Background There are abundant brackish water resources in China, and the use of brackish water for irrigation poses a great danger to soil quality and sustainable use. Salinization has been a major threat to soil quality deterioration and sustainable agricultural development in arid and semi-arid regions. Combining biochar, which has strong adsorption properties and soil improvement effects, with brackish water irrigation and using brackish water resources in a scientific and rational manner is of great significance to alleviate water shortages and achieve sustainable agricultural development.

Methods 4 biochar blends(0, 1%, 2% and 4%) and 4 water mineralization levels(0, 1, 3 and 5 g/L) were set. Soil moisture characteristic curves were measured by centrifuge method and fitted with Van-Genuchten model to analyze the soil moisture characteristic curves. The transport distance between the marsupial readings and the wetting front of soil column was recorded by indoor one-dimensional vertical constant head infiltration test, and the relationship between cumulative infiltration and time was fitted with Kostiakov model, and the relationship between the transport distance of wetting front and time was fitted with power function.

Results 1) The volumetric water content of soil treated with 2% biochar was greatest when the mineralization of brackish water was 0. The volumetric water content of soil treated with 0 biochar was greatest when the mineralization of brackish water was 1, 3 and 5 g/L. The soil moisture characteristic curves of different treatments could be accurately described using the Van-Genuchten model, with R² > 0.98. 2) In the same period, the cumulative infiltration time for each treatment was less than that of the control treatment. The length of infiltration time was 4% < 2% < 1% < 0 for brackish water mineralization of 0, and 2% < 4% < 1% < 0 for brackish water mineralization of 1, 3 and 5 g/L. The cumulative infiltration was described by the Kostiakov model with R²> 0.97. 3) Biochar significantly increased the transport distance of wetting fronts. When the mineralization of brackish water was 0, the shortest time was required to treat the transport distance of wetting front with 4% biochar; when the mineralization of brackish water was 1, 3 and 5 g/L, the shortest time was required to treat the transport distance of wetting front with 2% biochar. The transport distance of wetting front can be described by a power function with R² > 0.99. 4)Biochar significantly increased the mean mass water content of the soil, but it increased and then decreased, with the peak occurring in the 2% treatment; the soil conductivity increased with the increase of biochar, and the most significant effect of salt adsorption in the soil was observed in the 4% treatment of biochar.

Conclusions The combined use of biochar and brackish water can help alleviate the contradiction between water supply and demand. Biochar can significantly improve the water-holding capacity of the soil and has a strong adsorption effect on salt ions in the soil, improving the distribution of soil water and salt and effectively alleviating the problem of land salinization.