Kaushal Sujay S, Groffman Peter M, Mayer Paul M, Striz Elise, Gold Arthur J
University of Maryland, Center for Environmental Science, Appalachian Laboratory, 301 Braddock Road, Frostburg, Maryland 21532, USA.
Ecol Appl. 2008 Apr;18(3):789-804. doi: 10.1890/07-1159.1.
Increased delivery of nitrogen due to urbanization and stream ecosystem degradation is contributing to eutrophication in coastal regions of the eastern United States. We tested whether geomorphic restoration involving hydrologic "reconnection" of a stream to its floodplain could increase rates of denitrification at the riparian-zone-stream interface of an urban stream in Baltimore, Maryland. Rates of denitrification measured using in situ 15N tracer additions were spatially variable across sites and years and ranged from undetectable to >200 microg N x (kg sediment)(-1) x d(-1). Mean rates of denitrification were significantly greater in the restored reach of the stream at 77.4 +/- 12.6 microg N x kg(-1) x d(-1) (mean +/- SE) as compared to the unrestored reach at 34.8 +/- 8.0 microg N x kg(-1) x d(-1). Concentrations of nitrate-N in groundwater and stream water in the restored reach were also significantly lower than in the unrestored reach, but this may have also been associated with differences in sources and hydrologic flow paths. Riparian areas with low, hydrologically "connected" streambanks designed to promote flooding and dissipation of erosive force for storm water management had substantially higher rates of denitrification than restored high "nonconnected" banks and both unrestored low and high banks. Coupled measurements of hyporheic groundwater flow and in situ denitrification rates indicated that up to 1.16 mg NO3(-)-N could be removed per liter of groundwater flow through one cubic meter of sediment at the riparian-zone-stream interface over a mean residence time of 4.97 d in the unrestored reach, and estimates of mass removal of nitrate-N in the restored reach were also considerable. Mass removal of nitrate-N appeared to be strongly influenced by hydrologic residence time in unrestored and restored reaches. Our results suggest that stream restoration designed to "reconnect" stream channels with floodplains can increase denitrification rates, that there can be substantial variability in the efficacy of stream restoration designs, and that more work is necessary to elucidate which designs can be effective in conjunction with watershed strategies to reduce nitrate-N sources to streams.
城市化进程加快以及溪流生态系统退化导致氮输送量增加，这正促使美国东部沿海地区富营养化。我们测试了涉及将溪流与洪泛区进行水文“重新连接”的地貌恢复措施，是否能提高马里兰州巴尔的摩市一条城市溪流河岸带与溪流界面处的反硝化速率。利用原位添加15N示踪剂测量的反硝化速率在不同地点和年份存在空间变异性，范围从检测不到到>200微克氮×（千克沉积物）-1×天-1。溪流恢复段的平均反硝化速率为77.4±12.6微克氮×千克-1×天-1（平均值±标准误差），显著高于未恢复段的34.8±8.0微克氮×千克-1×天-1。恢复段地下水和溪水中硝酸盐氮的浓度也显著低于未恢复段，但这也可能与水源和水流路径的差异有关。为促进洪水泛滥和消散雨水侵蚀力而设计的低水文“连通”河岸的河岸区域，其反硝化速率明显高于恢复后的高“非连通”河岸以及未恢复的低河岸和高河岸。对潜流地下水流动和原位反硝化速率的联合测量表明，在未恢复段，每升地下水通过一立方米沉积物在河岸带与溪流界面处平均停留时间为4.97天的情况下，可去除高达1.16毫克硝酸根氮，恢复段硝酸盐氮的去除量估计也相当可观。未恢复段和恢复段中硝酸盐氮的去除量似乎受水文停留时间的强烈影响。我们的结果表明，旨在使河道与洪泛区“重新连接”的溪流恢复措施可提高反硝化速率，溪流恢复设计的效果可能存在很大差异，并且需要开展更多工作来阐明哪些设计与流域策略相结合能有效减少溪流中的硝酸盐氮来源。
