不同类型氮输入对三峡库区消落带紫色潮土氮赋存形态的影响

陆源氮输入是三峡水体富营养化的主要原因之一，外源氮在消落带土壤中的赋存形态是决定其进一步向三峡水体释放的关键.为此，以三峡典型土壤-紫色潮土为研究对象，进行4种氮输入〔分别添加NaNO₃、NH₄NO₃、(NH₄)₂SO₄、CO (NH₂)₂〕和淹水-落干两个水文条件处理，利用连续分级提取法测定土壤氮赋存形态含量，分析氮添加类型及水文条件对消落带紫色潮土氮赋存形态的影响.结果表明:NaNO₃、NH₄NO₃、(NH₄)₂SO₄和CO (NH₂)₂添加下，土壤培养结束后，离子交换态氮(IEF-N)含量在落干期分别为93.88、79.42、59.02和46.80 mg/kg，在淹水期分别为65.60、56.95、42.46和32.94 mg/kg；有机及硫化物结合态氮(OSF-N)含量在落干期分别为122.18、126.21、137.53和148.19 mg/kg，在淹水期分别为142.22、149.09、156.43和161.76 mg/kg；IEF-N含量变化占比在落干期分别为45.20%、35.56%、21.96%和13.82%，在淹水期分别为36.57%、30.80%、16.21%和7.26%；OSF-N含量变化占比在落干期分别为12.33%、15.02%、22.57%和29.68%，在淹水期分别为16.76%、21.84%、26.73%和30.29%.落干期和淹水期，IEF-N含量及其变化在氮添加总量中的占比均表现为NaNO₃添加下最高、CO (NH₂)₂添加下最低，且落干期大于淹水期；OSF-N含量及其变化占比则与IEF-N表现相反.落干期和淹水期，外源氮主要表现为向OSF-N转化，此外，外源氮在落干期还存在向铁锰氧化物结合态氮转化的过程.研究显示，控制落干期消落带土壤氮输入(特别是硝态氮)是预防三峡水库富营养化的有效途径. 

Effects of Different Types of Nitrogen Inputs on Purple Alluvial Soil Nitrogen Fractions in Riparian Zone of Three Gorges Reservoir Area

Terrestrial nitrogen (N) input is one of the main reasons for the eutrophication in the Three Gorges Reservoir (TGR). The chemical form of exogenous N in the riparian zone soil plays a crucial role in its further release into the TGR. Purple alluvial soils were collected in the riparian zone of the TGR, and soil N fractions were measured using the sequential fractionation method. Soil N fractions were investigated under the treatments of four types of N input (NaNO₃, NH₄NO₃, (NH₄)₂SO₄, CO(NH₂)₂) and two hydrological conditions (dry and flood). The results showed that with the addition of NaNO₃, NH₄NO₃, (NH₄)₂SO₄ and CO(NH₂)₂, the contents of soil ion-exchangeable form (IEF-N) were 93.88, 79.42, 59.02 and 46.80 mg/kg after the dry incubation, respectively, and were 65.60, 56.95, 42.46 and 32.94 mg/kg after the flood incubation, respectively; the contents of soil organic matter-sulfide form (OSF-N) were 122.18, 126.21, 137.53 and 148.19 mg/kg after the dry incubation, respectively, and were 142.22, 149.09, 156.43 and 161.76 mg/kg after the flood incubation, respectively. Meanwhile, with the addition of NaNO₃, NH₄NO₃, (NH₄)₂SO₄ and CO(NH₂)₂, the proportions of the content of IEF-N change in the amount of total N addition were 45.20%, 35.56%, 21.96% and 13.82% after the dry incubation, respectively, and were 36.57%, 30.80%, 16.21% and 7.26% after the flood incubation, respectively; the proportions of the content of OSF-N change in the amount of total N addition were 12.33%, 15.02%, 22.57% and 29.68% after the dry incubation, respectively, and were 16.76%, 21.84%, 26.73% and 30.29% after the flood incubation, respectively. The content of IEF-N and the proportion of its change in the amount of total N addition were the highest with NaNO₃ addition, and the lowest with CO(NH₂)₂ addition during both the dry and flood periods. Moreover, they were much higher in the dry period than that in the flood period. However, the opposite results were observed for that of OSF-N. We concluded that the exogenous N was mostly transformed to OSF-N during both the dry and the flood periods. In addition, it was also transformed to iron-manganese oxide form during the dry period. Therefore, we suggested that the exogenous N control, especially for nitrate in the riparian zone during the dry period, was an effective way to prevent the eutrophication of the TGR.