農(nóng)田施用沼液的重金屬污染評價及承載力估算——以江蘇濱海稻麥輪作田為例

湯逸帆,汪玲玉,吳 旦,戴 成,韓建剛*

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農(nóng)田施用沼液的重金屬污染評價及承載力估算——以江蘇濱海稻麥輪作田為例

湯逸帆1,2,汪玲玉1,吳 旦3,戴 成3,韓建剛1,2*

(1.南京林業(yè)大學(xué)生物與環(huán)境學(xué)院,江蘇 南京 210037；2.南京林業(yè)大學(xué)江蘇省南方現(xiàn)代林業(yè)協(xié)同創(chuàng)新中心,江蘇 南京 210037；3.中糧肉食(江蘇)有限公司,江蘇 東臺 224200)

以江蘇濱海稻麥輪作田為對象,研究沼液施用0, 3, 5a對土壤和作物籽粒重金屬(Cu、Zn、Pb、Cd)含量的影響,評價其污染風(fēng)險并估算農(nóng)田沼液承載力.結(jié)果表明:沼液施用3,5a后,土壤和作物籽粒中Cu、Zn、Pb、Cd均未超標,內(nèi)梅羅指數(shù)與土壤和農(nóng)產(chǎn)品綜合質(zhì)量指數(shù)顯示農(nóng)田重金屬污染程度屬于清潔.土壤Cu和Zn顯著富集,沼液施用5a后,小麥季土壤Cu、Zn含量分別為22.59,63.08mg/kg,較未施用分別提高了19.52%和28.89%.水稻季土壤Cu、Zn含量分別為26.12,78.74mg/kg,較未施用分別提高了27.73%和31.80%.小麥和水稻籽粒Zn含量隨沼液施用年限增加而增加(< 0.05),沼液施用5a分別達到25.07,30.98mg/kg,較未施用分別提高了23.50%和16.29%.小麥季和水稻季0~15cm土壤中Cu的累積速率分別為0.74,1.13mg/(kg·a),Zn的累積速率分別為2.83,3.80mg/(kg·a).基于土壤重金屬累積速率,江蘇濱海稻麥輪作田沼液安全施用年限為63a.

沼液；重金屬；施用年限；濱海農(nóng)田；稻麥輪作

隨著我國畜禽養(yǎng)殖業(yè)生產(chǎn)水平的提升,養(yǎng)殖量和規(guī)?；潭炔粩嗵岣?畜禽糞污帶來的生態(tài)環(huán)境問題日益嚴重[1-3].沼氣工程是畜禽糞污處理和資源化利用最有效的途徑,但同時會產(chǎn)生連續(xù)、大量、集中的副產(chǎn)物沼液[4-6].沼液富含速效養(yǎng)分,是一種優(yōu)質(zhì)液態(tài)有機肥,施用于農(nóng)田有節(jié)水減肥的效益[7-8].研究表明,沼液還田能夠有效改善土壤肥力、提高作物產(chǎn)量[9-10].然而,由于防病促長、毛發(fā)提亮、飼料利口等目的,重金屬被廣泛應(yīng)用于畜禽養(yǎng)殖飼料添加劑中,不可避免的造成其在畜禽糞污和沼液中殘留[11-13].衛(wèi)丹等[14]對嘉興市10家規(guī)?；i場沼氣工程沼液進行了水質(zhì)調(diào)查,結(jié)果表明沼液重金屬以Cu、Zn為主,平均值分別為1.88,7.63mg/L,Pb和Cd含量分別為0.21和0.01mg/L.已有學(xué)者對沼液還田的重金屬污染風(fēng)險進行了研究,但結(jié)果卻大相徑庭.例如,Bian等[15]檢測了中國太湖周邊施用沼液的農(nóng)田土壤重金屬含量,發(fā)現(xiàn)土壤Zn、Pb含量超過中國土壤環(huán)境二級質(zhì)量標準(GB15618-2008)[16];陳瑤等[17]的研究卻表明沼液施用對土壤Cu、Zn、Pb含量影響不顯著;Win等[18]的水稻田間試驗表明沼液施用不會顯著增加土壤Cu含量,但籽粒Cu含量卻顯著提升.以往的研究多為關(guān)注沼液用量、施用方式等單季作物控制實驗,而沼液連續(xù)施用對農(nóng)田重金屬累積和污染風(fēng)險的研究卻鮮有報道.

