Seed-Zone Salinity and Regional Ionic Signatures Associated with Cotton Emergence Failure Under Mulched Drip Irrigation in Southern Xinjiang

Soil salinity can constrain cotton (Gossypium hirsutum L.) establishment in southern Xinjiang, where emergence may vary sharply within uniformly managed fields. We conducted a paired observational field survey of 130 commercial cotton fields, comparing well-established microsites (A; live-seedling stand ≥ 80%) with non-emerged or nearly non-emerged microsites (B; emergence < 5%) in Kashgar (90 pairs) and Aksu (40 pairs). Soil was sampled 14–28 d after sowing, after establishment status was evident. B microsites had higher total water-soluble salts (13.54 vs. 5.95 g kg−1), EC1:5 (2.98 vs. 1.21 mS cm−1), and SAR1:5 (4.17 vs. 1.32; all q < 0.001). Matched models showed that EC1:5 remained associated with the B class after adjustment for soil texture and either Na+ or Cl−. Direct pair-level tests confirmed regional differences in ionic-composition change (permutation p < 0.001), with relatively stronger Na+–Cl− enrichment in Kashgar and Ca2+–SO42− enrichment in Aksu. Exploratory in-sample EC1:5 diagnostic cut-points were 2.51 mS cm−1 for Kashgar and 1.17 mS cm−1 for Aksu. Because sampling was post-outcome and the cut-points were internally derived from a contrast-selected dataset, causal interpretation and field application require longitudinal evidence and external validation.