鹽漬化環境下秸稈還田對稻米汞及甲基汞累積的影響

鄭順安,韓允壘,袁宇志,倪潤祥,吳澤嬴,黃宏坤,師榮光

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鹽漬化環境下秸稈還田對稻米汞及甲基汞累積的影響

鄭順安1,2,3,韓允壘4,袁宇志5,倪潤祥1,2,吳澤嬴1,2,黃宏坤1,2,師榮光3*

(1.農業農村部農業生態與資源保護總站,北京 100125；2.農業農村部資源循環利用技術與模式重點實驗室,北京 100125；3.農業農村部環境保護科研監測所,天津 300191；4.中國農學會,北京 100125；5.廣東省生態環境技術研究所,廣東 廣州 510650)

通過盆栽試驗探討了鹽漬化環境下秸稈還田對土壤中總汞(THg)、甲基汞(MeHg)含量和轉化以及對水稻吸收汞的影響.試驗用土采集自天津典型污灌地區,人為添加不同梯度的鹽分(0、0.2%、0.5% NaCl)及外源汞(0、5mg/kg~Hg(NO3)2),秸稈還田按照0.1%進行處理.結果顯示:1.秸稈還田促進稻田系統中無機汞的甲基化.與未添加秸稈的處理相比,秸稈還田后土壤MeHg含量提高了56.8%~76.8%,水稻MeHg含量增加了127%~171.6%.2.在輕度鹽漬化稻田開展秸稈還田,會進一步提高稻田土壤中汞的甲基化水平,進而增加水稻籽粒中甲基汞含量.與不添加鹽分處理相比,輕度鹽漬化環境中(0.2% NaCl),秸稈還田處理導致土壤MeHg含量提高了92.2%~101.2%,水稻籽粒MeHg含量增長了52.8%~132.1%.更高的鹽漬化水平會抑制土壤汞甲基化趨勢,水稻籽粒中甲基汞含量降低.在中度鹽漬化環境中(0.5% NaCl),秸稈還田導致土壤MeHg含量降低了57.9%~88.6%,水稻籽粒MeHg含量降低了72.9%~86.8%.以上研究結果表明,在鹽漬化且汞污染稻田開展秸稈還田可能大幅度增加該地區汞食物鏈暴露風險,因此在中輕度鹽漬化的污灌區,對秸稈還田等農藝措施需要格外慎重.

鹽分；汞；甲基汞；水稻；秸稈還田

天津污灌區是我國北方最大的污灌區[1],也是汞(Hg)污染重災區.王祖偉等[2]對天津污灌區水田和菜田調查顯示,灌區內土壤-作物系統中汞的污染等級達到了重度污染,部分農作物中汞的質量分數遠超國家食品衛生標準.王婷等[3]調查顯示,天津污灌區內采樣的22個蔬菜樣品100%都受到Hg污染,且都處于重污染的情況.濕地土壤中,隨著濕度增加,充滿水的土壤微孔增加,有利于形成硫酸鹽還原菌和鐵還原菌等甲基化微生物需要的還原環境.稻田生態系統是濕地的一種類型,水稻在生長期內因季節性灌溉,使其也成為一種特殊的濕地生態系統,為硫酸鹽還原菌和鐵還原細菌等提供理想的生存條件,因此稻田也存在較強的汞甲基化能力,而成為陸地生態系統的甲基汞“源”[4-5],這在之前的研究中已有報道[6],甚至有研究者認為,食用大米而非水產品已經成為汞污染嚴重地區人體甲基汞暴露的主要途徑[5, 7-8].2013~2014年,本課題組對天津污灌區內的稻田汞污染狀況進行了調查,在29個稻田中,土壤甲基汞(MeHg)平均含量為(0.87±0.77)μg/kg,占總汞(THg)的比例為0.12%~0.38%,個別汞污染較嚴重地塊存在甲基汞暴露風險[9].武超等[10]也對天津污灌區內水稻土壤及農產品汞污染狀況進行了調查,結果顯示,北塘、大沽和北京污灌區稻田土壤THg和MeHg濃度顯著高于對照區海河土壤THg濃度,水稻籽粒MeHg富集系數為1.63-3.70,污灌區食用稻米MeHg暴露對居民健康存在較大風險,人體MeHg每天攝入量超標率達到20.83%.