江蘇沿海地區(qū)是我國重要的糧食生產(chǎn)基地,土地后備資源豐富,畜禽養(yǎng)殖沼液受納潛力大.然而,濱海農(nóng)田土壤鹽堿度高、離子遷移行為復(fù)雜,沼液農(nóng)田消納的重金屬污染風(fēng)險尚不明確.因此,本文以江蘇濱海稻麥輪作田為對象,研究不同沼液施用年限(0, 3, 5a)對土壤和作物籽粒重金屬(Cu、Zn、Pb、Cd)含量的影響,評價其污染風(fēng)險并估算農(nóng)田沼液承載力,以期為畜禽養(yǎng)殖沼液安全、持續(xù)的農(nóng)田消納提供科學(xué)依據(jù).

1 材料與方法

1.1 研究區(qū)概況

研究區(qū)位于江蘇省東臺市弶港鎮(zhèn)金東臺農(nóng)場(32°55′24″N,120°52′50″E),屬于亞熱帶和暖溫帶過渡區(qū)域,年均氣溫14.4℃,降水量1059.9mm,日照時間221.38h,四季分明,雨量多集中在夏季,冬冷夏熱.研究區(qū)域農(nóng)田于1995年由灘涂圍墾改良成稻麥輪作田,土壤砂粒、粉粒、黏粒含量分別為36.2%、56.7%、7.1%,質(zhì)地為粉砂質(zhì)壤土,pH = 7.8,電導(dǎo)率(EC)為0.75mS/cm,有機碳含量為10.55g/kg.水稻種植周期為6月中旬~10月下旬,小麥為11月上旬~次年6月上旬,長期種植品種分別為淮稻5號和揚麥16號.

根據(jù)當?shù)馗鱾鹘y(tǒng),水稻常規(guī)施肥量為基肥磷酸二銨300kg/hm2,分蘗肥、穗肥、粒肥分別為尿素150, 300, 225kg/hm2.小麥常規(guī)施肥量為基肥磷酸二銨375kg/ha,苗肥和穗肥分別為尿素187,150kg/ hm2.該農(nóng)場于2012年起,逐年增加田塊施用豬場沼液(水稻沼液全量替代化肥,施用量為450m3/hm2,基肥:分蘗肥:穗肥比例為2:1:2;小麥在原有化肥基礎(chǔ)上額外增施沼液,施用量為45m3/hm2,基肥:穗肥比例為1:1).

沼液來自中糧肉食(江蘇)有限公司金東臺豬場沼氣工程.豬場糞污經(jīng)水泡糞工藝收集,通過勻漿池后(總固體含量TS為2.0%~3.0%)進入發(fā)酵罐,36~ 38℃下全混合厭氧反應(yīng)器(CSTR)發(fā)酵15d.產(chǎn)生的沼氣并網(wǎng)發(fā)電,液體進入存貯池.存貯池中液體穩(wěn)定1~2個月后,底部為沼渣,中上部為沼液(TS為0.7%~ 1.0%).沼液基本性狀見表1.

表1 沼液基本性狀

注:除pH值外,單位均為mg/L.

1.2 實驗方案設(shè)計

根據(jù)沼液施用歷史,設(shè)置未施用沼液(BS0)、施用3a(BS3)、施用5a(BS5)3個處理,每個處理設(shè)3個重復(fù)田塊,每個田塊面積為1hm2.于小麥和水稻收獲期采集耕層土壤和籽粒樣品,測定分析其重金屬(Cu、Zn、Pb、Cd)含量,采用內(nèi)梅羅指數(shù)法與土壤和農(nóng)產(chǎn)品綜合質(zhì)量指數(shù)法評價其污染風(fēng)險,并基于土壤重金屬累積速率估算農(nóng)田沼液承載力.

