近年来，河流上覆水体被视为脱氮的热点地区。然而，由于上覆水中的悬浮颗粒和水本身的物化性质能够造成光衰减现象，太阳光不能总是到达河流上覆水深处，这会使上覆水体形成光照和黑暗带。而且，沉积物通过再悬浮作用进入上覆水体后，沉积物上附着的微生物也由沉积物中的黑暗环境进入上覆水体的光照（或黑暗）环境。已有研究发现光照会对包括氮转化微生物在内的大多数微生物造成影响，因此推测会进一步影响各种氮转化过程。然而，上覆水体辐照度对微生物和脱氮会造成怎样和多大程度的影响目前尚不清楚。针对上述问题，本研究利用15N同位素示踪技术，模拟研究光照对含沙（1 g L-1）河流上覆水体脱氮作用和氧化亚氮的产生，通过分析光照对含沙上覆水体中的细菌群落结构及氮转化功能基因丰度的影响来探究光照对脱氮和氧化亚氮产生的影响机制，主要研究结果如下：

河流上覆水体悬浮泥沙的主要来源之一是沉积物的再悬浮作用，沉积物的再悬浮降低了物种丰富度，改变了原有的微生物群落结构，降低了原有微生物群落中的硝化、反硝化和固氮的潜力。首先，沉积物再悬浮后，泥沙上的微生物群落发生了改变。微生物总丰富度下降了33.1–44.6%。其中厚壁菌门（Firmicutes）、绿弯菌门（Chloroflexi）的相对丰度在14天的悬浮培养结束后分别下降了83.3%和55.6%，而放线菌门（Actinobacteria）的相对丰度上升了97.2%。其次，微生物的群落组成发生了改变，沉积物组中11.1%的OTU是上覆水组中没有的。再者，氮转化功能基因丰度下降，反硝化菌nirS、nirK、nosZ，固氮菌nifH基因丰度分别下降了87.3%（p < 0.001）、19.7%（p < 0.001）、16.2%（p < 0.01）和47.5%（p < 0.001）。造成这些现象的原因可能是不同微生物对沉积物再悬浮导致的环境条件变化的适应能力不同。另外，氨氧化β变形菌（细菌）16s rRNA基因丰度没有发生显著变化，但氨氧化古菌amoA显著下降了93.3%（p < 0.001）。这是因为AOA更加适应沉积物的低氧环境，好氧上覆水环境可能对AOA的生长不利。

光照不利于含沙上覆水体N2O的产生。与黑暗条件相比，光照能够显著抑制含沙上覆水中N2O的产生（p < 0.05），且在0–800W m-2光辐照度（相当于日均辐照度0–266.67W m-2）范围内，光辐照度越强，N2O累积产生量越低（p < 0.05）。进一步分析光照的影响机制发现，光照条件下AOA amoA和AOB amoA的基因丰度分别降低为黑暗条件的1/4和1/2，这会导致硝化过程释放的N2O降低。此外，随着光辐照度的增强，反硝化nirS和nirK基因丰度显著降低（分别为p < 0.05，p < 0.01），且nirS/nirK基因比值显著升高（p < 0.05），这可能造成NO2-还原为N2O的潜力降低以及完全反硝化作用的增加，从而降低N2O的产生。

光照能同时影响含沙上覆水体的脱氮和固氮作用，因此光照对这两个过程的综合作用决定了光照对N2的产生是否具有促进或抑制作用。在较高氨氮浓度（2 mg-N L-1和5 mg-N L-1）和较大悬浮泥沙粒径（75–150 μm）条件下，光照对N2产生具有抑制作用。这是由于较高氨氮浓度有利于固氮菌的生长，以及较大粒径悬浮泥沙具有较弱的光衰减能力，光照条件下的固氮菌nifH基因丰度显著高于黑暗条件（p < 0.001），蓝细菌（Cyanobacteria）在所有细菌中的相对丰度占比较高（42.6%），从而导致光照条件下固氮潜力更强；与之相反，光照条件下的反硝化菌nirS基因丰度显著低于黑暗条件（p < 0.01），表明反硝化潜力较低。在未添加氨氮和较小悬浮泥沙粒径（<75 μm）条件下，由于贫营养条件和更弱的光照环境，蓝细菌在所有组所有细菌中的相对丰度占比均未超过1.2%，这可能是由于其他畏光固氮菌占固氮菌的绝大多数，而且光辐照度的增强导致nifH基因丰度显著下降（p < 0.01），这就意味着固氮潜力随光辐照度的增强而下降。另外，虽然光辐照度增强显著降低了反硝化nirS、nirK和nosZ基因丰度（分别为p < 0.05，p < 0.01，p < 0.01），但nirS/nirK基因比值随光辐照度的增强而显著提高，意味着完全反硝化能力可能随光辐照度的增强而增强。在以上固氮和脱氮的综合作用下，光辐照度越强，N2产生量越多。

本研究剖析了光照对含沙河流上覆水体脱氮作用和氧化亚氮产生的影响，并揭示了相关影响机制，对含沙河流上覆水脱氮过程提供了更深刻的认识，能为河流脱氮的过程模型建立提供参考。然而未来对于光照对河流上覆水脱氮过程的影响，需从固氮的角度进行进一步的探究。

