THE IMPACT OF SUBMERGED AQUATIC MACROPHYTES ON DISSOLVED ORGANIC MATTER IN LAKES: A CASE STUDY OF DONGHU LAKE IN WUHAN
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摘要:
为探究沉水植物水生态修复对湖泊水体碳循环的影响, 以武汉东湖沉水植物水生态修复区和未修复区作为研究对象, 采用傅里叶变换离子回旋共振质谱(Fourier Transform Ion Cyclotron Resonance Mass Spectrometry, FT-ICR-MS)技术对比沉水植物水生态修复区与未修复区水体溶解性有机质(Dissolved Organic Matter, DOM)的化学多样性, 揭示沉水植物对水体DOM稳定性的影响。结果表明, 沉水植物水生态修复显著改善水体理化性质的同时改变了DOM分子的化学多样性。修复区与未修复区分别含有7814和7282种DOM分子, 其中修复区特有的1768种, 主要以CHO、CHON和CHOP类型为主; 未修复区特有的1236种, 主要以CHO和CHON类型为主。进一步分析发现, 修复区含P元素相对难降解的DOM分子类型(CHOP、CHONP、CHOPS和CHONPS)是未修复区的1.4—2.29倍且强还原态的DOM含量更高, 表明修复区DOM在热力学上比未修复区更稳定。综上所述, 沉水植物不仅改变了水体DOM分子组成, 同时提高了水体难降解DOM分子丰度。研究将有利于加深湖泊稳态碳转换过程对水体碳循环影响的认识, 为未来准确评估湖泊的碳汇潜力提供科学依据。
Abstract:The growth and senescence processes of submerged aquatic macrophytes can affect the dynamic balance of dissolved organic matter (DOM) in lakes, thereby reshaping the carbon cycling patterns of lakes. To explore the impact of submerged aquatic macrophytes-based ecological restoration on lake water carbon cycling, this study focused on the submerged aquatic macrophytes restoration area and the unrestored area in Donghu Lake, Wuhan. Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS) was employed to compare the chemical diversity of DOM in the restored and unrestored areas, revealing the effects of submerged aquatic macrophytes on DOM stability in water. The results showed that submerged aquatic macrophyte plant restoration significantly improved the physicochemical properties of the water while altering the chemical diversity of DOM molecules. The restored and unrestored areas contained 7814 and 7282 types of DOM molecules, respectively. Among them, 1768 types were unique to the restored area, primarily consisting of CHO, CHON, and CHOP types, while 1236 types were unique to the unrestored area, mainly consisting of CHO and CHON types. Further analysis revealed that the restored area had 1.4 to 2.29 times more relatively recalcitrant DOM molecules containing phosphorus (CHOP, CHONP, CHOPS, and CHONPS) than the unrestored area, Additionally, the content of highly reduced DOM was higher, indicating that DOM in the restored area was more thermodynamically stable than that in the unrestored area. In summary, submerged aquatic macrophytes not only changed the composition of DOM molecules in the water but also increased the abundance of recalcitrant DOM molecules. This study enhances our understanding of the impact of lake carbon transformation processes on the carbon cycle and provides a scientific basis for accurately assessing the carbon sink potential of lakes in the future.
