改良劑連續施用對農田水稻Cd吸收的影響

冉洪珍,郭朝暉,肖細元,史 磊,封文利

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改良劑連續施用對農田水稻Cd吸收的影響

冉洪珍,郭朝暉*,肖細元,史 磊,封文利

(中南大學冶金與環境學院,湖南 長沙 410083)

通過連續2a田間試驗,研究了在Cd重度污染土壤上施用有機肥、石灰、石灰與有機肥配施1a后,第2a連續施用和不再施用改良劑對稻田土壤有效態Cd含量和水稻Cd吸收的影響.結果表明,有機肥、石灰單施及石灰與有機肥配施均能顯著提高稻田土壤pH值,降低土壤中有效態Cd含量和水稻各部位Cd含量,第1a有機肥、石灰、石灰與有機肥配施處理的糙米中Cd含量較對照分別顯著降低35.9%、69.2%和65.4%.與對照相比,第2a連續施用有機肥、石灰及石灰與有機肥配施處理下,稻田土壤pH值分別顯著升高0.27、0.57和1.05個單位,土壤有效態Cd分別顯著降低26.6%、29.7%和59.4%;糙米中Cd含量較對照分別顯著降低63.1%、79.5%和83.6%,其中,第2a連續石灰與有機肥配施處理下糙米中Cd含量為0.20mg/kg,達到《食品安全國家標準食品中污染物限量》(GB2762-2017)中糙米限量值.第2a不再施用有機肥、石灰及石灰與有機肥配施處理的糙米中Cd含量較對照分別顯著降低49.2%、69.7%和75.4%.雙因素方差分析結果表明其值與連續施用改良劑的處理無顯著性差異.上述結果表明,石灰與有機肥配施可有效降低污染稻田土壤中有效態Cd含量和水稻Cd含量,施加后一年內可不施或減少改良劑施用量.

土壤改良劑；鎘；有效性；水稻；連續施用

我國南方部分地區農田土壤存在Cd超標問題,給作物生產帶來嚴重不利影響[1-3],因此解決農田Cd污染問題,實現糧食安全生產意義重大.化學原位鈍化修復技術具有操作簡單、見效快、可以實現邊修復邊生產等優勢而被廣泛應用[4-6].近年來,選擇一些廉價有效的改良劑,如石灰、泥炭、羥基磷石灰、有機肥、沸石等[7-8],進行原位穩定土壤中Cd是實現Cd污染稻田安全生產的有效措施.施用石灰可顯著提高土壤pH值和陽離子交換量,增加了土壤微生物對碳源的利用能力,提高了其功能多樣性,進而降低土壤中重金屬有效態含量[9-10];史磊等通過施用石灰顯著降低了稻田土壤中重金屬Cd的弱酸可提取態和可還原態比例,增加其殘渣態比例,從而降低了稻米中Cd含量[11].但連續施用石灰容易破壞土壤團粒結構,造成土壤板結[9],同時,土壤再次酸化會導致改良劑的效果退化[12].有機肥可以通過影響土壤pH值,土壤團聚體數量和穩定性,土壤吸附性能,土壤微生物豐度和活性等改變土壤中重金屬賦存形態,從而降低重金屬對植物的毒性[13-14].然而,目前大部分以畜禽糞便為主要原料的市售有機肥重金屬含量超標[15].石灰與低重金屬含量的菜籽餅有機肥施用能夠有效鈍化重金屬,同時減少土壤結構破壞[16-18]; Zhou等研究石灰和海泡石、腐植酸與沸石復配能顯著抑制水稻植株Pb、Cd、Cu和Zn的吸收積累,從而減少重金屬的毒性[19].

土壤改良劑對土壤中重金屬的影響因改良劑的種類、施用量、土壤自身理化性質的差異而不同[20-21].湖南省部分耕地土壤重金屬污染嚴重,存在石灰施用不合理且利用率低導致農業資源浪費,土壤理化性質被破壞等問題,制約了農業的可持續發展,因此研究改良劑修復Cd污染土壤及后效,對改善改良劑的不合理施用及其利用率低的問題具有重要作用.研究表明在Cd輕度污染土壤上,石灰、石灰與海泡石配施、石灰與甘蔗渣配施處理對抑制水稻各部位Cd的吸收均有一定的后效[22-23],石灰、木炭等處理對銅鎘由活性態向非活性態和潛在活性態轉化也有一定的后效[24],但缺少與連續施用改良劑處理對比試驗,因此不能較好地說明施加改良劑的科學性.同時,研究Cd重度污染土壤上施用改良劑后效的報道相對較少.因此,本試驗針對湖南省典型Cd重度污染土壤開展田間試驗,通過對比連續施用和單季施用石灰、有機肥及石灰與有機肥配施,分析稻田土壤pH值、土壤有效態Cd含量以及水稻植株各部位對Cd的吸收特征,以期為進一步優化Cd污染稻田土壤治理效果和合理利用農業資源提供科學依據.

