Collaborative Research: Controls on hyporheic nitrate retention - discriminating among transport, reaction-rate, and substrate limitation

[409534]氮在水系中的运输和命运是了解流域氮输出的关键。硝酸盐（NO3 -）的保留，特别是反硝化作用，已知取决于多种因素：地球化学条件（主要是氧化还原电位），底物（主要是NO3 -和溶解有机碳（DOC）的有效性），以及硝酸盐和DOC向有利于反硝化的位置的运输。我们将研究在俄勒冈州和怀俄明州的河流中硝酸盐滞留的控制：4条森林河流，4条农业河流和4条城市河流。在Lotic生物间氮实验II （LINX II）项目的合作下，每个河段将注射15次NO3 -，该项目使用氮-15 （15n）技术来研究吸收和保留。与LINX II合作，可以协同和经济有效地检查流和潜流氮循环。本研究的目的是研究控制小河流潜流区硝酸盐滞留和反硝化的因素，并量化这些河流中由于潜流交换而导致的硝酸盐滞留的比例。我们将检验以下假设：(1)由于底物和速率的限制，水源、森林溪流的低潜反硝化作用将较低，但由于无机氮的限制，相对于农田和城市溪流，生物同化作用将较高；(2)在周围土地利用以农业为主的中游网络位置，潜流反硝化作用将最大，但由于氮的限制较小，NO3 -的总损失将受到运输限制，生物同化将减少；(3)城市水系潜流带的潜在反硝化速率较高，但潜流带的总硝酸盐滞留量较低，因为潜流带的反硝化和生物同化都将受到严重的运输限制。河流河段纵剖面的高程测量将用于量化河道形态，并为地下水流动和反应性输运模拟提供基础。我们已经成功地使用这种技术来模拟地下水流动和计算地下停留时间分布[例如，Kasahara和wonzell, 2003；wonzell et al., 2003]。我们将基于对示踪剂测试的新理解，使用示踪剂测试来测量次高区的停留时间分布[Gooseff等人，2003；Haggerty et al., 2000；哈格蒂等人，2002]。我们将把我们的工作与LINX II NSF项目结合起来，在俄勒冈州和怀俄明州的12个LINX II实验河段的地下带安装一个采样井和压力计网络。采样井将提供地下水通道，我们将从中获得样品，用于测量15个NO3 -， 15个N2(g)和15个N2O(g)以及DO， DOC和其他物理和地球化学参数，这些参数将用于量化潜流区的生物同化，反硝化和地下水流量。我们将获得这些和其他（例如，15个NH4 +） n物种的LINX II流数据。这项工作将建立并增强当前的LINX II项目。LINX II项目将受益于其12个地点的潜流区氮循环的量化，并将获得一种改进氮动力学模型的方法。其中三个实验区位于俄勒冈州立大学校园内，这使我们能够将橡树溪作为一个实践性的、主动学习的实验室，有3门课程，每年有300多名学生注册。通过这些课程，本科生和研究生的研究项目，以及本科生参与我们的实地工作，大量学生将获得与经验相适应的水文学实地经验。USU研究团队将继续通过提顿科学学校与中学生和高中生进行互动，展示实地活动，并为这些学生提供实际的实地体验。俄勒冈州立大学和USUresearch团队将共同开发一个网站，将我们的数据和与项目相关的科学活动以及结果传播给K-12学生和教师，并将通过nsf赞助的GLOBE计划促进当地学校对地表水水文和水质的学习。

0409534The transport and fate of nitrogen in stream networks is critical to understanding watershed exportsof nitrogen. Nitrate (NO3 - ) retention, and denitrification in particular, is known to be dependent upona variety of factors: geochemical conditions (chiefly RedOx potential), substrate (largely availabilityof NO3 - and dissolved organic carbon (DOC)), and transport of nitrate and DOC to favorablelocations for denitrification. We will examine the controls of nitrate retention in the hyporheic zonesof streams in Oregon and Wyoming: 4 forested stream reaches, 4 agricultural streams, and 4 urbanstreams. Each reach will be the subject of 15 NO3 - injections in partnership with the Lotic InterbiomeNitrogen eXperiment II (LINX II) project which is using nitrogen-15 ( 15 N) techniques to studyuptake and retention. Collaborating with LINX II allows synergistic and cost-effective examinationof stream and hyporheic nitrogen cycling. The objectives of our research are to examine the factorscontrolling nitrate retention and denitrification in hyporheic zones of small streams and to quantifythe fraction of nitrate retention in these streams due to hyporheic exchange. We will test thefollowing hypotheses: (1) hyporheic denitrification in headwater, forested streams will be lowbecause of substrate and rate limitations, yet biotic assimilation will be high, relative to agriculturaland urban streams, because of inorganic N-limitation; (2) hyporheic denitrification will be greatestin mid-network locations where surrounding land use is predominantly agricultural, however, totalloss of NO3 - will be transport-limited and biotic assimilation will be reduced because nitrogen is lesslimiting; and (3) potential rate of denitrification in the hyporheic zone will be high in the urbanstream reaches, but total nitrate retention in the hyporheic zone will be low because bothdenitrification and biotic assimilation will be severely transport-limited.Elevation surveys of the longitudinal profiles of the stream reaches will be used to quantify channelmorphology and provide a base for groundwater flow and reactive transport simulations. We havesuccessfully used this technique to model groundwater flow and calculate hyporheic residence timedistributions [e.g., Kasahara and Wondzell, 2003; Wondzell et al., 2003]. We will use tracer testsbased on new understanding of their use for measuring residence time distributions in the hyporheiczone [Gooseff et al., 2003; Haggerty et al., 2000; Haggerty et al., 2002].We will couple our work to the LINX II NSF project by installing a sampling well andpiezometer network in the hyporheic zone of 12 LINX II experimental reaches in Oregon andWyoming. The sampling wells will provide access to hyporheic water, from which we will obtainsamples for measurement of 15 NO3 - , 15 N2(g), and 15 N2O(g) as well as DO, DOC and other physical andgeochemical parameters, which will be used to quantify biotic assimilation, denitrification, andgroundwater flow in the hyporheic zone. We will have access to LINX II stream data on these andother (e.g., 15 NH4 + ) N-species. The work will build upon, and enhance the current LINX II project.The LINX II project will benefit from quantification of nitrogen cycling in the hyporheic zone of 12of its sites and will gain a method to refine models of nitrogen dynamics.Three of the experimental reaches are on OSU campus, which allows us to use Oak Creek as a hands-on, active-learning laboratory in 3 courses with a combined annual enrollment of more than 300students. Through these courses, undergraduate and graduate research projects, and the participationof undergraduates in our field work, a large number of students will gain field experience inhydrology at experience-appropriate levels. The USU research team will continue to interact withmiddle school and high school students through the Teton Science School, demonstrating fieldactivities and facilitating hands-on field experience for these students. Together, the OSU and USUresearch teams will develop a web site with our data and project-related science activities anddissemination of results for K-12 students and teachers, and will promote learning about surfacewater hydrology and water quality in local schools through the NSF-sponsored GLOBE program.