Robertson William D, Schiff Sherry L
Dep. of Earth and Environmental Sciences, Univ. of Waterloo, Waterloo, ON, Canada.
J Environ Qual. 2008 Mar-Apr;37(2):669-79. doi: 10.2134/jeq2007.0102.
Streamside vegetated buffer strips (riparian zones) are often assumed to be zones of ground water nitrate (NO3(-)) attenuation. At a site in southwestern Ontario (Zorra site), detailed monitoring revealed that elevated NO3(-) -N (4-93 mg L(-1)) persisted throughout a 100-m-wide riparian floodplain. Typical of riparian zones, the site has a soil zone of recent river alluvium that is organic carbon (OC) rich (36 +/- 16 g kg(-1)). This material is underlain by an older glacial outwash aquifer with a much lower OC content (2.3 +/- 2.5 g kg(-1). Examination of NO3(-), Cl(-), SO4(2-), and dissolved organic carbon (DOC) concentrations; N/Cl ratios; and NO3(-) isotopic composition (delta15N and delta18O) provides evidence of four distinct NO3(-) source zones within the riparian environment. Denitrification occurs but is incomplete and is restricted to a narrow interval located within ~0.5 m of the alluvium-aquifer contact and to one zone (poultry manure compost zone) where elevated DOC persists from the source. In older ground water close to the river discharge point, denitrification remains insufficient to substantially deplete NO3(-). Overall, denitrification related specifically to the riparian environment is limited at this site. The persistence of NO3(-) in the aquifer at this site is a consequence of its Pleistocene age and resulting low OC content, in contrast to recent fluvial sediments in modern agricultural terrain, which, even if permeable, usually have zones enriched in labile OC. Thus, sediment age and origin are additional factors that should be considered when assessing the potential for riparian zone denitrification.
溪流边植被缓冲带（河岸带）通常被认为是地下水硝酸盐（NO₃⁻）衰减的区域。在安大略省西南部的一个地点（佐拉地点），详细监测发现，在一个100米宽的河岸洪泛平原上，NO₃⁻ -N（4 - 93毫克/升）持续处于较高水平。该地点是典型的河岸带，有一个近期河流冲积层的土壤区域，富含有机碳（OC）（36 ± 16克/千克）。该物质下方是一个较老的冰川冰水沉积含水层，有机碳含量低得多（2.3 ± 2.5克/千克）。对NO₃⁻、Cl⁻、SO₄²⁻和溶解有机碳（DOC）浓度、N/Cl比值以及NO₃⁻同位素组成（δ¹⁵N和δ¹⁸O）的检测提供了河岸环境中四个不同NO₃⁻源区的证据。反硝化作用确实发生，但并不完全，且仅限于冲积层与含水层接触处约0.5米范围内的一个狭窄区间以及一个DOC从源头持续升高的区域（家禽粪便堆肥区）。在靠近河流排放点的较老地下水中，反硝化作用仍不足以大量消耗NO₃⁻。总体而言，该地点与河岸环境相关的反硝化作用有限。与现代农业地形中的近期河流沉积物不同，该地点含水层中NO₃⁻的持续存在是其更新世年龄以及由此导致的低有机碳含量的结果，现代农业地形中的近期河流沉积物即使具有渗透性，通常也有富含不稳定有机碳的区域。因此，在评估河岸带反硝化潜力时，沉积物年龄和来源是应考虑的额外因素。
