Impact of Monoculture and Various Ratios of Intercropped Oats and Daikon Radish Cover Crops on Soil Properties, Weed Suppression, and Spinach Yield

Optimizing seeding ratios in mixed cover crop species can maximize their ecological benefits, such as soil properties and weed suppression. A two-year field study assessed seven oat (O) and daikon radish (D) ratios (100:0 to 0:100) for their effects on soil quality, weed pressure, and subsequent spinach yield. Measured parameters included cover crop biomass, C:N ratio, land equivalence ratio (LER), soil organic carbon (SOC), microbial population, soil enzyme activities, bulk density, porosity, moisture, and water infiltration time. The impact of intercrop residues and two weeding strategies (hand weeding and no weeding) on weed pressure and spinach yield was also assessed. Oat monoculture produced the highest biomass (338.7 g m−2), while radish monoculture biomass was the lowest (256.1 g m−2). Yet the 30:70 (O:D) ratio contributed to the highest SOC (0.96). The C:N ratio of all intercropped combinations was below the critical threshold (25:1) that causes N immobilization, with oat monoculture having the highest value (23:1). The microbial population was highest with the 10:90 (O:D) ratio, with 12.8 × 10−4 most probable number per g−1 soil. While urease and dehydrogenase enzyme activities were not affected by intercrop ratios, β-glucosidase and alkaline phosphatase activities were up to 30% higher in daikon radish-dominated intercrops. Bulk density decreased by 31.7% in oat monoculture, whereas infiltration time was shortened in daikon radish monoculture by 41.7% (4.6 s). Weed suppression was strongest in oat monoculture and the 90:10 (O:D) intercropping, reducing weed populations by over 30%. Spinach yield was highest in oat monoculture with hand weeding (842.9 g m−2), with a 40.2% increase over weeding alone. Overall, daikon radish-dominated intercropping ratios were more effective in enhancing soil properties, whereas oat-dominated intercropping improved spinach yield, mainly due to slower decomposition, thus better suppressing weeds.