Impacts of diverse walnut intercropping models on fine root biomass and nutrient stocks

Intercropping impacts on the plant fine root growth by improving soil fertility and productivity. This study assessed the seasonal dynamics of fine root biomass and nutrient stock of the different intercropping models by planting ginger (WGI), sweet potato (WSP), soybean (WSB), and potato (WPO) in a walnut plantation as compared to pure walnut (CK). Soils in the 0–20 cm and 20–40 cm layers were collected in four seasons, and a membership function was applied to integrate multiple fine-root traits and soil fertility indicators into a comprehensive evaluation value (CEV) for intercropping models. All intercropping models in the same soil layer and season showed increase in biomass and carbon, nitrogen, phosphorus, and potassium stocks of fine root, which clearly showed mean annual fine root nutrients stock in the 0–40 cm layer in autumn and summer and declines in winter; soil organic matter (SOM), alkali-hydrolysable nitrogen (AHN), available phosphorus (AVP), and available potassium (AVK) content in the 0–40 cm layer were highest in winter and spring. All intercropping models significantly (p< 0.05) increased fine root biomass and the accumulation of carbon (C), nitrogen (N), phosphorus (P), and potassium (K) in the 0–40 cm layer as compared to the CK; these indices in the 0–20 cm layer were higher than those in the 20–40 cm layer. Fine root biomass and nutrient stocks were maximum in summer and autumn, particularly the WGI system showed the highest nutrient accumulation. CEV ranged from 0.33 to 2.79 and ranked as WGI > WSB > WPO > WSP > CK. Correlation analysis revealed significant (p< 0.05 or p< 0.01) positive relationships among the CEV, fine root traits, and soil fertility indicators of the 0–40 cm layer. The WGI (walnut + ginger) model was identified as the most effective intercropping of walnut plantation for improving fine root nutrient stock and soil fertility. This highlights the critical role of seasonal fine root dynamics and intercropping in enhancing nutrient retention and cycling in the walnut agroforestry systems.