孫軍娜, 徐 剛, 邵宏波

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黃河三角洲新生濕地磷分布特征及吸附解吸規(guī)律

孫軍娜1, 2, 徐 剛1*, 邵宏波1

(1. 中國科學院 煙臺海岸帶研究所, 山東 煙臺 264003; 2. 中國科學院大學, 北京 100049)

采用改進的Hedley磷分級方法研究了黃河三角洲新生濕地由河向海過渡帶表層土壤磷形態(tài)變化和分布特征, 并通過等溫吸附解吸實驗闡明了沿程土壤對外源磷的持留能力和釋放風險。結(jié)果表明, 各樣點無機磷占總磷93%以上, 是磷的主要存在形態(tài)。土壤中有機磷含量較低, 可能與較低的有機質(zhì)含量有關(guān)。無機磷中稀鹽酸磷是最主要存在形態(tài), 與各樣點Ca/Al含量密切相關(guān)。有效磷含量在18.6~33.4 mg/kg之間, 僅占總磷的3.2%~5.9%, 可能會限制濕地植物的生長。覆有植被的土壤中有效磷含量顯著高于河灘和海灘土壤, 說明植被存在對有效磷的積累有一定促進作用。由吸附解吸實驗可知, 加入較低濃度(0.05~5 mg/L)的外源磷時, 隨著初始磷濃度的升高, 土壤對磷的吸附量增加, 吸附率為70%~99%, 解吸率小于7%, 這說明各樣點土壤的除磷能力較強, 且流失風險較低。

磷; Hedley分級; 吸附; 解吸; 黃河三角洲

0 引 言

磷是植物生長所必需的養(yǎng)分元素之一, 在濕地中往往成為一種主要的限制性養(yǎng)分。濕地土壤不同形態(tài)磷的相互轉(zhuǎn)化會影響磷的有效性, 從而影響著濕地植物的生長[1], 因此研究濕地土壤各形態(tài)磷的分布特征對評價濕地土壤-植物系統(tǒng)磷的吸收轉(zhuǎn)化能力十分重要。傳統(tǒng)的Chang-Jackson磷分級方法及其改進方法都存在很多缺陷[2–4], Hedley分級方法是目前被認為較為合理, 能及時反映土壤中磷素形態(tài)的動態(tài)變化, 且能兼顧無機磷和有機磷的分級方法[5–6]。后來Tissen.[7]對Hedley分級方法做了進一步的修正使其可操作性更強。目前國內(nèi)利用改進的Hedley分級方法對土壤磷分布特征進行研究的較少,主要集中在三江平原濕地土壤[8–9]。

濕地土壤對磷吸附解吸性能的也影響著濕地系統(tǒng)磷形態(tài)的轉(zhuǎn)化和植物磷素營養(yǎng)[10–11]。另外, 黃河三角洲濕地地處下游, 上游工農(nóng)業(yè)廢水進入濕地, 影響了濕地磷的循環(huán), 過多的磷會對水質(zhì)造成一定威脅。濕地土壤對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體富營養(yǎng)化的風險[14]。因而研究濕地土壤磷的形態(tài)及吸附解吸特性, 對農(nóng)業(yè)生產(chǎn)及磷循環(huán)等具有重要意義[10–11]。目前對濕地系統(tǒng)中磷素吸附與解吸附的研究相對較少[15–16]。本文將對黃河三角洲新生濕地由河向海過渡帶布點, 研究各樣點土壤磷的形態(tài)分布及對磷的吸附解吸規(guī)律, 為評價濕地土壤磷形態(tài)變化趨勢及濕地對磷的去除能力提供理論依據(jù)。

1 材料與方法

1.1 供試土壤

根據(jù)鹽分梯度和植被分布, 在黃河三角洲新生濕地由河向海過渡帶, 分別設(shè)置了S1至S5總共5個采樣點 (各點經(jīng)緯度見表1)。S1為黃河河灘, S2植被為檉柳, S3植被為鹽地堿蓬, S4為檉柳和鹽地堿蓬, S5為海灘。在每個樣點利用三角形取樣法, 選取3個0~20 cm的土樣混合后形成這一樣點的代表性土壤, 風干過10目篩備用。采用常規(guī)土壤農(nóng)化分析方法[17],測定土壤pH值、含鹽量 (Salinity)、總有機碳 (TOC)、Al/Fe/Ca金屬含量。土壤黏粒(Clay)、粉粒 (Slit)、沙粒(Sand)采用Marlvern Mastersizer 2000F激光粒度儀進行測定。5個采樣點的物理化學性質(zhì)見表1。

