Photosynthetic and growth characteristics of apple and soybean in an intercropping system under different mulch and irrigation regimes in the Loess Plateau of China

The shortage of and intense competition for water and light are the main factors restricting the productivity of agroforestry systems in the Loess Plateau of western Shanxi, China. The combination of soil mulch and irrigation in agroforestry systems can regulate the interspecific relationships and increase the productivity of the component crops. However, the effects of mulch and irrigation regimes on the photosynthetic physiological characteristics of the agroforestry system and its synergistic mechanisms are still unclear. A 3-year experiment with two factors, mulch and irrigation, was conducted to investigate the effects of different mulch practices and irrigation levels on the photosynthetic characteristics, growth indices, and interspecific relationships of a fruit tree-crop intercropping system. An apple–soybean intercropping system was selected as the research object, and two mulch types [straw mulch (M1) and plastic mulch (M2)] and four maximum irrigation levels [no irrigation (W0), 55% (W1), 70% (W2), and 85% (W3) field capacity (Fc)] were tested. No mulch together with no irrigation was the control treatment (CK). The results showed that there was no photosynthetic siesta phenomenon in the intercropping system, and the daily variations of the net photosynthetic rate (Pn) and transpiration rate (Tr) were unimodal. The mean values for Pn, Tr, and leaf photosynthetic efficiency (PUE) of the apple and soybean under the M2 treatment were generally higher than under the M1 treatment, whereas Tr and PUE increased as the amount of irrigation increased. In 2017–2018, the maximum mean Pn values for both apple and soybean were observed under the M2W2 treatment, whereas in 2019, they were observed under the M2W3 treatment. The Pn, Tr, and leaf intrinsic water use efficiency (WUEi) of soybean correlated positively with the distance from the trees, whereas PUE correlated negatively with it. Mulch and water replenishment increased apple shoot growth and the height, leaf area index, and yield of soybean, especially under plastic mulch. A principal components analysis found that the M2W2 treatment had the highest comprehensive benefits in 2017–2018, whereas in 2019, the highest were achieved with the M2W3 treatment. Therefore, to optimize the photosynthetic efficiency and yield of the apple–soybean intercropping system in the loess region of western Shanxi, we suggest a combination of plastic mulch and maximum irrigation of 70% Fc in normal years, but maximum irrigation should be increased to 85% Fc in dry years. • Both mulch and irrigation enhanced photosynthesis and yield in apple-soybean intercropping system. • The photosynthesis parameters under plastic mulch was greater than those under straw mulch. • Plastic mulch and 70–85% Fc irrigation was the most efficient regimes for the apple-soybean intercropping system.