A strong relationship between dissolved organic carbon (DOC) and
sulphate (SO42-) dynamics under drought conditions has been revealed
from analysis of a 10-year time series (1993-2002). Soil solution from
a blanket peat at 10 cm depth and stream water were collected at
biweekly and weekly intervals, respectively, by the Environmental
Change Network at Moor House-Upper Teesdale National Nature Reserve in
the North Pennine uplands of Britain. DOC concentrations in soil
solution and stream water were closely coupled, displaying a strong
seasonal cycle with lowest concentrations in early spring and highest
in late summer/early autumn. Soil solution DOC correlated strongly with
seasonal variations in soil temperature at the same depth 4-weeks prior
to sampling. Deviation from this relationship was seen, however, in
years with significant water table drawdown (>-25 cm), such that DOC
concentrations were up to 60% lower than expected. Periods of drought
also resulted in the release of SO42-, because of the oxidation of
inorganic/organic sulphur stored in the peat, which was accompanied by
a decrease in pH and increase in ionic strength. As both pH and ionic
strength are known to control the solubility of DOC, inclusion of a
function to account for DOC suppression because of drought-induced
acidification accounted for more of the variability of DOC in soil
solution (R-2=0.81) than temperature alone (R-2=0.58). This statistical
model of peat soil solution DOC at 10 cm depth was extended to
reproduce 74% of the variation in stream DOC over this period.
Analysis of annual budgets showed that the soil was the main source of
SO42- during droughts, while atmospheric deposition was the main source
in other years. Mass balance calculations also showed that most of the
DOC originated from the peat. The DOC flux was also lower in the
drought years of 1994 and 1995, reflecting low DOC concentrations in
soil and stream water. The analysis presented in this paper suggests
that lower concentrations of DOC in both soil and stream waters during
drought years can be explained in terms of drought-induced
acidification. As future climate change scenarios suggest an increase
in the magnitude and frequency of drought events, these results imply
potential for a related increase in DOC suppression by episodic
acidification.