Effects of biochar on coastal saline soil and maize yield under alternate irrigation with brackish and freshwater

The coastal areas possess substantial brackish water resources. The agricultural utilization of saline soil and brackish water resources in coastal regions is crucial to guarantee food security and can be conducive to alleviate increasing land demands and water shortages. Nonetheless, suitable irrigation and field management is essential to improve agricultural production of coastal saline soil and brackish water. In this study, alternate irrigation with brackish and freshwater combined with biochar was proposed to promote the agricultural utilization of these low-quality soil and water resources. A maize planting experiment in coastal saline soil was carried out using field plots under the condition of rain shelter in 2017 and 2018, respectively. We investigated the effects of alternate irrigation with brackish and freshwater and biochar application on coastal soil properties and maize yield parameters. The maize growth season was separated into three periods, that is, the six leaves stage to the tasseling stage, the tasseling stage to the silking stage, and the silking stage to the maturity stage. The alternate irrigation with brackish and freshwater was carried out by using brackish water irrigation during one of the three periods and freshwater irrigation during the remaining stages. The check treatment was conducted by using freshwater irrigation throughout the whole growing season. Biochar with three application rates (0, 15, 30 t/hm2) was incorporated into the surface layer of coastal saline soil in the first experiment year, respectively. Maize leaf area index was observed during the growing season. Maize dry matter accumulation and yield parameters were measured at harvest. Soil properties related to soil salinization, porosity, aggregate, and nutrient content were determined after harvest. The electrical conductivity and exchangeable sodium percentage of coastal saline soil remarkably increased under alternate irrigation with brackish and freshwater. The soil salinization was related to the proportion and order of brackish water use. The brackish water irrigation during the six leaves stage to the tasseling stage severely inhibited maize leaf growth and dry matter accumulation, and lead to a decline in grain number and grain weight, resulting in a 27.2%-32.8% yield reduction. The reduction in maize growth and production by the brackish water irrigation during the tasseling stage to the silking stage was less, but the reduced grain number still resulted in a 11.4%-14.0% yield reduction. The brackish water irrigation during the silking stage to the maturity stage did not have a significant adverse effect on maize growth and yield. Under alternate irrigation with brackish and freshwater, biochar application reduced the electrical conductivity and exchangeable sodium percentage of coastal saline soil by 3.7%-21.7% and 9.2%-45.2%, respectively. The total porosity and water-stable aggregate with biochar applications were increased by 3.1%-11.9% and 40.0%-168.9%, respectively. Biochar application also promoted the soil nutrient status and increased available nitrogen, available phosphorus, and available potassium content by 34.9%-104.0%, 21.0%-58.1%, and 13.6%-57.8%, respectively. With the improvement in soil conditions, biochar application was helpful to enhance salt tolerance in the early and middle stages of maize growth, thus alleviating the damage of salt stress under brackish water irrigation. The maize maintained a good condition of leaf area index, dry matter accumulation, and yield characteristics when brackish water irrigation was applied during the six leaves stage to the silking stage. Therefore, biochar application promoted the feasibility and applicability of alternate irrigation with brackish and freshwater. Compared to the treatments without biochar application, the grain yield of the treatments with biochar application increased by 10.9%-32.3% under the same alternate irrigation with brackish water and freshwater. The results could be helpful to improve the agricultural utilization of saline soil and brackish water resources in coastal regions.