秸稈還田是秸稈綜合利用的重要途徑之一.當前,我國每年還田的作物秸稈超過1.5億t,且在快速增加.對農業生產而言,秸稈還田具有增加土壤有機質含量、為植物生長提供營養、使作物增產等多種良性效果,但秸稈還田對土壤中重金屬等污染物,特別是汞的環境化學行為的影響報道相對較少.南京大學的鐘寰課題組研究表明,秸稈還田總體會提高土壤中汞的甲基化水平[11-13],在稻麥輪作體系中,秸稈還田后會顯著提高產地土壤中甲基汞含量,同時水稻和小麥各部位甲基汞含量也會明顯升高[14].天津污灌區承受汞污染與鹽漬化的雙重壓力,在這種特殊生境條件下,秸稈還田后是否會提高稻田土壤-水稻籽粒中總汞和甲基汞含量,增加汞食物鏈暴露風險,是人們迫切希望搞清楚的問題.綜上,本研究采集天津典型污灌地區土壤,通過盆栽試驗,添加不同梯度的鹽分及外源汞,探討秸稈還田對鹽漬化汞污染稻田土壤-作物系統Hg及MeHg含量的影響,為北方鹽漬化且汞污染風險較高地區合理開展秸稈還田提供支撐.

1 材料與方法

1.1 樣品準備

土壤:盆栽試驗所用土壤樣品采集自天津市郊東北方向李明莊的菜地(潮土,0~20cm).該菜地距離天津三大排污河之一的北(塘)排污河約400m,污灌歷史約30年,屬于間歇性清污混灌,污灌口位于菜地的西南角.土壤樣品經自然風干后過2mm尼龍篩冷凍儲存,基本理化性質分析參照中國土壤學會的分析方法[15],結果如下:pH值為8.03,CaCO3含量為1.03g/kg,有機質含量為12.43g/kg,CEC為16.27cmol/ kg,游離鐵含量為8.71g/kg,無定形鐵含量為0.88g/kg,黏粒(<0.002mm)含量為191.05g/kg.受長期污灌的影響,供試土壤的Hg含量(0.601mg/kg)顯著高于區域土壤Hg背景含量(0.073mg/kg),但未超過土壤環境質量二級標準(pH>7.5為1.0mg/kg,GB 15618- 1995).鹽分總量為0.875g/kg(低于0.1%),按照鹽土重量比劃分標準[15],不屬于鹽漬化土壤.

供試秸稈:水稻秸稈采自未受汞污染的天津津南地區.秸稈洗凈,30℃烘干至恒重,磨碎過2mm篩備用.水稻秸稈總汞含量為0.129mg/kg,甲基汞含量為0.017μg/kg.

水稻栽培品種:津原89(具有較強耐鹽漬化能力,經預實驗,該品種在實驗設置的鹽度梯度下可以生長).

1.2 實驗設計

表1 實驗處理設計方案

根據實驗目標,設置8個處理,每個處理設置6次重復,具體設置見表1.

添加時,將10kg原土按比例添加鹽分和外源汞,混勻采用逐級混勻的方法,先將鹽分和重金屬溶液與少量土壤混勻,再將少量土壤與大量土壤混勻,直到所有土壤.混勻后放置老化180d后(預備試驗證明180d后污染土壤內重金屬老化趨于穩定)自然風干,過2mm篩后保存.制備結束后,采用IonPac AS11- HC分析柱及30mmol/L氫氧化鉀溶液分離Cl-,測定鹽處理后土壤樣品中Cl-含量(Dionex ICS-3000型離子色譜儀),同時測定土壤總汞含量(方法見1.4),結果見表2.總體來看,外源加入的鹽分和總汞回收率在90.0%~110.5%之間.

