Biological measures significantly improve the salt discharge effect of subsurface pipes in saline soil of the agro-pastoral ecotone in Inner Mongolia

Objectives Biological measurements were studied to improve the desalting effect of engineering in saline soil of the lower reaches of agro-pastoral ecotone in the Inner Mongolia Hetao irrigation area.

Methods Field experiments were carried out in 2020 and 2021. Drainage pipes were buried at 0.8 m depth, running 20 cm apart. The biological treatments were salt-tolerant plants, including Sorghum bicolor, Sorghum sudanense, Medicago sativa, and no plant plot was used as the control (CK). After harvest, plants' aboveground and belowground biomass were recorded. The soil bulk density, total salt and individual ion content in 0−20 cm, 20−40 cm, and 40−60 cm soil layers were measured.

Results Combined with subsurface pipe, the bulk density of 0−20 cm soil layer in M. sativa, S. bicolor, and S. sudanense decreased after harvest by 16.77%, 13.57%, and 14.73%, respectively, compared with the values recorded before the experiment. The bulk density of the soil treated with the three forage plants decreased from the surface to the deeper layer, with an average of 6.37−7.88 times that of CK. Compared with the values recorded before sowing, the desalting rates of M. sativa, S. sudanense, S. bicolor, and CK in 0−20 cm soil layer after harvest were 61.42%, 56.69%, 39.96%, and 23.43%, respectively. The desalting rates of the three engineering and biological dual measures were 16.53%−37.99% higher than that of subsurface pipe single measure control. The desalting rate in 20−40 cm and 40−60 cm soil layers decreased successively. The desalting rates of SO₄²⁻、Na⁺+K⁺、Cl⁻ were relatively high. The average desalting rates of SO₄²⁻, Na⁺+K⁺, and Cl⁻ were 41.91%−57.47%, 33.4%−55.39% and 31.05%−53.49%, respectively. The desalting rates of all ions in the same treatment were similar. The contents of Na⁺, Ca²⁺, Mg²⁺, SO₄²⁻ and Cl⁻ in the aboveground biomass of M. sativa were significantly higher than those in S. bicolor and S. sudanense. The yields of M. sativa, S. bicolor, and S. sudanense increased in 2021 by 51.81%, 28.51% and 24.35% compared with 2020. The salt content of two years mowed M. sativa, S. bicolor, and S. sudanense was 30.0 g/m², 66.92 g/m², and 68.05 g/m², accounting for 1.59%, 5.13%, and 3.32% of the total soil desalination, respectively.

Conclusions Plant measures could improve the engineering measures effect n reducing soil bulk density and total salt content, especially in 0−20 cm soil depth. The salt content taken by plants through mowing is negligible compared with that lost through leaching. Plants improve the soil structure through roots and promote salt leaching out of the tillage layer. Therefore, suitable salt-tolerant plants are mainly selected according to the strength of the promoting leaching effect of soil salt. Planting M. sativa produced the best result in reducing soil bulk density and total salt content. Therefore, planting M. sativa is an effective measure of improving salinized soil with subsurface drainage in the agro-pastoral ecotone in the lower reaches of the Hetao irrigation area.