Water-extractable soil organic matter characterization by chromophoric indicators: Effects of soil type and irrigation water quality

Use of treated wastewater (TWW) for irrigation may affect the concentration and composition of dissolved organic matter (DOM) in the soil solution. Our objectives were (i) to characterize the water-extractable fraction of soil organic matter (SOM) in terms of chromophoric DOM (CDOM) for different soil types and irrigation water qualities, and (ii) to examine the possible relations between DOM composition and various soil properties. Aqueous extracts from four different soil types irrigated with either secondary TWW or fresh water (FW) were characterized for dissolved organic carbon (DOC) concentration and absorbance at 254 nm (Abs254). Excitation–emission matrices (EEM) of fluorescence were determined in these aqueous extracts and in extracts where DOM was separated into acid-soluble and acid-precipitated fractions. Parallel factor (PARAFAC) analysis of the EEMs of water-extractable DOM and their fractions identified two humic-like fluorescent components and one tryptophan-like component and provided their concentration scores. The concentration scores revealed that the acid-soluble DOM fraction contained the greater part of the humic-like fluorescent components present in the bulk DOM samples. Statistical examination demonstrated that the ratio of the concentration score of the humic-like substances that emit light at shorter wavelengths to that of humic-like substances that emit light at longer wavelengths was higher in the acid-soluble DOM fraction compared to the acid-precipitated DOM fraction. Among the FW-irrigated soils, the soil extracts varied in terms of concentration of DOM components. Irrigation of the coarse-textured soils with TWW generally increased the concentrations of DOC, fluorescent components and Abs254 in comparison to irrigation with FW. In the fine textured soils TWW application led to a decrease or had no impact on the concentration of DOC and the CDOM components in water-extractable SOM. Thus, the influence of TWW irrigation on water-soluble SOM relative to that of FW irrigation was also soil dependent. Strong linear relations (/r/>0.7) were found within CDOM indicators including concentration scores of fluorescent components and Abs254. Weak or no relations were found (i) between DOC concentration and CDOM indicators, and (ii) between attributes related to DOM properties and basic soil properties. Hence, nor DOC concentration neither SOM content can be used for estimating changes in concentrations of chromophoric components in water-extractable DOM.

► Chromophoric dissolved organic matter (CDOM) from soils was studied.
► Three fluorescent components were identified in the whole CDOM.
► The same components were found in the acid-separated CDOM fractions.
► Impact of treated wastewater irrigation on CDOM indicators was soil dependent.
► CDOM indices show no strong links with organic carbon contents in soils and extracts.