Effects of chemical fertilizer reduction and organic substitution on water and salt characteristics of high salinity soil and water and nitrogen use efficiency of sunflower

Objectives The effects of different organic fertilizer ratios in total nutrient input on improving soil fertility and crop growth in heavily salinized farmland were studied for efficient and sustainable agricultural production in Hetao Irrigation District.

Methods A two-year field experiment was conducted from 2019 to 2020 in high salinity soil in Hetao Irrigation District, taking sunflower as test crop. On the basis of recommended total N input (180 kg/hm²) for sunflower, organic fertilizer was used to replace chemical N by ratio of 0 (OF0), 50% (OF1) and 100% (OF2) respectively, while no fertilizer was taken as control (CK). The soil bulk density, moisture content and electrical conductivity of saturated extract (ECe) were measured before the experiment and during the main growth stages of sunflower in 2019 and 2020. The seed yields and yield traits of sunflower were measured at harvest.

Results Application of organic fertilizers reduced soil bulk density and increased soil porosity significantly. After harvest in 2020, the soil bulk density of 0−40 cm depth treated with OF1 and OF2 was 1.46 and 1.43 g/cm³, respectively, which were 3.97% and 5.92% lower than that before sowing in 2019. The soil porosity was 4.94% and 7.90% higher than that before sowing in 2019 (P < 0.05). Organic fertilizer significantly increased the water storage and decreased ECe values of saline soil. The soil water content in OF1 was 5.34%, 3.65%, and 10.55% higher than in OF0, OF2 and CK, respectively (P < 0.05). The mean ECe of 0−100 cm depth at harvest of the two growing seasons in OF2 was 6.77 dS/m, which was 44.10% and 11.61% lower than those in OF0 and OF1, respectively (P < 0.05). The seed yield under OF1 was 9.47%, 7.52%, and 62.90% higher than that in OF0, OF2 and CK, respectively (P < 0.05). The net income was 7.02%, 23.11%, and 65.00% higher than that in OF0, OF2 and CK, respectively (P < 0.05). The water use efficiency was 17.50%, 9.52%, and 73.82% higher than that in OF0, OF2 and CK, respectively (P < 0.05). In addition, OF1 significantly increased the partial factor productivity and agronomic efficiency of applied nitrogen compared with OF0 and OF2 (P < 0.05). OF2 and OF0 had similar yield and water use efficiency, but OF2 had lower fertilizer efficiency and net income compared with OF0.

Conclusions Organic fertilizer could improve soil structure and decrease salt content of high salinity soil, ameliorate soil water and salt environment in root zone. However, organic fertilizer could substitute about 50%, not 100% of chemical fertilizers for high sunflower yield, water and nitrogen use efficiency and economic benefit of high salinity soils in Hetao Irrigation District.