Arbuscular mycorrhizal fungi stabilize ammonium-rich nitrogen removal in constructed wetlands through plant-mediated microbial functional shifts.

Arbuscular mycorrhizal fungi (AMF) are increasingly proposed as a low-energy means of intensifying constructed wetlands (CWs), but whether their benefits depend on influent nitrogen speciation has not been tested. We compared inoculated and uninoculated CWs across three influent N forms at equal N loading. CWs planted with Iris pseudacorus were inoculated with Rhizophagus irregularis (AMF+) or left uninoculated (AMF-) and fed NO3--only (N1), NH4+-only (N2), or mixed NO3-/NH4+ (N3) influents at equal total N. Compared with AMF-, AMF+ systems showed higher plant height and chlorophyll content, 30-70% higher SOD/POD activities, and 30-50% lower MDA content and O2·- production, especially under N2 and N3. Under N2, AMF prevented TN-removal collapse (83% vs. 26%) and lowered effluent NH4+-N (∼3 vs. ∼9 mg N L-1). Under N3, AMF maintained ∼88% TN removal (vs. 72%). Selective NH4+-N accumulation with negligible effluent NO2--N and NO3--N indicated reduced ammonia-oxidation capacity as the primary N2 bottleneck. Only N2 showed significant AMF-associated community separation (ANOSIM R = 0.77, p = 0.03), accompanied by higher ammonia and nitrite-oxidation potentials (log2FC = 2.20 and 3.99) and increased metagenomic abundances of amoABC, hao, and nxrB. PLS-PM accounted for 68% of the variation in N-removal performance and identified N-cycling functional potential as its strongest positive correlate (β = 0.81), while significant colonization-plant and plant-microbiome paths highlighted host physiological maintenance within the proposed association framework. Overall, AMF benefits were more closely aligned with N-form-specific bottlenecks than with colonization intensity, supporting context-dependent application to stabilize N removal in low-energy CWs treating ammonium-rich or compositionally variable wastewater.