Photosynthetic characteristics, soil nutrients, and their interspecific competitions in an apple–soybean alley cropping system subjected to different drip fertilizer regimes on the Loess Plateau, China

Abstract

Imprecise irrigation hinders the sustainable development of the fruit tree–crop intercropping system in the Loess Gully Area of China. Drip fertigation was applied for two years as part of an apple–soybean intercropping system to optimize local water and nutrient management patterns. The effects of different drip irrigation and fertilizer levels on photosynthetic characteristics, soil nutrients, and their interspecific competitions were studied. The treatments were four irrigation levels, namely 60% (W1), 70% (W2), 80% (W3), and 90% (W4) of field capacity (Fc), and three nitrogen fertilization levels, namely 59 (F1), 92 (F2), and 124 kg∙ha–1 (F3). The control (CK) was a rain-fed (neither irrigation nor fertilizer) crop. The net photosynthetic rate (Pn), transpiration rate (Tr), and soybean yield (GY) increased at first, but then decreased as irrigation and fertilizer application levels rose. The partial factor productivity (PFP) decreased as fertilization increased, but the soil available nitrogen content showed the opposite trend. The soil nutrients, and soybean Pn and Tr were positively correlated with distance from the apple tree row. The interspecific competition intensity for light and soil water decreased from 2018 to 2019, while root and soil nitrate nitrogen (NN) increased. The interspecific competition correlation between the aboveground and belowground parts was weak and varied between the two intercropping years. A principal component analysis showed that treatment W3F2 had the highest comprehensive score. Thus, a combination of irrigation at 80% Fc and 92 kg∙ha–1 nitrogen fertilizer application is optimal for the system. A multiple regression analysis indicated a water input range of 4624.49–4795.75 m3·ha−1·a−1 combined with 84.10–107.27 kg·ha−1·a−1 fertilizer application was suitable for optimal irrigation and fertilization management based on the integrated benefits to root length density, GY, Pn, and PFP during the first 4–5 years of the intercropping system.