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近年来,由于生活污水、化肥农药等面源污染物质的排放,太湖水体富营养化日益突出,受到了社会广泛关注。有研究认为,过量使用化肥农药是造成太湖水质低下的主要原因之一,农田径流中氮磷流失量与肥料投入水平显著相关,施肥量远大于作物需求量,氮磷一直处于盈余状态[1]。在土壤-作物系统中,随降水径流和渗漏排出农田的氮素中有20%~25%是当季施用的氮素,而当季作物对磷肥的利用率只有5%~15%,约75%~90%的磷滞留在土壤中。长期过量施用磷肥会导致农田耕层土壤处于富磷状态,磷可通过地表径流等加速向水体迁移[2]。2010−2017年以来太湖水体总氮含量呈大幅度下降趋势,而总磷含量基本呈上升趋势[3]。河岸植被缓冲带是一个由土壤、水、植被等构成的生态系统,可通过植物吸收、土壤吸附等一系列物理、化学和生物等方式,阻止径流水中的磷进入水体[4],被认为是面源污染防治的最佳管理措施[5-6]。不同缓冲带截留磷的效率差异较大,截留功能受河岸带植物状况、宽度、坡度、土壤类型等影响[7]。国外对河岸植被缓冲带开展了较多的研究,主要以森林缓冲带为研究对象[8-9]。中国关于植被缓冲带的研究开展较晚,研究对象多以灌木、草本为主[10-12],而乔木缓冲带研究较少[13],且关于平缓坡度缓冲带的研究相对较少。本研究以太湖流域河岸人工林为研究对象,对比分析平缓坡地上不同宽度、植被组成和林龄河岸植被缓冲带对上游径流水中总磷和可溶性磷的去除差异,以期为太湖流域适宜河岸植被缓冲带的构建提供科学依据。
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研究区位于江苏省宜兴市周铁镇沙塘港村(31°07′~31°37′N,119°31′~120°03′E)东部,太湖西部沿岸,农田下游,与太湖间相隔一个沿湖大堤。气候类型属亚热带季风气候,四季分明,雨量充沛,气候温和湿润,年均气温为15.7 ℃,无霜期为239 d,年均降水量为1 277.1 mm,多集中于夏季(6−8月)。土壤类型为中性重壤质黄土,土质均匀,土壤容重为1.38 g·cm−3。周边居民主要以农业为主,种植水稻Oryza sativa、小麦Triticum aestivum和油菜Brassica napus 等农作物。树种为太湖流域常见的中山杉Taxodium hybrid‘Zhongshanshan’和‘南林95’杨Populus × euramericana‘Nanlin 95’,林龄6 a,平均树高4.6 m,胸径5.2 cm,林下为自然更新的草本植物,以芦苇Phragmites australis为主。人工林样地共设置400、1 000和1 600株·hm−2等3个林分密度(表1)。
表 1 样地植被配置
Table 1. Allocation of vegetation in the experiment plots
样地设置 植被配置 林分密度/(株·hm−2) 样地设置 植被配置 林分密度/(株·hm−2) 样地1 荒地 0 样地5 中山杉林 1 000 样地2 中山杉林 400 样地6 ‘南林95’杨林 1 000 样地3 ‘南林95’杨林 400 样地7 中山杉林 1 600 样地4 ‘南林95’杨-中山杉混交林 1 000 样地8 ‘南林95’杨林 1 600 -
缓冲带分为8个小区(20 m×50 m),坡度比为1∶250。各样地平行排列,与地表径流方向垂直,每个小区相互间隔1 m,用宽60.0 cm、厚1.5 cm的胶合板分隔[14],以减少样地间干扰。在每块样地不同宽度处(沿径流方向分别距离样地起点15、30和40 m)分别埋设PVC淋溶管,收集径流水。每个宽度设3组淋溶管作为重复,每组2个深度(20和40 cm)。每个淋溶管间隔1 m,组与组间隔3 m(图1)。
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随径流流失是农田土壤中的磷进入水体的主要途径[2],因此在2014−2017年分别选择当地的雨季(4−9月)进行采样,利用降雨溶解肥料,模拟污染物进入植被缓冲带。在每个样地起始端0~0.5 m处进行施肥,施肥量参考当地农田施肥量,为1 200 kg·hm−2的复合肥(N∶P∶K为16∶8∶16)。在降雨前施肥,降雨结束后24 h内采集样品。用小型水泵抽取淋溶水,装入250 mL塑料瓶中,带回实验室,放入−4~0 ℃冰箱内保存,测定磷。每次采完水样,排空取水管内水,排空的水样在远离采样点之处倾倒。
采用钼锑抗-紫外分光光度法测定水样中总磷(TP)和可溶性磷(DP)[15]。其中待测DP的水样先经过0.45 μm滤膜抽滤预处理后,采用钼锑抗-紫外分光光度法进行测定[16]。
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径流水中磷去除率计算公式为rP=(P0−Pi)/P0×100%。其中,rP为缓冲带不同宽度径流水中磷累计去除率;Pi为缓冲带i宽度处径流水中磷质量浓度(mg·L−1);P0为缓冲带起始处径流水中磷质量浓度(mg·L−1);i为宽度值(15、30和40 m)。
采用Microsoft Office 2016和SPSS 22.0进行数据处理与分析,所有数据均采用多次重复的平均值±标准误。
Effects of riparian plantation buffer strips on phosphorus interception in runoff water
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摘要:
目的 含磷等农业面源污染物的大量排放,已成为太湖水体富营养化的主要原因之一。河岸植被缓冲带能通过吸收、截留,减少面源污染物进入水体。在平缓坡地上构建适宜的河岸植被缓冲带,研究河岸植被缓冲带对磷的截留效率,为减缓太湖农业面源污染提供科学依据。 方法 以太湖河岸人工林缓冲带为研究对象,分析缓冲带在不同宽度(15、30、40 m)、不同林分密度(400、1 000、1 600株·hm−2)、不同植物组成(‘南林95’杨Populus × euramericana‘Nanlin 95’林、中山杉Taxodium hybrid ‘Zhongshanshan’林、‘南林95’杨-中山杉混交林)以及不同林龄(3、4、5、6 a)方面对径流水中总磷和可溶性磷截留效果的差异,确定适宜的人工林缓冲带结构。 结果 30 m宽的河岸缓冲带可很好地截留径流水中磷素,总磷截留率在30 m处最高,为77.30%,缓冲带对可溶性磷的截留率随宽度增加而提高,截留率在不同宽度间差异不显著(P>0.05);对径流水中总磷和可溶性磷截留效果较好的河岸缓冲带的林分密度为1 000株·hm−2,截留率分别为84.29%和93.25%;杨树林缓冲带对径流水中总磷的截留率较高,达82.37%,中山杉林缓冲带对可溶性磷截留率略高于杨树林,不同植物组成缓冲带对可溶性磷的截留效果差异不显著(P>0.05),河岸缓冲带对径流水中可溶性磷的截留效果较总磷好;随林龄增加,缓冲带对径流水中磷的截留率呈上升趋势(P<0.01),5年生时截留率最高。 