1 材料與方法

1.1 區域概況

田間試驗選擇在湖南某退役老工業基地周邊污染稻田進行.試驗田區域屬湘江下游河谷及丘陵帶,成土母質為第四紀紅土,屬亞熱帶季風性濕潤氣候,年平均氣溫16℃至18℃,年降水量在1500mm左右.供試田塊土壤基本理化性質見表1.

表1 供試土壤基本理化性質(mg/kg) Table1 Basic properties of tested soil sample (mg/kg)

1.2 試驗材料與設計

2016年6月,在該Cd污染田塊開展田間試驗,試驗田翻耕后設置對照(CK)、施用有機肥(OM)、石灰(L)和石灰與有機肥配施(LOM)共4個處理,每個處理重復6次.每個處理小區面積為5m′6m=30m2,隨機排列.試驗小區周邊設保護行,并用聚乙烯加厚塑料膜對小區田埂進行保護用來消除各區之間的干擾.按照試驗設計要求將改良劑施入相應田塊小區中(石灰施加量為1500kg/hm2,有機肥施加量為2250kg/hm2,組配改良劑施用前先將兩種改良劑混合),隨后將改良劑和土壤充分混勻,老化7d.2016年7月15日移栽秧苗,11月8日收獲并采集土壤和相應水稻樣品(2016T).2017年6月任選每個處理的三個小區繼續按照2016年的試驗設計施加改良劑(2017T),其余三個小區不再施加改良劑(2017NT),并按照對照處理進行移栽秧苗.整個水稻生長期間均按照當地一般農田的管理模式.

兩年水稻種植品種均為天優華占,為秈型三系雜交水稻.田間試驗用改良劑均為市售商品,其中石灰購自農資市場,氧化鈣質量分數達80%,Cd含量為0.40mg/kg.有機肥購自湖南省湘暉農業技術開發有限公司的菜籽餅有機肥,N+P2O5+K>5%,有機質質量分數>45%,Cd含量為0.43mg/kg.

1.3 樣品采集及分析

水稻成熟期采集田間長勢均勻的代表性水稻植株樣品(每個處理3株),用自來水洗凈后,再用去離子水洗凈,然后按水稻根、莖葉以及籽粒分開,在105℃殺青30min,60℃烘干至恒重,記錄生物量.將樣品粉碎后分別放入聚乙烯封口袋中密封備用.采集水稻植株相應的0~20cm表層土壤,經自然風干,除去動植物殘體及碎石等雜物后研磨,分別過10目和100目尼龍篩后放入聚乙烯封口袋中備用.

土壤pH值采用1:2.5土水比浸提,用pH計(上海雷磁,PHS-3C)測定;土壤有機質含量采用重鉻酸鉀容量法測定;堿解氮采用堿解擴散-硫酸滴定法測定;有效磷采用碳酸氫鈉提取-釩鉬黃比色法測定;速效鉀采用醋酸銨提取-火焰光度計法測定[25].土壤中有效態Cd采用DTPA提取法提取[25].土壤樣品用HNO3-HF-HClO4法消解,水稻植株和糙米樣品采用HNO3-HClO4法消解,浸提液和消解液中Cd含量采用ICP-MS(美國,Thermo Fisher X2)測定.

1.4 數據處理

數據統計采用Microsoft Excel2013進行分析,采用origin 9.0軟件作圖.單因素方差分析(One-way ANOVA)和相關性分析均采用SPSS 19.0完成,<0.05表示處理間有顯著性差異,<0.01表示處理間有極顯著差異.

