『Abstract
The effect of chronic high groundwater nitrate loading on riparian
forests is poorly understood. The growth patterns of northern
white cedar (Thuja occidentalis) and related plant-soil
processes were examined at four riparian sites in southern Ontario,
Canada which have similar vegetation, soils, and hydrology but
have differed in adjacent land use for ＞60 years. Fertilized cropland
at two riparian sites produced groundwater-fed surface flows with
high mean NO3-N concentrations of 9 and 31
mg l-1, whereas mean concentrations were ＜0.5 mg l-1
at two control sites down slope from forest. Tree-ring analysis
at the two nitrate-rich sites indicated a positive growth trend
in 1980-2004 and an absence of a positive growth trend in the
1945-1970 period that preceded high rates of synthetic nitrogen
fertilizer use on cropland. However, a significant increase in
growth also occurred in 1980-2004 at the two control riparian
sites suggesting that high groundwater nitrate inputs did not
influence tree growth. Cedar foliar and litter N content did not
differ significantly between the high nitrate and control sites.
Litter decomposition rates measured by the litterbag technique
at a nitrate-enriched and control site were similar. Litter from
a high nitrate and a control site produced a similar rate of potential
denitrification in lab incubations of riparian surface peat. This
study indicates that prolonged nitrate inputs in groundwater did
not increase nitrogen uptake and growth of white cedar or stimulate
decomposition and denitrification as a result of changes in the
quality of plant material. In the absence of anthropogenic nitrate
inputs, riparian wetland soils are typically high in ammonium
and low in nitrate, and as a consequence, white cedar may have
a limited ability to utilize nitrate.
Keywords: Foliar chemistry; Nitrogen saturation; Riparian forest;
Tree rings; Thuja occidentalis』
1. Introduction
2. Materials and methods
2.1. Study area
2.2. Water chemistry
2.3. Tree core sampling and management
2.4. Foliage and litter chemistry
2.5. Litter decomposition rates
2.6. Soil laboratory incubations
2.7. Statistical analysis
3. Results
4. Discussion
5. Conclusions
Acknowledgements
References