典型滨海湿地干湿交替过程氮素动态的模拟研究

滨海湿地是海陆相互作用的交错过渡地带,具有敏感而复杂的环境过程与功能价值.以典型滨海湿地崇明东滩为原型,采集湿地沉积物及海水样品,通过土柱模拟方法,研究了半月潮(15 d左右为周期的"大潮")与日潮(一个太阴日内出现的涨潮和落潮)水分生态过程沉积物NO3--N、NO2--N、NH4+-N与溶解性有机氮(DON)含量以及硝酸还原酶(Nar),亚硝酸还原酶(Nir),羟胺还原酶(Hyr)活性等的变化,旨为揭示滨海湿地潮汐驱动下沉积物周期性干湿交替过程氮素动态变化规律及还原机制.半月潮过程中,沉积物变干期间(含水量从35%降至5%～7%),硝化作用占主导地位.随着干燥程度的加剧(含水量从5%～7%降至0%～3%),沉积物中数量可观的NO3--N和NO2--N转化为DON.然而,随着干湿交替频次的增加,NO3--N和NO2--N向DON的转化率显著降低.干燥沉积物淹水变湿后(含水量从0%～3%升至37%～45%),NO3--N、NO2--N、NH4+-N与DON含量平均增加1～3倍.淹水后Nar、Nir活性的迅速升高表明,NO3--N和NO2--N的还原明显改善.3个培养周期沉积物Nar与Nir活性、Hyr活性与NH4+-N含量之间呈现极显著的正相关性,NO3--N、NO2--N含量的减少与NH4+-N含量的增加也显著相关.结合湿地沉积物干湿交替过程"低氮高碳"特征,可以认为,氨化途径主导了半月潮过程NO3--N的还原.相比较而言,日潮过程中,NO3--N、NO2--N、NH4+-N以及DON的含量均较为稳定,分别为(3.0±0.3)、(1.2±0.1)、(133.3±4.3)和(41.1±10.6)mg·kg-1.因此,日潮过程对沉积物氮素动态变化的影响较小.

Effects of Drying-rewetting Alternation on Nitrogen Dynamics in a Typical Coastal Wetland: A Simulation Study

The coastal wetland, a key transitional zone between land and ocean, is a complex and sensitive ecosystem with special environmental processes and functional values. Sediment and sea water samples were taken from Chongming east intertidal flat, a typical coastal wetland in China. Sediment cores were incubated under simulated spring tides (approximately twice a month) and semi-diurnal tides (two almost equal high tides and two low tides in a lunar day), to define the responses of nitrogen to periodicalternation of drying and re-flooding. The contents of NO₃^--N, NO₂^--N, NH₄⁺-N, dissolved organic N (DON), and the activities of nitrate reductase (Nar), nitrite reductase (Nir), and hydroxylamine reductase (Hyr) in sediments were determined during the incubation. Nitrification stood in a dominant position during the drying period with sediment moisture decreasing from 35% to 5%-7% in simulated spring tides. The NO₃^--N and NO₂^--N were found to be substantially converted into DON following a further exsiccation (the sediment moisture decreased from 5%-7% to 0%-3%). However, the conversion rates decreased with the increase of drying and rewetting cycles. Following the rewetting of dried sediments (from 0%-3% to 37%-45%), the contents of NO₃^--N, NO₂^--N, NH₄⁺-N and DON increased to 1 to 3 times that of the dry sediments. Nar and Nir activities in sediments rapidly increased after the rewetting, indicating that the reduction of NO₃^--N and NO₂^--N were substantially enhanced. Significant positive correlations were found between Nar and Nir activities, and Hyr activities and NH₄⁺-N contents during the simulated spring tides. Moreover, the decreases of NO₃^--N and NO₂^--N contents were observed to be significantly correlated to the increases of NH₄⁺-N contents. Combined with the fact that the sediment had a high organic carbon content but a very low NO₃^--N content, it can be concluded that the reduction of NO₃^--N and NO₂^--N following the rewetting was dominated by nitrate ammonification. In contrast, NO₃^--N, NO₂^--N, NH₄⁺-N and DON contents in sediments were relatively stable during semi-diurnal tides, which were (3.0±0.3), (1.2±0.1), (133.3±4.3) and (41.1±10.6) mg·kg^-1, respectively. This indicated that semi-diurnal tides slightly affected the variations of nitrogen contents in the wetlands.