Lignin Degradation and Humus Formation in Alluvial Soils and Sediments

The contribution of lignin to the formation of humic compounds was examined in different environments of the terrestrial-aquatic interface in the Garonne River valley in southwestern France. Alluvial soils and submerged or nonsubmerged river and pond sediments containing alder, poplar, or willow [¹⁴C-lignin]ligno-celluloses were incubated. After a 49-day incubation period, 10 to 15% of labeled lignins in alluvial soils was recovered as evolved ¹⁴CO₂. In nonsubmerged sediments, 10% of the applied activity was released as ¹⁴CO₂, and in submerged sediments, only 5% was released after 60 days of incubation. In the different alluvial soils and sediments, the bulk of residual activity (70 to 85%) remained in the two coarsest-grain fractions (2,000 to 100 and 100 to 50 μm). Only 2 to 6% of the residual activity of these two coarse fractions was recovered as humic and fulvic acids, except in the case of alder [¹⁴C-lignin]lignocellulose, which had decomposed in a soil collected beneath alders. In this one 55% of the residual activity was extracted as humic substances from the 2,000- to 100-μm fraction. Humic and fulvic acids represented from 6 to 50% of the residual activity in the finest-grain fractions (50 to 20 and 20 to 0 μm). The highest percentages were obtained in soil collected beneath alders and in submerged pond sediment. The contribution of different groups of microorganisms, as well as nutrients and clay content, may influence humic-substance formation in such environments. Physical stability also may be an important factor for complex microbial activity involved in this process.