Factors Controlling the Dissolved Organic Matter Concentration in Pore Waters of Agricultural Soils

Dissolved organic matter (DOM) affects a wide range of soil processes, but it is generally unknown which factors control its concentration. Here we report a survey of soil solution dissolved organic carbon (DOC) concentrations in agricultural soils with contrasting properties. Eighty‐seven agricultural topsoils were sampled throughout Europe. Soil solution was isolated by centrifugation (3500 g) from soils sampled field moist and analyzed directly ( n = 30), from air‐dried soils that were rewetted and incubated for 5 wk ( n = 30), or from soils stored moist at 4°C during 1.5 yr ( n = 39). The soil storage and treatment effects on DOC concentrations in membrane filtered soil solution (0.45 μm), tested on identical samples, showed that air‐drying followed by wetting increased DOC 2.5‐ to 7.8‐fold compared with freshly sampled field moist samples, whereas cold storage had inconsistent effects. In the entire data set, DOC in the soil solution ranged from 12 to 104 mg C L −1 (10th– 90th percentile), with a median of 33 mg C L −1 . Soil or soil solution properties (including ionic strength) only explained 14% of the variance of log[DOC] in freshly sampled soils, whereas 41% was explained in the entire dataset. The stepwise regression models predict that DOC increases with increasing soil organic matter content and decreasing pH or % base saturation. The specific UV‐absorbance of DOM at 254 nm (SUVA) as an indicator of the aromaticity of DOM ranged sevenfold among samples. Soil drying and rewetting decreased the SUVA of DOM, suggesting that DOM released after rewetting originates from decaying biomass rather than from humified organic matter. The DOC concentrations are largely affected by drying–rewetting processes. In freshly sampled soils, DOC concentrations correlate poorly to physicochemical properties of the soil, likely due to biological processes involved.