1.3 樣品采集與分析

分別于2017年6月1日和10月24日采集小麥季和水稻季樣品.各田塊按S形布點法多點采集0~15cm土壤制成混合土樣,同步“點對點”采集作物籽粒[19].土樣自然風(fēng)干后去除沙礫、植物殘體,過100目篩,采用HCl-HNO3-HF-HClO4體系消煮[20].作物籽粒于105℃殺青0.5h,80℃烘干至恒重后研磨過100目篩,采用HNO3-HF-HClO4體系消煮[21].消煮液用電感耦合等離子體質(zhì)譜儀(ICP-MS; NexION 300X, PerkinElmer, USA)測定重金屬含量.

1.4 污染評價方法

目前,農(nóng)田重金屬污染評價方法多樣,其中包括單因子指數(shù)法、內(nèi)梅羅指數(shù)法、地積累指數(shù)法和模糊判別法等,而內(nèi)梅羅指數(shù)法是最為常用的多種重金屬復(fù)合污染綜合評價方法[22].此外,王玉軍等[23]將土壤和農(nóng)產(chǎn)品重金屬緊密聯(lián)系,構(gòu)建了土壤和農(nóng)產(chǎn)品綜合質(zhì)量指數(shù)法,顯著提升了農(nóng)田重金屬污染評價的科學(xué)性和可靠性.

1.4.1 內(nèi)梅羅指數(shù)法 內(nèi)梅羅指數(shù)法是以土壤元素實測值和環(huán)境質(zhì)量標準計算各因子的污染指數(shù),再通過其平均值和最大值綜合評價重金屬污染程度[24-25].計算公式如下:

式中:為內(nèi)梅羅綜合指數(shù);C為第種重金屬的實測值;S為第種重金屬在環(huán)境質(zhì)量標準中的限值,Cu、Zn、Pb、Cd分別為100, 300, 170, 0.6mg/kg(GB 15618-2018)[26].內(nèi)梅羅指數(shù)法評價標準為￡0.7清潔(安全),0.7 <￡1.0尚清潔(警戒限),1.0 <￡2.0輕度污染,2.0 <￡3.0中度污染,> 3.0重污染.

1.4.2 土壤和農(nóng)產(chǎn)品綜合質(zhì)量指數(shù)法 土壤和農(nóng)產(chǎn)品綜合質(zhì)量指數(shù)法基于土壤環(huán)境質(zhì)量標準、土壤元素背景值、農(nóng)產(chǎn)品污染物限量標準和元素價態(tài)效應(yīng)等,通過計算土壤相對影響當量、土壤偏離背景值程度、土壤標準偏離背景值程度和農(nóng)產(chǎn)品品質(zhì)指數(shù),在此基礎(chǔ)上得到土壤綜合質(zhì)量影響指數(shù)和農(nóng)產(chǎn)品綜合質(zhì)量指數(shù),最后求和構(gòu)建綜合質(zhì)量影響指數(shù).計算公式如下:

式中:RIE為土壤相對影響當量;DDDB為土壤偏離背景值程度;DDSB為土壤標準偏離背景值程度; QIAP為農(nóng)產(chǎn)品品質(zhì)指數(shù);為測定重金屬的種類數(shù)量;為各重金屬在土壤中的穩(wěn)定態(tài)氧化數(shù);C為土壤第種重金屬的實測值;S為第種重金屬在土壤環(huán)境質(zhì)量標準中的限值,Cu、Zn、Pb、Cd分別為100, 300, 170, 0.6mg/kg(GB 15618-2018)[26];B為第種重金屬的土壤背景值,Cu、Zn、Pb、Cd分別為22.3, 62.6, 26.2, 0.126mg/kg(江蘇)[27];APi為農(nóng)產(chǎn)品第種重金屬的實測值;APi為第種重金屬在農(nóng)產(chǎn)品污染物限量標準中的限值,Cu、Zn、Pb、Cd分別為10, 50, 0.2, 0.1mg/kg(GB 2762-2017)[28].

