卞永榮,朱 波,程 虎,谷成剛,宋 洋,楊興倫,王 芳,葉 茂,蔣 新
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有機肥對水稻中汞/甲基汞累積的影響①
卞永榮1,2,3,朱 波1*,程 虎2,3,谷成剛3,宋 洋3,楊興倫3,王 芳3,葉 茂3,蔣 新3
(1 中國科學院水利部成都山地災(zāi)害與環(huán)境研究所,中國科學院山地表生過程與生態(tài)調(diào)控重點實驗室,成都 610041;2 中國科學院大學,北京 100049;3中國科學院土壤環(huán)境與污染修復(fù)重點實驗室(南京土壤研究所),南京 210008)
稻米對甲基汞的累積危害居民身體健康。本文通過施用厭氧腐熟有機肥的盆栽試驗,探討有機肥施用對水稻中汞-甲基汞累積的影響。結(jié)果表明,有機肥施用水稻土中甲基汞含量顯著增加,對照(10.43 μg/kg)<1% 豆餅粉肥(16.80 μg/kg)<1% 魚粉肥(24.10 μg/kg)<2% 豆餅粉肥(33.53 μg/kg)<2% 魚粉肥(38.46 μg/kg),可能是施加有機肥后,增加了土壤微生物數(shù)量,提高了酶活性,導致水稻土中甲基汞含量增加。有機肥施用后,水稻不同部位總汞累積差異顯著,根部最高(2 812.83 μg/kg),其次是糙米(336.78 μg/kg)和莖葉(300.44 μg/kg)。有機肥施用后,水稻不同部位累積甲基汞的能力不同,表現(xiàn)為糙米(180.06 μg/kg)>根(59.71 μg/kg)>莖葉(38.97 μg/kg)。不同有機肥的施用均增加籽粒中甲基汞的含量,與對照相比,各處理增加量表現(xiàn)為1% 豆餅粉肥(16.1%)< 1% 魚粉肥(19.3%)<2% 豆餅粉肥(41.5%)<2% 魚粉肥(57.9%)。同時稻米中甲基汞含量與水稻土中甲基汞含量呈正相關(guān)關(guān)系。有機肥施用增加汞污染土壤水稻中汞與甲基汞累積,其可為合理施肥提供科學依據(jù)和理論指導。
汞;甲基汞;水稻;有機肥;水稻土
汞在環(huán)境中分布廣泛,具有生物富集和生物放大等特性,呈現(xiàn)不同的賦存形態(tài),其中甲基汞具有神經(jīng)毒性。由于水生食物鏈中甲基汞易于生物放大,尤其是食物鏈頂端魚類[1-2],因此通常情況下水產(chǎn)品,如魚和貝殼等被看作是甲基汞重要的污染暴露途徑。而在汞污染土壤環(huán)境中,農(nóng)作物如稻米會累積甲基汞而嚴重影響農(nóng)產(chǎn)品質(zhì)量安全,可能對諸如我國貴州礦區(qū)以大米為主食的居民身體健康造成危害,如誘發(fā)胎兒神經(jīng)發(fā)育缺陷[3-6]。調(diào)查研究顯示,汞礦區(qū)農(nóng)產(chǎn)品中總汞含量達到260 μg/kg,可食部分超過國家標準20 μg/kg;由于水稻生長在淹水和厭氧環(huán)境中,稻米對水稻土中甲基汞的生物累積含量也比較高,平均含量為4.2 ~ 18 μg/kg,如清鎮(zhèn)稻米的甲基汞含量甚至達到41.4 μg/kg,嚴重威脅人體健康[7-9]。雖然甲基汞在稻米中含量通常比魚類低10倍[3, 4, 9-10],但稻米是亞洲和其他一些地區(qū)的主要糧食作物(2012年,全球水稻種植總量為1.63億hm2),全球大米產(chǎn)量為7.29億t,其中90% 在亞洲生產(chǎn)(FAO,2013),基于汞污染水稻土種植的水稻,其甲基汞攝入量可能與魚類相當。
水稻土長期淹水缺氧環(huán)境為微生物汞甲基化提供條件,其中汞甲基化受微生物(如硫還原菌和產(chǎn)甲烷菌等)、有機質(zhì)和硫等影響,并且半胱氨酸影響甲基汞向稻米轉(zhuǎn)運[11]。有機肥能夠改良土壤結(jié)構(gòu),肥效長,并且有機質(zhì)礦化分解過程中形成許多可溶性巰基化合物,進而與汞發(fā)生絡(luò)合,影響無機汞生物有效性。有研究報道,汞的絡(luò)合作用能夠降低無機汞的生物有效性[12],從而減少微生物汞甲基化過程[13-15];但也有相反報道,當汞與可溶性小分子量巰基化合物(如半胱氨酸)絡(luò)合時,顯著提高無機汞的吸收和汞甲基化能力[16-17]。
近年來,由于稻米中大量甲基汞累積,受汞污染的農(nóng)田土壤成為汞研究的一個新熱點[18]。然而,有機肥施用影響水稻土中汞的形態(tài)轉(zhuǎn)化和生物有效性,特別是厭氧腐熟有機肥含有大量還原性硫?qū)ν寥乐泄谆绊懙难芯旷r有報道。以及前期研究表明,可溶性有機質(zhì)在汞生物地球化學循環(huán)過程中扮演著重要角色[19]。本研究主要是探討節(jié)水灌溉自然落干、選用厭氧腐熟豆餅粉肥與魚粉肥對土壤中汞形態(tài)轉(zhuǎn)化以及對水稻不同部位總汞和甲基汞累積的影響,為合理施肥提供科學依據(jù)和理論指導。
