Controls on dissolved organic carbon export through surface runoff from loamy agricultural soils

Dissolved organic carbon (DOC) is one of the most active and mobile carbon pools, and thus an important component of the global carbon cycle. Previous research on DOC transport in the soil and on factors controlling DOC export towards the river system focused mainly on forest and wetland areas, with only limited information available from agricultural soils. We carried out rainfall simulations on agricultural field sites to identify the effect of soil properties, field characteristics and hydrological conditions on DOC export by surface runoff from loamy agricultural soils. Furthermore, the temporal evolution of DOC concentrations and specific UV absorbance (SUVA) values in runoff water during a rainfall event was monitored. Additional rainfall simulations in the lab allowed to investigate the effects of drop impact, crop residue incorporation and drying–rewetting of the soil on DOC concentrations and SUVA values in both runoff and percolation water.DOC concentrations were the highest and SUVA values the lowest at the start of a rainfall event, both in runoff and percolation water. Afterwards, DOC concentrations diminished and SUVA values rose to steady values towards the end of the experiments. Overall, rainfall conditions prior to the experiment showed to be a major control on DOC concentrations and quality in runoff water from agricultural fields. Smaller rainfall depths before the experiment and lower initial soil moisture content led to high concentrations of low aromatic DOC in the runoff water. This drying–rewetting effect on DOC concentrations and quality was also observed in the lab for percolation water. For the range of considered soil types, only a limited effect of soil and field characteristics on DOC concentrations and quality in runoff was observed. The effect of reduced tillage on DOC concentrations in surface runoff was ambiguous, with effects differing between experimental field sites.