Investigation on the Effects of Combined Application of Nitrogen Fertilizer and Humic Acid on the Root Morphology and Physiological Characteristics of Quinoa in Dryland Environment

This experiment examined the effects of different nitrogen fertilizer rates in combination with humic acid on quinoa roots and physiology, aiming to address the issues of soil quality decline and water eutrophication caused by excessive nitrogen fertilizer application. The study tested four nitrogen levels (0, 120, 150, 180 kg ha⁻¹) and three humic acid levels (0, 1500, 2250 kg ha⁻¹), resulting in 12 treatments. The N2H1 treatment (150 kg ha⁻¹ nitrogen + 1500 kg ha⁻¹ humic acid) exhibited the largest root length (RL), diameter (RD), volume (RV) and surface area (RS), with respective increases of 64.2%, 15.7%, 103.7%, and 112.2% compared to the control (N0H0). Humic acid application reduced malondialdehyde content and increased proline content, particularly at medium nitrogen levels (26.3–51.5% increase). At maturity, soil total nitrogen (TN) and soil organic matter (SOM) increased in all fertilized treatments compared to pre-sowing levels. Fertilization raised SOM by 4.57–37.85% compared to the control. SOM decreased with higher nitrogen rates when applied alone, with the lowest rate having the highest increase (16.46%). The medium nitrogen treatment combined with humic acid had the highest SOM (33.31–37.85% increase). This treatment also promoted quinoa root development, enhanced drought resistance by reducing malondialdehyde and increasing proline, and improved soil moisture and nutrient content. Excessive nitrogen or humic acid alone can inhibit root growth, but medium nitrogen combined with humic acid balanced yield and environmental safety, reducing chemical fertilizer use and increasing efficiency. This provides a basis for further research on root-soil interactions.