黄河三角洲新生湿地磷分布特征及吸附解吸规律

孙军娜, 徐 刚, 邵宏波



黄河三角洲新生湿地磷分布特征及吸附解吸规律

孙军娜1, 2, 徐 刚1*, 邵宏波1

(1. 中国科学院 烟台海岸带研究所, 山东 烟台 264003; 2. 中国科学院大学, 北京 100049)

采用改进的Hedley磷分级方法研究了黄河三角洲新生湿地由河向海过渡带表层土壤磷形态变化和分布特征, 并通过等温吸附解吸实验阐明了沿程土壤对外源磷的持留能力和释放风险。结果表明, 各样点无机磷占总磷93%以上, 是磷的主要存在形态。土壤中有机磷含量较低, 可能与较低的有机质含量有关。无机磷中稀盐酸磷是最主要存在形态, 与各样点Ca/Al含量密切相关。有效磷含量在18.6~33.4 mg/kg之间, 仅占总磷的3.2%~5.9%, 可能会限制湿地植物的生长。覆有植被的土壤中有效磷含量显著高于河滩和海滩土壤, 说明植被存在对有效磷的积累有一定促进作用。由吸附解吸实验可知, 加入较低浓度(0.05~5 mg/L)的外源磷时, 随着初始磷浓度的升高, 土壤对磷的吸附量增加, 吸附率为70%~99%, 解吸率小于7%, 这说明各样点土壤的除磷能力较强, 且流失风险较低。

磷; Hedley分级; 吸附; 解吸; 黄河三角洲

0 引 言

磷是植物生长所必需的养分元素之一, 在湿地中往往成为一种主要的限制性养分。湿地土壤不同形态磷的相互转化会影响磷的有效性, 从而影响着湿地植物的生长[1], 因此研究湿地土壤各形态磷的分布特征对评价湿地土壤-植物系统磷的吸收转化能力十分重要。传统的Chang-Jackson磷分级方法及其改进方法都存在很多缺陷[2–4], Hedley分级方法是目前被认为较为合理, 能及时反映土壤中磷素形态的动态变化, 且能兼顾无机磷和有机磷的分级方法[5–6]。后来Tissen.[7]对Hedley分级方法做了进一步的修正使其可操作性更强。目前国内利用改进的Hedley分级方法对土壤磷分布特征进行研究的较少,主要集中在三江平原湿地土壤[8–9]。

湿地土壤对磷吸附解吸性能的也影响着湿地系统磷形态的转化和植物磷素营养[10–11]。另外, 黄河三角洲湿地地处下游, 上游工农业废水进入湿地, 影响了湿地磷的循环, 过多的磷会对水质造成一定威胁。湿地土壤对磷的吸附作用能去除一定量的磷[12–13], 但其解吸也可能增加水体富营养化的风险[14]。因而研究湿地土壤磷的形态及吸附解吸特性, 对农业生产及磷循环等具有重要意义[10–11]。目前对湿地系统中磷素吸附与解吸附的研究相对较少[15–16]。本文将对黄河三角洲新生湿地由河向海过渡带布点, 研究各样点土壤磷的形态分布及对磷的吸附解吸规律, 为评价湿地土壤磷形态变化趋势及湿地对磷的去除能力提供理论依据。

1 材料与方法

1.1 供试土壤

根据盐分梯度和植被分布, 在黄河三角洲新生湿地由河向海过渡带, 分别设置了S1至S5总共5个采样点 (各点经纬度见表1)。S1为黄河河滩, S2植被为柽柳, S3植被为盐地碱蓬, S4为柽柳和盐地碱蓬, S5为海滩。在每个样点利用三角形取样法, 选取3个0~20 cm的土样混合后形成这一样点的代表性土壤, 风干过10目筛备用。采用常规土壤农化分析方法[17],测定土壤pH值、含盐量 (Salinity)、总有机碳 (TOC)、Al/Fe/Ca金属含量。土壤黏粒(Clay)、粉粒 (Slit)、沙粒(Sand)采用Marlvern Mastersizer 2000F激光粒度仪进行测定。5个采样点的物理化学性质见表1。

1.2 磷分级实验

采用改进的Hedley磷分级方法[4]连续提取稳定性由弱到强的土壤各形态磷。称取0.5 g样品于50 mL离心管中, 加入30 mL不同提取液, 用钼锑抗方法[15]测定提取液中的磷。主要步骤为: (1) 树脂交换磷(Resin-P): 提取剂为水和树脂, 主要提取土壤溶液中的无机磷; (2) NaHCO3提取态磷: 提取剂为0.5 mol/L NaHCO3溶液, 主要提取吸附在土壤表面的无机磷(NaHCO3-Pi)和有机磷(NaHCO3-Po); (3) NaOH提取态磷: 提取剂为0.1 mol/L NaOH, 主要提取土壤铁铝化合物表面的无机磷(NaOH-Pi)和有机磷(NaOH-Po); (4) 稀盐酸提取态磷(Dil.HCl-P): 提取液为1 mol/L HCl, 主要提取部分闭蓄态磷; (5) 浓盐酸提取态磷: 提取剂为浓盐酸, 主要提取残留的部分无机磷(Conc.HCl-Pi)和有机磷(Conc.HCl-Po); (6)残留态磷(Residual-P): 提取剂为H2SO4和H2O2, 主要提取一般条件下极难被植物利用的那部分磷。土壤中总磷含量为各形态磷的加和, 其中Resin-P、NaHCO3-Pi、NaHCO3-Po为有效磷(Available-P, AP), NaOH-Pi、NaOH-Po为中等活性磷, Dil.HCl-P、Conc.HCl-Pi、Conc.HCl-Po为中稳态磷, Residual-P为稳态磷。

