三峡水库消落带优势草本植物残体淹水后温室气体排放特征

三峡水库消落带淹水后植物腐烂分解，向水体释放碳、氮养分，可能导致二氧化碳（CO₂）、甲烷（CH ₄）和氧化亚氮（N₂O）等温室气体（GHGs）排放，消落带可能成为大气GHGs的排放源，但目前植物淹水腐解导致的GHGs排放还缺乏定量研究。选择三峡水库消落带优势草本植物狗牙根（Cynodon dactylon (Linn.) Pers.）、水蓼（Polygonum hydropiper）、鬼针草（Bidens bipinnata Linn.）和苍耳（Xanthium sibiricum Patrin ex Widder.）的茎叶，开展为期120 d的室内浸泡模拟淹水实验，测定植物残体干重、上覆水的水化学指标和养分浓度并监测其动态变化，同时测定水-气界面GHGs排放速率，旨在查明消落带植物残体淹水后的GHGs排放过程和通量及其植物种间差异。结果表明:植物残体淹水初期（前15 d）干重下降较快，随后下降变缓，其中苍耳干重损失最大，狗牙根干重损失最小；植物残体淹水初期（前15 d）水-气界面的GHGs快速排放，淹水第4~7 d达到峰值，中后期（第30 d起）平稳排放，鬼针草和苍耳淹水后水-气界面GHGs排放速率显著高于狗牙根和水蓼（P<0.05），鬼针草和苍耳淹水后CO ₂和CH₄累积排放量显著高于狗牙根和水蓼，苍耳、鬼针草和水蓼淹水后N₂O累积排放量显著高于狗牙根，总体而言，狗牙根淹水后GHGs排放量最低；消落带优势草本植物残体淹水后的CO₂和CH ₄排放速率随上覆水的DOC浓度升高而增加， N₂O排放速率与上覆水的NO^-₃-N浓度具有正相关关系，受NO^-₃-N浓度驱动。同时，植物形态和其碳氮含量影响植物淹水分解速率、进而影响养分释放和GHGs排放。

Characteristics of Greenhouse Gases Emission under Inundation from Dominant Herbaceous Plants in the Water-level Fluctuation Zone of the Three Gorges Reservoir

Soaking plants decomposed in the Water-level Fluctuation Zone（WLFZ）of the Three Gorges Reservoir (TGR), while nutrients released into the overlying water. Then, greenhouse gases (GHGs) including carbon dioxide (CO₂), methane (CH ₄) and nitrous oxide (N₂O) were formed and emitted. The WLFZ may be an important GHGs source to the atmosphere. In this study, typical herbaceous plants were collected from WLFZ, while plant soaking experiments were conducted with detritus of leaves and stems of Cynodon dactylon (Linn.) Pers.，Polygonum hydropiper，Bidens bipinnata Linn.，Xanthium sibiricum Patrin ex Widder. under indoor incubation conditions. Indexes were determined including the dry weight losses in detritus, physicochemical properties, nutrient concentrations of overlying water and GHGs emission rate of water-gas interface GHGs, in order to investigate the GHGs emission processes, fluxes and interspecific differences of soaking plant detritus. Results showed that: (1) after the plants were inundated, the dry weight was declined rapidly in the first 7 days. The dry weight loss of Xanthium sibiricum Patrin ex Widder. was the highest, while Cynodon dactylon (Linn.) Pers. was the lowest. The pH and DO of the overlying water were reduced at the early stage and increased rapidly in the middle and late stages. The Eh and EC value increased in early days, and decreased later. (2) At the initial stage of plants detritus inundation, GHGs emission from the plant-water-gas interface were rapid, reaching the peak on the 4th to 7th day of inundation, then get stable in the middle and late stages. The release rate of GHGs from Bidens bipinnata Linn. and Xanthium sibiricum Patr ex Widder. were higher than that of Cynodon dactylon (Linn.) Pers. and Polygonum hydropiper. There was a significant positive correlation between CO₂, CH₄ emission rates and DOC concentrations in the overlying water (P<0.01), which was mainly due to the large amount of DOC released by plant soaking. The positive correlation between the N₂O emission rate and the NO^-₃-N concentration of the overlying water was significant (P<0.01), which was mainly due to NO^-₃-N released from inundated plant detritus. The initial carbon and nitrogen contents and plant types may affect plant soaking decomposition rate, leading to different pattens of nutrient release and GHGs emissions. (3) Cumulative emissions of CO₂ and CH₄ from the inundatedBidens bipinnata Linn.’s and Xanthium sibiricum Patrin ex Widder.’s were significantly higher than those from Cynodon dactylon (Linn.) Pers. and Polygonum hydropiper. The cumulative emissions of N₂O from Xanthium sibiricum Patrin ex Widder., Bidens bipinnata Linn. and Polygonum hydropiper were significantly higher than that of Cynodon dactylon (Linn.) Pers. As a whole, the amount of GHGs emission of Cynodon dactylon (Linn.) Pers. was the lowest under inundation. So, Cynodon dactylon (Linn.) Pers. may be recommended as ecological restoration species in the WLFZ of TGR.