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

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)center dot kg(-1), respectively, and the N2O emission were 6.1 x 10(-2)-6.9 x 10(-2), 5.6 x 10(-2)-6.3 x 10(-2) and 5.6 x 10(-2)-6.0 x 10(-2) mg center dot kg(-1), respectively. In addition, the accumulation of mineral nitrogen was 16.60-24.34 mg center dot kg(-1) (0-cycle), 13.57-15.85 mg center dot kg(-1) (5-cycle) and 12.96-14.85 mg center dot 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

Effect of iron oxide on nitrification in two agricultural soils with different pH

Iron (Fe) affects soil nitrogen (N) cycling processes both in anoxic and oxic environments. The role of Fe in soil N transformations including nitrification, mineralization, and immobilization, is influenced by redox activity, which is regulated by soil pH. The effect of Fe minerals, particularly oxides, on soil N transformation processes depends on soil pH, with Fe oxide often stimulating nitrification activity in the soil with low pH. We conducted lab incubations to investigate the effect of Fe oxide on N transformation rates in two subtropical agricultural soils with low pH (pH 5.1) and high pH (pH 7.8). ¹⁵N-labeled ammonium and nitrate were used separately to determine N transformation rates combined with Fe oxide (ferrihydrite) addition. Iron oxide stimulated net nitrification in low-pH soil (pH 5.1), while the opposite occurred in high-pH soil (pH 7.8). Compared to the control, Fe oxide decreased microbial immobilization of inorganic N by 50 % in low-pH soil but increased it by 45 % in high-pH soil. A likely explanation for the effects at low pH is that Fe oxide increased NH₃-N availability by stimulating N mineralization and inhibiting N immobilization. These results indicate that Fe oxide plays an important role in soil N transformation processes and the magnitude of the effect of Fe oxide is dependent significantly on soil pH