有機肥選用活性硫含量較高的豆餅粉和魚粉,其中豆餅粉質(zhì)量組成:蛋白質(zhì)45%,脂0.8%,碳水化合物33.2%,纖維素6.5%,碳、氮和硫分別是39.0%、4.86% 和0.329%;魚粉質(zhì)量組成:蛋白質(zhì)60%,脂肪10%,灰分約20%,碳、氮和硫分別是43.2%、11.6% 和1.11%。
采集湖南桂陽水稻田紅壤表土,風干過2 mm篩,土壤基本理化性質(zhì)為pH 6.2(土水比1︰2.5),礦物含量67.4%(>2 μm)和32.6%(<2 μm),陽離子交換量(CEC)14.9 cmol/kg,有機質(zhì)38g/kg,硫0.8 g/kg,全氮1.42 g/kg,總磷0.42 g/kg[20],總汞0.36 mg/kg。添加氯化汞制備模擬污染土,其中汞含量達10 mg/kg。
盆栽試驗過程中,豆餅肥和魚粉肥施入前須經(jīng)腐熟處理。豆餅肥/魚粉肥與水混合(1︰2,/),在18 ~ 33 ℃厭氧發(fā)酵30 d完成。隨著發(fā)酵時間延長豆餅粉逐漸腐化并伴有臭味,而魚粉發(fā)酵液亦變得粘稠,呈黑色并有臭味。
每盆裝入5 kg模擬污染土(8 L水桶),加腐熟豆餅肥、魚粉肥并淹水一周后移栽水稻秧苗。水稻秧苗生長過程中,除了干濕交替處理外,其他長期淹水。CK:污染土(不加有機肥),保持長期淹水;AWD:污染土(不加有機肥,干濕交替-自然落干);BOM1:污染土壤+腐熟豆餅粉(1%,/),保持長期淹水;BOM2:污染土壤+腐熟豆餅粉(2%,/),保持長期淹水;FOM1:污染土壤+腐熟魚粉(1%,/),保持長期淹水;FOM2:污染土壤+腐熟魚粉(2%,/),保持長期淹水。
水稻品種:鎮(zhèn)稻#1,先將水稻浸泡發(fā)芽,幼苗生長約一個月,約10 cm長,然后將秧苗移栽到提前一周配好基肥的土壤。每個處理3個重復(fù),維持土壤表面1 cm長期淹水直到抽穗,干濕交替處理是在秧苗移栽10 d后,自然落干(約每周補充500 ml水)。水稻生長期為136 d,收集籽粒、莖葉和水稻根,同時采集土壤樣品裝入聚乙烯自封袋中,帶回實驗室冷凍干燥,測定土壤和水稻不同部位總汞和甲基汞含量。
硝酸/硫酸(4︰1,/)水浴消化水稻籽粒、莖葉和根樣品,BrCl氧化,冷原子熒光光譜測定水稻不同部位總汞含量;萃取土壤及水稻根、莖葉和籽粒中甲基汞,其含量測定參照美國EPA1630,蒸餾-乙基化GC-CVAFS測定(Model Ⅲ,Brook rand,USA)[21]。
質(zhì)量控制:汞與甲基汞質(zhì)量控制使用空白,方法添加和基質(zhì)添加即標準加入法和盲樣重復(fù),對于總汞最低檢測限10 ng/kg,甲基汞最低檢測限3 ng/kg。添加總汞、甲基汞樣品回收率分別為84% ~ 110%、80% ~ 116%。
為總體比較數(shù)據(jù)差異性,使用統(tǒng)計分析軟件IBM SPSS 22進行單因素方差分析(ANOVA,α=0.05),組間差異選用LSD多重比較檢驗。
如表1所示,干濕交替水分管理與有機肥施用均能促進水稻的生長過程,提升水稻產(chǎn)量。相比單一的水分管理措施,有機肥施用能夠提高土壤有機質(zhì)比例,改善土壤環(huán)境質(zhì)量,從而更好地提供水稻營養(yǎng)基質(zhì),提升水稻產(chǎn)量。
表1 干濕交替與有機肥施用對水稻產(chǎn)量的影響
圖1給出了水稻土中甲基汞隨干濕交替和有機肥施用的變化情況。比較發(fā)現(xiàn),干濕交替處理不能顯著改變水稻土中的甲基汞含量(>0.05),相反,單因素方差分析表明施肥處理能夠顯著增加水稻土中甲基汞含量,不同施肥處理造成水稻土中甲基汞的含量變化順序為CK(10.43 μg/kg) (圖中小寫字母不同表示處理間差異達到P<0.05顯著水平,下圖同) 圖2顯示水稻根、莖葉和糙米對總汞累積差異較大,具體表現(xiàn)為根部最高,平均2 812.83 μg/kg,其次是糙米平均336.78 μg/kg和莖葉300.44 μg/kg。水稻根富集總汞能力也受干濕交替和有機肥施用的影響,其根部富集總汞含量的變化順序為AWD 水稻莖和根系具有發(fā)達的通氣組織,將氧氣輸送至根部,同時其生長過程長期處于淹水厭氧環(huán)境,水稻具有根系泌氧、形成鐵膜的能力。由于水稻根表鐵膜主要由結(jié)晶鐵和無定形鐵氧化物或氫氧化物構(gòu)成[26],鐵膜具有吸附和固定重金屬作用,同時水稻根中半胱氨酸植物螯合素與Hg2+形成絡(luò)合物,均會抑制無機Hg(II)從植物的根部到地上部分的轉(zhuǎn)運[27],影響汞在根部富集。可以推斷,水稻中不同部位總汞含量變化與水稻根中植物絡(luò)合素和根表鐵膜緊密相關(guān),水稻對總汞富集主要集中在根部。 