Degradation of softwood [14C lignin] lignocelluloses and its relation to the formation of humic substances in river and pond environments

Abstract

The fate of lignin in water and sediment of the Garonne river (France) and of a pond in its floodplain was examined using specifically labeled [14C-lignin] lignocelluloses. No significant differences appeared in the mineralization rate of alder, poplar or willow [14C-lignin] in running water samples. Conversion of total radioactivity to 14CO2 ranged between 18.7% and 24.4% after 120 days of incubation. Degree of 14C-labeled lignin mineralization in standing water and sediments was clearly lower, especially in submerged sediments, and was correlated with oxygen supply. After 60 days of incubation 3.3% to 7.9% of the 14C-labeled lignin was recovered in water samples as dissolved organic carbon originating from microbial metabolism. In water extracts from sediment the percentage of dissolved organic 14C was only 0.4% to 1.3% of the applied activity. In the humic fraction extracted from sediments it did not exceed 4.4% which was much lower than in soils. No significant difference appeared between river and pond conditions for humic substances formation.