Applying fluorescence and 2D-correlation spectroscopy to elucidate changes in the chemical composition of DOM from soils with different vegetation types

To understand the decomposition of the dissolved organic matter (DOM), four different soils (from grassland, spruce forest, oak forest, and agricultural land) were incubated at 15 °C, and the DOM changes were monitored using UV-Vis, synchronized fluorescence and 2D-correlation spectroscopy. The DOM fraction of the agricultural and grassland soils showed a significant increase in both SUVA values in the middle of the incubation ranging from an initial 3–5 L mg−1 m−1 to 40–50 L mg−1 m−1. Synchronous fluorescence spectroscopy revealed that at the beginning of incubation, on day 3, there was an increase in the proportion of low molecular-weight, readily biodegradable compounds (PLF, protein-like fluorescence), while from day 8 onwards, microbial degradation generated a large number of medium-molecular-weight molecules (MHLF, microbial humus-like fluorescence). At further sampling dates, the proportion of compounds of microbial origin decreased systematically, due to a decrease in microbial activity caused by the lack of substrate. 2D-correlation measurements revealed further details of the changes in the quality of DOM, revealing two distinct patterns in the DOM dynamics of the soils, the sequence MHLF→PLF being observed for grassland and arable land and PLF→HLF for forest soils, probably due to the different N supplies of the soils.