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Although nitrogen (N) transformations have been widely studied under oxic or anoxic condition, few studies have been carried out to analyze the transformation accompanied with NO 2 - -N accumulation. Particularly, the control of mixed N species in N-transformation remains unclear in an oxic-anoxic transition zone (OATZ), a unique and ubiquitous redox environment. To bridge the gap, in this study, OATZ microcosms were simulated by surface water and sediments of a shallow lake. The N-transformation processes and rates at different NH 4 + -N/NO 3 - -N ratios, and NO 2 - -N accumulations in these processes were evaluated. N-transformation process exhibited a turning point. Simultaneous nitrification and denitrification occurred in its early stage (first 10 days, dissolved oxygen (DO) ≥ 2 mg/L) and then denitrification dominated (after 10 days, DO < 2 mg/L), which were not greatly affected by the NH 4 + -N/NO 3 - -N ratio, on the contrary, the transformation rates of NH 4 + -N and NO 3 - -N were distinctly affected. The NH 4 + -N transformation rates were positively correlated with the NH 4 + -N/NO 3 - -N ratio. The highest NO 3 - -N transformation rate was observed at an NH 4 + -N/NO 3 - -N ratio of 1:1 with organic carbon/NO 3 - -N of 3.09. The NO 2 - -N accumulation, which increased with the decrease in NH 4 + -N/NO 3 - -N ratio, was also controlled by organic carbon concentration and type. The peak concentration of NO 2 - -N accumulation occurred only when the NO 3 - -N transformation rate was particularly low. Thus, NO 2 - -N accumulation may be reduced by adjusting the control parameters related to N and organic carbon sources, which enhances the theoretical insights for N-polluted aquatic ecosystem bioremediation., (Copyright © 2020 Elsevier Inc. All rights reserved.)