『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 day-1 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 16× larger in autumn 2007
in comparison to 2006. During high water tables in November 2006
groundwater in the gravel layer had low NO3-N
concentrations of <0.1 mg l-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 l-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.
Keywords: Riparian zone; Net N mineralization; Nitrification;
Water table; Soil organic matter; Groundwater』
Introduction
Study sites
Methods
Soil sampling
Nitrogen mineralization and nitrification
Groundwater hydrology and chemistry
Results
Soil organic carbon and nitrogen contents
Water table fluctuations and groundwater flow patterns
Net nitrogen mineralization and nitrification
Ammonium and nitrate pools
Groundwater nitrogen
Discussion
Buried stores of organic matter
Impact of water table fluctuations on soil n dynamics
Nitrate mobilization into riparian groundwater
Export of mobilized groundwater nitrate to streams
Riparian zone characteristics in relation to subsurface N mineralization
and nitrate mobilization
Conclusion
Acknowledgements
References