表1 各采样点物理化学性质

1.3 吸附和解吸实验

称取各点土样1.0 g于50 mL已知质量的离心管中, 分别加入10 mL含磷量为0.05 mg/L、0.1 mg/L、0.5 mg/L、1 mg/L、2 mg/L、5 mg/L (用KH2PO4配制)的0.01 mol/L的KCl溶液, 在(27±2) ℃振荡器上震荡24 h后, 取出离心(5000 转/min, 10 min), 过滤。取上清液用钼锑钪比色法测定滤液中磷的含量, 同时做空白实验, 由吸附前后磷含量变化计算吸附量, 所有实验处理均重复3次。

在吸附实验完成后, 将含磷量为0.05 mg/L和1 mg/L的离心管中的上清液倒掉, 称量离心管、土及残留液的质量(扣除土壤间隙残留磷对磷解吸量计算的影响)。然后向离心管内各加入10 mL的0.01 mol/L KCl溶液, 在(27±2) ℃振荡器上震荡24 h后, 取出离心 (5000 转/min, 10 min)。用钼锑钪比色法测定滤液中磷的含量, 计算磷解吸量。

1.4 数据分析及处理

磷的吸附率为吸附浓度与初始浓度的比值, 解吸率(des)由吸附量与解吸量比值求得; 分配系数(d)为达到吸附平衡时, 磷在固液两相中浓度的比值;d值高表明土壤颗粒对磷的吸附能力强[18–19]。实验数据由Microsoft Excel 2010、OriginPro 7.5和Spss 13.0软件进行分析处理。

2 结果与讨论

2.1 土壤各形态磷含量

各点土样中总磷(TP)含量为558.5~702.3 mg/kg (表2), 其中S5点TP含量最高。各样点有机磷(OP) 含量较低, 仅为18.6~33.4 mg/kg。无机磷(IP)是各点磷的主要存在形态, 占TP的93%~98% (表2)。以往研究发现三江平原湿地[20]及向海湿地[21]磷主要以有机磷为主, 杭州湾湿地[22]磷含量主要以无机磷为主, 这与湿地土壤母质、成土作用和耕作施肥密切相关[23–25]。

由表2可知, 各样点AP含量在18.4~25 mg/kg之间, 仅占TP的3.2%~5.9%。按照全国第2次土壤普查分级标准, 各样点中可被植物利用的磷约为3级, 含量较低。这可能由于黄河三角洲土壤主要来源于黄河上游土壤, 连续的冲刷作用造成了有效磷的大量流失。中等活性磷含量和稳态磷含量也较低, 分别为6.8~18.1 mg/kg和28.6~58.4 mg/kg。中稳态磷含量较高为454.4~498.5 mg/kg, 其中Dil.HCl-P含量最高, 平均占TP的61.5%~83.6%, 这说明各样点磷主要以中稳态形式存在。另外我们发现, 有效磷含量在覆有植被的S2、S3点比较高, 其中S3点有效磷含量比海边S5点高82%, 因此, 我们推断植被(如盐地碱蓬和柽柳)的存在有利于植物有效磷的形成。这与前人的研究结果一致, Tuchman.[26]研究发现, 香蒲、青冈的入侵使湿地土壤有效磷的含量增加。Chen.[27]也发现了辐射松的存在有利于土壤有机磷的矿化。但同样覆有植被的S4样点有效磷含量却较低, 这可能由于该样点植被量较少且离海较近, 涨落潮淋洗会损失大量有效磷。

2.2 磷的吸附及解吸

从表3可知, 各样点对磷的吸附量随着初始加入磷浓度的升高而增加。各样点对磷的吸附较强, 特别是在0.05~0.5 mg/L区域内, 吸附率达到99%。虽然随着初始浓度的增加, 各样点对磷的吸附率逐渐降低, 但仍在70%以上。因此当携带大量磷的污水进入湿地系统时, 湿地土壤可以通过吸附作用除去大量的磷, 这与Sakadevan.[1]、黄树辉[28]等的研究结果一致。通过计算可知, 各采样点d均值的顺序为: S3 (837) > S2 (666) > S5 (551) > S4 (486) > S1 (389)。这说明S3样点土壤对磷的吸附能力最强, S1点最弱。