1.2 磷分級實驗

采用改進的Hedley磷分級方法[4]連續(xù)提取穩(wěn)定性由弱到強的土壤各形態(tài)磷。稱取0.5 g樣品于50 mL離心管中, 加入30 mL不同提取液, 用鉬銻抗方法[15]測定提取液中的磷。主要步驟為: (1) 樹脂交換磷(Resin-P): 提取劑為水和樹脂, 主要提取土壤溶液中的無機磷; (2) NaHCO3提取態(tài)磷: 提取劑為0.5 mol/L NaHCO3溶液, 主要提取吸附在土壤表面的無機磷(NaHCO3-Pi)和有機磷(NaHCO3-Po); (3) NaOH提取態(tài)磷: 提取劑為0.1 mol/L NaOH, 主要提取土壤鐵鋁化合物表面的無機磷(NaOH-Pi)和有機磷(NaOH-Po); (4) 稀鹽酸提取態(tài)磷(Dil.HCl-P): 提取液為1 mol/L HCl, 主要提取部分閉蓄態(tài)磷; (5) 濃鹽酸提取態(tài)磷: 提取劑為濃鹽酸, 主要提取殘留的部分無機磷(Conc.HCl-Pi)和有機磷(Conc.HCl-Po); (6)殘留態(tài)磷(Residual-P): 提取劑為H2SO4和H2O2, 主要提取一般條件下極難被植物利用的那部分磷。土壤中總磷含量為各形態(tài)磷的加和, 其中Resin-P、NaHCO3-Pi、NaHCO3-Po為有效磷(Available-P, AP), NaOH-Pi、NaOH-Po為中等活性磷, Dil.HCl-P、Conc.HCl-Pi、Conc.HCl-Po為中穩(wěn)態(tài)磷, Residual-P為穩(wěn)態(tài)磷。

表1 各采樣點物理化學性質(zhì)

1.3 吸附和解吸實驗

稱取各點土樣1.0 g于50 mL已知質(zhì)量的離心管中, 分別加入10 mL含磷量為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) ℃振蕩器上震蕩24 h后, 取出離心(5000 轉(zhuǎn)/min, 10 min), 過濾。取上清液用鉬銻鈧比色法測定濾液中磷的含量, 同時做空白實驗, 由吸附前后磷含量變化計算吸附量, 所有實驗處理均重復(fù)3次。

在吸附實驗完成后, 將含磷量為0.05 mg/L和1 mg/L的離心管中的上清液倒掉, 稱量離心管、土及殘留液的質(zhì)量(扣除土壤間隙殘留磷對磷解吸量計算的影響)。然后向離心管內(nèi)各加入10 mL的0.01 mol/L KCl溶液, 在(27±2) ℃振蕩器上震蕩24 h后, 取出離心 (5000 轉(zhuǎn)/min, 10 min)。用鉬銻鈧比色法測定濾液中磷的含量, 計算磷解吸量。

1.4 數(shù)據(jù)分析及處理

磷的吸附率為吸附濃度與初始濃度的比值, 解吸率(des)由吸附量與解吸量比值求得; 分配系數(shù)(d)為達到吸附平衡時, 磷在固液兩相中濃度的比值;d值高表明土壤顆粒對磷的吸附能力強[18–19]。實驗數(shù)據(jù)由Microsoft Excel 2010、OriginPro 7.5和Spss 13.0軟件進行分析處理。

2 結(jié)果與討論

2.1 土壤各形態(tài)磷含量

各點土樣中總磷(TP)含量為558.5~702.3 mg/kg (表2), 其中S5點TP含量最高。各樣點有機磷(OP) 含量較低, 僅為18.6~33.4 mg/kg。無機磷(IP)是各點磷的主要存在形態(tài), 占TP的93%~98% (表2)。以往研究發(fā)現(xiàn)三江平原濕地[20]及向海濕地[21]磷主要以有機磷為主, 杭州灣濕地[22]磷含量主要以無機磷為主, 這與濕地土壤母質(zhì)、成土作用和耕作施肥密切相關(guān)[23–25]。

由表2可知, 各樣點AP含量在18.4~25 mg/kg之間, 僅占TP的3.2%~5.9%。按照全國第2次土壤普查分級標準, 各樣點中可被植物利用的磷約為3級, 含量較低。這可能由于黃河三角洲土壤主要來源于黃河上游土壤, 連續(xù)的沖刷作用造成了有效磷的大量流失。中等活性磷含量和穩(wěn)態(tài)磷含量也較低, 分別為6.8~18.1 mg/kg和28.6~58.4 mg/kg。中穩(wěn)態(tài)磷含量較高為454.4~498.5 mg/kg, 其中Dil.HCl-P含量最高, 平均占TP的61.5%~83.6%, 這說明各樣點磷主要以中穩(wěn)態(tài)形式存在。另外我們發(fā)現(xiàn), 有效磷含量在覆有植被的S2、S3點比較高, 其中S3點有效磷含量比海邊S5點高82%, 因此, 我們推斷植被(如鹽地堿蓬和檉柳)的存在有利于植物有效磷的形成。這與前人的研究結(jié)果一致, Tuchman.[26]研究發(fā)現(xiàn), 香蒲、青岡的入侵使?jié)竦赝寥烙行Я椎暮吭黾?。Chen.[27]也發(fā)現(xiàn)了輻射松的存在有利于土壤有機磷的礦化。但同樣覆有植被的S4樣點有效磷含量卻較低, 這可能由于該樣點植被量較少且離海較近, 漲落潮淋洗會損失大量有效磷。