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沉水植物作为初级生产者在湖泊生态系统中发挥着关键作用, 一方面其通过吸收水体的氮磷营养盐能有效减缓水体富营养化进程[1]。边园琦等[2]对莫愁湖的研究发现, 沉水植物对湖泊水体氮、磷的去除率分别达到79.9%和48%。对沉积物中氮、磷去除量可达到1136和387 mg/(kg·a)[3]。另一方面其通过光合作用能将水体溶解氧浓度提高2～4 mg/L, 为水生生物提供适宜的生存环境[4]。此外, 沉水植物通过改善湖泊生态环境和增强微生物碳泵机制能减少湖泊温室气体排放[5]。郑冰清等[6]的研究表明, 沉水植物能使湖泊水体的水气界面CO2通量下降165%。因此, 当前研究主要集中在沉水植物对水体净化和温室气体排放等方面, 但对水体DOM影响的研究相对较少。
沉水植物对湖泊DOM的组成和动态变化密切相关。Gracea等[7]的研究表明, 沉水植物的根系能减少因水流冲刷而造成的沉积物再悬浮, 进而降低湖泊底泥中DOM的释放。此外, 沉水植物能够通过改变湖泊的微生态环境, 提高微生物碳、氮和硫转化相关功能基因的相对丰度[8]。沉水植物产生的化感物质能够抑制藻类的生长, 从而改变湖泊内源有机质的来源。Wang等[9]对华阳河湖泊研究表明, 沉水植物恢复后能使蓝藻的比例下降21%。同时, 沉水植物的生长和代谢过程也会向水体不断释放大量DOM, 致使湖泊水体中的内源DOM及碳、氮和磷等元素的储量发生变化[10]。糖类、氨基酸、有机酸等小分子物质及腐殖质等大分子物质是沉水植物释放的主要DOM, 它们的组成受植物的种类、生长阶段、环境条件等多种因素影响[11]。An等[12]的研究表明, 光降解后产生的DOM微生物利用效率低, 使水体中难降解的溶解性有机碳(Dissolved Organic Carbon, DOC)含量增加20.04%。此外, 沉水植物为微生物提供附着基质, 能促进微生物对DOM的降解利用[13]。
DOM是由C、H、O、N、P、S等多种元素组成的复杂混合物, 它们在水体中主要以碳水化合物、氨基酸和腐殖质等有机分子的形式存在[14]。作为湖泊生态系统中微生物的重要营养源, 通过微生物的分解和转化释放能量和营养物质能促进湖泊的初级生产力。DOM的组成和浓度不仅能降低湖泊温室气体的排放, 还能直接影响微生物的代谢活动和群落结构, 因此其在调节湖泊的生态环境和生态健康起着至关重要的作用 [15−17]。尽管应用光谱学技术能对湖泊DOM进行表征[18−20], 但由于分辨率的限制导致其难以有效揭示DOM的分子组成, 因此在分子水平上对其化学组成进行解析的研究相对较少。FT-ICR-MS技术作为最前沿的质谱技术之一, 能从分子水平解析DOM类型且具有高分辨率和质量精度, 已成为评估DOM来源及其生物地球化学反应性和生物利用度的有力工具, 提供了基于DOM分子组成变化推断元素循环过程的线索, 已成功应用于河流、湖泊和海洋等环境[21−23]。本研究利用FT-ICR-MS技术对武汉东湖水生态修复区及未修复区DOM分子组成及其化学多样性进行解析, 探究种植沉水植物对湖泊DOM分子组成影响; 通过对比修复区与未修复区DOM分子稳定性差异, 揭示沉水植物生态恢复对湖泊碳循环的潜在影响。
1. 材料与方法
1.1 研究区域概况
东湖(30°31'—30°36'N, 114°21'—114°28'E)位于武汉市东南部, 由于冰后期全球变暖, 长江水位上升, 两岸天然河堤发育、壮大, 导致侵蚀洼地的出口被淤塞而形成的沟谷壅塞湖。其湖形极其不规则, 南北最大纵距17 km, 东西最大横距18 km, 湖泊岸线长119.9 km, 现水域面积33.63 km2[24]。东湖地处北亚热带季风性湿润气候内, 年均温度为16.7℃, 多年平均降水量为1180 mm。20世纪60年代以前, 由于人为影响较少水质相对较好, 但随着城市化发展, 每年有近20余万吨城市污水排入东湖, 其中每年总氮和总磷的排入量分别为536.3 t和87.8 t[25, 26]。导致东湖面临着严重的水生态环境问题。为改善东湖水生态环境实施了基于沉水植物恢复的水生态修复工程, 通过在浅水区(水深2 m)建设围格, 同时采用扦插、沉床种植和抛种等方法种植沉水植物进行水生态修复[27]。2021−2023年东湖水生态修复区的沉水植物覆盖面积已经从0.1 km2扩大到3.2 km2(图 1a)[27]。
