Root Oxygen Microzones and Nitrogen Cycling in Constructed Wetlands: A Mechanistic Review of Radial Oxygen Loss, Pathway Partitioning, and Scale-Bridging

Constructed wetlands rely on microscale redox heterogeneity to couple ammonium oxidation with downstream nitrogen loss, yet the causal role of radial oxygen loss (ROL) is often inferred from planting effects, bulk redox potential, functional genes, or outlet performance rather than measured directly. This narrative mechanistic review aims to determine when ROL-generated root oxygen microzones become functionally relevant, how they alter competition and coupling among nitrification, denitrification, anammox, DNRA, and NO/N2O branch points, and whether present evidence supports scaling from individual roots to bed-scale total-nitrogen (TN) performance. We conduct a structured narrative evidence synthesis that separates direct root-zone O2/ROL measurements from redox-resolved contextual evidence and mechanistic analogy, and appraises studies by measurement directness, resolution, nitrogen-endpoint resolution, operating-context reporting, control of competing explanations, and scale relevance. The broader mechanistic corpus is extensive, but only a small subset contains spatially resolved root-zone oxygen measurements coupled with nitrogen-related endpoints. Direct measurements show strong heterogeneity: reported lateral-root DO is 0.64–2.04 mg L−1 with 0.76–1.16 mm oxygen layers, root-surface oxygen declines from about 2.0 to 0.5 mg L−1 with depth in one Acorus system, and potential radial oxygen diffusion in Typha middle-root sections spans 0.003–0.316 µmol O2 L−1 µm−1. We therefore treat ROL as a context-dependent oxygen budget rather than a fixed species property. Root-associated oxygen gradients are demonstrated and strongly heterogeneous; their effects on pathway feasibility and spatial coupling are conditionally supported; however, a transferable ROL coefficient that predicts bed-scale TN removal is not yet demonstrated. Progress requires direct root-zone mapping, pathway-rate measurements, complete nitrogen mass balances, internally constrained reactive-transport models, and seasonal field validation.