Organic substitution improves soil structure and water and nitrogen status to promote sunflower (Helianthus annuus L.) growth in an arid saline area

Organic substitution (partial substitution of chemical fertilizer with organic fertilizer) is an effective approach to address soil degradation caused by excessive application of chemical fertilizers, particularly in saline areas. This study aimed to evaluate the effects of different organic substitution proportions by using organic fertilizer made from agricultural wastes on saline-sodic soil properties and sunflower (Helianthus annuus L.) growth in an arid saline area. Additionally, the study tried to reveal the possible mechanism of organic substitution on soil quality improvement from soil structure. A two-year field experiment was conducted in the Hetao Irrigation District, northwest China during the sunflower growing season in 2019 and 2020. The study soil type is saline-sodic soil with ECe of 18.3 dS m−1, pH of 8.3, and exchange sodium percentage (ESP) of 18 % in the 0–0.2 m soil layer, with a silt loam soil texture. Based on the recommended total N input (180 kg ha−1) for sunflower, four treatments were designed with three replicates: (i) CK (no fertilization), (ii) OF0 (chemical fertilization only), (iii) OF1 (organic fertilizer substituting 50 % of the urea N), and (iv) OF2 (organic fertilizer substituting 100 % of the urea N). Totally 12 plots were arranged in a randomized complete block design. The results showed that in the 0–0.4 m soil player, compared with OF0, OF1 and OF2 significantly decreased bulk density by 4 % and 6 %, respectively, and turbidity of soil suspension by 36 % and 66 %, respectively. Additionally, OF1 and OF2 significantly increased soil total porosity in 0–0.4 m soil player, mainly in macro-porosity (> 30 µm) and meso-porosity (0.2–30 µm), and increased soil water content compared with OF0 and CK. The improvement of soil structure was attributed to the amelioration of sodicity (reduction of sodium adsorption ratio (SAR) or cation ratio of soil structural stability (CROSS)), and as its consequence, leaching of salt out of the surface layers. Furthermore, compared with OF0 and OF2, OF1 maintained a stable NO3-N supply and significantly increased agronomic efficiency of applied nitrogen by 29 % and 22 %, respectively. Moreover, OF1 on average significantly increased grain yield by 9 % and 8 %, crop water productivity by 12 % and 4 %, and net income by 7 % and 23 %, respectively. Mantel tests revealed that soil water content in the root zone and NO3-N concentration in the maturity stage were key indicators significantly affecting sunflower growth under the organic substitution. In conclusion, from the perspective of improving the soil properties, crop water-fertilizer productivity, and economic benefits, organic fertilizer substituting 50 % of the urea N was recommended for sunflower production under the arid saline environment.