表2 不同處理中土壤Cl-與總Hg含量

1.3 盆栽試驗

供試土壤中按照總氮100mg/kg、總磷100mg/kg和總鉀150mg/kg施加底肥后混勻,根據表1實驗方案開展水稻種植(16年6月到11月).具體方法為:每盆3.5kg土壤添加35.0g水稻秸稈(即土壤和秸稈質量比為100:1,秸稈長度為0.2cm,Hg含量低于0.1 μg/kg),加入去離子水后淹水靜置14d插播水稻秧(在無汞污染土壤上培育秧苗).盆栽試驗露天進行,采用有機玻璃板遮擋防止雨水.作物生長期間,淹水層保留3cm,進行常規蟲害和肥料管理.

水稻盆栽周期為(120±7)d.盆栽試驗結束后采集根區土壤,測定總汞、甲基汞含量.水稻收獲后,采集植株地上部分,去離子水洗凈后,用0.8mmol/L半胱氨酸溶液浸泡20.0min,去除植株表面吸附的MeHg,測定水稻籽粒(脫殼后糙米)中總汞與甲基汞含量.土壤和水稻籽粒中總汞和甲基汞含量測定參考鄭順安等[9]的測定方法.

1.4 統計制圖

土壤和水稻籽粒中總汞和甲基汞含量變化采用單因素方差分析(One-way ANOVA,LSD法)進行統計學檢驗.制圖采用Origin 8.6SR2軟件.

2 結果與討論

2.1 不同處理下根際土壤總汞與甲基汞含量

圖1 盆栽后各處理土壤總汞(THg)與土壤甲基汞(MeHg)含量

CK:對照,T1:添加0.1%水稻秸稈,T2:添加0.2% NaCl和0.1%水稻秸稈,T3:添加0.5% NaCl和0.1%水稻秸稈,T4:添加5mg/kg Hg,T5:添加5mg/kg Hg和0.1%水稻秸稈,T6:添加5mg/kg Hg、0.2% NaCl和0.1%水稻秸稈,T7:添加添加5mg/kg Hg、0.5% NaCl和0.1%水稻秸稈.不同小寫字母表示處理間差異達到顯著性水平(<0.05)

圖1為水稻收獲后不同處理根際土壤總汞與甲基汞含量.對于土壤總汞,未添加外源汞的處理——對照(CK)、秸稈(T1)、低鹽秸稈(T2)和中鹽秸稈(T3),土壤THg含量平均為0.655mg/kg,處理間無顯著性差別(<0.05,下同);添加外源汞的處理——高汞(H)、高汞低鹽秸稈(HLS)和高汞中鹽秸稈(HHS),土壤THg均值為5.62mg/kg.對于土壤甲基汞,不同處理之間差異顯著,土壤MeHg含量依次為:T6(高汞低鹽秸稈,102.37μg/kg)>T5(高汞秸稈,53.27μg/kg)> T4(高汞,30.13μg/kg)>T2(低鹽秸稈,8.55μg/kg)> T7(高汞中鹽秸稈,6.09μg/kg)>T1(秸稈,4.25μg/kg)> CK(對照,2.71μg/kg)>T3(中鹽秸稈,1.79μg/kg).可以看出,高汞低鹽秸稈處理的土壤MeHg含量最高,是高汞秸稈處理的1.92倍、高汞處理的3.4倍、高汞中鹽處理的16.8倍、對照的37.8倍;低鹽秸稈處理下土壤MeHg含量是秸稈處理的2.01倍、對照的3.2倍;中鹽秸稈處理下土壤MeHg含量最低,僅為對照的66.1%.