圖2 總汞在水稻不同部位的累積與分配 有機肥施用總體上能夠降低水稻籽粒中總汞含量水平。Zhong等人[28]的研究也有類似發(fā)現(xiàn),即半胱氨酸降低莖葉中總汞含量。這可能是由于有機肥中含有大量具有絡(luò)合功能團的有機化合物能夠與汞絡(luò)合,從而阻礙無機汞向莖葉及籽粒遷移。 如圖3所示,水稻不同部位累積甲基汞平均含量是糙米(180.06 μg/kg)>根(59.71 μg/kg)>莖葉(38.97 μg/kg),糙米富集甲基汞的能力最強。與對照相比,干濕交替可能增加土壤中氧氣含量,使得土壤中汞甲基化效率減小,降低了水稻根、莖葉和籽粒中甲基汞含量(30%、40% 和26%)。相反,有機堆肥施用增加甲基汞在根、莖葉和籽粒中甲基汞積累。不同的有機肥類型比較可看出,施用相同百分比豆餅粉堆肥后的水稻根部甲基汞積累大于施用魚粉堆肥,與對照相比,提高甲基汞累積的順序為FOM1(3.6%)< BOM1 (10.4%)< FOM2(20.7%)< BOM2(50.4%);而施用相同百分比豆餅粉堆肥后籽粒中甲基汞積累小于施用魚粉堆肥,即BOM1(16.1%)< FOM1(19.3%)< BOM2 (41.5%) 圖3 甲基汞在水稻不同部位的累積與分配 圖4直觀給出了不同處理下水稻土中的汞甲基化程度。如圖所示,甲基汞與土壤總汞比例AWD(1.29‰) 圖4 土壤中甲基汞與總汞千分比 圖 5 水稻糙米中甲基汞含量與土壤中甲基汞含量相關(guān)性分析 圖6顯示,無論是干濕交替處理還是調(diào)控有機肥類型,稻米對甲基汞生物富集系數(shù)在7 ~ 15之間。雖然添加有機肥增加了稻米中甲基汞富集量,但與對照相比等量添加有機肥卻降低了富集系數(shù),主要與為稻米對甲基汞具有最大富集量有關(guān);另外,有可能緣于甲基汞含量增加,降低了CNS芳基硫酸酯酶活性[31],土壤硫是作物產(chǎn)量和質(zhì)量的重要元素,而CNS芳基硫酸酯酶的活性與植物有效硫量顯著正相關(guān)[32]。 圖6 水稻糙米中甲基汞的生物富集系數(shù) 1)由于施加有機肥會改善土壤微生物和酶活性,水稻土中甲基汞含量隨之顯著增加:對照(10.43 μg/kg) <1% 豆餅粉肥(16.80 μg/kg)<1% 魚粉肥(24.10 μg/kg) <2% 豆餅粉肥(33.53 μg/kg)<2% 魚粉肥(38.46 μg/kg)。魚粉肥具有更高硫含量,而硫還原菌是水稻土中汞甲基化關(guān)鍵微生物,因此魚粉肥水稻土中甲基汞含量顯著高于豆餅肥水稻土。此外,有機肥施用增加水稻土中汞甲基化比例。 2)有機肥施用后,水稻不同部位總汞累積差異顯著,根部最高(2 812.83 μg/kg),其次是糙米(336.78 μg/kg)和莖葉(300.44 μg/kg)。 3)有機肥施用后,水稻不同部位累積甲基汞的能力表現(xiàn)為糙米(180.06 μg/kg)>根(59.71 μg/kg)>莖葉(38.97 μg/kg)。不同有機肥施用均增加籽粒中甲基汞含量,魚粉肥增加幅度要大于豆餅粉肥。稻米中甲基汞含量與水稻土中甲基汞含量呈正相關(guān),說明糙米中甲基汞主要來自于土壤中甲基汞向上傳輸。 [1] Morel F M M, Kraepiel A M L, Amyot M, et al. 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Pharmacology & Toxicology,1991, 69(1): 71–74 [32] 張玉蘭, 陳利軍. 土壤芳基硫酸酯酶及其活性和農(nóng)業(yè)措施影響[J]. 土壤通報, 2006, 37(4): 792–798 Effects of Organic Fertilizer on Accumulation of Mercury/Methylmercury in Rice BIAN Yongrong1,2,3, ZHU Bo1*, CHENG Hu2,3, GU Chenggang3, SONG Yang3, YANG Xinglun3, WANG Fang3, YE Mao3, JIANG Xin3 (1 Key Laboratory of Mountain Surface Processes and Ecological Regulation, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610041, China; 2 University of Chinese Academy of Sciences, Beijing 100049, China; 3 Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China) Accumulation of methylmercury