The long-term regularities of the interaction between mineralized irrigation water and soil: Agroecological patterns and strategies for soil health restoration

Abstract

The objectives of the study were to establish long-term patterns of transformation in the “irrigation water– soil” system under irrigation with saline water, determine the temporal consistency of water and soil physicochemical dynamics, quantitatively assess changes in humus, nutrient, biological, and physical conditions, and substantiate an integrated model for adaptive agricultural amelioration management of water and soil resources. For these purposes, results of stationary field experiments, agrochemical surveys, and operational monitoring conducted from 2001 to 2020 under the agroecological conditions of the Dry Steppe zone in southern Ukraine were synthesized. The baseline soil state from 1966 was used as a reference benchmark for comparative analysis. The study examined dark chestnut soils, meadow-chestnut soils, southern chernozems, irrigation water from the Inhulets Irrigation System, and fields with varying post-irrigation durations. In irrigation water, total dissolved solids (TDS/mineralization), pH, toxic ions, HCO₃⁻, CO₃²⁻, Cl⁻, Ca²⁺, Mg²⁺, and Na⁺ were determined; the proportion of Na⁺ in total cations, Mg²⁺/ Ca²⁺, and Ca²⁺/Na⁺ ratios were calculated. In soil samples, total concentration and composition of water-soluble salts, pH, exchangeable Ca²⁺, Mg²⁺, Na⁺, and K⁺, total humus content and storage, available nitrogen, mobile phosphorus, exchangeable potassium, bulk density, porosity, and aggregate composition were measured. Analytical and synthetic processing of the data included descriptive statistics, trend and contrast evaluations, scenario modeling, and calculation of a multi-criteria Ecological-Ameliorative State Index. During 2001–2020, irrigation water mineralization ranged between 1.425 and 1.661 g/dm3 (mean: 1.531 g/dm3; coefficient of variation: 4.35%), while the Ca²⁺/Na⁺ ratio varied from 0.20 to 0.50 (coefficient of variation: 20.02%). Annually, water quality corresponded to Class II regarding secondary salinization and sodification; in 15% of the years, the integrated state deteriorated to Class III due to toxicity or alkalinization. Under a scenario-based irrigation norm of 300 mm, potential gross salt intake was estimated at 4.275–4.983 t/ha. The Ca²⁺/Na⁺ trend in water was positive, but statistically insignificant in the 0–30 cm and 0– 100 cm soil layers, with noncoinciding temporal extrema. The average proportion of Na⁺ and K⁺ in total exchangeable bases increased from 4.31% under nonirrigated conditions to 6.29% under unfertilized irrigation. In the 1-meter soil layer following long-term irrigation, total salt content exceeded 1966 baseline values by 0.031–0.040 percentage points. Weighted average humus content across the studied Inhulets area decreased from 2.43% to 1.70% between 2003 and 2018; the proportion of area with low and very low humus levels reached 80.6%. Following the cessation of 45-year irrigation, recovery was asynchronous: after 29 years, humus reached 2.5% compared with 2.6% under rainfed conditions, whereas the Ca²⁺/Na⁺ ratio remained at 1.43 versus 2.03. The integral recovery index was 28.1, 58.3, and 69.2 points after 11, 18, and 29 years, respectively. Water mineralization alone is insufficient for long-term forecasting of soil conditions following amelioration. Cation ratios, cumulative salt loading, soil profile status, and temporal lags hold key diagnostic value. Based on the evaluation of interactions within the “irrigation water–soil” system, a two-tier decision-making system for reclamation management was developed. Soil recovery following irrigation cessation is not a passive reversal to the baseline state; it requires long-term profile monitoring alongside targeted, scientifically grounded physicochemical and biological reclamation.