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图 1 研究区域位置及采样点分布Figure 1. Location of research area and distribution of sampling points本研究以2021年在东湖修建的一片水生态修复区及相对应的未修复区作为研究对象。二者之间采用不透水围格将水体分隔开(图 1b)。修复区最高水深不超过2 m, 平均水深1.66 m, 建设面积14600 m2, 主要种植苦草(Vallisneria natans)、金鱼藻(Ceratophyllum demersum)、黑藻(Hydrilla verticillate)、穗状狐尾藻(Myriophyllum spicatum)等沉水植物。为使采集的样品具有代表性, 综合考虑修复区的形状、离岸距离和水深等综合因素在修复区设置了X1 (114°21'27"E, 30°32'45"N)、X2 (114°21'25"E, 30°32'46"N)和X3 (114°21'23"E, 30°32'47"N)三个采样点, 以及相对应的未修复区W1 (114°21'30"E, 30°32'45"N)、W2 (114°21'25"E, 30°32'49"N)和W3 (114°21'20 "E, 30°32'46"N)三个采样点(图 1b)。
1.2 野外采样
野外采样时间为2023年10月11日早上9−11点, 采样时天气晴朗。现场利用美国YSI ProQuatro多参数水质分析仪测定修复区与未修复区的水温、溶解氧(Dissolved Oxygen, DO)、电导率(Electrical Conductivity, EC)、盐度(Salinity, SAL)、pH和氧化还原电位(Oxidation Reduction Potential, ORP), 其精度分别为0.1℃、0.01 mg/L、0.1 μS/cm、0.01 ppt、0.01 pH和0.1 mV。水深使用Speed tech便携式测深仪SM-5A进行测定, 其精度为0.1 m。浊度(Turbidity, 为行文方便后文均用NTU表示)采用HACH 2100P手持浊度计测定, 精度为0.01 NTU。透明度(Transparency, 为行文方便后文均用SD表示)采用直径20 cm透明度盘进行测定, 精度为1 cm。使用2 L聚乙烯材质的采样瓶用采样点位水样润洗后采集水面以下30 cm处的水样用于检测总氮(Total Nitrogen, TN)、总磷(Total Phosphorus, TP)、亚硝氮(${\rm{NO}}^-_2 $-N)、氨氮(${\rm{NH}}^+_4 $-N)、硝态氮(${\rm{NO}}^-_3 $-N)、碱度(Alkalinity, ALK)、溶解性无机碳(Dissolved Inorganic Carbon, DIC)和DOC。分别采集修复区X1、X2和X3处水面30 cm以下水样各150 mL, 混合后装入经475℃高温预处理的500 mL棕色玻璃瓶, 混合水样用于FT-ICR-MS检测[28]。未修复区的采样方法同上。采集的水样带回实验室放入4℃冰箱, 水质指标在1周内测完。
1.3 室内分析
水化学测定 水质TN采用碱性过硫酸钾消解紫外分光光度法检测(HJ636-2012); TP采用钼酸铵分光光度法检测(GB11893-89); ${\rm{NO}}^-_2 $-N采用分光光度法测定(GB/T7493-87); ${\rm{NH}}^+_4 $-N采用纳氏剂分光光度法测定(HJ535-2009); ${\rm{NO}}^-_3 $-N采用紫外分光光度法测定(HJ/T346-2007); ALK采用盐酸滴定法测定; 将水样过0.45 μm玻璃纤维滤膜, 再使用总有机碳/总氮分析仪(Elementar Inc, Germany)测定DOC。
固相萃取和FT-ICR-MS测定 采用固相萃取法提取水样中DOM, 将采集的水样过孔径0.45 μm玻璃纤维滤膜过滤, 再将过滤后的水样用优级纯盐酸酸化至pH=2。接着使用120 mL甲醇、24 mL pH=2酸化水和24 mL纯水活化固相萃取柱(Oasis HLB), 再用固相萃取仪将酸化后的水样通过活化后的固相萃取柱吸附DOM, 使用50 mL pH=2酸化水和50 mL纯水进行脱盐, 接着使用氮气将固相萃取柱吹干, 最后使用15 mL甲醇洗脱固相萃取柱上的DOM。
将提取的DOM洗脱液用50%的甲醇稀释50倍后使用配备7 T超导磁体(Solari X 2XR, Bruker, Germany)、四极(20)探测器和负离子模式的电喷雾电离[ESI(-)]的FT-ICR-MS仪进行检测。在检测前所有光谱均用甲酸钠的峰簇进行外部校准。选择信噪比(S/N)不小于4的峰进行进一步的数据处理。仪器设置的参数: 样品以250 μL/h的流速注入仪器; FT-ICR-MS谱图质量范围在107.5−1100 Da; 离子累积时间为0.006s, 飞行时间为0.5s; 平均扫描次数为450; 分子式的质量误差小于2 ppm。
