Effect of combined frozen saline water irrigation and soil amendment on coastal saline land reclamation

Frozen saline water irrigation (FSWI) in winter is a kind of irrigation that utilizes natural cold conditions and abundant saline water. Natural melting of frozen brackish water separates salt from soil via salt water leaching that leaves behind fresh water, leading to saline soil desalinization. In accordance with water desalinization via freezing and thawing of saline water, a field experiment was conducted to study the principle of water and salt transfer following FSWI and soil amendment application. The results showed that FSWI in winter reduced rhizosphere soil salinity. The amount of irrigation water was positively correlated with soil moisture. FSWI caused soil alkalization in initial stage of saline water ice melt, although this trend gradually weakened with ice melt under different treatments. Application of phosphogypsum decreased soil HCO₃^- content, increased soil SO₄^2- and Ca²⁺ contents, and effectively reduced ratios of Cl^- and Na⁺ in total salt content. With increasing phosphogypsum application rate, rhizosphere soil pH decreased while soil water retention capacity increased (7 500 kg·hm^-2 phosphogypsum > 4 500 kg·hm^-2 phosphogypsum). High FSWI along with phosphogypsum application enhanced the growth of Chinese tamarisk. The best treatment combination was FSWI in winter at 1 350 m³·hm^-2 saline water + 7 500 kg·hm^-2 phosphogypsum. Combined FSWI and phosphogypsum application improved coastal saline land reclamation efficiency. It also amended soil alkalization induced by FSWI and provided favorable conditions for early seed germination and plants growth.