Modeling strategies to balance salt leaching and nitrogen loss for drip irrigation with saline water in arid regions

Soil salinization has severely constrained agricultural sustainability in arid and semi-arid regions. Generally, using additional irrigation water to leach excessive salts is a widely used strategy for alleviating soil salt accumulation in the root zone. Nevertheless, this practice simultaneously increases the risk of nitrogen loss. To address this contradiction, a new index, namely, the leaching balance index of soil salinity and nitrogen (LBI), which was defined by multiplying the nitrogen leaching fraction (NLF) by the absolute value of the discrepancy between the crop tolerance salinity threshold and soil salinity (|∆SSC|), was proposed. The lower the index value is, the better the soil salt and nitrogen leaching can reach an optimal equilibrium state. Soil salt leaching and nitrogen loss were simulated by the HYDRUS-2D model. The two-year field observation data of soil moisture, salinity, ammonium, and nitrate nitrogen contents were collected for calibration and validation. A model case was presented using the LBI to investigate the appropriate mulched drip irrigation and fertigation management practices with different levels of salty waters for cotton in Xinjiang in this study. In the testing process, the normalized root mean square error (nRMSE) provided by this model was 2–13 %, 5–14 %, 20–40 %, and 14–27 % for soil water content (SWC), soil salinity (EC1:5), NH4+–N, and NO3––N, respectively, and the index of agreement (d) was 0.63–0.94, 0.83–0.99, 0.69–0.97, and 0.86–0.96, respectively. Then, the simulation scenarios were established to minimize the NLF and the |∆SSC| and thus the LBI. The results showed that increasing the number of fertigation splits could reduce the LBI. Specifically, five fertigation splits during the growing season could result in a decrease in the LBI of 21–32 %. Furthermore, when applying an irrigation amount of 100% ETc, the alternate use of water salinities of 6.0 dS m−1 and 10.0 dS m−1 at the squaring stage and flower boll stage produced a lower LBI. The irrigation scheme of a higher water salinity and irrigation amount during the squaring stage, coupled with a lower water salinity and irrigation amount during the flower boll stage, was beneficial for decreasing the LBI. The lower the value of LBI is, the more sustainable the irrigation and soil management will be. The new index of the LBI proposed by our study represents one of the first efforts to solve the contradiction between salt accumulation and nitrogen loss, which will ultimately aid in the salt control and efficient utilization of water and nitrogen for arid and semi-arid regions.