环境科学  2016, Vol. 37 Issue (12): 4615-4625   PDF    
洱海入湖河口湿地干湿季沉积物氮、磷、有机质垂向分布特征及污染风险差异性
王书锦1 , 刘云根1,2 , 王妍1,2 , 侯磊1,2 , 张超3     
1. 西南林业大学环境科学与工程学院, 昆明 650224;
2. 西南林业大学农村污水处理研究所, 昆明 650224;
3. 西南林业大学林学院, 昆明 650224
摘要: 以云南洱海罗时江河口湿地为典型对象,利用柱状底泥分层采样器采集罗时江河口湿地表层(0~10 cm)、中层(10~30 cm)和底层(30~60 cm)沉积物样品,分析干、湿季沉积物总氮(TN)、总磷(TP)、有机质(OM)垂向分布特征,并对沉积物进行污染风险评价.结果表明:①干季罗时江河口湿地表层、中层和底层沉积物TN平均含量分别为1.734、1.453和1.255 g·kg-1,TP平均值分别1.085、1.034和0.992 g·kg-1,OM平均值分别为59.051、47.730和42.133 g·kg-1;湿季罗时江河口湿地表层、中层和底层沉积物TN平均含量分别为1.147、0.948和0.895 g·kg-1,TP平均值分别0.599、0.523和0.519 g·kg-1,OM平均值分别为53.098、46.897和43.395 g·kg-1;干、湿季各指标含量垂向分布随沉积深度的增加呈下降趋势,表层富集明显;三层沉积物各指标含量均为干季高于湿季,且除OM外,TN和TP均达到显著差异(P < 0.01). ②单因子指数(PI)、有机氮指数(ON)和有机指数(OI)评价结果表明,整体上干季沉积物氮磷属于重度污染,有机质属于中度污染;湿季沉积物氮和有机质均为中度污染,磷为轻度污染;总体上沉积物氮、磷、有机质污染水平为干季高于湿季,且干、湿季各污染指数均为表层最高,中层次之,底层最低,表层污染最为严重. ③干、湿季罗时江河口湿地污染来源存在差异,干季以外源为主,湿季以内源为主;干季沉积物营养盐潜在释放风险较湿季大,且以表层释放风险最大.
关键词: 洱海流域      罗时江河口湿地      沉积物氮、磷、有机质      干、湿季垂向分布特征      污染风险评价     
Vertical Distribution and Pollution Risk Assessment of Nitrogen, Phosphorus, and Organic Matter in Sediment of Inflowing Rivers of Erhai Lake Estuarine Wetland in Wet and Dry Seasons
WANG Shu-jin1 , LIU Yun-gen1,2 , WANG Yan1,2 , HOU Lei1,2 , ZHANG Chao3     
1. College of Environmental Science and Engineering, Southwest Forestry University, Kunming 650224, China;
2. Research Institute of Rural Sewage Treatment, Southwest Forestry University, Kunming 650224, China;
3. College of Forestry, Southwest Forestry University, Kunming 650224, China
Abstract: The estuarine wetland of Luoshijiang River, which flows into Erhai Lake in Yunnan province, was regarded as the typical research object. Sediment samples in the estuarine wetland of Luoshijiang River were collected by the cylindrical sediment sampler and the samples were from surface layer (0-10cm), middle layer (10-30 cm), and bottom layer (30-60 cm). The vertical distribution of total nitrogen, total phosphorus, and total organic material in dry and rainy season was analyzed and contamination risk assessment of sediment was accomplished. The results showed that: ① In dry season, average concentrations of total nitrogen in sediment on surface, middle and bottom layer at estuarine wetland of Luoshijiang River were respectively 1.734, 1.453 and 1.255 g·kg-1, the average concentrations of total phosphorus were respectively 1.085, 1.034 and 0.992 g·kg-1, the average concentrations of organic material were respectively 59.051, 47.730 and 42.133 g·kg-1. In rainy season, average concentrations of total nitrogen in sediment on surface, middle and bottom layer at estuarine wetland of Luoshijiang River were respectively 1.147, 0.948 and 0.895 g·kg-1, the average concentrations of total phosphorus were respectively 0.599, 0.523 and 0.519 g·kg-1, the average concentrations of organic material were respectively 53.098, 46.897 and 43.395 g·kg-1. ② Single factor pollution index, organic nitrogen and organic index assessment were used to evaluate the contamination level of sediment, the assessment results showed that: In dry season, the contamination level of nitrogen and phosphorus in sediment was on severe level, organic material contamination was on medium level. In rainy season, the contamination levels of ON and OI were medium, and that of PI was mild. Generally, the contamination levels of nitrogen, phosphorus and organic material in sediment tendency were as follows: dry season>rainy season, and the pollution index of each layer followed the sequence of surface layer > middle layer > bottom layer, and the most extraordinary contamination of them on surface layer was uncovered whatever the season was. ③ Pollution origin of estuarine wetland of Luoshijiang River in dry and wet season had great differences, it was mainly the exogenous source in dry season, but it was the internal source in wet season; The potential risk of release of nutrient salt of sediment in dry season was higher than the wet, and the risk in the surface layer was the highest.
Key words: Erhai River basin      the estuarine wetland of Luoshijiang River      nitrogen, phosphorus and organic material in sediment      vertical distribution characteristics in dry and rainy season      pollution risk assessment     

