Potential for fluorescence spectroscopy to assess the quality of soil water-extracted organic matter

Fluorescence spectroscopy has potential for characterizing the water soluble fraction of soil organic matter. However, some questions remain unanswered: (i) can this technique be applied to a complex and evolving mixture of compounds and (ii) can the quality parameter obtained from fluorescence analysis be linked to climatic parameters and soil microbial activity. To answer these questions, a set of 98 soil water extracts, from a study of a 2.50-m-thick soil section in the vadose zone during a 7-month experiment was analyzed. Water-extracted organic matter (WEOM) content and the potential aerobic respiratory activity were measured in the laboratory, whereas environmental factors (rainfall soil temperature) were monitored on site. The humification index (HIX) was also calculated for the 98 samples. Results showed a significant positive correlation (R-2 = 0.87, P < 0.001) between the fluorescence signal and the WEOM content. A higher proportion of low molecular weight and size compounds was observed in the deepest layers than in the surface soils. The decrease in aerobic respiratory activity (from 0.9 to 0.4 mugO(2) g(-1)dw h(-1)) and in HIX (from 3.7 to 2.1) from the surface to the 1.5-m depth suggest that the small fluorescent compounds were more recalcitrant to biodegradation in the deep vadose zone. Heavy rainfall events (60 mm) resulted in a sharp decrease of HIX values (from 2.6 to 1.3) at the 1.5-m depth, probably the result of leaching of small compounds, whereas the WEOM content was stable (2 mg C L-1). Fluorescence spectroscopy was demonstrated to be useful for analyzing the fate of agricultural organic residues in soils.