Controls on fluxes and export of dissolved organic carbon in grasslands with contrasting soil types

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.