Effects of intercropping rape on soil moisture and quality of apple fruit in an apple orchard in the Loess Plateau

Objectives Planting grass in an orchard is one of the models of partially replacing chemical fertilizers with an organic fertilizer in apple production areas. Due to low and uneven inter-season variation in precipitation in the Loess Plateau, planting grass may increase water uptake and deteriorate soil water storage. Therefore, we tested rapes with different water consumption patterns capable of serving as orchard grass.

Methods The experiment was conducted in 2018 and 2019 in Yan’an City, Shaanxi Province. Three intercropped rape varieties; namely, napus rape Shaanyou 2013 (A-G), spring cabbage type rape Haoyou 21 (A-H), strong winter cabbage type rape Yanyou 2 (A-Y), and clean soil surface control (CK), were used as treatments. Soil moisture, rape growth index and apple fruit quality were measured.

Results The growth index of rape was (P < 0.05) different among different rape types in two years. The leaf and branch biomass per area were as follows: cabbage type rape (A-G) > strong winter cabbage type rape (A-Y) > spring cabbage type rape (A-H). The soil moisture variation within the apple-growing period was similar in the four treatments, and the most significant variation was observed in the 0–20 cm soil layer. Compared with CK, the soil water content of A-G, A-H, and A-Y in 0–20 cm soil layer decreased by 19.2%, 3.3%, and 8.5%, respectively. In the 20–100 cm soil layer, the soil moisture content increased by –4.9%, 12.1%, and 6.4%. The coefficient of variation of average soil water content in the 20–100 cm soil layer was higher in A-G and smaller in A-H. The three intercropping treatments improved the fruit taste. However, the most significant effect was observed in A-H, where the contents of soluble solids, soluble sugar and V_C in the apple of intercropping spring rape was higher than other treatments, improving by 1 and 1.57 percent point, and 47.58%.

Conclusions Intercropping of cabbage type rape, strong winter cabbage type rape, and spring cabbage type rape are all efficient in increasing soil water storage in 20–100 cm soil depth, thereby decreasing the consumption of soil water during apple dormancy. Consequently, the treatments stimulate the growth and development of apple trees and improve the fruit quality. Spring cabbage type rape performed better than the other two types, and thus, it is recommended for intercropping with dry apple orchards on the Loess Plateau.