School of Life Sciences, Arizona State University, P.O. Box 874501, Tempe, Arizona 85287, USA.
Ecol Appl. 2011 Oct;21(7):2618-36. doi: 10.1890/10-1613.1.
The Indian Bend Wash (IBW) flood-control project relies on a greenbelt to carry floods through Scottsdale, Arizona, USA. The greenbelt is characterized by a chain of shallow artificial lakes in a larger floodplain of irrigated turf, which has been protected from encroaching urban development. As such, this urban stream-floodplain complex can be divided into three subsystems: artificial lakes, channelized stream segments, and floodplain. We conducted experiments to evaluate which, if any, of these subsystems were important sites of denitrification, and to explore factors controlling denitrification rates. Denitrification enzyme activity (DEA) bioassays were conducted on sediments from eight lake and six stream segments as well as soil samples from eight floodplain transects. Mass-specific potential denitrification rates were significantly higher in lakes than in streams or floodplains. Nutrient limitation bioassays revealed that nitrate (NO3-) limited denitrification in lake sediments, a surprising finding given that NO3(-)-rich groundwater additions frequently raised lake NO3(-) concentration above 1 mg N/L. Experiments on intact lake cores suggested that denitrification was limited by the rate NO3(-) diffused into sediments, rather than its availability in overlying water. Floodplain denitrification was limited by water content, not NO3(-) or C, and irrigation of soils stimulated denitrification. We constructed a N budget for the IBW stream-floodplain complex based on our experimental results. We found that both lakes and floodplains removed large quantities of N, with denitrification removing 261 and 133 kg N ha(-1) yr(-1) from lake sediments and floodplain soils, respectively, indicating that lakes are hotspots for denitrification. Nevertheless, because floodplain area was >4.5 times that of lakes, floodplain soils removed nearly 2.5 times as much N as lake sediments. Given the desert's low annual precipitation, a finding that floodplain soils are active sites of denitrification might seem implausible; however, irrigation is common in urban landscapes, and it elevated annual denitrification in IBW. Based on our results, we conclude that construction of artificial lakes created hotspots while application of irrigation water created hot moments for denitrification in the stream-floodplain complex, demonstrating that management decisions can improve the ability of urban streams to provide critical ecosystem services like N retention.
印第安弯冲沟（IBW）防洪工程依赖绿化带将洪水输送穿过美国亚利桑那州斯科茨代尔。绿化带的特点是在灌溉草坪的更大洪泛区中有一连串的浅人工湖，这些人工湖免受不断侵占的城市发展的影响。因此，这个城市溪流-洪泛平原综合体可以分为三个子系统：人工湖、渠道化的溪流段和洪泛平原。我们进行了实验，以评估这些子系统中哪些（如果有）是反硝化的重要场所，并探讨控制反硝化速率的因素。对来自八个湖泊和六个溪流段的沉积物以及来自八个洪泛平原横截面上的土壤样本进行了反硝化酶活性（DEA）生物测定。湖泊沉积物的比质量潜在反硝化速率明显高于溪流或洪泛平原。养分限制生物测定表明，硝酸盐（NO3-）限制了湖泊沉积物中的反硝化作用，这一发现令人惊讶，因为富硝酸盐的地下水添加物经常将湖泊中的 NO3-浓度提高到 1mg N/L 以上。对完整湖泊岩芯的实验表明，反硝化作用受到 NO3-扩散到沉积物中的速率限制，而不是其在覆盖水中的可用性。洪泛平原的反硝化作用受到含水量的限制，而不是 NO3-或 C 的限制，并且土壤的灌溉刺激了反硝化作用。我们根据实验结果为 IBW 溪流-洪泛平原综合体构建了一个 N 预算。我们发现，湖泊和洪泛平原都去除了大量的 N，其中湖泊沉积物和洪泛平原土壤中的反硝化作用分别去除了 261 和 133kgNha-1yr-1，这表明湖泊是反硝化作用的热点。然而，由于洪泛平原的面积是湖泊的 4.5 倍以上，因此洪泛平原土壤去除的 N 几乎是湖泊沉积物的两倍。考虑到沙漠的年降水量较低，洪泛平原土壤是反硝化作用的活跃场所这一发现似乎不太可信；然而，灌溉在城市景观中很常见，并且它提高了 IBW 的年度反硝化作用。根据我们的结果，我们得出结论，人工湖的建设创造了热点，而灌溉水的应用则为溪流-洪泛平原综合体中的反硝化作用创造了热点时刻，这表明管理决策可以提高城市溪流提供关键生态系统服务（如 N 保留）的能力。
