碳氮比改变对崇明东滩湿地反硝化与硝态氮氨化的影响

【目的】分析自然和人为活动加速影响下沿海湿地土壤碳氮比变化对硝态氮还原过程的影响。【方法】以崇明东滩典型滨海湿地为例,采集4种不同覆被类型下沉积物样品,添加C₆H₁₂O₆或KNO₃溶液,使沉积物有机碳与硝态氮比例($C/NO_3^--N$)增大30%和减小30%,借助 ¹⁵N同位素稀释技术,研究反硝化(Den)与硝态氮氨化(DNRA)的变化特征。【结果】$C/NO_3^--N$的升高或降低均引起芦苇和互花米草覆被下沉积物Den和DNRA速率的显著下降(P<0.05)。芦苇覆被下Den速率从原土的10.1 μg/(kg·h)降至1.0~3.1 μg /(kg·h),互花米草覆被下Den速率从原土的3.4 μg /(kg·h)降至0.3~0.4 μg /(kg·h)。相比较而言,芦苇植被下DNRA速率从原土的21.9 μg /(kg·h)降至12.7~14.5 μg /(kg·h),互花米草覆被下从原土的42.6 μg /(kg·h)降至3.1~5.8 μg /(kg·h)。【结论】4种覆被下沉积物DNRA/Den值均大于1,表明DNRA是湿地硝态氮还原的主要途径。与$C/NO_3^--N$减小相比,$C/NO_3^--N$增大使$NO_3^--N$的还原更趋向DNRA过程。崇明东滩湿地$C/NO_3^--N$的波动(±30%)可能并不会导致沉积物N₂O排放的显著增加。

Effects of $C/NO_{3}^{-}-N$ change on denitrification and dissimilatory nitrate reduction to ammonium in the Chongming Dongtan wetland

【Objective】 Fluctuations in the ratio of carbon and nitrogen in the wetland soil have an important impact on the nitrate nitrogen reductions under the influence of accelerated natural and human activities. 【Method】 Sediment samples were collected under four vegetation types in the Chongming Dongtan wetland. The ratio of organic carbon to nitrate ($C/NO_3^--N$) in sediments was adjusted to increase or decrease by 30% by adding C₆H₁₂O₆ or KNO₃ solution, respectively. The ¹⁵N isotope dilution technique was used to define the changes in denitrification (Den) and dissimilatory nitrate reduction to ammonium (DNRA). 【Result】 The results showed that the increase or decrease in $C/NO_3^--N$ caused a significant decrease in the rates of Den and DNRA in the sediments under Phragmites australis and Spartina alterniflora (P < 0.05). The Den rate under P. australis decreased from 10.1 μg/(kg·h) (CK) to 1.0-3.1 μg/(kg·h), whereas under S. alterniflora, it decreased from 3.4 μg/(kg·h) (CK) to 0.3-0.4 μg/(kg·h). In contrast, the DNRA rate decreased from 21.9 μg/(kg·h) (CK) to 12.7-14.5 μg/(kg·h) under P. australis, and for S. alterniflora, it decreased from 42.6 μg/(kg·h) (CK) to 3.1-5.8 μg/(kg·h). This indicates that the impact of changes in $C/NO_3^--N$ on nitrate reduction should be considered in the assessment of N₂O emissions in the wetland. Particularly, the sediments under P. australis had a higher denitrification rate, whereas the DNRA rate under S. alterniflora was higher. 【Conclusion】 This study showed an essential difference in nitrate utilization between the two vegetation types. All DNRA/Den value under the four vegetation types were greater than 1, indicating that DNRA is the main pathway for nitrate reduction in the wetland. Compared with the decrease in $C/NO_3^--N$, the increase in $C/NO_3^--N$ resulted in the nitrate reduction in sediments being biased toward the DNRA process. Therefore, the fluctuations in $C/NO_3^--N$ in the Chongming Dongtan wetland (± 30%) may not cause a significant increase in N₂O emissions from sediments.