Riverine overlying water has been regarded as a hot spot for nitrogen removal. However, because of suspended particles in the overlying water and water’s physicochemical properties causing light attenuation, sunlight cannot always reach the overlying water deeply, which will form euphotic and aphotic layers. Moreover, after sediments enter overlying water through resuspension, the microorganisms attached to the sediments are exposed to light (or dark) conditions of overlying water from a dark environment. It has been found that light impacts most microorganisms, including nitrogen transformation microorganisms, so it is speculated that it will further affect various nitrogen transformation processes. However, how and to what extent the irradiance of overlying water will affect microorganisms and nitrogen removal is unclear. In response to the above issues, this study investigated nitrogen removal and nitrous oxide production in simulated riverine overlying water systems with suspended sediments under (1 g L-1) via 15N isotope tracing technology. Furthermore, this study explored the underlying mechanisms of light exposure on nitrogen removal and nitrous oxide production by analyzing the effects of light exposure on bacterial communities and the abundance of functional genes for nitrogen transformation in simulated overlying water systems. The main results are as follows:

One of the major sources of suspended sediment in riverine overlying water is sediment resuspension. In this study, sediment resuspension reduced species richness, changed the original microbial communities, and weakened nitrification, denitrification, and nitrogen fixation potential. The total microbial richness decreased by 33.1–44.6%. The relative abundances of Firmicutes and Chloroflexi decreased by 83.3% and 55.6%, respectively, at the end of the 14 days’ incubation, while the relative abundance of Actinobacteria increased by 97.2%. Second, the community composition of microorganisms was altered, and 11.1% of the OTUs in the sediment group were absent from the overlying water group. Then again, the abundance of nitrogen transformation genes nirS, nirK, nosZ and nifH decreased by 87.3% (p < 0.001), 19.7% (p < 0.001), 16.2% (p < 0.01) and 47.5% (p < 0.001), respectively. The reason for this may be the different adaptive ability of different microorganisms to the changes in environmental conditions resulting from the resuspension of sediment. In addition, beta proteobacteria ammonia oxidizers 16S rRNA gene abundance did not change significantly, but AOA amoA significantly decreased by 93.3% (p < 0.001). This may be because AOA is more adapted to the low oxygen environment of sediment, and the aerobic overlying water environment is unfavorable for the growth of AOA.

Light is not conducive to N2O production in riverine overlying water with suspended sediments. Light can significantly inhibit N2O production in overlying water with suspended sediments (p < 0.05) compared with dark conditions. In 0–800W m-2 light irradiance (equivalent to 0–266.67W m-2 daily average irradiance), the stronger the light irradiance, the lower the cumulative N2O production (p < 0.05). Further analysis of the mechanism caused by light showed that the gene abundance of AOA amoA and AOB amoA decreased to 1/4 and 1/2 of that of the dark condition, respectively, which would lead to the decrease of N2O released during nitrification. In addition, with the increase in light irradiance, the abundance of denitrifying nirS and nirK genes decreased significantly (p < 0.05, p < 0.01, respectively), and the ratio of nirS/nirK gene increased significantly (p < 0.05), which may reduce the potential of NO2- reduction to N2O and promote the complete denitrification, leading to reduce the N2O production.

Light can simultaneously affect the nitrogen removal and fixation of overlying water with suspended sediments. Therefore, the comprehensive effect of light on these two processes determines whether light can promote or inhibit the production of N2. At higher ammonium concentrations (2 mg-N L-1 and 5 mg-N L-1) and larger suspended sediment sizes (75–150 μm), light can inhibit N2 production. Higher ammonium concentration is conducive to the growth of diazotrophs, and the suspended sediment with larger particle size has a weaker light attenuation. The abundance of nifH gene under the light condition is significantly higher than that under the dark condition (p < 0.001), and the relative abundance of Cyanobacteria in all bacteria is relatively high (42.6%), resulting in a more substantial nitrogen fixing potential under the light condition. On the contrary, the nirS gene abundance under the light condition was significantly lower than that under the dark conditions (p < 0.01), indicating low denitrification potential. The system with no added ammonium and smaller suspended sediment particle size (<75 μm) was the condition of poor nutrition and weaker light condition. The relative abundance of Cyanobacteria in all bacteria of all groups did not exceed 1.2%, which may be because other photophobic diazotrophs account for the vast majority of diazotrophs. The enhancement of light irradiance leads to the significant decrease of nifH gene abundance (p < 0.01), which means that the nitrogen fixation potential decreases with the enhancement of light irradiance. In addition, although enhanced light irradiance significantly reduced the abundance of nirS, nirK, and nosZ genes (p < 0.05, p < 0.01, p < 0.01, respectively), the ratio of nirS/nirK genes increased significantly with the increase of light irradiance, which means that the complete denitrification potential may increase with the increased light irradiance. Under the comprehensive effects of nitrogen fixation and removal above, the stronger the light irradiance is, the more N2 is produced.

This study analyzed the influence of light on the nitrogen removal and nitrous oxide production of riverine overlying water with suspended particles, revealed the relevant mechanism, provided a deeper understanding of the nitrogen removal process of overlying water with suspended sediment, and can provide a reference for construct a process model of nitrogen removal on riverine overlying water. However, the effect of light on the nitrogen removal process in riverine overlying water needs to be further explored from the perspective of nitrogen fixation.