2 結果與分析

2.1 土壤改良劑對土壤pH值,有機質和有效態Cd含量的影響

由圖1可知,不同改良劑處理對土壤pH值影響程度不同.但與對照相比,施加有機肥、石灰及石灰和有機肥配施均能處理能顯著(<0.05)提高土壤pH值.2016年施用OM、L和LOM的處理比對照土壤pH值顯著(<0.05)提高0.46~1.20,施用石灰和有機肥均能在短期內不同程度提高土壤pH值[26-27]. 2017年連續施用OM、L和LOM處理的土壤pH值較對照顯著提高0.27~1.05,僅2016年水稻季施用土壤改良劑的土壤pH值分別較對照提高0.19、0.37和0.75.說明連續施加土壤改良劑能夠更有效維持土壤pH值.改良劑提高土壤pH值的效果表現為LOM>L>OM,說明對于Cd污染土壤,石灰和有機肥配施能進一步提高土壤pH值[28].有機肥的施用將大量的有機物帶入土壤中.一方面,有機肥中有機物腐解過程產生的有機酸對土壤中重金屬產生活化效應,增加重金屬的生物有效性[29];但另一方面,有機質中大量的官能團吸附重金屬[30],腐殖質分解產生的腐植酸與重金屬離子形成絡合物均能降低土壤重金屬的有效性[31].從圖1可知,與對照相比,2016年施用OM、L和LOM,2017年連續施用和不施用OM、L和LOM的土壤有機質質量分數均無顯著性差異.僅2017年連續施用改良劑的土壤有機質質量分數略高于單季施用改良劑的處理.這可能與土壤有機質本底值較高,該種菜籽餅有機肥最佳肥效期為45d,其后有機質含量下降[32]及有機肥的施用量較少有很大的關系.

與對照相比,連續或單季施用OM、L和LOM處理均能顯著(<0.05)降低土壤有效態Cd含量,2016年水稻收獲后施用土壤改良劑的處理比對照土壤有效態Cd含量分別顯著(<0.05)降低25.1%、20.8%和55.8%.與對照相比,2017年水稻季連續施用土壤改良劑土壤有效態Cd分別顯著降低26.6%、29.7%和59.4%;不繼續施加土壤改良劑的土壤有效態Cd含量也顯著(<0.05)降低.土壤有效態Cd含量與土壤pH值的變化趨勢一致,相比于石灰和有機肥單施,石灰和有機肥配施處理能進一步顯著(<0.05)降低土壤有效態Cd含量.施用石灰能顯著提高土壤pH值,增加土壤膠體表面的負電荷,同時促進土壤中鐵錳氧化物的形成,進而增加土壤重金屬的吸附位點和增強重金屬的吸附能力,減少土壤重金屬有效態[10,33].由于有機肥自身較高的pH值以及分解過程中釋放的少量NH3,添加有機肥能一定程度提高土壤pH值,同時有機肥所含的有機質含有較大的比表面積和大量的官能團,使得土壤有效態Cd向其他形態轉化[34-36].從圖1中可以看出,連續施用土壤改良劑并不會進一步降低土壤有效態Cd含量,說明土壤改良劑的施用能有效降低土壤有效態Cd含量,單季施加的土壤改良劑有較強的后效[37].可見有機、無機改良劑配施能有效降低土壤有效態Cd含量,施加改良劑后的一定年限內可不施或適當減少改良劑的施用量以便降低農業生產投入成本和避免改良劑施用不合理導致的利用效率低的問題.