式中:IICQS為土壤綜合質(zhì)量影響指數(shù);IICQAP為農(nóng)產(chǎn)品綜合質(zhì)量指數(shù);IICQ為綜合質(zhì)量影響指數(shù);、、分別為實測值超過土壤環(huán)境質(zhì)量標準限值、土壤背景值和農(nóng)產(chǎn)品污染物限量標準限值的重金屬種類數(shù)量;為背景校正因子,一般取5.土壤和農(nóng)產(chǎn)品綜合質(zhì)量指數(shù)法評價標準為IICQ￡1清潔,1 < IICQ￡2輕微污染,2 < IICQ￡3輕度污染,3 < IICQ￡5中度污染,IICQ > 5重度污染.

1.5 農(nóng)田沼液安全承載力計算

采用累積速率法計算農(nóng)田沼液安全承載力[29],計算公式如下:

式中:HMI為土壤重金屬累積速率,mg/(kg·a);HMT為土壤重金屬超標時間,a;HM和HM分別為第和年的土壤重金屬含量,mg/kg;HMs為土壤環(huán)境質(zhì)量標準限值,Cu、Zn、Pb、Cd分別為100, 300, 170, 0.6mg/kg(GB 15618-2018)[26];HM0為初始年限土壤重金屬含量,mg/kg.

1.6 數(shù)據(jù)處理

采用Excel 2016進行數(shù)據(jù)整理,SPSS 22.0進行單因素方差分析和Duncan法比較不同處理間土壤和作物籽粒重金屬含量的差異顯著性(< 0.05),并用Origin 2017進行作圖.

2 結(jié)果與分析

2.1 沼液施用年限對土壤重金屬含量的影響

由圖1可見,濱海稻麥輪作田小麥季土壤Cu、Zn含量隨沼液施用年限增加而增加(<0.05),其中BS5分別為22.59,63.08mg/kg,較BS0分別提高了19.52%和28.89%.各處理小麥季土壤Pb和Cd含量無顯著差異.水稻季土壤Cu、Zn含量的處理間差異和變化趨勢與小麥季保持一致(圖2),其中BS5分別為26.12, 78.74mg/kg,較BS0分別提高了27.73%和31.80%.各處理水稻季土壤Pb含量無顯著差異,BS5的水稻季土壤Cd含量顯著高于BS0和BS3.

圖1 不同沼液施用年限下濱海稻麥輪作田小麥季土壤重金屬含量

不同字母表示差異顯著,下同

圖2 不同沼液施用年限下濱海稻麥輪作田水稻季土壤重金屬含量

2.2 沼液施用年限對作物籽粒重金屬含量的影響

由圖3可見,小麥籽粒Zn含量隨沼液施用年限增加而增加(<0.05),其中BS5為25.07mg/kg,較BS0提高了23.50%.各處理小麥籽粒Cu和Pb含量無顯著差異,BS5的小麥籽粒Cd含量顯著高于BS0和BS3.水稻籽粒Zn含量的處理間差異和變化趨勢與小麥籽粒保持一致(圖4),其中BS5為30.98mg/kg,較BS0提高了16.29%.各處理水稻籽粒Pb含量無顯著差異,BS3和BS5的水稻籽粒Cu含量顯著高于BS0.此外,BS0的水稻籽粒Cd含量最高,與BS3存在顯著性差異.

圖3 不同沼液施用年限下濱海稻麥輪作田小麥籽粒重金屬含量

圖4 不同沼液施用年限下濱海稻麥輪作田水稻籽粒重金屬含量

2.3 不同沼液施用年限下的農(nóng)田重金屬污染評價

表2 不同沼液施用年限下濱海稻麥輪作田重金屬內(nèi)梅羅評價指數(shù)(小麥季/水稻季)

注:為內(nèi)梅羅綜合指數(shù).