河口湿地对入湖河流污染物具有很好的截留、贮存和净化功能,对消减入湖污染负荷具有积极作用[1, 2].云贵高原受东亚和南亚季风的共同影响,形成冬干、夏湿,干湿季节较为分明的区域降水季节特征.这种独特的干湿季节使流域水文过程[3]、湿地植被适生性[4]、沉积物营养物质迁移转化[5, 6]等发生明显变化.当前,国内对沉积物营养物质空间分布、形态组成及转化机理之间的关系进行了研究,如赵海超等[7~9]对洱海沉积物氮、磷、有机质含量及其形态时空分布进行了研究,结果表明不同季节沉积物氮、磷、有机质含量空间分布存在差异,但总体呈表层富集明显;向速林等[10]研究了鄱阳湖沉积物中磷的赋存形态及分布特征,认为表层沉积物总磷主要由无机磷组成,且以Fe-P含量最高,垂向分布上总磷和各形态磷含量随沉积深度增加而降低;Smolders等[11]和王圣瑞等[12]研究认为,氮在沉积物中以不同形态存在,其在地球化学循环中的作用有所不同;李如忠等[13]对沉积物中氮、磷、有机质的迁移规律、释放风险和生物有效性进行了研究,结果表明十五里河沉积物中各形态氮磷在纵向和垂直方向上表现出一定的规律性,且主要氮磷形态之间还具有显著的相关性,不同沉积深度生物有效性氮磷含量存在差异.这些研究主要集中在平原湿地或湖泊沉积物,而对高原河口湿地干、湿季下沉积物营养盐的垂向分布特征及污染风险差异性的研究鲜见报道.本文以高原洱海罗时江河口湿地为研究对象,分析了干、湿季不同沉积深度沉积物氮、磷、有机质的含量特征,并运用单因子指数、有机氮和有机指数法对沉积物营养盐进行污染风险评价,研究结果对罗时江河口湿地的保护及洱海富营养化防控具有重要意义.

1 材料与方法 1.1 研究区概况

罗时江河口湿地是削减罗时江携带的农业面源污染物、保护洱海水质的最后屏障,该湿地位于洱海北部湖湾(E 100°05′59.9″~100°06′5.9″, N 25°56′52.8″~25°57′24.9″),面积约0.5 km2.研究区属北亚热带高原季风气候区,光照充足,四季不明显,冬春干旱,夏秋多雨,雨季旱季分明,年均温度为15.6℃,年均降雨量约为942 mm.湿地西北和东北部分别有罗时江、黑泥沟携带大量的泥沙、氮磷污染物注入.湿地内植被多为人工栽植的水生植物,其中挺水植物主要有:芦苇(Phragmites australis (Cav.) Trin. ex Steud.)、荷花(Nillumbik nucifera Gaertn.)、梭鱼草(Pontederia cordata L.)、再力花(Thalia dealbata Fraser ex Roscoe.)、香蒲(Typha orientalis Presl.)、菰(Zizania latifolia (Griseb.) Stapf.);沉水植物主要有:金鱼藻(Ceratophyllum demersum Linn.)、微齿眼子菜(Potamogeton maackianus A. Bennett.)、黑藻(Hydrilla verticillata (Linn.f.) Royle.);浮叶植物主要有:睡莲(Nymphaea tetragona Georgi.)等.

