Direct and indirect impacts of ionic components of saline water on irrigated soil chemical and microbial processes

The characteristics of ions in irrigation water can be significantly different in response to the changes in the salinity gradient. The chemical and biological processes of soils which were irrigated with this kind of water would therefore, be remarkably influenced. Based on the field sampling data and utilizing the Structural Equation Model (SEM), this paper aims to investigate the ionic characteristics of irrigation water and evaluate their direct and indirect impacts on soil salt ions, nutrients and microbial metabolic activities. The present results demonstrated that the ionic components of irrigation water had a direct influence on the soil salt ions. In particular, the Na⁺ concentrations in irrigation water were not only seen to significantly increase the soil Na⁺ concentrations but also change the other soil ions' concentrations. The contents of soil total nitrogen (TN), available phosphorus (Pₐᵥₐ) and available potassium (Kₐᵥₐ) appeared to change slightly with the increase of water salinity and meanwhile, no significant impact was found from any of the ions present in the irrigation water. Although high concentrations of Na⁺, Cl⁻, SO₄²⁻, Mg²⁺ and Ca²⁺ in irrigation water significantly drove the reduction of the soil organic carbon (OC), the direct influences were actually relatively small in this respect, and it was the increase of soil salt ions that dominantly led this reduction. Irrigation water with high salinity contents could significantly restrain the soil microbial metabolic activities. In addition, high concentrations of SO₄²⁻, Mg²⁺, Cl⁻, Ca²⁺ and Na⁺ in irrigation water (accounting for about 11% of the total effects), increased concentrations of soil salt ions (i.e., 74%) and reduced contents of soil OC (i.e., 15%) all contributed to this restraint, which together were seen to affect the microbial processes under saline water irrigation. The present results revealed that the ionic effects of irrigation water on soil processes would be more complex than we currently understood, which should attract increasing concerns in the future.