表3 不同沼液施用年限下濱海稻麥輪作田重金屬土壤和農(nóng)產(chǎn)品綜合質(zhì)量指數(shù)(小麥季/水稻季)

2.4 江蘇濱海稻麥輪作田沼液承載力估算

由于沼液中重金屬以Cu和Zn為主(表1),且在小麥季和水稻季土壤均出現(xiàn)顯著富集(圖1、2),故以二者為約束條件計算江蘇濱海稻麥輪作田沼液承載力(累積速率以BS0和BS5為間隔計算).由表4可見,Zn的累積速率高于Cu,水稻季的累積速率高于小麥季.江蘇濱海稻麥輪作田小麥季和水稻季分別在沼液施用110,70a后出現(xiàn)土壤Cu超標,在89,63a后出現(xiàn)Zn超標.

表4 Cu、Zn約束下的江蘇濱海稻麥輪作田沼液安全施用年限(小麥/水稻)(a)

3 討論

作為畜禽糞污厭氧發(fā)酵副產(chǎn)物,沼液農(nóng)田施用的重金屬安全問題近年來備受關(guān)注[30-32].研究表明,豬糞沼液中重金屬以Cu、Zn為主[12],這與本研究供試沼液性狀吻合(表1).江蘇濱海稻麥輪作田沼液施用3,5a后,小麥季和水稻季土壤Cu、Zn、Pb、Cd均未超過農(nóng)用地土壤質(zhì)量標準限值(GB 15618- 2018)[26],但Cu、Zn出現(xiàn)了顯著富集(圖1、2),這與宋三多等[33]的研究一致.本研究中,小麥和水稻籽粒Cu、Zn、Pb、Cd均未超過食品中污染物限量標準限值(GB 2762-2017)[28],而籽粒Zn含量和土壤保持一致,隨沼液施用年限的增加而增加(圖3、4),Cu、Pb、Cd則無此現(xiàn)象.究其原因,沼液中Zn含量較其他重金屬最高,并且土壤中Zn的作物可利用度也高于其他重金屬[34-35].此外,水稻籽粒Cd含量在沼液施用3a后顯著下降,而施用5a后無顯著變化.董同喜等[36]認為是由于有機肥中重金屬與有機質(zhì)結(jié)合緊密復(fù)雜,隨著有機質(zhì)分解和組分變化,導(dǎo)致了重金屬形態(tài)和生物有效性發(fā)生了改變.根據(jù)內(nèi)梅羅評價指數(shù)以及土壤和農(nóng)產(chǎn)品綜合質(zhì)量指數(shù),沼液施用3和5a后,江蘇濱海稻麥輪作田重金屬污染均屬于未受污染(表2、3).一方面是由于江蘇濱海農(nóng)田土壤本底較為清潔(BS0),另一方面也表明沼液農(nóng)田施用的重金屬污染風(fēng)險較低.內(nèi)梅羅評價指數(shù)顯示,沼液施用的小麥季土壤重金屬污染程度變化不大,這可能是由于小麥季沼液施用量小的原因.值得注意的是,水稻季重金屬土壤和農(nóng)產(chǎn)品綜合質(zhì)量指數(shù)逐年增加而小麥季無規(guī)律性趨勢,但沼液施用5a后小麥季卻有較大提升且高于水稻季.因此,今后的研究不僅要以沼液施用造成的土壤重金屬累積為主,還要關(guān)注于稻麥輪作田沼液施用的重金屬形態(tài)和環(huán)境行為變化.

已有較多研究基于土壤重金屬累積速率和環(huán)境質(zhì)量標準對農(nóng)田有機肥安全施用年限進行了估算.例如,孫國峰等[37]的研究中,當豬糞替代化學(xué)氮肥50%時的安全施用年限為20a;Yang等[38]的研究中,當有機肥用量為45t/hm2時的安全施用年限為15a.在沼液施用63a后,江蘇濱海稻麥輪作田水稻季土壤Zn含量會超出環(huán)境質(zhì)量標準(表4).本研究中沼液安全施用年限遠大于上述二者,可能的原因是其研究對象為豬糞有機肥,重金屬含量較高,而經(jīng)過厭氧發(fā)酵和存貯過程,重金屬由于有機質(zhì)絡(luò)合和沉淀作用大多存在于沼渣當中,沼液中含量則較低[13,39].為降低沼液農(nóng)田施用的重金屬污染風(fēng)險和增加安全施用年限,一方面可從源頭控制飼料重金屬添加劑投入從而降低沼液重金屬含量[40],另一方面,積極地向土壤中配施生物炭等重金屬鈍化材料顯得尤為必要[41-42].

