Effects of mulch and irrigation regimes on water distribution and root competition in an apple–soybean intercropping system in Loess Plateau, China

Water scarcity seriously hinders the sustainable development of the fruit tree–crops intercropping system in the Loess Gully Area of western Shanxi. The combination of mulch measures and appropriate irrigation would help to maintain soil moisture and improve crop yield and water use efficiency (WUE). This study investigated the combined effects of mulch and irrigation on soil water and root distribution, yield, and water use of an apple–soybean intercropping system. A three-year field trial was performed with two factors: two types of mulch, namely straw mulch (M1), and plastic mulch (M2), and three levels of maximum irrigation (55%, 70%, or 85% of field capacity [Fc]), which were designated W1, W2, and W3, respectively. Three groups of non-irrigation controls i.e., CK0 (no mulch), CK1 (straw mulch), and CK2 (plastic mulch) were also included. These results indicated that the soil water contents (SWC) in each treatment initially decreased and then increased with increasing distance from the tree, whereas it increased gradually with depth. The SWCs in various sampling points under straw mulch was generally higher than under plastic mulch. The root length density (RLD) of apple tree decreased with increasing distance from the tree in the horizontal direction, while the opposite trend was observed for soybean. The RLD of both apple tree and soybean in the soil layer (0–60 cm) under plastic mulch was higher than that under straw mulch, and increased with increased irrigation level. The underground interspecific competition intensity index (UICII) of the apple–soybean intercropping system under straw mulch was higher than that under plastic mulch, and increased with the increased irrigation amount. Water consumption, soybean yield, and WUE under plastic mulch were significantly higher than under straw mulch. The results of this study suggested that the combination of plastic mulch and irrigation water supplied at 70–85% Fc could effectively increase the yield and WUE of the apple–soybean intercropping system.