Hill Alan R, Vidon Philippe G F, Langat Jackson
Department of Geography, York University, Toronto, ON, Canada M3J 1P3.
J Environ Qual. 2004 May-Jun;33(3):911-9. doi: 10.2134/jeq2004.0911.
The influence of riparian zone lithology on nitrate dynamics is poorly understood. We investigated vertical variations in potential denitrification activity in relation to the lithology and stratigraphy of five headwater riparian zones on glacial till and outwash landscapes in southern Ontario, Canada. Conductive coarse sand and gravel layers occurred in four of the five riparian areas. These layers were thin and did not extend to the field-riparian perimeter in some riparian zones, which limited their role as conduits for ground water flow. We found widespread organic-rich layers at depths ranging from 40 to 300 cm that resulted from natural floodplain processes and the burial of surface soils by rapid valley-bottom sedimentation after European settlement. The organic matter content of these layers varied considerably from 2 to 5% (relic channel deposit) to 5 to 21% (buried soils) and 30 to 62% (buried peat). Denitrification potential (DNP) was measured by the acetylene block method in sediment slurries amended with nitrate. The highest DNP rates were usually found in the top 0- to 15-cm surface soil layer in all riparian zones. However, a steep decline in DNP with depth was often absent and high DNP activity occurred in the deep organic-rich layers. Water table variations in 2000-2002 indicated that ground water only interacted frequently with riparian surface soils between late March and May, whereas subsurface organic layers that sustain considerable DNP were below the water table for most of the year. These results suggest that riparian zones with organic deposits at depth may effectively remove nitrate from ground water even when the water table does not interact with organic-rich surface soil horizons.
河岸带岩性对硝酸盐动态的影响尚不清楚。我们调查了加拿大安大略省南部冰川冰碛和冰水沉积地貌上五个源头河岸带的潜在反硝化活性与岩性和地层的垂直变化关系。五个河岸带中有四个出现了传导性粗砂和砾石层。这些层很薄，在一些河岸带并没有延伸到野外河岸边界，这限制了它们作为地下水流通道的作用。我们发现，在40至300厘米深处广泛存在富含有机物的层，这些层是自然洪泛平原过程以及欧洲人定居后谷底快速沉积掩埋表层土壤的结果。这些层的有机质含量差异很大，从2%至5%（残留河道沉积物）到5%至21%（掩埋土壤）再到30%至62%（掩埋泥炭）。通过乙炔阻断法在添加硝酸盐的沉积物浆液中测量反硝化潜力（DNP）。在所有河岸带中，最高的DNP速率通常出现在顶部0至15厘米的表层土壤层。然而，DNP通常不会随深度急剧下降，在深层富含有机物的层中也会出现高DNP活性。2000 - 2002年的地下水位变化表明，地下水仅在3月下旬至5月期间经常与河岸表层土壤相互作用，而维持相当高DNP的地下有机层在一年中的大部分时间都处于地下水位以下。这些结果表明，即使地下水位不与富含有机物的表层土壤层相互作用，深处有有机沉积物的河岸带也可能有效地从地下水中去除硝酸盐。
