Comparison of the Chemical Composition of Soil Solutions in Subsurface Drip and Furrow Irrigation Methods in Southwest Iran

In order to examine the chemical behavior of major cations and anions in the soil solution and the solid phases controlling salt concentrations under subsurface drip irrigation (SDI) and to compare these with furrow irrigation at the field scale, a study was conducted at Hakim Farabi Company on Haplic Calcisols (Loamic, Aridic) soil in Khuzestan province, southwest Iran. The evaluation of soil solution in two irrigation methods carried out in area 22 ha (220 × 1000 m) for each field from November 2021 to June 2022 at four depths (0–15, 15–30, 30–60 and 60–90 cm). Sugarcane (variety CP69-1062) was planted in a double-row arrangement and the total water usage in the surface irrigation method was 620 026 m3, whereas in the SDI method, it was 259 940 m3. The results showed that at a depth of 30 cm, soil salinity increased by 85% under SDI compared to surface irrigation (SI). Soil salinity is affected by the salinity of irrigation water, evaporation and transpiration, and the amount of water used. Under SDI, the concentrations of Ca2+, Mg2+, Na+, and K+ cations increased on average by 188, 137, 71, and 6%, respectively, at a depth of 30 cm compared to SI. Similarly, the concentrations of the anions Cl− and $${\text{SO}}_{4}^{{2 - }}$$ increased by an average of 110 and 68% respectively, while $${\text{HCO}}_{3}^{ - }$$ concentration was higher under SI compared to SDI. At a depth of 30 cm, the negative logarithm of activity of cations in both irrigation methods followed the decreasing trend: pK > pMg > pCa > pNa. For anions in the soil solution, the decreasing trend was: pCO3 > pHCO3 > pSO4 > pCl. Across all sampling times in both irrigation methods, the sequence of minerals ranked by their saturation indices, from the most positive to the most negative, was dolomite, calcite, aragonite, gypsum, and halite. The saturation index of gypsum was more negative under SI than SDI, indicating higher gypsum solubility in the soil under SI in sugarcane cultivation. In conclusion, SDI, compared to SI, affects the chemical behavior of cations and anions and dissolution and precipitation of solid phases controlling salt concentrations.