Response of Maize Root Endophytes to Carbon-Nitrogen Coupling in the Irrigated Area of the West Liaohe Plain, Inner Mongolia

Based on a five-year positioning experiment conducted in the irrigated area of the West Liaohe Plain, this study employed a two-factor split-plot design. Two factors were set: the straw stover return rate (no stover return; half stover return, 4500 kg/ha; full stover return, 9000 kg/ha) and the nitrogen application rate (conventional nitrogen, 450 kg/ha; reduced nitrogen, 300 kg/ha), resulting in six treatment combinations: NN (no stover return + conventional nitrogen), NR (no stover return + reduced nitrogen), HN (half stover return + conventional nitrogen), HR (half stover return + reduced nitrogen), TN (full stover return + conventional nitrogen), and TR (full stover return + reduced nitrogen). Root tissue samples were collected at the R1 stage (silking) of maize for analysis of bacterial and fungal community structures. High-throughput sequencing of the 16S rRNA gene V5-V7 region and 18S rRNA gene ITS2 region, combined with Accu 16S™ and Accu ITS™ absolute quantification technologies for bacteria and fungi, was used to systematically investigate the diversity, community structure, functional potential, and coupling relationships with the soil nutrient factors of maize root endophytes. The results revealed that the dominant bacterial phylum was Pseudomonadota, and the dominant genus was Pseudomonas . Unlike the NN treatment, which maintained relatively high bacterial diversity and complex structure, the NR treatment led to increased dominance of Pseudomonas and extreme community simplification, with high total fungal abundance but a uniform structure. In the HN treatment, the bacterial communities were dominated by Pseudomonadota, enriching the organic matter-degrading groups, whereas the fungal communities included opportunistic pathogens such as Fusarium . The HR treatment significantly enriched Bacteroidota, inhibited Pseudomonas , had the highest number of unique bacterial OTUs but decreased diversity, and it presented the strongest fungal heterogeneity, with an increase in functional groups such as Mortierella . The TN treatment had a conserved bacterial structure enriched with Enterobacterales , which presented the lowest total fungal abundance but was enriched with yeast-like groups. The TR treatment significantly inhibited Pseudomonas ; increased the abundance of Rhizobium , Agrobacterium , and Actinomycetota; maintained high functional differentiation potential; and enriched Mortierella and Penicillium with a stable fungal structure. The dominant fungal phylum was Ascomycota. Under reduced nitrogen, stover return amplified community differentiation and increased the contribution of Mortierella . A comprehensive correlation analysis revealed that alkali-hydrolyzable nitrogen and available potassium were the core environmental factors driving changes in the root endophytic microbial community structure. The TR treatment was more conducive to constructing a highly functionally diverse community with both nitrogen fixation and organic matter transformation capabilities, providing a microbiological basis for the synergistic management of maize stover utilization and nitrogen fertilizer reduction in the irrigated area of the West Liaohe Plain.