Objectives The effects of intercropping green manures on maize yield and soil fertility were compared, to provide reference for efficient maize production in Hexi Oasis irrigation area.

Methods The 12-year old experiment was established in 2009 and the intercropped green manure crops were melilotus, needle leaf pea, and sweet pea, and each crop was returned to the field in two patterns, incorporating whole fresh plants or just stubbles. Mono maize cropping was used as control (CK). The yield, yield components, and nutrient accumulation of maize and green manure were analyzed, the soil samples were collected at the same time for the determination of fertility and enzyme activities.

Results The incorporation of whole plants and stubble of melilotus, whole plants of needle leaf pea and sweet pea significantly increased maize yield by 28.06%, 16.74%, 13.99%, and 9.86%, compared with CK, the incorporation of only stubbles of needle leaf pea and sweet pea did affect yield in 2020. Incorporating the whole fresh melilotus plants significantly increased maize plant height, ear diameter, ear length, kernel number, and aboveground weight, thus, led to high grain yield, and the lowest difference of annual maize yield variation coefficient. Compared with CK, incorporating whole plant of fresh needle leaf pea and sweet pea significantly increased soil inorganic N and available K concentrations by 15.39%–22.48% and 9.61%–15.92%, respectively. Incorporating needle leaf pea elicited the highest soil total N concentrations and maize N accumulations, which were 12.7% and 76.89% higher than that of CK. Intercropping needle leaf pea and sweet pea improved the soil alkaline phosphatase activity by 13.46%–45.03%, meanwhile incorporating sweet pea enhanced urease activity by 19.75%. The aggregated boosted tree (ABT) analysis indicted that soil total N, available K and urease activity were the top three contributors to maize yield, accounting for 41.08%, 10.15%, and 12.24%, respectively.

Conclusions In the maize-green manure intercropping system, incorporating whole plants or just stubbles of melilotus are efficient in increasing maize yield, while incorporating whole fresh plant is better than just returning stubbles. Incorporating the whole plants of either needle leaf pea or sweet pea could significantly improve maize yield, but they are more efficient in improving soil fertility.