Soil Moisture, Nutrients, Root Distribution, and Crop Combination Benefits at Different Water and Fertilizer Levels during the Crop Replacement Period in an Apple Intercropping System

Uneven soil moisture and nutrient distribution before and after intercropping limits apple cropping system productivity in the western Shanxi loess area. To address the problem, a field experiment was conducted between 2020 and 2021 to investigate the effects of different water and fertilizer management practices on soil moisture, nutrients, root distribution, and overall benefits of the intercropping system during the crop replacement period. The experimental set up included three factors: irrigation method, irrigation level, and fertilizer application; irrigation methods included drip (D) and flood (M) irrigation; irrigation levels included rain-fed without irrigation (W0), 50% of the field water holding capacity (Fc)-W1, and 80% of the field water holding capacity (Fc)-W2; fertilizer treatments included F0 (no additional fertilizer application), F1 (N 206.2 kg∙hm^-2 + P₂O₅ 84.4 kg∙hm^-2 + K₂O 84.4 kg∙hm^-2), F2 (N 412.4 kg∙hm^-2 + P₂O₅ 168.8 kg∙hm^-2 + K₂O 168.8 kg∙hm^-2), and control (CK) without irrigation and fertilization, for a total of 15 treatments. According to the results, soil water content (SWC) decreased after the crop replacement. Besides, nitrate nitrogen (NN), ammonium nitrogen (AN), and organic matter (OM) contents in all treatments increased, whereas total phosphorus (TP) content decreased. The main soil aggregate layer with crop roots shifted downwards (from the 0–40 cm soil layer before crop replacement to the 0–60 cm soil layer) after crop replacement, and partial fertilizer productivity (PFP), irrigation water use efficiency (IWUE), and water use efficiency (WUE) under both irrigation treatments were decreased. Principal component analysis showed that the W2F2 treatment had the highest combined benefits both irrigation treatments during the crop replacement period. Structural equation modeling showed that apple tree and maize RLDs had no significant effects on water use (ET) and WUE before crop replacement. Apple tree and soybean RLDs had significant positive correlations with ET and significant negative correlations with WUE after crop replacement. According to our results, to optimize the benefits of apple-crop intercropping, drip irrigation with complete water supply and flood irrigation with incomplete water supply are recommended during crop replacement. In addition, an upper irrigation limit of 80% of the field water holding capacity and a fertilizer application rate of N 412.4 kg∙hm^-2 + P₂O₅ 168.8 kg∙hm^-2 + K₂O 168.8 kg∙hm^-2 are recommended for optimal water and fertilizer regulation.