in rice is harmful to the health of consumers. A pot experiment was conducted to study the effects of organic fertilizer application on the accumulation of mercury and methylmercury in rice in mercury- methylmercury contaminated soil. The treatments included CK, no organic fertilizer + long term flooding; AWD, no organic fertilizer + dry/wet alternation; BOM1, rotten soybean meal (1%,/) + long term flooding; BOM2, rotten soybean meal (2%,/) + long term flooding; FOM1, rotten fish meal (1%,/) + long term flooding; FOM2, rotten fish meal (2%,/) + long term flooding. The results showed that organic fertilizer significantly increased CH3Hg+content in paddy soil: CK (10.43 μg/kg) < BOM1 (16.80 μg/kg) < FOM1 (24.10 μg/kg) < BOM2 (33.53 μg/kg) < FOM2 (38.46 μg/kg). After the application of organic fertilizers, total Hg accumulation in various rice organs were significantly different, the highest in root (2 812.83 μg/kg), followed by brown rice (336.78 μg/kg) and stem and leaf (300.44 μg/kg), the abilities to accumulate CH3Hg+by different rice organs were different: grain (180.06 μg/kg) > root (59.71 μg/kg) > stem and leaf (38.97 μg/kg). Organic fertilizers increased the content of CH3Hg+in rice grains compared with the CK treatment: BOM1 (16.1%) < FOM1 (19.3%) < BOM2 (41.5%) < FOM2(57.9%). Significant positive correlation was found between CH3Hg+concentration in rice grains and CH3Hg+content in paddy soil. This study proved organic manure can increase the accumulation of mercury and methylmercury in rice in mercury contaminated soil, and it provided scientific basis and theoretical guidance for rational fertilization. Mercury; Methylmercury; Rice; Organic fertilizer; Paddy soil 國家自然科學基金項目(41271464)、江蘇省自然科學基金項目(BK20131463)、中國科學院戰(zhàn)略性先導科技專項(XDA05050506)和中國科學院前沿科學重點研究計劃項目(QYZDJ-SSW-DQC035)資助。 通訊作者(bzhu@imde.ac.cn) 卞永榮(1974—),男,江蘇姜堰人,助理研究員,博士研究生,主要研究方向為環(huán)境化學與污染控制。E-mail: yrbian@issas.ac.cn X131.3 A 10.13758/j.cnki.tr.2019.01.0152.3 有機肥對水稻不同部位總汞累積與分配的影響
2.4 有機肥對水稻不同部位甲基汞累積與分配的影響
2.5 有機肥對水稻土中甲基汞與總汞千分比的影響
2.6 有機肥對糙米吸收甲基汞的生物富集系數(shù)的影響
3 結(jié)論