Brugia Malayi - Clark Science Center - Smith College

Comparison of Nutrient Cycling in Secondary and Mature
Deciduous Forest
Theo Sweezy
This study examines the impact of forest succession, induced by invasive species, on soil chemistry. The hemlock wooly adelgid
(Adelges tsungae, HWA) is causing the decline of eastern hemlock trees (Tsuga canadensis) in the northeast. To ascertain the effect
of this decline on soil nutrient cycling, we compared soil chemistry between a mature hemlock stand and a black birch stand - a
regrowth of hemlock trees logged twenty years ago. We were interested to know if differences in soil chemistry between hemlock
and black birch growth were representative of long-term or short-term differences resulting from forest disturbance. Therefore,
this years study included analysis of soil chemistry beneath black birch trees in a nearby mature hardwood forest.
Concentrations of ammonium and nitrate were measured by ion chromatography after preparing extract solutions by mixing
soil with 0.02M strontium chloride. Concentrations of exchangeable base cations (Mg, Ca, Na, K) were measured on the ICP with
soil extract solutions prepared with 1M ammonium chloride. Exchangeable acid cations, Al and H, were measured on extracts,
prepared with 1M KCl, by tritrations of NaOH and HCl and using a phenolphtalein color indicator. Results were normalized by
soil weight and corrected for soil moisture.
In a general summary of our findings, the mature black birch mineral horizon, similar to young black birch soils, has more
aluminum and hydrogen than base cations. However, the mature black birch organic horizon had a base saturation of 58.8%,
which is significantly higher than the 29% base saturation in young black birch organic horizon.
We measured large nitrogen mineralization rates in mature black birch organic horizons for both incubation periods (May
21-June 19 and June 26 - July 23; total nitrogen= 7.0 mg N/kg soil*days; nitrification=3.8 mg N/kg soil*days). Nitrogen
mineralization and nitrification rates in the young black birch organic horizons for the May-June incubation period (3.3 and 0.1 mg
N/kg soil*days, respectively) were lower compared to mature black birch rates, but are high relative to young black birch data from
data from the summer of 2011.
Findings indicate that the soil chemistry of the young black birch plot does not resemble that of the mature deciduous
forest. Differences in soil chemistry might be explained by the young black birches’ logging history, but geographic and physical
differences (eg. the mature black birch is on a slope and soils drier) could also effect soil chemistry. We anticipate differences due
to location and soil moisture would be small. (Supported by the S. D. Bechtel Fund)
Advisor: Amy Rhodes
2012
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