Temporal changes in fractions and loading of sediment nitrogen during the holistic growth period of <i>Phragmites australis</i> in littoral Lake Chaohu, China<sup>*</sup>

Abstract

水生植被对于维持水生态系统结构和功能稳定性具有举足轻重的作用，而重建水生植物被认为是污染湖泊生态修复的重要手段.氮素是水生态系统重要的限制性元素之一，根着挺水植物生长发育无疑将深刻地影响着沉积物氮的迁移转化过程，但水生植物不同生长阶段对沉积物氮的需求和植物代谢强度均不同，目前对挺水植物完整生长过程中沉积物氮组分及含量变化认识仍十分不足.本研究通过为期120 d的沉积物柱芯培养和水槽模拟试验，探究巢湖芦苇恢复完整生长过程中沉积物总氮（TN）、无机氮（TIN）与可转化态氮（TF-N）的变化及其关键调控因子.结果表明，芦苇完整生长过程将持续激发沉积物氮活性，沉积物TIN与TF-N含量逐渐增加，而沉积物TN和非可转化态氮（NTF-N）含量显著降低.模拟试验期间，指数型增长的芦苇生物量提高了沉积物铵态氮（NH₄⁺-N）和硝态氮（NO₃^--N）含量，但亚硝态氮（NO₂^--N）含量却逐渐降低；与第0天相比，第120天沉积物离子交换态氮（IEF-N）、碳酸盐结合态氮（CF-N）、铁锰氧化态氮（IMOF-N）和有机态及硫化物结合态氮（OSF-N）含量分别增加了1.10、3.40、3.60和1.40倍，这主要受芦苇吸收利用、根系代谢强化根际沉积物氧化还原电势和改变pH微环境共同驱动.在第120天，沉积物NH⁺₄-N和NO₃^--N含量急剧升高，分别是第90天的9.43和2.22倍，表明芦苇衰亡凋落过程将向沉积物释放大量的TIN，故需要综合采取湖泊物理-生态工程手段来有效管控芦苇枯落物，从而提升水生植被修复效果并构建长效稳态机制.;Aquatic vegetation's important structuring function in shallow freshwater ecosystems has been increasingly recognized as an important ecological restoration measure to rehabilitate heavily polluted water. Nitrogen (N) is one of the most important limiting elements in aquatic ecosystems, and the growth of rooted aquatic macrophytes profoundly affected sediment N biogeochemistry. Due to variable requirements of N for the different growth phases of Phragmites australis, as well as metabolism extensity of macrophytes is changing, it is thereby unclear how the holistic growth period of aquatic macrophytes affects sediment N cycling in eutrophic lakes. In this study, combined intact sediment core microcosm batch experiment and flume modelling, a 120-d simulation study was conducted to investigated changes of total nitrogen (TN), inorganic nitrogen and total exchangeable form of nitrogen (TF-N) in sediments surrounding rhizosphere of P. australis during the whole growth of P. australis. The results showed a priming effect of sediment N by P. australis was observed, with the contents of total inorganic nitrogen (TIN) and TF-N in sediments gradually increasing, while TN and non-exchangeable form of nitrogen (NTF-N) declining. During the experiment, the exponential-increasing biomass of P. australis remarkably promoted the contents of ammonium nitrogen (NH⁺₄-N) and nitrate nitrogen (NO^-₃-N), but not for nitrite nitrogen (NO^-₂-N). Compared with the initial values of TF-N, on day 120, the contents of the exchangeable form (IEF-N), carbonate form (CF-N), iron-manganese oxides form (IMOF-N) and organic matter-sulfide form (OSF-N) in surface sediments increased by 1.10, 3.40, 3.60 and 1.40 times. This phenomenon could be attributed to the root metabolisms-driven redox condition and pH changes in rhizospheric microsites. On day 120, the contents of NH⁺₄-N and NO^-₃-N in sediments dramatically increased, of which were 9.43 and 2.22 times in comparison with those on day 90, suggesting that massive TIN was released into the sediments caused by senescent processes of litter from P. australis. In brief, to improve the restoration efficiency and long-term stabilization of pollution control, it is of more significance to adopt comprehensive lake physico-ecological engineering measures to manage the litters derived from P. australis.