1.4 数据分析及作图
FT-ICR-MS公式分配 FT-ICR-MS的化学方程式利用FTMSCalibirate算法进行内部校准, 使用FTMSDeu代码通过以下条件进行分配[29]: 非卤代和卤代配方的信噪比分别大于6和10; 等价双键(Double Bond Equivalents, DBE)为≥0的整数; 对于C≥5的分子, 0.3≤H/C≤2.25和0≤O/C≤1.2; 对于C≤4的分子, H/C≤4和0≤O/C≤1.2; 1≤12C≤50和13C≤2; 18O≤1; -10≤DBE-O≤10; 14N≤5; 15N≤1; 32S≤3; 33S≤1; 34S≤1; P≤1; 35Cl≤5; 37S≤5; 79Br≤5和81Br≤5。
范氏图可视化 DOM分子的氧碳比(O/C)和氢碳比(H/C)分别反应其氧化程度和氢化程度, 使用范氏图可将DOM分为8类: 饱和化合物(0≤O/C≤0.52, 1.5≤H/C≤2.2), 氨基糖(0.52＜O/C≤0.71, 1.5≤H/C≤2.2), 碳水化合物(0.71＜O/C≤1.2, 1.5≤H/C≤2.4), 不饱和碳氢化合物(0≤O/C≤0.1, 0.7≤H/C＜1.5), 木质素(0.1＜O/C≤0.67, 0.7≤H/C＜1.5), 单宁酸(0.67＜O/C≤1.2, 0.5≤H/C＜1.5)和缩合芳香烃化合物(0≤O/C≤0.67, 0.2≤H/C＜0.7)和其他类型有机质[30, 31]。
分子特征参数 碳的标准氧化态(Nominal Oxidation State of Carbon, NOSC)表示DOM分子潜在能量, 具有较高NOSC的有机分子在热力学上更容易被微生物被降解利用(式1) [32]。每个DOM分子的相对强度(Relative Intensity, RI)计算公式为每个分子的峰值强度除以全部分子的峰值强度之和(式2)。通过每个DOM分子的相对峰值强度进行加权计算出样品的碳的标准氧化态加权平均值(NOSCwa; 式3)。而分子不稳定性边界(Molecular Lability Boundary, MLB)是基于有机质的分子式计算出的氢饱和度(H/C)值。当H/C≥1.5时, MLB将有机质的组分进行分类, 其对应的是生物利用度更高的化学类区域中更不稳定的分子类型。而H/C＜1.5时, 在所有的O/C比值中, 代表相对稳定, 更顽固的化学性质[33]。
$$ \text{NOSC=4}-\frac{{4{\mathrm{C}} + {\mathrm{H}}-3{\mathrm{N}}-2{\mathrm{O}} + 5{\mathrm{P}}-2{\mathrm{S}}}}{\text{C}} $$ (1) $$ {\mathrm{R}\mathrm{I}}_{i}\text={\mathrm{I}}_{i}{/}\sum {\mathrm{I}}_{i} $$ (2) $$ {\text{NOSC}}_{\mathrm{w}\mathrm{a}}\text=\sum ({\text{NOSC}}_{i}\text{×}{\mathrm{R}\mathrm{I}}_{i}{)} $$ (3) 式中, C、H、O、N、P和S分别代表分子式中的碳、氢、氧、氮、磷和硫原子的数目, I为每种DOM分子的峰值强度, i表示DOM分子类型。
作图 采样点分布图利用CorelDRAW X7进行绘制。用Excel 2019进行数据整理和分析, Origin 2021进行绘图。FT-ICR-MS仪测得的质谱数据利用Data Analysis软件进行处理, 再用Matlab2021a对数据进行解析。
2. 结果
2.1 水化学特征
采样期间气温的变化在21.8—29.7℃, 修复区采样点的水深在1.5—2.1 m, 平均水深为1.8 m, 未修复区采样点水深在1.5—2.3 m, 平均水深为1.9 m。除${\rm{NO}}^-_2$-N、DO和DOC等指标无显著差异外, 修复区与未修复区其余水体指标均具有显著差异。其中修复区的SD和pH提升了1.25 m和0.4, 而NTU、ORP、ALK和 EC分别降低了8.49、19.9 mV、0.44 mmol/L、42.5 μS/cm。在总溶解碳方面, ${\rm{CO}}^{2-}_3 $含量升高了61.5%, 而${\rm{HCO}}^-_3 $、CO2和DIC的含量分别下降了25%、65%和25%。在营养盐方面, 水体中TP和TN分别下降了47.9%和26.2%, 而水体中溶解性无机氮则是${\rm{NO}}^-_3 $-N和${\rm{NH}}^+_4 $-N含量分别降低了0.269 mg/L和0.01 mg/L, 虽然两区域${\rm{NO}}^-_2 $-N差异不显著, 但修复区平均升高了0.012 mg/L。由此可见, 种植沉水植物进行湖泊修复能有效改善水体的理化指标, 降低水体无机碳和氮、磷营养盐含量。