1.2 样点布设

采用网格法布点(图 1),网格大小为100 m×100 m,去除无效网格(如整个网格中为小岛、或无水淹没区),罗时江河口湿地中共划分45个网格,在网格中心处取样,其中23、28、30、31、33、34、35、36号样点网格,由于网格被小岛分割,未在网格中心取样,根据地形条件及水流状况增加1~2个采样点,增加后采样点共有55个.

图 1 罗时江河口湿地区位及样点布置示意 Fig. 1 Locations of sediments sampling sites in estuarine wetland of Luoshijiang River

1.3 样品采集与分析

采样时间分别为2013年11月20日(干季),2014年6月20日(湿季),利用柱状底泥采样器自上而下分层取样,其中表层为0~10 cm、中层为10~30 cm、底层为30~60 cm,分层取样后放入便携式冷恒温箱带回实验室分析沉积物中总氮(TN)、总磷(TP)、有机质(OM)含量.沉积物TN、TP、OM分析方法分别采用高氯酸-硫酸消化法(LY-T 1228-1999)、酸熔-钼锑抗比色法(LY/T 1232-1999)和重铬酸钾氧化-外加热法(LY/T 1237-1999).

数据分析采用SPSS 19.0和Excel 2013,空间分布图采用Surfer 12.0软件中的等值线功能绘制.

1.4 污染风险评价方法及标准

(1)单因子指数法

单因子指数法常被用于评价污染物的污染状况,本研究采用单因子指数法对罗时江河口湿地沉积物TP的污染状况进行评价,计算公式如下[14]:

(1)

式中,PI为单项评价指数或标准指数,PI大于1表明含量超过评价标准值;Ci为评价因子i的实测值,g ·kg-1Cs为评价因子i的评价标准值,g ·kg-1.本研究采用TP的评价标准(TP=0.60 g ·kg-1),与加拿大安略省环境和能源部发布的指南[15]中沉积物中能引起最低级别生态风险效应的TP的含量相一致.沉积物单因子指数评价标准见表 1.

表 1 沉积物单因子指数评价标准 Table 1 Evaluation standards of single factor index in sediments

(2)有机指数及有机氮指数法

单因子指数法忽略了OM指标,因此本研究用有机污染指数法对罗时江河口湿地沉积物污染现状进一步评价,使评价结果更完善.有机指数法常被用于评价沉积物的营养状况,而有机氮则是评价沉积物所受氮污染程度的重要指标.计算公式(2)~(4)和评价标准(表 2)如下[16~18]:

表 2 沉积物有机指数、有机氮评价标准 Table 2 Evaluation standards of organic index and organic nitrogen in sediments

(2)
(3)
(4)

式中,ON为有机氮,%;OC为有机碳,%;OI为有机指数,其中TN换算成质量分数形式,即1.000 g ·kg-1=0.1%.

2 结果与分析 2.1 干、湿季沉积物总氮垂向分布特征

图 2可知,干季罗时江河口湿地表、中、底3层沉积物TN含量分别在0.335~3.720、0.500~3.067和0.265~2.622 g ·kg-1之间,平均值分别为1.734、1.453和1.255 g ·kg-1. 55个样点中有75.93%的样点沉积物TN含量随着沉积深度增加而减少,各样点沉积物TN含量降幅在1.00%~84.54%之间,平均降幅达39.92%.

图 2 干、湿季沉积物TN含量垂向分布 Fig. 2 Vertical distribution of total nitrogen concentration in sediments of dry and wet seasons

湿季罗时江河口湿地表、中、底3层沉积物TN含量分别在0.339~2.742、0.213~2.029和0.292~0.895 g ·kg-1之间,平均值分别为1.147、0.948和0.895 g ·kg-1. 55个样点中有70.8%的样点沉积物TN含量以表层最高并向底层递减,各样点上沉积物TN含量降幅在8.5%~87.1%之间,平均降幅达45.8%.

干、湿季罗时江河口湿地沉积物TN含量随着沉积深度增加而减少,表层富集明显,且各层沉积物TN含量为干季高于湿季(P<0.01,见表 3).

