Oat and Forage Pea Mixed Sowing Improves Soil Chemical Fertility and Fresh and Dry Mass Yield in Light Saline–Alkali Land: Preliminary Results

Saline–alkali land is widely distributed worldwide, and poses significant challenges to agricultural productivity and ecological restoration, particularly in the Qaidam Basin. This study investigates the effects of intercropping and mixed sowing systems of oats (Avena sativa L.) and forage peas (Pisum sativum L.) on hay yield and soil chemical fertility in mildly saline–alkali soils of the Qaidam Basin. Through a comprehensive analysis of hay yield, soil chemical properties, and enzyme activities under different sowing methods (mixed sowing, intercropping, and monoculture) and row spacings (30, 25, 20 and 15 cm), an optimal planting pattern was identified. The 25 cm intercropping treatment produced the highest amount of fresh biomass (2242 kg hm⁻¹), with an increase ranging from 3.17% to 70.44% compared to other treatments. Additionally, this treatment led to significant reductions in soil pH and electrical conductivity (by 3.87–8.31% and 5.13–6.05%, respectively), while enhancing organic matter content and enzyme activities. The intercropping system also demonstrated higher ammonium nitrogen and available phosphorus contents compared to the mixed sowing system. Structural equation modeling revealed that row spacing indirectly influenced both hay yield and enzyme activities, with the 25 cm row spacing proving to be the most effective. A comprehensive evaluation using the TOPSIS model further affirmed that the 25 cm intercropping system significantly enhanced hay yield, soil chemical properties, and enzyme activities, making it the most suitable planting pattern for sustainable land management in the study area. Therefore, the 25 cm intercropping model not only optimizes hay yield, but also improves soil quality and ecological functions in saline–alkali land, offering valuable insights and practical recommendations for the sustainable use of saline–alkali land in the Qaidam Basin.