Transformation and mineralization of synthetic C-14-labeled humic model compounds by soil-feeding termites

The majority of termite species are considered to be humivorous, but the ex act nature of their carbon and energy source, the mechanisms involved in di gestion and the impact of this feeding habit on the humification of soil or ganic matter are largely obscure. We performed feeding trials with soil-fee ding termites (Termitidae: Termitinae), using C-14-labeled humic model comp ounds as substrates. In the case of Cubitermes orthognathus, the components of chemically identical synthetic humic acids (HA), labeled either in thei r proteinaceous or aromatic building blocks, and synthetic fulvic acids (FA ), labeled in the aromatic building blocks, all had similarly low mineraliz ation rates (2.5, 2.2 and 2.7%, respectively) when incubated in soil for 10 days in the absence of termites. When termites were present, the mineraliz ation rate of the aromatic component of HA and FA increased only slightly b ut significantly (2.4 and 3.1%, respectively; P < 0.05), whereas that of th e proteinaceous component increased more than 10-fold (30.8%). Similar resu lts were obtained when the protein was peptonized prior to polymerization ( 9.7 vs. 27.4% mineralization). Mineralization of HA was accompanied by a tr ansformation of the residual peptide label to FA, whereas the aromatic labe l of HA and FA was partly transformed to humin during gut passage. High-per formance gel permeation chromatography showed a strong shift in the size-cl ass distribution of peptide label towards low-molecular-weight products, es pecially of the material ingested by the termites; the smallest molecules w ere recovered from the termite bodies. Similar results were obtained in fee ding trials with Cubitermes umbratus and Thoracotermes macrothorax. Togethe r with previous findings, the current results provide strong evidence that during gut passage, the combined action of extreme alkalinity in the anteri or hindgut, autoxidative processes, and probably also proteolytic activitie s, renders a large reservoir of potential substrates accessible. While pept idic components of humic substances are selectively digested, aromatic comp onents are apparently not an important food source for soil-feeding termite s. These findings have important implications for the mobilization of organ ic nitrogen in tropical soils. (C) 2000 Elsevier Science Ltd. All rights re served.