表 3 干、湿季各层沉积物TN方差分析 Table 3 Analysis of variance for total nitrogen in each layer of sediments in dry and wet seasons

2.2 干、湿季沉积物总磷垂向分布特征

图 3可知,干季表层沉积物TP含量在0.628~1.860 g ·kg-1之间,平均值为1.085 g ·kg-1;中层TP含量在0.548~2.472 g ·kg-1之间,平均值为1.034 g ·kg-1;底层TP含量在0.526~2.788 g ·kg-1之间,平均值为0.992 g ·kg-1. 55个样点中有65.45%的样点沉积物TP含量随着沉积深度增加而减少,各样点沉积物TP含量降幅在3.64%~53.28%之间,平均降幅达23.93%.

图 3 干、湿季沉积物TP含量垂向分布 Fig. 3 Vertical distribution of total phosphorus concentration in sediments of dry and wet seasons

湿季表层沉积物TP含量在0.170~1.371 g ·kg-1之间,平均值为0.589 g ·kg-1;中层TP含量在0.213~1.047 g ·kg-1之间,平均值为0.513 g ·kg-1;底层TP含量在0.292~0.789 g ·kg-1之间,平均值为0.511 g ·kg-1. 55个样点中有59.18%的样点沉积物TP含量以表层最高并向底层递减,各样点上沉积物TP含量降幅在3.3%~71.1%之间,平均降幅达33.2%.

与TN分布规律一致,罗时江河口湿地各层沉积物TP含量为干季大于湿季(P<0.01,见表 4),垂向分布以表层含量最高,中层次之,底层最低.

表 4 干、湿季各层沉积物TP方差分析 Table 4 Analysis of variance for total phosphorus in each layer of sediments in dry and wet seasons

2.3 干、湿季沉积物有机质垂向分布特征

图 4可知,干季罗时江河口湿地表、中、底3层沉积物OM含量分别在17.947~233.493、2.615~166.008和10.043~115.757 g ·kg-1之间,平均值分别为59.051、47.730和42.133 g ·kg-1. 55个样点中有66.67%的样点沉积物OM含量随着沉积深度增加而减少,各样点沉积物OM含量降幅在1.41%~88.79%之间,平均降幅达44.1%.

图 4 干、湿季沉积物OM含量垂向分布 Fig. 4 Vertical distribution of total organic matter concentration in sediments of dry and wet seasons

湿季罗时江河口湿地表、中、底3层沉积物OM含量分别在8.199~98.962、11.944~232.678和10.049~159.800 g ·kg-1之间,平均值分别为53.098、46.897和43.395 g ·kg-1. 55个样点中有79.17%的样点沉积物OM含量以表层最高并向底层递减,各样点上沉积物OM含量降幅在0.2%~72.7%之间,平均降幅达34.0%.

与氮磷的规律一致,干、湿季罗时江河口湿地3层沉积物OM垂向分布均为表层>中层>底层,但干、湿季各层沉积物OM含量无显著性差异(见表 5).

表 5 干、湿季各层沉积物OM方差分析 Table 5 Analysis of variance for organic matter in each layer of sediments in dry and wet seasons

2.4 沉积物氮、磷、有机质污染风险差异性

(1)干、湿季单因子(PI)指数评价结果

采用公式(1)~(4)对干、湿季罗时江河口湿地沉积物表、中和底层氮、磷、有机质进行评价,结果如表 6~8图 5~7所示.

图 5 干、湿季三层沉积物PI指数分布 Fig. 5 Distribution of single factor index in sediments of dry and wet seasons

表 6 干、湿季沉积物单因子指数评价结果 Table 6 Result of single factor index evaluation in sediments of dry and wet seasons

根据表 6可知,干季罗时江河口湿地3层沉积物PI在0.877~4.647之间,平均值分别为1.809、1.723和1.654;湿季3层沉积物PI在0.283~2.285之间,平均值分别为0.946、0.808和0.759.总体上,各层沉积物PI为干季高于湿季(P<0.01),干季属于重度污染,污染风险达到Ⅳ级,湿季属于轻度污染,污染风险达到Ⅱ级.此外,干季和湿季3层沉积物PI均呈表层>中层>底层的趋势(见图 5).综上所述,各层沉积物污染风险从高到低依次为表层、中层、底层,且干季污染风险较高.

