Characteristics of soil water and heat, physiological growth and interspecific competition in apple–soybean alley cropping for different water and heat coupling on the Loess Plateau, China

Soil water and heat are critical factors for the sustainable development of alley cropping systems in the Loess Plateau, west Shanxi, China. Experiments were conducted from 2021 to 2023 to explore the impacts of water and heat coupling regulation on crop root distribution, physiological growth, and the interrelationships between soil water and heat. The treatments consisted of three irrigation upper limit levels, set at 50% (W1), 65% (W2), and 80% (W3) of field capacity for the top 0–60 cm soil layer, and two film mulching durations: from soybean sowing to the podding stage (M1) and throughout the entire growth period (M2). Additionally, three non-irrigation control groups were included: CK0 (no mulch), CK1 (film mulching from branching to podding stage), and CK2 (full-period film mulching). In years with ample rainfall (2021 and 2023), half-period mulching was found to effectively preserve soil water compared to full-period mulching, resulting in higher underground interspecific competition coefficient; conversely, in the drier year of 2022, full-period mulching proved more effective at retaining soil water, with a lower underground species competition coefficient observed. Elevated soil moisture increased root length density (RLD) across species, intensified belowground interspecific competition, and consequently reduced soil temperature. The underground interspecific competition index initially increased and then gradually decreased as irrigation levels rose, with the peak intensity of competition occurring at the W2 irrigation level. Principal component and multiple regression analyses indicated that a water input range of 1015.63–1093.75 m³·ha⁻¹·a⁻¹ combined with half-period mulching was most advantageous during the initial 3–5 years of the alley cropping system under local production conditions.