4 結(jié)論

4.1 江蘇濱海稻麥輪作田沼液施用3和5a后,土壤和作物籽粒中Cu、Zn、Pb、Cd均未超標,土壤Cu、Zn和籽粒Zn含量隨沼液施用年限增加而顯著增加.沼液施用5a后,小麥季和水稻季重金屬內(nèi)梅羅評價指數(shù)分別為0.28和0.23,土壤和農(nóng)產(chǎn)品綜合質(zhì)量指數(shù)分別為0.35和0.31,均顯示清潔或未受污染.

4.2 江蘇濱海稻麥輪作田連續(xù)施用沼液,小麥季和水稻季0~15cm土壤中Cu的累積速率分別為0.74, 1.13mg/(kg·a),Zn的累積速率分別為2.83,3.80mg/ (kg·a),安全施用年限為63a.

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Assessment of heavy metal pollution and bearing capacity estimation of continuous biogas slurry application on cropland: A case study of the coastal rice-wheat rotated farmland in Jiangsu, China.

TANG Yi-fan1,2, WANG Ling-yu1, WU Dan3, DAI Cheng3, HAN Jian-gang1,2*

(1.College of Biology and the Environment, Nanjing Forestry University, Nanjing 210037, China；2.Collaborative Innovation Center of Sustainable Forestry in Southern China of Jiangsu Province, Nanjing Forestry University, Nanjing 210037, China；3.COFCO Meat (Jiangsu) Co., Ltd, Dongtai 224200, China)., 2019,39(4)：1687~1695

In order to evaluate the pollution risk and estimate the bearing capacity, the coastal rice-wheat rotated farmland in Jiangsu Province was studied to investigate the effects of biogas slurry application for 0, 3, and 5years on heavy metal (Cu, Zn, Pb, Cd) contents in soil and crop grains. Cu, Zn, Pb, and Cd contents in soil and crop grains did not exceed the national standard limits after 3 and 5years of biogas slurry application, and the degree of heavy metal pollution in farmland met the requirements of cleaning according to Nemerow index and the Influence Index of Comprehensive Quality. The Cu and Zn contents in soil enriched significantly after 5years of application of biogas slurry and the contents of Cu and Zn in wheat and rice season were 22.59 and 63.08mg/kg (26.12 and 78.74mg/kg), which were 19.52% and 28.89% (27.73% and 31.80%) higher than those grains without application. As the increasing of application age of biogas slurry, the content of Zn in crop grains increased significantly (< 0.05) and the Zn contents in wheat and rice grains reached up to 25.07 and 30.98mg/kg after 5years of biogas slurry application, which was 23.50% and 16.29% higher than those grains without application. The accumulation rate of Cu and Zn in soil (0~15cm depth) was 0.74mg/(kg·a), 2.83mg/(kg·a) and 1.13mg/(kg·a), 3.80mg/(kg·a) for wheat and rice season, respectively. Based on the accumulation rate of heavy metals in soil, the safe application period of biogas slurry in coastal rice-wheat rotated farmland was 63years.

biogas slurry；heavy metal；application age；coastal farmland；rice-wheat rotation

X53

A

1000-6923(2019)04-1687-09

2018-09-03

國家重點研發(fā)計劃項目(2017YFC0505803);江蘇省農(nóng)業(yè)科技自主創(chuàng)新資金項目(CX(16)1003-8);江蘇省“六大人才高峰”高層次人才資助項目(2016-NY-064)

*責(zé)任作者, 教授, hjg@njfu.edu.cn

湯逸帆(1993-),男,江蘇鎮(zhèn)江人,南京林業(yè)大學(xué)博士研究生,主要從事種養(yǎng)結(jié)合與土壤生態(tài)方面研究.發(fā)表論文4篇.