2.2 磷的吸附及解吸

從表3可知, 各樣點對磷的吸附量隨著初始加入磷濃度的升高而增加。各樣點對磷的吸附較強, 特別是在0.05~0.5 mg/L區(qū)域內(nèi), 吸附率達到99%。雖然隨著初始濃度的增加, 各樣點對磷的吸附率逐漸降低, 但仍在70%以上。因此當攜帶大量磷的污水進入濕地系統(tǒng)時, 濕地土壤可以通過吸附作用除去大量的磷, 這與Sakadevan.[1]、黃樹輝[28]等的研究結(jié)果一致。通過計算可知, 各采樣點d均值的順序為: S3 (837) > S2 (666) > S5 (551) > S4 (486) > S1 (389)。這說明S3樣點土壤對磷的吸附能力最強, S1點最弱。

表2 不同采樣點各形態(tài)磷的含量 (mg/kg)

表3 各樣點在不同濃度下吸附磷量(mg/kg)

從表4可知, 在吸附-解吸動態(tài)平衡過程中, 解吸率隨著磷加入量的增加而增加, 但土壤對磷的吸附作用強于解吸作用。如處理濃度為1 mg/L時, 各樣點中超過90%的磷留在土壤固相中, 這可能由于實驗設(shè)定濃度范圍下, 土壤對磷的吸附大多以共價鍵的化學性吸附為主[29–31], 很難被解吸下來, 這也減少了吸附在土壤中的磷發(fā)生遷移的風險。

表4 各樣點在不同濃度下磷解吸率Pdes (%)

2.3 相關(guān)性分析

土壤理化性質(zhì)與土壤磷形態(tài)及吸附解吸參數(shù)的相關(guān)關(guān)系見表5。通過相關(guān)分析發(fā)現(xiàn), 各樣點OP含量與土壤中TOC含量有關(guān) (< 0.01)。這可能由于黃河三角洲成土時間短, 土壤含鹽量高造成植被生物量少, 因此土壤中有機磷含量較低。Dil.HCl-P受Ca/Al含量影響較大, 因為Dil.HCl-P主要提取磷灰石型磷及部分閉蓄態(tài)磷[5]。AP含量與各樣點有機質(zhì)、黏粒含量顯著正相關(guān) (< 0.01), 這與前人研究一致[32–33]。與其他樣點相比, 覆有植被的S2、S3點Dil.HCl-P較低, AP較高, 這說明植被的存在可能有利于穩(wěn)態(tài)磷向生物可利用磷轉(zhuǎn)化[34]。各樣點對磷的吸附解吸能力與土壤黏粒含量密切相關(guān) (< 0.01)。S3點黏粒含量較高, 因此吸附量遠大于S1點。這主要由于黏粒表面有較大的表面積[35], 能夠迅速吸附磷素于土壤顆粒外表面的吸附點位上。

3 結(jié) 論

各樣點土壤中總磷含量為558.5~702.3 mg/kg, 有效磷含量僅占TP的3.2%~5.9%。各樣點中無機磷占總磷的93%~97%, 有機磷含量較低, 這與土壤植被生物量少有機質(zhì)含量低有關(guān)。無機磷中Dil.HCl-P含量最高, 平均占總磷的75.8%, 是各樣點磷的主要存在形態(tài), 與各樣點Ca/Al含量密切相關(guān)。在吸附解吸實驗中設(shè)定的濃度范圍內(nèi), 土壤對磷的吸附能力較高, 吸附率在70%~99%之間。吸附磷的解吸量較低, 在處理濃度為1 mg/L時, 解吸率僅為2.5%~6.1%, 吸附解吸特性與土壤物理化學性質(zhì)密切相關(guān)。因此我們推斷, 實驗選取的濕地各樣點土壤中較低的有效磷含量可能成為濕地植物生長的限制性因子, 植被的存在有利于有效磷的積累, 采樣點土壤對外源磷的輸入有一定的吸附能力且釋放風險較低。

注: *和**分別代表顯著性水平為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

國家自然科學基金項目(41001137, 41171216); 中國科學院煙臺海岸帶研究所“一三五”發(fā)展規(guī)劃項目(Y254021031); 中國科學院創(chuàng)新團隊國際合作伙伴計劃(KZCX2-YW-T14)

孫軍娜(1984–), 女, 博士研究生, 環(huán)境科學專業(yè)。E-mail: jnsun@yic.ac.cn

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