2.2. Experimental design
Four treatments were arranged as a completely randomized design with three replications: four straw interlayers with application rates of 0, 6, 12, and 18 Mg ha-1 (i.e., CK, SL6, SL12, and SL18) (Figure 2). The plot was 3.24 m2 (1.8 m × 1.8 m) and separated by double-plastic sheets buried from soil surface to the soil depth of 100 cm to ensure no salt and water transports between adjacent plots. The interlayer material used in this study was chopped maize straw (C% = 42%, and N% = 0.65%). The straw was uniformly placed at 40 cm depth according to the amount required for each treatment in the first year, no additional straw input in subsequent years. The plots without straw interlayer (i.e., CK) were also ploughed to ensure similar soil disturbance. The plots used 180 kg N ha-1, 53 kg P ha-1, and 62 kg K ha-1 as base fertilizers, with forms of urea (N% = 46%), diammonium phosphate (N% = 18%, and P% = 20%), and potassium sulphate (K% = 41%). All plots were flood-irrigated as 185 mm from Yellow River (salt conc. 0.58 g L-1) before sowing. All plots were mulched with plastic film with a width of 70 cm and a film spacing of 20 cm every year. The tested crop was sunflower (Helianthus annuus L.), which was seeded at a row spacing of 60 cm and a plant spacing of 20 cm on plastic film. The dates of sowing and harvest were shown in Table S1.