Applying biochar and flue gas desulfurization gypsum in the root zone to improve saline-alkali soil quality and sunflower yield

More than two-thirds of the arable land in the Hetao Irrigation District of Inner Mongolia, China, is suffering from various soil salinization and alkalization, which has seriously restricted the local agriculture and animal husbandry economy. Among them, biochar and flue gas desulfurization (FGD) gypsum have been widely used as soil amendments in the saline-alkali soils. The amendments can often be applied on the surface and then mixed with the topsoil. However, the large investment and high cost of the traditional practice cannot fully meet the large-scale production in recent years. There is a high demand to develop more effective practices in soil amendments. In this study, a field experiment was conducted in Wuyuan County, Bayannur City, Inner Mongolia Autonomous Region, China, in order to cultivate healthy and fertile soil for less soil salinization. Three treatments were designed: 1) covering the planting holes with sand (S), 2) strip application of biochar in topsoil and then covering the planting holes with sand (B+S), and 3) strip application of biochar in topsoil and then covering the planting holes with FGD gypsum (B+G). In biochar treatments, 3750 kg biochar per hectare was buried in strips at a depth of 5-10 cm below the soil surface, and 5-10 cm away from the planting strip. In coverage treatments, the planting holes were covered completely with 750 kg sand or FGD gypsum per hectare. The soil samples were collected at a depth of 0-40 cm from the sunflower planting strip after harvest. The soil quality was then determined among the three treatments, in terms of salinity, alkalinity, organic matter, and nutrient status. Meanwhile, the sunflower yield and its components were also analyzed to deduce their relationship with soil quality. The results showed that the biochar and FGD gypsum in the root zone improved the water-soluble cations composition of the 0-40 cm soil layers, resulting in a higher Ca²⁺ concentration, while lower Na+ concentration after the experiment. As expected, the B+S and B+G treatments significantly decreased the sodium adsorption ratio (SAR) of the 0-40 cm soil layers and the pH value of the 20-40 cm soil layer, but increased the soil desalination rate (D_s), compared with the S treatment. In addition, the B+S and B+G treatments shared the higher contents of organic matter (OM), nitrate nitrogen (NO₃^--N), available phosphorus and available potassium (AK) in the 0-40 cm soil layers, indicating the better soil environment for sunflower emergence and growth. The entire dataset of soil parameters showed that the soil quality index (SQI) of B+S and B+G treatments increased by 35.7% and 88.5%, respectively, compared with the S treatment. The SQI parameters (such as SAR, D_s, Ca²⁺, Mg²⁺, AK, NO₃^--N and OM) achieved high scores after the B+S and B+G treatments. The reason was that the higher soil quality was obtained to reduce the salinity stress for the high nutrient levels. By contrast, the B+G treatment showed better performance than the B+S treatments, resulting in a higher seed yield of sunflower. Therefore, the combination of biochar strip application and FGD gypsum covering planting holes in saline-alkali land can be expected to intensively regulate the root zone soil, for the high soil quality and sunflower yield. The findings can also provide technical support to the amelioration and utilization of saline-alkali soils in the Hetao Irrigation District.