Organic matter composition in soils irrigated with treated wastewater: FT-IR spectroscopic analysis of bulk soil samples

- Treated wastewater (TWW) effects on soil content of organic groups were quantified.
- TWW irrigation resulted in enrichment of hydrophilic organic groups in loamy sand.
- Irrigation water quality may have a greater effect on SOM than crop type.
- Irrigation with TWW did not increase the relative abundance of aliphatic CH groups.

Irrigation water quality can affect the composition and properties of soil organic matter (SOM). This study used a direct analysis of bulk soil samples with transmission Fourier transform infrared (FT-IR) spectroscopy to assess the possible SOM compositional changes induced by irrigation water quality in different soil types. Soils were sampled in Israeli fields irrigated over 5 to 12 years with freshwater (FW) or secondary treated wastewater (TWW) characterized by a biochemical oxygen demand and total suspended solids not exceeding 20 and 30 mg L− 1, respectively. A peak height ratio analysis was performed using the absorbance (at 800-795 cm− 1) of quartz present in the soils as an internal reference to identify possible changes in soil content of hydrophobic (aliphatic) CH groups (absorbing at 2968-2838 cm− 1) and hydrophilic components (absorbing at 1645-1626 cm− 1). The results revealed that (1) short-term TWW irrigation (5 years), as compared with FW irrigation, had no statistically significant effect on the presence of aliphatic CH and hydrophilic components in three of the four soils studied; (2) only loamy sand irrigated with TWW for both short- and long-term (10 years) was enriched by hydrophilic components as compared with loamy sand irrigated with FW. No relation was established between the accumulation of hydrophobic components in SOM and clay or sand content in soils. For samples of a sandy clay collected “in-row” near the dripper line, long-term irrigation (12 years) with either FW or TWW resulted in a SOM composition similar to that of samples collected “between rows”, midway between two neighboring rows. Conversely, for “in-row” samples of a clay, short-term TWW irrigation (5 years) resulted in the depletion of SOM and of both the aliphatic CH and hydrophilic components as compared to “between rows” samples. The results showed no evidence to suggest that irrigation with TWW affected the relative abundance of hydrophobic aliphatic to hydrophilic components in the soils studied. However, no definite conclusions can be drawn regarding (i) the relative changes in soil contents of aromatic moieties and (ii) the contribution of various functional groups to the hydrophilic components.