODCr. The apparatus did not purify NO3--N in influent, furthermore, the NO3--N concentrations in effluent flowing through the apparatus increased significantly resulting in the average removal rate of-1150.0%.(9) The treatment capability of apparatus improved with the increasing experiments, and the treatment loads of NH3-N, TN, TP and CODcr in the last experiment (the sixth experiment) were1483.33mg·m-2·h-1,84.28mg·m-2·h-1,28.93mg·m-2·h-1and2220.84mg·m-2·h-1, respectively. Significantly positive correlations were found between removal capability and concentrations of NH3-N and TP in influent indicating that the concentrations of NH3-N and TP in influent might be the main factors influencing the purification effect, but it is not the same with TN, NO3--N and NO2--N. |
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