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Buried organic-rich horizons: their role as nitrogen sources in stream riparian zones

Alan R. Hill
Biogeochemistry
Vol. 104, No. 1/3 (July 2011), pp. 347-363
Published by: Springer
Stable URL: http://www.jstor.org/stable/41490480
Page Count: 17
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Buried organic-rich horizons: their role as nitrogen sources in stream riparian zones
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Abstract

The effect of water table fluctuations on the interaction between nitrogen mineralization in buried organic-rich soil horizons and nitrate mobilization into groundwater was examined in the riparian zone of a small headwater agricultural catchment in southern Ontario, Canada. Riparian soils contained buried organic layers and lenses overlying a poorly sorted gravel layer at a depth of 0.8-1.0 m. The riparian water table in summer 2006 generally remained within 0.4 m of the surface, whereas during a major drought in 2007 the water table declined to > 1.9 m in August and riparian soils above the gravel layer inland from the stream remained unsaturated from mid-June to early November. Mean daily net N mineralization and nitrification rates during 2-4 week in situ soil incubations were 0.2-1.35 mg N kg-¹ day-₁ in 0-10 cm surface soils in May-September 2006, whereas N mineralization and nitrification were negligible at 30— 45 and 60-75 cm soil depths. In summer and fall 2007, high daily rates of N mineralization and nitrification of 0.3-0.8 mg N kg-¹ day¹ occurred at 30-45 and 60-75 cm depths that were similar to surface soil rates. The soil nitrate pool at 60-75 cm depth was 16X larger in autumn 2007 in comparison to 2006. During high water tables in November 2006 groundwater in the gravel layer had low N₃ – N concentrations of < 0.1 mg 1 –1 . In contrast, after the drought in 2007 nitrate was flushed into groundwater as the water table rose to within 30-50 cm of the surface in December. An extensive area of high NO₃–N concentrations (3-18 mg 1 –1 ) occurred inland from the stream bank. This zone of high nitrates declined gradually by April 2008 probably as a result of denitrification. These results indicate that buried organic deposits at depth within riparian areas can be important nitrogen sources during major water table drawdowns. The influence of these episodes of mineralization at depth during droughts on riparian groundwater chemistry and the emissions of greenhouse gases to the atmosphere merit further research.

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