漂浮水生植物对富营养化水体中N2O产生及输移过程的调节作用

漂浮水生维管束植物具有发达的通气组织,然而关于其传输水体中产生温室气体N_2O方面的研究还很匮乏.本研究以漂浮水生植物凤眼莲为代表,利用稳定氮同位素示踪技术,设计能够分隔根室和叶室的水生植物生长系统,通过微宇宙实验定量追踪N-15标记的氮素在凤眼莲根系介导下的转化途径、N_2O产生规律及N_2O通过通气组织向空气的传输过程.研究结果表明,加入水体的~(15)NO_3~-有少部分通过异化还原成为铵(DNRA)过程转化为NH_4~+-~(15)N,主要通过反硝化反应生成N_2O;加入的~(15)NH_4~+主要发生了耦合硝化-反硝化反应.种植凤眼莲均使叶室中N_2O-~(15)N原子百分超和~(15)N_2O浓度明显高于无植物的对照,一方面说明凤眼莲根系能够促进反硝化、硝化-反硝化反应过程,同时也说明水体中的~(15)N_2O有相当一部分通过植株体传输到空气中.凤眼莲通气组织主要通过分子扩散从高浓度空间向低浓度空间输送~(15)N_2O.在标记NO_3~--~(15)N的水体中,凤眼莲在前期促进了~(15)N_2O向顶空排放,但并未在整个生长期持续促进N_2O释放.在标记NH_4~+-~(15)N的水体中,植株体富集是NH_4~+-~(15)N的一个主要归趋途径,但同时也有部分NH_4~+-~(15)N转化为N_2O通过植株通气组织持续、缓慢地释放到顶空当中.研究结果阐明了漂浮植物对水体氮转化过程及N_2O输移途径的调节作用,可为全面理解水体生态系统氮循环过程提供理论基础.

Mediation of production and transportation of N2O in eutrophic water with the free-floating aquatic plant, Eichhornia crassipes

The aquatic vascular plants, including free-floating plants, have well-developed vascular tissues. However, little is known about the transportation of N₂O, produced from the biological nitrogen transformation process in water, via the vascular tissues of the free-floating plants. In this study, water hyacinth was used as a representative for free-floating plant. In order to distinguish between two N₂O diffusion sources (i.e. coming from water and from the vascular tissues), a plant growth system that can separate the root compartment from the aerial compartment was designed. The stable isotope N-15 tracing method was used to investigate the fate of N in water with or without growth of water hyacinth using microcosm experiments, including the transformation of N-15 in water, the production of ¹⁵N₂O and the transfer of ¹⁵N₂O via vascular tissues of water hyacinth. The results show that dissimilatory nitrate reduction to ammonium (DNRA) process occurred in water with addition of ¹⁵NO₃^- although a large proportion of ¹⁵NO₃^- was transformed via denitrification. The coupled nitrification and denitrification was the major process for biological transformation of ¹⁵NH₄⁺ in water. N-15 excess percentage of N₂O and ¹⁵N₂O concentrations in the headspace of aerial apartment from treatments with water hyacinth were significantly higher than those without water hyacinth, indicating that the roots could promote the denitrification of NO₃^--¹⁵N and nitrification/nitrification-denitrification of NH₄⁺-¹⁵N to produce ¹⁵N₂O. This could also illustrate that a large proportion of ¹⁵N₂O produced in water would diffuse into air via the aerating tissues of water hyacinth. In water with NO₃^--¹⁵N, the growth of water hyacinth could only promote the ¹⁵N₂O emission during the early stage of incubation, while relatively slow and continuous emission of ¹⁵N₂O via the aerating tissues were observed in water with NH₄⁺-¹⁵N during the whole experimental period. In addition, plant assimilation was one of the major fates of NH₄⁺-¹⁵N in water. The results could help to elucidate the role of free-floating aquatic plants in mediating the production and transportation of gaseous nitrogen in eutrophic water, providing the theoretical basis for comprehensive understanding of nitrogen cycling process in aquatic ecosystem.