蓝藻固氮对人工湿地颗粒物有机氮的贡献

Contribution of nitrogen fixation by cyanobacteria to particulate organic nitrogen in a constructed wetland

固氮蓝藻能为水生态系统输入新氮,生物固氮对水体氮的贡献观点不一。人工湿地中固氮蓝藻输入的新氮对除氮效率的影响仍不明晰。本研究通过连续监测,比较了北京野生动物救护中心水禽栖息的人工湖和净化其水质的人工湿地中的浮游植物组成、颗粒物有机氮(PON)和水体固氮速率(Rn),分析固氮蓝藻鱼腥藻属(Anabaena)和非固氮蓝藻微囊藻属(Microcystis)藻细胞密度的季节变化,及其与水体PON和Rn相关性。通过IsoSource软件分析15N自然丰度,计算了各个来源(大气N_2、鸟粪、沉积物和水体硝酸盐)对人工湖和人工湿地中PON的贡献率。结果显示,人工湖中鱼腥藻和微囊藻交替爆发,Rn与PON和鱼腥藻细胞密度均呈正相关(P<0.05),说明鱼腥藻输入的新氮可能被微囊藻利用转化为PON。大气N_2对生长季人工湖和人工湿地PON贡献率分别为0.5%~82.0%和50.0%~86.0%,9月水体固氮速率达到最高时大气N_2对PON贡献率可达80%以上。本研究表明,人工湖中的鱼腥藻固氮能为微囊藻提供可利用氮,可能是人工湖夏季微囊藻水华的原因之一。由于浮游植物在表流人工湿地较难沉淀和吸附,浮游植物增多使人工湿地去除PON的能力降低,因此鱼腥藻固氮可能间接降低人工湿地除氮能力。

N-fixing cyanobacteria can create extra nitrogen for aquatic ecosystems. Previous studies reported inconsistent patterns of the contribution of biological nitrogen fixation to the nitrogen pools in aquatic ecosystems. However, there were few studies concerning the effect of N-fixation by cyanobacteria on the nitrogen removal efficiency in constructed wetlands. This study was performed at Beijing Wildlife Rescue and Rehabilitation Centre, where a constructed lake for the habitation of waterfowls and a constructed wetland for purifying water from the lake are located. The composition of phytoplankton communities, the concentrations of particulate organic nitrogen （PON） and nitrogen fixation rates （Rn） in the constructed lake and the constructed wetland were compared throughout a growing season. We counted the densities of Anabaena and Microcystis cells, and explored their relationships with PON and Rn in water. The proportions of PON from various sources, including the ambient N2, waterfowl faeces, wetland sediments and nitrates in water, were calculated by the natural abundance of 15N with the IsoSource software. The results revealed that the constructed lake was alternately dominated by Anabaena and Microcystis throughout the growing season, and the Rn was positively correlated with PON and the cell density of Anabaena （P〈0.05）. This implied that the fixed nitrogen by N-fixing Anabaena might be utilized by nonN-fixing Microcystis, maintaining the fixed nitrogen with PON form. The contributions of ambient N2 to the PON in the constructed lake and wetland were 0.5%-82.0% and 50.0%-86.0% respectively during the growing season. The proportions of PON from N2 increased to more than 80% when the Rn reached the highest in September. The results demonstrated that the nitrogen fixed by Anabaena might be utilized by nonN-fixing Microcystis, which formed water blooms in summer. Therefore, the decline of the removal efficiency of PON in the constructed wetland in summer might indirectly result from the nitrogen fixation, since the proliferated algae were difficult to sediment in surface flow wetlands.