Retention of Dissolved Organic Carbon and Sulfate in Aggregated Acid Forest Soils

Abstract

AbstractAdsorption of dissolved organic carbon (DOC) and sulfate was examined in mineral A and B horizons of aggregated acid forest soils from declining (Oberwarmensteinach) and healthy (Wülfersreuth) Norway spruce [Picae abies (L.) Karst.] stands in northeastern Bavaria, Germany. Adsorption studies were conducted in the laboratory on undisturbed, aggregated soil samples under nonequilibrium conditions using a miscible displacement method. Initial mass relationships suggest a net release of DOC in the A horizons of both stands at all DOC concentration levels added. In contrast, the B horizons retained DOC from solutions containing more than 4 to 5 g C m−3 in the Oberwarmensteinach B horizon, and 2 to 3 g m−3 in the Wülfersreuth B horizon, respectively. These concentrations correspond to mean annual DOC concentrations in the B horizon solutions of both sites. Addition of sulfate (1.60 molc SO4 m−3) significantly reduced DOC retention, suggesting competition between SO4 and parts of DOC for adsorption sites. Retention of sulfate from solution occurred concentrations with more than 0.38 to 0.42 molc SO4 m−3 in Oberwarmensteinach, and 0.25 to 0.31 molc SO4 m−3 in Wülfersreuth, respectively. Compared with batch experiments, the miscible displacement method resulted in lower affinities of the soils for DOC. Reduction of DOC and sulfate retention by the use of the flow‐generated technique is due to the consideration of sorption kinetics, which are influenced by soil structure and texture. Although this method underestimates the affinity of a soil for DOC to a certain extent, we suggest that it reflects the situation in the field quite well. Flow‐generated experiments showed that the decline phenomena of spruce in Oberwarmensteinach correspond with a reduced potential for DOC retention in the upper B horizon, as well as with lower sulfate retention throughout the profile.