2.2 DOM分子组成特征
修复区与未修复区分别检测出7814和7282种类型的DOM分子(图 3a), 其中相似的有6046种。修复区含有1768种特有的DOM分子, 占修复区DOM分子总量的22.9%; 而未修复区含有1236种特有的DOM分子, 占未修复DOM分子总量的17%。修复区与未修复区特有的DOM分子主要以类木质素为主, 分别占特有的DOM分子的61.7%和58%, 其次是类饱和化合物DOM, 分别占26%和26.6%(图 3b)。由图 2c可知水生态修复区特有的DOM分子组成以CHON、CHO和CHOP为主, 分别占26.4%、21.8%和20.7%; 而未修复区特有的DOM分子组成以CHON和CHO为主, 分别占44.5%和25.1%。结果表明, 种植沉水植物能引起湖泊DOM分子组成变化。
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图 2 修复区与未修复区水体理化性质对比(X为修复区, W为未修复区)Figure 2. Comparison of water physicochemical properties in the restored and unrestored areas(X represents the restored area, W represents the unrestored area)![]()
图 3 修复区与未修复区水体DOM对比(a. 修复区与未修复区Van Krevelen图; b. 修复区与未修复区特有的DOM分子不同类型有机化合物对比图; c. 修复区特有的DOM分子百分比; d. 未修复区特有的DOM分子的百分比)Figure 3. Comparison of DOM difference between restored and unrestored areas(a. Restored and unrestored areas Van Krevelen comparison chart; b. Comparison of different types of organic compounds of unique DOM molecules in the restored and unrestored areas; c. Percentage of unique DOM molecules in the restored area; d. Percentage of unique DOM molecules in the unrestored areas)2.3 DOM分子稳定性
水生态修复区DOM分子NOSC值在–2—0区间概率密度更高, 表明其富含更多强还原态有机物, 未修复区NOSC值在0−1区间概率密度更高, 表明未修复区的DOM分子更倾向于氧化态有机物(图 4)。强还原态的有机物相对于氧化态较高的有机物更难于被微生物降解利用。同时, 通过DOM分子的峰值强度进行加权计算出修复区与未修复区NOSCwa分别为–0.47和–0.44, 进一步表明水生态修复区的DOM分子在热力学上比未修复区更为稳定。此外, 基于DOM分子不稳定性边界对DOM分子分类发现(表 1), 修复区相较于未修复区含有更丰富相对难降解的DOM分子类型, 尤其是含磷(P)元素的DOM分子类型(CHOP、CHONP、CHOPS和CHONPS), 修复区的种类是未修复区的1.4—2.29倍。总的来说, 种植沉水植物能有效提高湖泊水体中相对难降解DOM分子的丰度, 进而增强湖泊水体DOM的稳定性。
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图 4 修复区与未修复区特有DOM分子NOSC值的密度图Figure 4. Density map of NOSC values of unique DOM molecules in the restored and unrestored areas表 1 修复区与未修复区DOM分子的稳定性Table 1. Stability of DOM molecules in restored and unrestored areas分子类型Molecular type 水生态修复区Restored area 未修复区Unrestored area 水生态修复区/未修复区 NOSCwa MLBL种类 MLBR种类 NOSCwa MLBL种类 MLBR种类 MLBR比率 CHO –0.5 901 2638 –0.51 981 2482 1.06 CHON –0.14 248 2129 –0.14 316 2145 0.99 CHOP –1.01 491 220 –0.76 259 96 2.29 CHOS –0.41 195 366 –0.45 170 316 1.15 CHONP –0.63 102 102 –0.58 97 70 1.45 CHONS –0.35 84 167 –0.24 77 165 1.01 CHOPS –1.13 61 31 –1.06 23 20 1.55 CHONPS –0.57 51 28 –0.62 45 20 1.4 总类型 –0.47 2133 5681 –0.44 1968 5314 1.07 注：MLBL是易降解的DOM，MLBR是相对难降解的DOM[29]Note: MLBL refers to DOM that is labile, while MLBR refers to DOM that is recalcitrant[29] 3. 讨论
修复区水体中的CO2浓度(0.014 mmol/L)显著低于未修复区(0.04 mmol/L), 不仅减少了湖泊向大气释放温室气体的可能性, 还可能使湖泊成为大气CO2的汇。相对于未修复区较高的CO2浓度可能导致其向大气中释放CO2, 进一步加剧温室效应。魏岩洁等[34]在盛家湾的研究发现, 与无沉水植物覆盖的区域相比有沉水植物区域能减少89%温室气体排放。因此, 沉水植物的种植对减缓湖泊温室气体排放具有重要作用。在营养盐方面, 修复区的TP和TN含量分别比未修复区降低了47.9%和26.2%。这表明沉水植物通过吸收、反硝化和微生物协同作用等多种机制, 有效地降低了水体的氮磷负荷[35]。此外, 沉水植物释放的酚类、萜类和含芳香族溶解有机酸通过抑制藻类的光合效率、改变微观结构、破坏氮磷代谢等生理生化过程抑制藻类生长[36, 37]。致使湖泊从藻类占优势转变为以沉水植物为主的生态系统, 湖泊内源DOM从藻源转向沉水植物来源[38], 从而改变湖泊水体DOM分子组成(图 5)。修复区特有的DOM分子主要以CHON、CHO和CHOP类型为主, 而未修复区特有的DOM分子则是以CHON和CHO类型, 这可能与草源型与藻源型DOM特性密切相关。沉水植物衍生的DOM富含多酚类物质, 具有较高的芳香性, 而浮游植物主要由多糖物质组成[39, 40]。章奇等[41]通过三维荧光平行因子分子也表明, 藻源DOM以类色氨酸组分为主, 草源DOM的荧光物质则以类酪氨酸组分为主。此外, 虽然本次研究并未涉及到微生物群落组成变化, 但有研究表明沉水植物对湖泊生态环境的改变会影响生物多样性[42−44], 在以高等水生植物为主的湖泊中, β变形菌门的细菌占优势; 而在微囊藻占优势的湖泊中, 放线菌门的浮游细菌占优势[45], 因此不同微生物对有机质的利用和转化也可能会引起有机质性质的差异。
水生态修复区含有更多难降解的DOM分子, 表明沉水植物能提高水体DOM的稳定性。姚昕等[46]研究显示, 藻源型DOM在2 d内能降解约70%, 而沉水植物源DOM的降解速率显著较低, 通常需要更长时间完成降解转化。高岩等[47]的研究也支持这一观点, 他们发现沉水植物凋亡与分解过程中释放的木质素、纤维素和半纤维素等物质具有较高的稳定性。王亚茹等[48]的研究也表明, 在沉水植物降解释放的有机质中, 大分子质量的DOM较多, 且较为稳定不易被微生物降解。从元素组成上看, 修复区含有更丰富含P元素(CHOP、CHONP、CHOPS和CHONPS)的相对难降解DOM分子, 可能与沉水植物释放的含P元素DOM分子的稳定性相关。藻类有机磷分解产生的正磷酸盐释放速率是水生植物4倍, 当有机物来源于藻类时有机磷的生物利用度更高, 而来自水生植物的有机磷的生物利用度相对较差, 往往被埋在沉积物中[49]。Guo等[50]的研究表明湖泊富营养化越严重, CHOP类型的不稳定有机物占比越高, 表明藻源型的有机磷活性较高。未修复区则有更多CHON类型的分子, 这一现象可能与微生物群落的结构和功能密切相关。Vila-Costa等[51]的研究提供了有力证据, 他们发现沉水植物能显著增强与氮循环相关的微生物群落中nirS和nirK功能基因的活性, 加速湖泊中有机氮的分解过程。因此, 修复区中未被检测出的CHON类型DOM分子可能是因为沉水植物促进了微生物对原湖泊水体中有机氮的降解。
4. 结论
(1)在水生态修复区和未修复区分别检测出7814和7282种DOM分子, 其中水生态修复含有1738种特有的DOM分子主要以CHON、CHO和CHOP类型的为主, 而未修复区特有的1236种DOM分子主要是CHON和CHO类型。这表明种植沉水植物能增加湖泊DOM分子的丰富度改变湖泊DOM的分子组成。