表2 不同采样点各形态磷的含量 (mg/kg)

表3 各样点在不同浓度下吸附磷量(mg/kg)

从表4可知, 在吸附-解吸动态平衡过程中, 解吸率随着磷加入量的增加而增加, 但土壤对磷的吸附作用强于解吸作用。如处理浓度为1 mg/L时, 各样点中超过90%的磷留在土壤固相中, 这可能由于实验设定浓度范围下, 土壤对磷的吸附大多以共价键的化学性吸附为主[29–31], 很难被解吸下来, 这也减少了吸附在土壤中的磷发生迁移的风险。

表4 各样点在不同浓度下磷解吸率Pdes (%)

2.3 相关性分析

土壤理化性质与土壤磷形态及吸附解吸参数的相关关系见表5。通过相关分析发现, 各样点OP含量与土壤中TOC含量有关 (< 0.01)。这可能由于黄河三角洲成土时间短, 土壤含盐量高造成植被生物量少, 因此土壤中有机磷含量较低。Dil.HCl-P受Ca/Al含量影响较大, 因为Dil.HCl-P主要提取磷灰石型磷及部分闭蓄态磷[5]。AP含量与各样点有机质、黏粒含量显著正相关 (< 0.01), 这与前人研究一致[32–33]。与其他样点相比, 覆有植被的S2、S3点Dil.HCl-P较低, AP较高, 这说明植被的存在可能有利于稳态磷向生物可利用磷转化[34]。各样点对磷的吸附解吸能力与土壤黏粒含量密切相关 (< 0.01)。S3点黏粒含量较高, 因此吸附量远大于S1点。这主要由于黏粒表面有较大的表面积[35], 能够迅速吸附磷素于土壤颗粒外表面的吸附点位上。

3 结 论

各样点土壤中总磷含量为558.5~702.3 mg/kg, 有效磷含量仅占TP的3.2%~5.9%。各样点中无机磷占总磷的93%~97%, 有机磷含量较低, 这与土壤植被生物量少有机质含量低有关。无机磷中Dil.HCl-P含量最高, 平均占总磷的75.8%, 是各样点磷的主要存在形态, 与各样点Ca/Al含量密切相关。在吸附解吸实验中设定的浓度范围内, 土壤对磷的吸附能力较高, 吸附率在70%~99%之间。吸附磷的解吸量较低, 在处理浓度为1 mg/L时, 解吸率仅为2.5%~6.1%, 吸附解吸特性与土壤物理化学性质密切相关。因此我们推断, 实验选取的湿地各样点土壤中较低的有效磷含量可能成为湿地植物生长的限制性因子, 植被的存在有利于有效磷的积累, 采样点土壤对外源磷的输入有一定的吸附能力且释放风险较低。

注: *和**分别代表显著性水平为0.05和0.01,= 15。

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Fractionation and adsorption-desorption characteristics of phosphorus in newly formed wetland soils of Yellow River Delta, China

SUN Jun-na1, 2, XU Gang1*and SHAO Hong-bo1

1. Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai 264003, China; 2. University of Chinese Academy of Sciences, Beijing 100049, China

A modified Hedley phosphorus (P) fractionation was used to study the P distribution seaward in the newly formed wetlands soils of Yellow River Delta. In addition, the adsorption-desorption experiments were performed to evaluate the ability of P retention and release in the sampled soils. The results showed that inorganic P remained the largest portion of total P (TP), accounting for more than 93% of TP. Due to the lower content of organic matter, the organic P was relatively low in these soils. Among the inorganic P, Dilute HCl-P was the dominated form, related with the content of Ca in soils. The content of available P was 18.6~33.4 mg/kg, accounting for only 3.2%~5.9% of TP in soils, which might restrict the growth of plants in the wetland system. Furthermore, it was found that the content of available P in the sampling site covered with plants was higher than that in the beach soils, indicating that the vegetation cover may enhance the accumulation of soil available P. According to the adsorption-desorption experiments, when the concentration of initial P addition was in the range of 0.05~5 mg/L, the P adsorption increased with the increase of initial P concentration. Moreover, the percentage of adsorption was 70%~99% while desorption rate was less than 7%. It could be concluded that in these soils, the capacity of P retention was high and the release potential was relatively low.

phosphorus; Hedley fractionation; adsorption; desorption; Yellow River Delta

P595; X142

A

0379-1726(2014)04-0346-06

2013-12-14;

2014-04-02;

2014-04-08

国家自然科学基金项目(41001137, 41171216); 中国科学院烟台海岸带研究所“一三五”发展规划项目(Y254021031); 中国科学院创新团队国际合作伙伴计划(KZCX2-YW-T14)

孙军娜(1984–), 女, 博士研究生, 环境科学专业。E-mail: jnsun@yic.ac.cn

XU Gang, E-mail: gxu@yic.ac.cn , Tel: +86-535-2109169