Objectives Drought and poor soil fertility restrict the apple production benefit in the semi-arid area of Loess Plateau. We screened suitable rapeseed varieties as green manure, and studied their mulching effect in improving soil moisture in spring and soil fertility.

Methods The experiment was carried out in a hilly apple orchard in Yan’an City, Shaanxi Province, where the rainfall was rare in winter and spring, and mainly happen from July to October of a year. Firstly, rapeseed varieties were screened for water conservation effect, including five Brassica campestris L. winter varieties (V1−V5), one Brassica campestris L. spring variety (V6), and four Brassica napus L. winter varieties (V7−V10). The rapeseeds were sown in unplanted land between apple tree lines in early August 2019, and the aboveground part were mown by the end of October and mulched on nearby root area of apple tree (0−115 cm radius around tree trunk). The water contents in 0−60 cm soil depth in the root area were measured in the following March. The rapeseed varieties V9 and V10 showed better water conservation effect than the other ones, so were chosen as the test materials for the following two years position mulching experiment (2020−2022), in which the rapeseed sowing, mowing, and mulching were the same as in the variety screening experiment. In rapeseed growing season (in October 2021), the soil water contents in 0−175 cm soil layer in both rapeseed planting area and apple tree root area were measured. After autumn mulching of rapeseed plant, soil in 0−175 cm soil layer in the mulched root area of apple tree was sampled in March and May 2022 for the measurement of water contents, and the 0−20 cm soil was sampled on August for nutrient measurement. At harvest season, the apple yield and quality were measured.

Results 1) Screening of rapeseed variety: The mulching of rapeseed variety V1, V2, V3, and V4 did not show water conservation effect, the V5 mulching only increased water content in 0−20 cm soil layer. The V6, V7, V8, V9 and V10 mulching significantly increased 0−20 or 0−60 cm soil water content. The V6, V9 and V10 were recorded significantly higher aboveground biomass than the V1, V2, V3, and V5. The water contents in 20−40 and 40−60 cm soil layers were positively (P<0.05) correlated with the mulching amount of rapeseed plants. 2) Mulching position experiment: Rapeseed planting decreased water contents in 0−50 cm soil layer in their growing season (P<0.05), and did not influence that in root area soil of apple tree. In the second year (2022) of continuous mulching, the water contents in 0−150 cm depth of root area soil of apple tree were increased by 17.6%−21.8% in March, and those in the 0−100 cm soil layer were increased by 12.5%−21.6% in May; the contents of available P, available K, total N, and organic matter in the 0−20 cm soil were significantly increased by 32.2%−44.9%, 95.4%−146.4%, 25.2%−25.6%, and 23.8%−37.7%, respectively. The rapeseed mulching significantly improved the fruit yield per tree and single fruit weight, and increased apple vitamin C content, total sugar content and decreased total acid content.

Conclusions Interplanting of rapeseeds in autumn and mulching with the aboveground shoots around root area soil of apple tree showed significant soil water conservation and fertility promotion effect, resulting in satisfactory yield and quality effect on apple fruit. As the mulching effect is correlated to the biomass of rapeseed shoots, Brassica campestris L. spring variety and Brassica napus L. winter variety are recommended as the mulching green manure of apple orchards.