(2)干、湿季有机氮(ON)指数评价结果

根据表 7可知,干季罗时江河口湿地3层沉积物ON在0.025~0.353之间,平均值分别为0.178、0.140和0.119;湿季3层沉积物ON在0.020~0.260之间,平均值分别为0.111、0.091和0.084.总体上,各层沉积物ON为干季高于湿季(P<0.01),干季介于中度污染和重度污染之间,污染风险达到Ⅲ~Ⅳ级,湿季属于中度污染,污染风险达到Ⅲ级.与PI评价结果一致,干、湿季3层沉积物ON亦呈表层>中层>底层的趋势(见图 6),且以干季表层ON污染指数最高.

图 6 干、湿季三层沉积物ON指数分布 Fig. 6 Distribution of organic nitrogen in sediments of dry and wet seasons

表 7 干、湿季沉积物有机氮指数评价结果 Table 7 Result of organic nitrogen evaluation in sediments of dry and wet seasons

(3)干、湿季有机指数(OI)评价结果

表 8可知,干季罗时江河口湿地3层沉积物OI在0.015~1.878之间,平均值分别为0.613、0.424和0.308;湿季3层沉积物OI在0.013~2.360之间,平均值分别为0.344、0.273和0.055.总体上,沉积物OI为干季高于湿季,且除底层外,表层和中层差异性显著(P<0.01),干季介于中度污染和重度污染之间,污染风险达到Ⅲ~Ⅳ级,湿季属于中度污染,污染风险达到Ⅲ级.与PI、ON评价结果一致,干季和湿季三层沉积物ON亦呈表层>中层>底层的趋势(图 7).

图 7 干、湿季三层沉积物OI指数分布 Fig. 7 Distribution of organic index in sediments of dry and wet seasons

表 8 干、湿季沉积物OI评价结果 Table 8 Result of organic index evaluation in sediments of dry and wet seasons

3 讨论

沉积物氮、磷、有机质时空分布主要受外源输入、沉积环境、人为因素、水动力条件及污染状况等影响[19].罗时江河口湿地干、湿季沉积物氮、磷、有机质垂向分布差异性,反映了罗时江河口湿地干、湿季外源输入、沉积环境、水动力条件及氮、磷、有机质污染来源各异.

OM主要来源于流域内生物残体的迁移和湿地内水生动植物残体的沉积,有机残体经过湿地生物的分解及矿化,不断与水体发生交换,逐步沉积和埋藏于沉积物中[20].湿地沉积物C/N是OM来源的有效指示指标,OM的来源不同,C/N比值也具有明显差异[21~23].一般认为,当C/N>10时,沉积物OM以外源为主,C/N<10时,以内源OM为主,C/N≈10时,外源与内源OM基本达到平衡状态[24, 25].不同的生物种类C/N的比值也存在差异性,高等植物C/N为14~23,水生生物为2.8~3.4,而浮游动植物为6~13,藻类为5~14[26].

表 9可知,干季罗时江河口湿地表层沉积物C/N值在1.236~9.803之间,平均值为3.276,31.6%的比值介于5~14之间,26.3%的比值介于2.8~3.4之间;湿季罗时江河口湿地表层沉积物C/N值在1.590~15.960之间,平均值为5.335,61.2%的比值介于5~14之间,30.6%的比值介于2.8~3.4之间.综上所述,表明干、湿季罗时江河口湿地表层沉积物OM主要受内源影响,且以藻类及浮游动植物为主.湿季某些点位的C/N值大于10,则说明该区域可能受到外源OM进入水体后不能迅速分解氧化而造成的影响.而罗时江河口湿地沉积物OM的垂直分布情况可能与湿地中植物枯萎死亡后在沉积物中的长期累积有关,也有可能与OM自身特点有关,沉积物中的OM在微生物作用下,不断发生矿化分解,尤其是其中的TOC,在甲烷菌的作用下,转化为CO2释放到大气中[24],导致了OM在垂向上呈逐渐降低的趋势.

表 9 干、湿季表层沉积物C/N比值 Table 9 Ratio of C/N in the surface layer sediment of dry and wet seasons

罗时江河口湿地干、湿季3层沉积物TN、TP和OM相关性如表 10所示.干季沉积物TN和OM呈显著相关,而到了湿季呈极显著正相关,这表明罗时江河口湿地沉积物中TN与OM之间具有同源性.干季沉积物TP与TN、OM均无相关性,到了湿季表层沉积物TP和TN、TP和OM均呈极显著正相关,表明干、湿季罗时江河口湿地沉积物中氮磷污染来源存在差异性,TN和TP主要受外源影响,OM主要受内源影响,且湿季以外源为主,干季以内源为主.

