Identifying the factors dominating the spatial distribution of water and salt in soil and cotton yield under arid environments of drip irrigation with different lateral lengths

The trend of secondary soil salinization has been increasingly concerned in the arid region under drip irrigation in recent decades in China. It remains unclear whether the potential increasing nonuniformity of irrigation water aroused by the increased acreage of subunit leads to salt harmfulness and yield reduction. Cotton field experiments were conducted in 2018 and 2019 to evaluate the effects of lateral length on the distribution of water and salt in soil as well as lint yield under mulched drip irrigation to identify the dominating factors using the random forest regression. Three lateral lengths of 40-, 80-, and 120-m were used. Along with a trigger point of 60–70% of field capacity (FC), irrigation amount was determined using three irrigation upper limits of 90%, 100%, and 110% of FC in the 2018 and of 90%, 110%, and 130% of FC in the 2019. The random forest analysis revealed that the nonuniform water applied by drip irrigation greatly affected the distribution of soil water content with an affecting weight exceeding 0.4. However, it imposed little impact on soil salt content along the lateral with the affecting weights less than 0.1. The relative importance of spatial variation of soil properties on soil water content decreased with increasing lateral length, while the nonuniformly applied water became the dominant factor affecting the distribution of soil water content for a field with lateral length of 120-m. The initial soil salt content dominated the spatial and temporal variation of soil salt content for treatments with all the three lengths of lateral. The long lateral did not produce obvious soil salt accumulation at the 0–60 cm soil depth along the lateral. The cumulative irrigation water and nitrogen applied imposed greater influence on the cotton lint yield than other factors. A medium lateral length is a promising selection to maintain sustainable production balancing possible salt accumulation and crop production in arid lands with relatively low initial soil salt content.