Leaching amount and timing modified the ionic composition of saline-alkaline soil and increased seed cotton yield under mulched drip irrigation

Salt leaching has been widely applied to increase crop production in saline-alkaline soils. However, leaching patterns affect the ionic composition of saline-alkaline soils and increase seed cotton yield under mulched drip irrigation have been poorly understood. The objective of this study was to determine why leaching patterns succeed to increase seed cotton yield in saline-alkaline soils. A two-year field experiment (2020–2021) was conducted on cotton under mulched drip irrigation to investigate how different leaching amounts and timings modified the ionic composition of saline-alkaline soils and improved the seed cotton yield. There were four leaching amounts (W1: 75, W2: 150, W3: 225 and W4: 300 mm) and three leaching timings (T1: once at seedling stage, T2: twice at seedling and budding stages and T3: thrice at seedling, budding and pollen setting stages) in addition to non-leaching treatment (CK: 85% crop evapotranspiration). Soil cations, soil anions, total dissolved salts (TDS), sodium adsorption ratio (SAR), dry matter accumulation and seed cotton yield were examined. The results showed that increasing the leaching amount significantly reduced soil cations, soil anions, TDS and SAR compared with CK, by 31.85%, 51.82%, 34.83% and 22.20% under W3 over the two years, respectively. The chloride and sulfate in the soil accounted for the majority of TDS (more than 81.03%), suggesting that decreasing chloride and sulfate content in saline-alkaline soils could reduce soil salinity effectively. Increasing leaching amount first increased and then decreased seed cotton yield. Compared with that under CK, the optimal seed cotton yield under W3T2 was increased by 139.42% and 99.06% in 2020 and 2021, respectively. Dry matter accumulation increased with the increase in leaching amount. The Pearson correlation analysis revealed that sodium had the greatest negative effect on seed cotton yield. According to the Technique for Order Preference by Similarity to Ideal Solution, the combination of 225 mm leaching amount and timing of leaching at seedling and budding stages was recommended as the optimal leaching pattern for saving water, controlling salts and increasing yield in this region. Overall, the optimal leaching pattern modified the ionic composition and significantly reduced soil ions in the root zone that were harmful to cotton growth. The study can provide a scientific basis for the rehabilitation of saline-alkaline soils in arid and semi-arid conditions under mulched drip irrigation.