结论 30 m宽的河岸缓冲带基本能满足截污需求,且杨树林缓冲带截留污染物效果较好。缓冲带在林龄较小时截留磷能力较强。图5表1参40 Abstract:Objective The discharge of agricultural non-point source pollutants such as phosphorus has become one of the major causes for the eutrophication in Taihu Lake. Riparian vegetation buffer strips can reduce non-point source pollution from agricultural area into water through absorption and interception. The riparian vegetation strips were built on gentle slopes to study the efficiency of retaining phosphorus so as to provide a scientific basis for mitigating the pollution of agricultural non-point sources in Taihu Lake. Method Taking the buffer strip of plantation along the bank of Taihu Lake as the research object, the interception effects of riparian plantation buffer strips with different widths (15, 30, 40 m), densities (400, 1 000, 1 600 trees ·hm−2 ), plantation types (Populus × euramericana ‘Nanlin 95’ plantation, Taxodium hybrid ‘Zhongshanshan’ plantation and ‘Nanlin 95’-‘Zhongshanshan’ mixed plantation) and ages (3, 4, 5, 6 a) on total phosphorus (TP) and dissolved phosphorus (DP) in surface runoff water were studied to determine the suitable structure of vegetation buffer strips. Result The 30 m wide buffer strips had the optimal effect on intercepting phosphorus in runoff water, and the TP interception rate was the highest at 30 m, 77.30%. The interception rate of DP increased with the increase of buffer strips width but there existed no significant difference between different widths of buffer strips in terms of DP interception rate (P>0.05). The stand density of riparian buffer strip with good interception effect on TP and DP in runoff water was 1 000 trees ·hm−2, and the interception rate was 84.29% and 93.25%, respectively. The ‘Nanlin 95’ plantation could intercept 82.37% TP in runoff water while the interception rate of ‘Nanlin 95’- ‘Zhongshanshan’, the mixed plantation on DP was a little higher than that of ‘Nanlin 95’ plantation, but the interception rate of DP in the buffer strips of different plant compositions was not significant different (P>0.05). The effect of buffer strips on interception of DP in runoff water was better than that on TP. With the increase of plantation age, the interception rate of phosphorus in runoff water increased gradually (P<0.01), and reached the highest at the age of 5. Conclusion The 30 m wide buffer strips can meet the requirement of pollution interception, and the ‘Nanlin 95’ plantation buffer strips have good interception effect on phosphorus. Besides, the buffer strips have stronger phosphorus interception capacity at a young age. [Ch, 5 fig. 1 tab. 40 ref.] -
表 1 样地植被配置
Table 1. Allocation of vegetation in the experiment plots
样地设置 植被配置 林分密度/(株·hm−2) 样地设置 植被配置 林分密度/(株·hm−2) 样地1 荒地 0 样地5 中山杉林 1 000 样地2 中山杉林 400 样地6 ‘南林95’杨林 1 000 样地3 ‘南林95’杨林 400 样地7 中山杉林 1 600 样地4 ‘南林95’杨-中山杉混交林 1 000 样地8 ‘南林95’杨林 1 600 -
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