表 10 干、湿季各层沉积物氮、磷和有机质相关性分析1) Table 10 Correlations of TN, TP and OM in the each layer of sediments in dry and wet seasons

罗时江流域以面源污染为主,农业面源已成为该流域的主要污染源[27].而大蒜种植是造成该流域农业面源污染的主要原因[28].资料显示干季大理地区大蒜和玉米等作物种植面积广、需肥量大且养分盈余量高[29],随着湿季雨季的到来,含氮磷化肥在土壤中随着大雨冲刷地表径流汇入河道最终进入湿地,因此湿季表层沉积物TP和TN、TP和OM均呈极显著正相关.罗时江河口湿地沉积物TN和TP含量均为干季显著高于湿季,一定程度上也反映了罗时江河口湿地受内源污染的影响较外源大.

有研究显示氮磷在表层沉积物富集的现象,主要是由于近年来外源污染输入增加[30, 31],而另有学者认为这种现象还有可能是由于氮磷在生物地球化学作用下迁移至沉积物表层所致[32].罗时江河口湿地氮磷在表层富集的现象说明近年来罗时江河口湿地沉积物中氮磷含量有明显增加趋势,可能的原因是由于近年来流域内农业和旅游业的发展,农村及农业面源污染物输入量增加[33],从而使得大量的氮磷物质向沉积物表层沉积;此外,随着沉积深度增加,生物对有机质矿化作用释放的氮素逐渐减弱[34],且表层以下厌氧环境使得磷从沉积物中释放至上覆水体[35],最终导致沉积物氮磷含量随沉积深度增加而减少.这与翟继红等[36]和岳维忠等[37]关于平原河口湿地底泥氮磷分布特征的研究类似.

PI、ON和OI的评价结果较为一致,即干季罗时江河口湿地沉积物的评价指数较高,湿季较低.总体趋势为干季污染程度较重,湿季污染程度较轻,且干、湿季均以表层污染程度最高,底层最低.与太湖流域[38]和西溪湿地[39]沉积物相比,罗时江河口湿地营养盐污染处于较高水平.

沉积物是湿地氮磷营养盐的蓄积库[40],在外源得到控制后,沉积物氮磷释放将逐渐成为水体富营养化控制的重点,尤其是对潜水湿地而言.虽然不是所有形态的氮磷都能释放至水体,但是沉积物TN和TP形态代表氮磷素在沉积物中的污染程度以及潜在的释放风险[41].罗时江河口湿地沉积物PI、ON指数呈干季大于湿季,且表层污染最为严重.据现场调查,干季研究区风力较大,在风力作用下易引起沉积物扰动使得沉积物处于再悬浮状态[42],有助于营养盐的释放或者再生.此外,OM对氮磷释放具有重要的影响.如王圣瑞等[43]和易文利[44]研究认为有机污染较重的情况下,沉积物氮磷素释放量变化幅度大于污染较轻的沉积物.由此可以推断,干季罗时江河口湿地沉积物氮磷的潜在释放风险较大,尤其是表层沉积物,更易对水体造成二次污染.

4 结论

(1)干、湿季罗时江河口湿地沉积物氮、磷、有机质含量存在差异,各指标含量为干季大于湿季,除OM外,TN和TP均存在显著性差异(P<0.01);干、湿季沉积物营养盐垂向分布规律一致,即以表层富集明显,且营养盐含量随沉积深度增加而降低.罗时江河口湿地沉积物TN和TP主要受湿季外源影响,OM主要受干季内源影响.

(2)运用单因子指数法、有机氮指数法和有机指数法对干、湿季沉积物氮、磷、有机质污染状况进行评价,干季3层沉积物磷污染均为重度污染;氮污染除底层为中度污染外,表层和中层均为重度污染;有机污染除表层属于重度污染外,中层和底层均为中度污染.湿季三层沉积物除磷属于轻度污染,氮和有机污染均为中度污染.总体而言,罗时江河口湿地沉积物营养盐干季污染水平较湿季高,且干、湿季均以表层污染风险最高,底层最低.

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