Redesign of dryland apple orchards by intercropping the bioenergy crop canola (Brassica napus L.): Achieving sustainable intensification

China's Loess Plateau (LOP) contains around 22% of all the world's apple trees, mainly on smallholdings run by farmers. However, business-as-usual (BAU) intensification has reduced the ecological resilience of orchards. Agroforestry is considered to be an important method of realizing sustainable intensification of agroecosystems. However, the design of practical agroforestry systems in drylands remains poorly researched. We hypothesized that crop density is a key factor in designing dryland apple tree-based agroforestry systems. We carried out an experiment in apple orchards in the semiarid LOP, where the bioenergy crop canola (Brassica napus L.) was intercropped at low, medium, and high densities with row spacings of 25, 50, and 100 cm, respectively, to study its effects on water, carbon, and production. We found that intercropping of canola at different densities clearly decreased soil water content (SWC) in the top 60 cm soil layer at all sampling locations, reduced midday leaf water potential (psi(m)) and leaf water content (LWC), and caused apple trees to produce more fine roots at greater depths. However, intercropping did not affect SWC in soil layers below 60 cm. The SWC profiles, psi(m) and LWC, progressively recovered over the mulching period. Introduction of canola also significantly (p < 0.05) increased SOC content in the surface layer, an effect less visible in deeper layers, and increased the quality of apple fruit to varying degrees while not affecting apple yield measurably. Overall, higher intercropping density does not lead to severer water deficit but have better carbon sequestration benefit than lower density, and therefore seems more appropriate in our study site. Our findings provide solid evidence that agroforestry helps achieve sustainable intensification of dryland orchards.