The Short-Term Effect of Olive Mill Wastewater Application on Humic Acid in Subsurface Soil Layers of the Arid Region of Tunisia

Few studies have assessed the changes in soil quality based on humic acids (HA) in relation to the fresh olive mill wastewater (OMW) application. Qualitative changes to soil organic matter (SOM) under the application of OMW in real conditions have been little investigated. To make a contribution in this context, an experimental field was established in an arid Tunisian area, which included a control soil, a soil treated with 10 L m(-2) OMW and another amended with 10 kg m(-2) manure. Soil samples were collected from soil depths: 0-20 cm, 20-40 cm and 40-60 cm on the first day of OMW and manure application (day 0) and after 4 months (120 days). Results revealed that soil treated with OMW had the highest SOM content (1.42 +/- 0.13%), cation exchange capacity (CEC) (3.03 +/- 0.51 (cmol(c) kg(-1))) and soil pH value. OMW mineralization supplied soluble ions which enhanced the electrical conductivity (EC) (1.44 +/- 0.34 dS m(-1)). Infrared (IR) spectroscopy showed higher aliphatic, aromatic and phenolic groups in OMW amended soil than the manured soil for initial and final dates. Spreading OMW on soils provides a favorable environment for improving soil humic acids (HAs), recycling organic matter (OM) and enhancing soil properties by transferring aromatic and aliphatic structures into deeper horizons, which in turn increased the sequestration of soil organic carbon (SOC).