Soil water, nutrient distribution and use efficiencies under different water and fertilizer coupling in an apple–maize alley cropping system in the Loess Plateau, China

Soil water and nutrients are the main factors that restrict the stable development of alley cropping systems in the Loess Plateau of west Shanxi, China. Field experiments were conducted during 2014–2016 to investigate the effects of water and fertilizer coupling regulation on soil water, nutrient distribution and their use efficiencies in a typical apple–maize alley cropping system in the area. There were three upper limits for irrigation according to appropriate ranges of soil moisture for apple and maize, 50% (W1), 65% (W2) and 85% (W3) of field capacity (Fc) for the 0–60 cm soil layer, coupled with three nitrogen/phosphorus/potassium compound fertilizer levels of F1 (289.0/118.0/118.0 kg∙ha−1), F2 (421.4/168.8/168.8 kg∙ha−1), and F3 (537.0/219.0/219.0 kg∙ha−1) and a CK (rain fed, no irrigation or fertilizer) as a control, making a total of 10 treatments. The results indicated that the horizontal and vertical soil water content (SWC) increased with distance from the tree row and with soil depth, respectively. The SWC near the maize belts was greater than near the tree line, and this was reversed with the increase of intercropping years. In the horizontal direction, the soil nutrient content (SNC) for each indicator increased with distance from the tree row, with the maximum value occurring at 2.5 m. In the vertical direction, SNC decreased with soil depth, with the maximum value at 0–20 cm. The 85% Fc combined with 70% empirical fertilizer application amount resulted in the maximum grain yield and partial factor productivity (PFP). Irrigation water use efficiency (IWUE) was highest for the F1 level. Multivariate and spatial analyses showed that a range of 50–65% Fc combined with 70% empirical fertilizer application was suitable for optimal irrigation and fertilization management of this alley cropping system considering the integrated benefits of maize yield, IWUE and PFP.