Fluorescence spectroscopy: A sensitive tool for identifying land-use and climatic region effects on the characteristics of water-extractable soil organic matter

Abstract

Water-extractable organic matter (WEOM) is an indicator of active and labile pools of soil organic matter (SOM). Land-use is expected to have a major effect on SOM, yet the effects of different land-uses on WEOM are not well understood. Reported trends in the changes in water-extractable organic C (WEOC) concentrations following changes in land-use are inconsistent or even contradictory. Fluorescent WEOM components, including humic-like and proteinaceous substances, may be useful for elucidating long-term effects of different land-uses on WEOM content and composition. Therefore, the objectives of this research were (i) to characterize SOM and WEOM in soils from different climatic regions that were subjected to different land-uses and (ii) to compare the sensitivity of fluorescent components in WEOM to that of other SOM/WEOM attributes for evaluating the impact of land-use and climatic region on SOM. Two soil types from different climatic regions (Mediterranean and semi-arid) that were collected from land subjected to different uses (field crops, orchards and non-cultivated plots) and characterized for total soil organic C and total N. The concentration of WEOC, UV absorbance at 254 nm and excitation-emission matrices (EEMs) of fluorescence were determined in diluted solution extracts. Parallel-factor (PARAFAC) analysis was applied to identify meaningful fluorescent components of WEOM and quantify their presence. Most of the quantitative measures used for SOM and WEOM characterization demonstrated insignificant and/or inconsistent relationships with the type of land-use and climatic region. However, EEM-PARAFAC-derived fluorescent components present in soil WEOM and, specifically, their relative fractions exhibited significant associations with the climatic region and type of land-use. These results may support the use of EEM-PARAFAC fluorescent components as indicators for the assessment of soil health.