Humic Acid Enhances Soil Fertility and Microbial Diversity Under Optimized Nitrogen Fertilization in Quinoa Rhizosphere

Excessive nitrogen fertilization can degrade soil quality by inducing nutrient leaching, disrupting the microbial balance, and impairing plant reproductive growth, ultimately reducing crop yields. Optimizing nitrogen application rates and integrating humic acid fertilizers are promising strategies for enhancing soil fertility and improving agricultural productivity. The experimental design included four nitrogen application rates (N0:0 kg ha^-1, N1:120 kg ha^-1, N2:150 kg ha^-1, and N3:180 kg ha^-1) with and without humic acid (H: 1500 kg ha^-1). Key findings revealed that: (1) The combined application of humic acid (1500 kg ha^-1) and medium nitrogen (150 kg ha^-1) significantly increased the contents of soil organic carbon (SOC), total nitrogen (TN), available phosphorus (AP), and available potassium (AK) by an average of 21.7% (<i>p</i> < 0.05), 90.5% (<i>p</i> < 0.01), 59.4% (<i>p</i> < 0.05), and 11.3% (<i>p</i> < 0.05), respectively (two-year mean), with significant interactive effects between nitrogen and humic acid on nutrient accumulation; (2) humic acid supplementation significantly increased soil bacterial abundance and diversity: under the combined treatment of medium nitrogen (150 kg ha^-1) and humic acid, the bacterial Ace index (indicating species richness) and Shannon index (indicating community diversity) increased by an average of 0.76% and 0.30%, respectively, compared with the single medium nitrogen treatment (<i>p</i> < 0.05), promoting a more balanced microbial community; and (3) quinoa yields improved by 24.62-66.83% with humic acid application, with the highest yield increase observed under the moderate nitrogen rate (150 kg ha^-1) in combination with humic acid. These results demonstrate that integrating humic acid with optimized nitrogen fertilization (150 kg ha^-1 N + 1500 kg ha^-1 HA) can effectively improve soil nutrients and enhance quinoa productivity. The increases in soil total nitrogen (TN, <i>p</i> < 0.01), available phosphorus (AP, <i>p</i> < 0.05), bacterial Shannon index (<i>p</i> < 0.05), and quinoa yield (<i>p</i> < 0.01) under this combined treatment were all significantly higher than those under single nitrogen fertilization or humic acid application, confirming the synergistic effect of the two fertilizers.