For understanding the effects of soil salinity and nitrogen (N) fertilizer on the emergence rate, yield, and nitrogen-use efficiency (NUE) of sunflowers, complete block design studies were conducted in Hetao Irrigation District, China. Four levels of soil salinity (electrical conductivity [EC_e] = 2.44–29.23 dS m⁻¹) and three levels of N fertilization (90–180 kg ha⁻¹) were applied to thirty-six microplots. Soil salinity significantly affected sunflower growth (P < 0.05). High salinity (EC_e = 9.03–18.06 dS m⁻¹) reduced emergence rate by 24.5 percent, seed yield by 31.0 percent, hundred-kernel weight by 15.2 percent, and biological yield by 27.4 percent, but it increased the harvest index by 0.9 percent relative to low salinity (EC_e = 2.44–4.44 dS m⁻¹). Application of N fertilizer alleviated some of the adverse effects of salinity, especially in highly saline soils. We suggest that moderate (135 kg ha⁻¹) and high (180 kg ha⁻¹) levels of N fertilization could provide the maximum benefit in low- to moderate-salinity and high- or severe-salinity fields, respectively, in Hetao Irrigation District and similar sunflower-growing areas.

Understanding salinity and fertilizer interaction is of great importance to improve crop production and fertilizer use efficiency in saline areas. To evaluate the interactive effects of different soil salinity levels and nitrogen (N) applications rates on the sunflower photosynthetic characteristics of N uptake and N use efficiency, a two-year field experiment was conducted in Hetao Irrigation District, China. The experiment consisted of three initial salinity (IS) levels expressed as the electrical conductivity of a saturated soil extract (EC_e) (S0: 1.72–2.61 dS/m; S1: 4.73–5.90 dS/m; S2: 6.85–9.04 dS/m) and four N rates (45, 90, 135, and 180 kg/ha), referred as N0–N3, respectively. The results indicated that the net photosynthetic rate (Pn) of sunflowers treated with S0 and S1 levels both had a significant decrease in the bud stage, and then reached their maximum at anthesis. However, during the crop cycle, the Pn at S2 level only had small fluctuations and still remained at a high level (>40 μmol CO₂/(m² s)) at the early mature stage. When increasing IS levels from S0 to S1, the plant N uptake (PNU) under the same N rates were only decreased by less than 10% at maturity, whereas the decline was expanded to 17.2–45.7% from S1 to S2. Additionally, though applying the N2 rate could not increase sunflower PNU at the S0 and S1 levels, its N use efficiency was better than those under N3. Meanwhile, at the S2 level, the application of the N0 rate produced a higher N productive efficiency (NPE) and N uptake efficiency (NUPE) than the other N rates. Therefore, our study proposed recommended rates of N fertilizer (S0 and S1: 135 kg/ha, S2: 45 kg/ha) for sunflowers under different saline conditions.

The dry drainage system (DDS) is an alternative technique for controlling salinization. To quantify its role in soil salinity control, a five-year field observation from 2007 to 2011 was completed in a 2900 ha experimental plot in Yonglian Experimental Station, Hetao Irrigation District, China. Results showed that the groundwater table depth in the fallow areas quickly responded to the lateral recharge from the surrounding croplands during irrigation events. The groundwater electrical conductivity (GEC) of fallow areas increased from 5 mS·cm⁻¹ to 15 mS·cm⁻¹, whereas the GEC below croplands produced small fluctuations. The analysis of water and salt balance showed that the excess water that moved to fallow was roughly four times that moved by an artificial drainage system and with 7.7 times the corresponding salt. The fallow areas act as a drainage repository to receive excess water and salt from surrounding irrigated croplands. Slight salt accumulation occurred in irrigated croplands and salts accumulated, with an accelerating trend over the final two years. The evaporation capability weakened, partly due to the salt crust in the topsoil, and the decrease in soil permeability in the soil column, which was almost impermeable to water. Using halophytes may be an effective method to remove salts that have accumulated in fallow areas, having great economic and ecological value. A DDS may be effective and sustainable in situations where the fallow areas can sustain an upward capillary flux from planted halophytes.

Crop coefficients (Kc) are important for the development of irrigation schedules, but few studies on Kc focus on saline soils. To propose the growth-stage-specific Kc values for sunflowers in saline soils, a two-year micro-plot experiment was conducted in Yichang Experimental Station, Hetao Irrigation District. Four salinity levels including non-salinized (ECe = 3.4–4.1 dS·m–1), low (ECe = 5.5–8.2 dS·m–1), moderate (ECe = 12.1–14.5 dS·m–1), and high (ECe = 18.3–18.5 dS·m–1) levels were arranged in 12 micro-plots. Based on the soil moisture observations, Vensim software was used to establish and develop a physically-based water flow in the soil-plant system (WFSP) model. Observations in 2012 were used to calibrate the WFSP model and acceptable accuracy was obtained, especially for soil moisture simulation below 5 cm (R2 > 0.6). The locally-based Kc values (LKc) of sunflowers in saline soils were presented according to the WFSP calibration results. To be specific, LKc for initial stages (Kc1) could be expressed as a function of soil salinity (R2 = 0.86), while R2 of LKc for rapid growth (Kc2), middle (Kc3), and mature (Kc4) stages were 0.659, 1.156, and 0.324, respectively. The proposed LKc values were also evaluated by observations in 2013 and the R2 for initial, rapid growth, middle, and mature stages were 0.66, 0.68, 0.56 and 0.58, respectively. It is expected that the LKc would be of great value in irrigation management and provide precise water application values for salt-affected regions.

Monitoring and improving environmental stress in crops is vital for the sustainable development of agriculture and food security. Traditional experimental methods are costly and time-consuming, yet crop growth models focus mainly only on water and nutrient stresses. In this study, a new World Food Studies (WOFOST) model, WOFOST-ES, was developed by the addition of a general environmental stress factor (ES). To calibrate and validate WOFOST-ES, two-year micro-plot experiments and one-year field experiments with sunflower were conducted in the Hetao Irrigation District, China. The results of the micro-plot experiments indicated that the WOFOST model failed to simulate sunflower growth correctly but that the WOFOST-ES model was highly accurate in simulating both yield (R² = 0.99, root mean square error (RMSE) = 56 kg/ha) and leaf area index (LAI) (R² = 0.86, RMSE = 0.44). A statistical method for estimating ESs based on the dominant stress factor (salt at our study site) was also proposed as a supplemental tool for WOFOST-ES, and micro-plot and field experiments conducted in 2013 and 2017 both proved acceptable accuracy of the statistical method when using WOFOST-ES. Comparison between ESs and the water and salt stress factors of Feddes-type stress reduction functions indicated that ESs failed to reveal actual environmental stresses during the sunflower seeding stage but did reflect other environmental stresses in addition to water and salt during the bud, flowering, and maturity stages. Although the present WOFOST-ES model proved to be accurate, stable, and practical, future studies should be performed, focusing on the physical separation of ESs, their mechanistic quantification, and their evaluation at small time steps using more observations.