Changes in understory vegetation during the reclamation of saline-alkali soil by drip irrigation for shelterbelt establishment in the Hetao Irrigation Area of China

A two-year experimental field study was conducted to monitor the changes in understory vegetation during reclamation of saline-alkali land by ridge cultivation with drip irrigation for the establishment of ecological shelterbelts in the Hetao Irrigation Area in northern China. The experiment carried out in 2019 and 2020, included four drip irrigation treatments, designated 0 (control, no irrigation, no trees), 1 (1 year of drip irrigation plus trees), 2 (2 years of drip irrigation plus trees), and 3 (3 years of drip irrigation plus trees). Treatments 1, 2, and 3 resulted in decreases in soil salinity (relative to the control) of 75.76%, 85.98%, and 85.58%, respectively, over a depth of 0-40 cm, and 51.65%, 81.85%, and 70.80%, respectively, over a depth of 0-120 cm. The pH of the saturated soil paste extract (pHe) of the 0-40 cm and 0-120 cm soil layer decreased from 9.68 and 9.54 in the first year to 9.07 and 8.44 in the third year, in response to drip irrigation. The coverage, plant species and plant density of understory vegetation (following natural germination of seeds from the seed bank under the shelterbelt trees) was increased significantly from treatment 0 to 3, with values of 5.7%, 2 and 15 plants/m2 (treatment 0), respectively, to 94.4%, 20 and 416 plants/m2, (treatment 3), respectively. The biomass of the understory vegetation increased dramatically (after treatment 1), then decreased slightly more (after treatment 2), until the maximum value occurred after treatment 3. The dominant species of the vegetation gradually changed from the halophytic annual herb Suaeda glauca to the halophytic perennial grass Leymus chinensis. The research showed that, when the electrical conductivity of the saturated soil paste extract (ECe) exceeded 4.5 dS/ m, there was sparse vegetation; At lower salinity (2.6 < ECe < 4.5 dS/m) and higher pH (8.9 < pHe < 9.2), the species diversity gradually increased but without a clearly dominant species; At ECe < 2.8 dS/m, and pHe < 8.6, the dominance of plants of the Poaceae and Asteraceae families became apparent with L. chinensis developing into the dominant plant species in this area. This study contributes to our understanding of revegetation of saline alkali soils vegetation construction by salt regulation using drip irrigation and can inform ecological reconstruction in saline alkali regions in the future.