Redox effect on carbon and nitrogen mineralization in the drawdown zone of the Three Gorges Reservoir

Abstract

Soil redox is a critical environmental factor affecting nutrient cycling, but little is known about how soil carbon and nitrogen mineralization accrue in response to repeated redox fluctuations. To study the effect of prolonged redox disturbances on soil carbon and nitrogen mineralization, soils from Wanzhou, Changshou and Fengdu areas of the Three Gorges Reservoir drawdown zone experiencing 0, 5 and 8 redox cycles within approximately six months were tested using the soil mineralization culture method. The repeated wetting–drying cycles for up to 6 months altered soil redox state compared to unflooded soil, thereby inhibiting soil carbon and nitrogen mineralization, and the trend was consistent across regions. During the 14-day incubation, the cumulative CO₂ emission of 0-cycle, 5-cycle and 8-cycle sites from all three areas were 155–176, 141–143 and 125–129 mg⁻¹·kg⁻¹, respectively, and the N₂O emission were 6.1 × 10⁻²–6.9 × 10⁻², 5.6 × 10⁻²–6.3 × 10⁻² and 5.6 × 10⁻²–6.0 × 10⁻² mg·kg⁻¹, respectively. In addition, the accumulation of mineral nitrogen was 16.60–24.34 mg·kg⁻¹ (0-cycle), 13.57–15.85 mg·kg⁻¹ (5-cycle) and 12.96–14.85 mg·kg⁻¹ (8-cycle), respectively. Results showed the emissions of CO₂ and N₂O were reduced and mineralized nitrogen decreased with the increase of redox frequency. Therefore, the analysis of the effects of prolonged and repeated redox disturbances on soil carbon and nitrogen mineralization will help to increase the accumulation of organic carbon and nitrogen in the drawdown zone of the Three Gorges Reservoir, and reduce the release of CO₂ and N₂O to alleviate ecological and environmental problems such as greenhouse gas emissions and water pollution.