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Abstract

Export of dissolved organic carbon
(DOC) from grassland ecosystems can be an important
C flux which directly affects ecosystem C
balance since DOC is leached from the soil to the
groundwater. DOC fluxes and their controlling factors
were investigated on two grassland sites with
similar climatic conditions but different soil types
(Vertisol vs. Arenosol) for a 2.5-year period. Parts of
both grasslands were disturbed by deep ploughing
during afforestation. Contrary to what was expected,
ploughing did not increase DOC export but surface
soil DOC concentrations decreased by 28% (Vertisol)
and 14% (Arenosol). DOC flux from the soil profile
was negatively influences by the clay content of
the soil with seven times larger DOC export in the
clay-poor Arenosol (55 kg C ha-1 a-1) than in the
clay-rich Vertisol (8 kg C ha-1 a-1). At the Arenosol
site, highest DOC concentrations were measured in
late summer, whereas in the Vertisol there was a time
lag of several months between surface and subsoil
DOC with highest subsoil DOC concentrations during
winter season. DOC export was not correlated
with soil organic carbon stocks. Large differences in
14C concentrations of 22–40 pMC between soil
organic carbon and DOC in the subsoil indicated that both C pools are largely decoupled. We conclude that DOC export at both sites is not controlled by the
vegetation but by physicochemical parameters such
as the adsorption capacity of soil minerals and the
water balance of the ecosystem. Only in the acidic
sandy Arenosol DOC export was a significant C flux of about 8% of net ecosystem production.