Size matters: biochemical mineralization and microbial incorporation of dicarboxylic acids in soil

The transformation and turnover time of medium- to long-chain dicarboxylic acids (DCA) in soil is regulated by microbial uptake and mineralization. However, the chain length of n-alkyl lipids may have a remarkable influence on its microbial utilization and mineralization and therefore on the formation of stable soil organic carbon from e.g. leave- needle- and root-derived organic matter during decomposition. To investigate their size dependent mineralization and microbial incorporation, four DCA of different chain lengths (12–30 carbon atoms), that were 13 C labeled at each of their terminal carboxylic groups, were applied to the Ah horizon of a Fluvic Gleysol. Incorporation of 13 C into CO 2 and in distinct microbial groups classified by phospholipid fatty acid (PLFA) analysis was investigated. Mineralization of DCA and incorporation into PLFA decreased with increasing chain length, and the mineralization rate was highest during the first days of incubation. Half-life time of DCA carbon in soil increased from 7.6 days for C 12 DCA to 86.6 days for C 18 DCA and decreased again to 46.2 days for C 22 DCA, whereas C 30 DCA had the longest half-life time. Rapid and efficient uptake of C 12 DCA as an intact molecule was observable. Gram-negative bacteria incorporated higher amounts of DCA-derived 13 C compared to other microbial groups, especially compared to actinomycetes and fungi during the first phase of incubation. However, the incorporation of C 12 DCA derived 13 C into the PLFA of actinomycetes, and fungi increased steadily during the entire incubation time, suggesting that those groups take up the 13 C label from necromass of bacteria that used the C 12 DCA for formation of their lipids before.