Upland streamwater nitrate dynamics across decadal to sub-daily timescales: a case study of Plynlimon, Wales
Upland streamwater nitrate dynamics across decadal to sub-daily timescales: a case study of Plynlimon, Wales
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DOI：
10.5194/bg-10-8013-2013
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发表时间：
2013-01-01
期刊：
影响因子：
4.9
通讯作者：
Kirchner, J. W.
中科院分区：
文献类型：
--
作者：
Halliday, S. J.;Skeffington, R. A.;Kirchner, J. W.
利用一系列统计技术对威尔士中部普林利蒙的哈夫伦河的溪水硝酸盐动态进行了十年到次日的研究。长期数据来自每周抓取样本(1984-2010年)和7小时样本的高频数据(2007-2009年)，这两个样本都是在两个地点测量的：源流排干荒地和人工林下的下游地点。这项研究是第一批分析如此广泛的时间范围内旱地溪水硝酸盐动态的研究之一，并报告了已确定的主要机制。数据分析没有提供明确的证据表明，流水硝酸盐浓度的长期下降仅与大气沉降量的下降有关，因为氮沉降量在研究期间先增加后减少。溪流温度升高和反硝化作用也可能是溪流硝酸盐浓度下降的原因，前者是通过增加N的吸收速率，后者是由于溶解有机碳浓度的增加。强烈的季节循环，浓度在夏季最低，是由季节性流量最小和季节性生物活动促进硝酸盐吸收所驱动的。观测到了复杂的日动态，周期的相位和幅度具有季节性变化，且日动态沿河流变化。在沼地场地，有规律的每日循环，最低浓度在下午早些时候，与最高气温相对应，表明内河生物处理的重要性。在下游，日动态是一个复合信号，由平流、弥散和下游土壤中硝酸盐的处理产生。干旱条件对河水硝态氮的日动态也有影响。2007年春季日间循环的加强归因于旱灾后硝酸盐可利用性的增加以及这一时期的低流速和高温。高频短期测量和长期监测相结合，为增进对地表水中元素通量和浓度控制的了解提供了一个强有力的工具。
Streamwater nitrate dynamics in the River Hafren, Plynlimon, mid-Wales were investigated over decadal to sub-daily timescales using a range of statistical techniques. Long-term data were derived from weekly grab samples (1984-2010) and high-frequency data from 7-hourly samples (2007-2009) both measured at two sites: a headwater stream draining moorland and a downstream site below plantation forest. This study is one of the first to analyse upland streamwater nitrate dynamics across such a wide range of timescales and report on the principal mechanisms identified. The data analysis provided no clear evidence that the long-term decline in streamwater nitrate concentrations was related to a decline in atmospheric deposition alone, because nitrogen deposition first increased and then decreased during the study period. Increased streamwater temperature and denitrification may also have contributed to the decline in stream nitrate concentrations, the former through increased N uptake rates and the latter resultant from increased dissolved organic carbon concentrations. Strong seasonal cycles, with concentration minimums in the summer, were driven by seasonal flow minimums and seasonal biological activity enhancing nitrate uptake. Complex diurnal dynamics were observed, with seasonal changes in phase and amplitude of the cycling, and the diurnal dynamics were variable along the river. At the moorland site, a regular daily cycle, with minimum concentrations in the early afternoon, corresponding with peak air temperatures, indicated the importance of instream biological processing. At the downstream site, the diurnal dynamics were a composite signal, resultant from advection, dispersion and nitrate processing in the soils of the lower catchment. The diurnal streamwater nitrate dynamics were also affected by drought conditions. Enhanced diurnal cycling in Spring 2007 was attributed to increased nitrate availability in the post-drought period as well as low flow rates and high temperatures over this period. The combination of high-frequency short-term measurements and long-term monitoring provides a powerful tool for increasing understanding of the controls of element fluxes and concentrations in surface waters.