2.2 不同處理下水稻籽粒中總汞與甲基汞含量

圖2為水稻收獲后不同處理水稻籽粒中總汞與甲基汞含量.對于THg,不同處理間存在顯著差異,含量依次為:T7(高汞中鹽秸稈,56.44μg/kg)> T6(高汞低鹽秸稈,42.15μg/kg)>T5(高汞秸稈, 29.79μg/kg)、T4(高汞,29.33μg/kg)>T3(中鹽秸稈, 16.59μg/kg)>T2(低鹽秸稈,12.77μg/kg)>CK(對照, 10.35μg/kg)、T1(秸稈,9.48μg/kg).對于MeHg,不同處理間同樣差異顯著,含量依次為:T6(高汞低鹽秸稈,20.54μg/kg)>T5(高汞秸稈,13.44μg/kg)>T2(低鹽秸稈,6.43μg/kg)、T4(高汞,5.92μg/kg)>T1(秸稈, 2.77μg/kg)>T7(高汞中鹽秸稈,1.78μg/kg)>CK(對照,1.02μg/kg)>T3(中鹽秸稈,0.71μg/kg).總體來看,高汞中鹽秸稈處理下水稻籽粒中THg含量最高,是對照的5.45倍;高汞低鹽處理下水稻籽粒中MeHg含量最高,是對照的20.13倍.

圖2 盆栽后各處理稻米籽粒中總汞(THg)與甲基汞(MeHg)含量

CK:對照,T1:添加0.1%水稻秸稈,T2:添加0.2% NaCl和0.1%水稻秸稈,T3:添加0.5% NaCl和0.1%水稻秸稈,T4:添加5mg/kg Hg,T5:添加5mg/kg Hg和0.1%水稻秸稈,T6:添加5mg/kg Hg、0.2% NaCl和0.1%水稻秸稈,T7:添加添加5mg/kg Hg、0.5% NaCl和0.1%水稻秸稈.不同小寫字母表示處理間差異達到顯著性水平(<0.05)

3 討論

對于未發生鹽漬化的土壤,與未添加秸稈的對照相比,秸稈施入后,土壤和水稻總汞含量未出現顯著性變化,這與還田的秸稈本身汞含量較低,且還田量不高有關.另一方面,秸稈還田后,土壤和水稻籽粒中甲基汞含量均顯著提高.低汞土壤中,加入秸稈的處理相比對照,土壤甲基汞含量提高了56.8%,水稻籽粒中甲基汞含量提高了171.6%;高汞土壤中,加入秸稈的處理與未加入秸稈的處理相比,土壤甲基汞含量提高了76.8%,水稻籽粒中甲基汞含量提高了127.0%.這種秸稈施入農田后土壤和作物甲基汞升高的趨勢,與之前報道的研究結果基本一致.如Zhu[16]等報道,水稻秸稈和根施入萬山汞礦區污染的稻田土壤后,土壤中甲基汞的濃度提高了2~8倍.陳宗婭等[14]研究顯示,小麥秸稈還田后土壤甲基汞含量增加127.1%,水稻秸稈還田后土壤甲基汞含量增加25.1%.Liu等[17]研究表明,秸稈還田大幅度提高重度汞污染土壤(含量高于5mg/kg)中汞的甲基化,對于汞含量相對較低的土壤(0.5mg/kg),效果弱于高汞土壤,原因可能是由于兩種土壤中微生物群落特征不同.不同的土壤微生物群落組成影響秸稈的降解、溶解性有機碳含量及其與汞結合形態,從而影響汞的微生物甲基化.這與本研究的結果也是相一致的,其他研究者[18-19]也有類似的結論.以往的報道[18,20-22]認為秸稈還田促進稻田生態系統汞甲基化,可能與秸稈還田后稻田土壤性質發生變化有關,如氧化還原電位的變化、土壤中有機質含量的升高、S和Fe的還原、無機汞活性的變化、甲基化微生物數量和活性的變化等.這些因素中,溶解性有機質比較關鍵,目前有4種解釋的機制,一是秸稈分解產生的活性有機碳直接作為速效碳源提高汞甲基化微生物的數量和活性,促進無機汞的甲基化[19],二是溶解性有機質與HgS絡合形成甲基化生物可利用態的復合物[23-24],三是溶解性有機質與土壤有機質競爭甲基汞的結合點位,促進甲基汞溶出[25],四是孔隙水中的溶解性有機質絡合汞,降低汞在土壤顆粒上的吸附量,附近土壤中固持的汞溶出,提高可甲基化的汞“原料”[26].