(2)水生态修复区与未修复区相比DOM分子更趋向于还原态, 而未修复区趋向于氧化态。水生态修复区与未修复区的NOSCwa分别为–0.47和–0.44, 表明水生态修复区DOM分子在热力学上更稳定, 同时按分子不稳定性边界分类表明修复区含有更丰富相对难降解的DOM分子类型, 尤其是含P元素相对难降解的DOM分子类型是未修复区的1.4—2.29倍。因此种植沉水植物进行湖泊生态修复能提高湖泊水体DOM的稳定性。
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图 1 研究区域位置及采样点分布
Figure 1. Location of research area and distribution of sampling points
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图 2 修复区与未修复区水体理化性质对比
(X为修复区, W为未修复区)
Figure 2. Comparison of water physicochemical properties in the restored and unrestored areas
(X represents the restored area, W represents the unrestored area)
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图 3 修复区与未修复区水体DOM对比
(a. 修复区与未修复区Van Krevelen图; b. 修复区与未修复区特有的DOM分子不同类型有机化合物对比图; c. 修复区特有的DOM分子百分比; d. 未修复区特有的DOM分子的百分比)
Figure 3. Comparison of DOM difference between restored and unrestored areas
(a. Restored and unrestored areas Van Krevelen comparison chart; b. Comparison of different types of organic compounds of unique DOM molecules in the restored and unrestored areas; c. Percentage of unique DOM molecules in the restored area; d. Percentage of unique DOM molecules in the unrestored areas)
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图 4 修复区与未修复区特有DOM分子NOSC值的密度图
Figure 4. Density map of NOSC values of unique DOM molecules in the restored and unrestored areas
表 1 修复区与未修复区DOM分子的稳定性
Table 1 Stability of DOM molecules in restored and unrestored areas
分子类型Molecular type 水生态修复区Restored area 未修复区Unrestored area 水生态修复区/未修复区 NOSCwa MLBL种类 MLBR种类 NOSCwa MLBL种类 MLBR种类 MLBR比率 CHO –0.5 901 2638 –0.51 981 2482 1.06 CHON –0.14 248 2129 –0.14 316 2145 0.99 CHOP –1.01 491 220 –0.76 259 96 2.29 CHOS –0.41 195 366 –0.45 170 316 1.15 CHONP –0.63 102 102 –0.58 97 70 1.45 CHONS –0.35 84 167 –0.24 77 165 1.01 CHOPS –1.13 61 31 –1.06 23 20 1.55 CHONPS –0.57 51 28 –0.62 45 20 1.4 总类型 –0.47 2133 5681 –0.44 1968 5314 1.07 注：MLBL是易降解的DOM，MLBR是相对难降解的DOM[29]Note: MLBL refers to DOM that is labile, while MLBR refers to DOM that is recalcitrant[29] 
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