| The sulphate (SO4) released by mineralisation and desorption from
soil can play an important role in determining concentrations of SO4 in
streams. The MAGIC model was calibrated for two catchments in the Black Forest, Germany
(Schluchsee and Villingen) and SO4 concentrations in the streams for the years
2016 and 2030 were predicted. Special emphasis was placed on the dynamics of soil sulphur
(S) pools. At Schluchsee, 90% of soil S is stored in the organic S (Sorg) pool,
whereas at Villingen, 54% is in the inorganic (Sinorg) pool. The Villingen
stream chemistry was modelled successfully by measured Langmuir isotherm parameters (LIPs)
for Sinorg. Schluchsee data could not be modelled satisfactorily using measured
or freely adapted LIPs only, as the Sinorg pool would have to be more than five
times larger than what was measured. With
60.5 mmolc SO4 m-2 yr-1 as internal soil
source by mineralisation and the measured LIPs, stream data was modelled successfully.
The modelling shows that in these two catchments pre-industrial concentrations of
SO4 in runoff can be reached in the next two decades if S deposition decreases
as intended under currently agreed national and international legislation. Sorg
is the most likely dominant source of SO4 released at Schluchsee.
Mineralization from the Sorg pool must be included when modelling
SO4 concentrations in the stream. As the dynamics and the controlling factors
of S release by mineralisation are not yet clear, this process remains a source of
uncertainty for predictions of SO4 concentrations in streams. Future research
should concentrate on dynamics of S mineralisation in the field, such that mathematical
descriptions of long-term S-mineralisation can be incorporated into biogeochemical
models.
Keywords: sulphate release, organic S, mineralisation, acidification, recovery, modelling,
MAGIC, catchments, predictions, Germany, forest |