對于鹽漬化土壤,秸稈對土壤和作物中汞累積的影響更為復雜.含0.2% NaCl和0.1%秸稈的輕度鹽漬化土壤中,與不添加鹽分僅加入秸稈的處理相比,土壤總汞無顯著性變化,但土壤甲基汞和水稻籽粒中總汞、甲基汞含量有顯著增長,其中土壤MeHg含量提高了92.2%(低汞)~101.2%(高汞),水稻籽粒中THg含量增長了24.2%(低汞)~36.9%(高汞),水稻籽粒中MeHg含量增長了52.8%(高汞)~132.1%(低汞).含0.5% NaCl和0.1%秸稈的中度鹽漬化土壤中,與不添加鹽分僅加入秸稈的處理相比,土壤總汞仍無顯著性區別,但土壤甲基汞含量與水稻籽粒中甲基汞含量顯著降低,其中土壤MeHg含量降低了57.9% (低汞)~88.6%(高汞),水稻籽粒中MeHg含量降低了72.9%(低汞)~86.8%(高汞);水稻籽粒中總汞含量則有所上升,上升幅度為75%(低汞)~83.3%(高汞).綜合來看,在鹽漬化土壤中這些升高或降低的效應不論在低汞或高汞土壤中均存在,這些效應可能與鹽漬化土壤中Cl含量有關.Cl-對于Hg(II)而言是最易移動和常見的結合劑,與Hg(II)之間存在強烈的絡合作用,可形成HgCl20及HgCl3-等絡合離子,使帶負電荷的腐殖物質和粘土礦物膠體對汞的專性吸附作用顯著降低,進而使土壤對Hg(II)的固持量及吸附速率迅速下降[27].根據報道[28-29],體系Cl-含量增加后,易被植物吸收的土壤交換態(含水溶態)、富里酸結合態汞顯著上升,增加了水稻總汞吸收風險,這與本研究的結果是相一致的.但Cl-對土壤-水稻體系中汞甲基化的影響與其濃度相關,根據之前的報道[30], NaCl處理下,隨著鹽度的增長,外源Hg進入土壤后汞甲基化程度總體呈現先增長后降低的趨勢.低鹽度水平下(0.2%~0.6%),土壤汞甲基化程度提高,增加了土壤-水稻體系中甲基汞含量;高鹽度(1.0%~5.0%)則會抑制汞的甲基化,且高鹽度環境下生成的MeHg不穩定,容易發生去甲基化,降低土壤-水稻體系中甲基汞水平.這是當NaCl鹽度較低時,Cl-與Hg(II)之間存在強烈的絡合作用,可形成HgCl3-和HgCl42-等多種負性絡合離子,使得帶負電荷的腐殖物質和粘土礦物膠體對汞的專性吸附作用顯著降低,進而使土壤對Hg(II)的固持量及吸附速率迅速下降,Hg在土壤中的移動性及活性增強,增加了汞甲基化的“供應量”,甲基化率上升;但當鹽度進一步提高后,微生物可能通過溶液體系中滲透壓變化而影響其物質運輸過程,引起細胞質壁分離,造成細胞死亡或活性下降,從而降低汞甲基化程度[31-32].這些報道與本研究結果總體一致,但在敏感鹽度水平區間上有所不同.在加入0.1%秸稈的基礎上,本研究在0.2% NaCl鹽度水平下出現稻田汞甲基化程度提高,在0.5%鹽度水平下就出現了顯著降低,再結合之前與僅加入秸稈的處理之間土壤-水稻體系汞含量的差異,表明鹽度和秸稈之間存在一些交互性作用.綜合來看,在鹽漬化環境中秸稈還田后的汞環境化學更加復雜,涉及到多環境因子的組合及眾多生物化學反應,對其機理機制有待更深入的研究.

4 結論

4.1 秸稈還田會促進稻田土壤中無機汞的甲基化.在輕度鹽漬化稻田開展秸稈還田,會進一步提高稻田土壤中汞的甲基化水平,進而增加水稻籽粒中甲基汞含量.但更高的鹽漬化水平會抑制土壤汞甲基化趨勢,水稻籽粒中甲基汞含量降低.

4.2 在輕度鹽漬化稻田開展秸稈還田,會大幅度增加該地區汞食物鏈暴露風險,因此在污灌區等存在鹽漬化問題的地區,對秸稈還田等農藝措施需要格外慎重.

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Influence of rice straw amendment on mercury/methylmercury accumulation in rice grains in the saline soils.

ZHENG Shun-an1,2,3, HAN Yun-lei4, YUAN Yu-zhi5, NI Run-xiang1,2, WU Ze-ying1,2, HUANG Hong-kun1,2, SHI Rong-guang3*

(1.Rural Energy & Environment Agency, Ministry of Agriculture, Beijing 100125, China；2.Key Laboratory of Technologies and Models for Cyclic Utilization from Agricultural Resources, Ministry of Agriculture, Beijing 100125, China；3.Agro-Environmental Protection Institute, Ministry of Agriculture, Tianjin 300191, China；4.Chinese Association of Agricultural Science Society, Beijing 100125, China；5.Guangdong Institute of Eco-environment and Soil Science, Guangzhou 510650, China)., 2019,39(1)：243~248

A pot experiment was conducted to simulate a mercury-contaminated paddy fields irrigated with wastewater. Soil sample was collected from a typical wastewater-irrigated area of Tianjin, and exogenous salinity (0%, 0.2%, 0.5% NaCl), mercury (0, 5mg/kg Hg(NO3)2) and rice straw (0, 0.1%) were added into soil manually to discuss the influence of rice straw amendment on total mercury (THg)/methylmercury (MeHg) accumulation in rice grain. Results showed that, (1) Methylation of inorganic mercury in paddy soil-rice system was promoted by rice straw amendment. After rice straw amendment, MeHg concentration in soil and rice grain increased by 56.8%~76.8% and 127%~171.6% respectively. (2) Stimulation of methylation of inorganic mercury in mild salinized paddy soil by rice straw amendment was even more serious, and which lead to an additional increment of MeHg in rice grain. Compared with un-salinized soil, MeHg in soil and rice grain increased by 92.2%~101.2% and 52.8%~132.1% respectively after rice straw amendment. However, in a more serious salinized soil (0.5% NaCl), methylation of inorganic mercury in soil was suppressed and MeHg accumulation in rice grain decreased. MeHg concentration in soil and rice grain decreased by 57.9%~88.6% and 72.9%~86.8% respectively in moderate salinized soil (0.5% NaCl) after treated with rice straw amendment, compared with in un-salinized soil. These results indicated that the treatment of rice straw amendment in a medium or low level salinized soil with Hg contamination may lead to adverse effect to human health. Therefore, treatment of rice straw amendment in this kind of area should be cautious.

salinity；mercury；methylmercury；rice；rice straw amendment

X53

A

1000-6923(2019)01-0243-06

鄭順安(1981-),男,安徽合肥人,副研究員,博士,主要從事產業環境監測與土壤污染防治工作研究.發表論文40余篇.

2018-06-08

國家重點研發計劃資助項目(2016YFD0201306,2016YFD0201200, 2017YFD0801401,2017YFD0801205);國家自然科學資助項目(41203084, 41371463)

* 責任作者, 研究員, shirongguang_aepi@126.com