2.2 土壤改良劑對水稻各部位Cd吸收的影響

水稻植株中Cd含量的變化如圖2所示.與對照相比,連續或單季施用OM、L和LOM處理均能顯著(<0.05)降低水稻植株各部位Cd含量.水稻植株各部位Cd含量差異明顯,大小順序為根＞莖葉＞糙米,這與植物各部位代謝程度有關[38].石灰和有機肥通過控制土壤中有效態Cd含量減少Cd向水稻植株各部位遷移[13,19,39-40].2016年施用OM、L和LOM處理比對照處理下水稻根中Cd含量顯著(<0.05)降低44.9%~85.8%.2017年連續施用OM、L和LOM處理的水稻根中Cd含量較對照分別顯著(<0.05)下降60.4%、77.3%和86.2%,略低于單季施加OM、L和LOM處理下水稻根中Cd含量,其下降幅度分別為56.0%、73.5%和87.1%(<0.05).與對照相比,2016年施用OM、L和LOM處理莖葉中Cd含量均顯著(<0.05)下降,尤其是LOM處理下水稻莖葉中Cd含量降低至0.58mg/kg.2017年連續施用OM、L和LOM處理水稻莖葉中Cd含量略低于單季施加土壤改良劑處理,與對照相比,水稻莖葉中Cd含量分別顯著(<0.05)下降68.8%~88.7%和54.7%~ 86.1%.與水稻根和莖葉中Cd含量變化趨勢相一致,與對照相比,單季或連續施用OM、L和LOM處理均能顯著(<0.05)降低糙米中Cd含量.2016年施用OM、L和LOM處理糙米中Cd含量比對照處理顯著(<0.05)降低35.9%、69.2%和65.4%.2017年連續施用OM、L和LOM處理的水稻糙米中Cd含量分別比對照顯著(<0.05)降低63.1%、79.5%和83.6%,其中石灰與有機肥配施處理下糙米中Cd含量降低至0.20mg/kg,達到《食品安全國家標準食品中污染物限量》(GB2762-2017)中Cd限量值.與對照相比,單季施加土壤改良劑處理的糙米中Cd含量分別顯著(<0.05)降低49.2%、69.7%和75.4%,雙因素方差分析結果表明(表2)不連續與連續施用土壤改良劑對糙米中Cd含量無顯著性影響.僅鈍化劑種類對水稻地上部Cd含量產生極顯著(<0.01)影響;就衡量因素作用大小的平方和(SS)來看,鈍化劑種類因素的影響最明顯,即水稻對Cd的吸收特征主要受鈍化劑種類的控制.上述結果表明,施用土壤改良劑對減少水稻Cd吸收有較強的后效.

綜上所述,石灰比有機肥能更有效地抑制水稻各部位對Cd的吸收.與石灰和有機肥單施相比較,石灰與有機肥配施效果最佳.石灰、有機肥和石灰與有機肥配施對抑制水稻各部位Cd的吸收均有較強的后效.因此從農業經濟成本和改良劑對土壤環境質量影響的角度考慮,在輕Cd污染土壤上施加改良劑后一年內不施改良劑或在重Cd污染土壤上一定年限內適當減少改良劑使用量也可達到顯著減少水稻植物體內重金屬含量的效果.

表2 鈍化劑種類與不同處理方式交互作用對水稻各部位的雙因素方差分析 Table 2 Two-way ANOVA of interactions of the soil amendments andthe application strategies of the soil amendments on the Cd contents in different organs of rice

注:表中數據自由度值=3;<0.05表示該變異來源因素對水稻部位中Cd含量影響顯著;CdG,CdS,CdR分別表示水稻糙米、莖葉和根中Cd含量(下同).

2.3 相關性分析

相關性分析結果(表3)表明,土壤DTPA提取態Cd含量、水稻根、莖葉和糙米中Cd含量與土壤pH值均呈極顯著負相關(0.01).土壤pH值與土壤DTPA-Cd含量的相關系數為0.834,與水稻根、莖葉和糙米中Cd含量的相關系數分別為0.790,0.773, 0.711,說明稻田土壤pH值升高首先會引起土壤有效態Cd含量降低,從而進一步抑制水稻根對Cd的吸收及其向水稻地上部遷移.土壤DTPA-Cd含量、水稻根、莖葉和糙米中Cd含量均與土壤有機質含量呈負相關.在中性偏堿性的土壤中,土壤有機物隨著pH值增加溶解度增大,絡合能力增強,故大量重金屬被絡合[41],土壤DTPA-Cd含量減少.表3結果表明,土壤DTPA-Cd含量與土壤有機質含量相關性不顯著,說明石灰和有機肥等堿性改良劑主要通過提高土壤pH值來降低土壤有效態重金屬含量.李忠義等[42]和Zeng等[43]的研究中,土壤有效態Cd、Cu、Pb和Zn等與土壤pH值呈極顯著負相關(<0.01),而與土壤有機質含量呈顯著正相關,這可能是由于土壤中有效態重金屬含量在短期內受外源添加物的影響較大.糙米中Cd含量與土壤DTPA-Cd含量、水稻根、莖葉中Cd含量均呈極顯著正相關(<0.01),相關系數分別為0.666,0.932和0.914,該結果說明Cd首先通過從土壤中向根部轉移,再經過木質部的蒸騰作用和功能葉中韌皮部隨著同化物一起從輸送到籽粒[44].因此,通過降低土壤中有效態Cd的含量,減少土壤Cd向水稻根和莖葉中轉移能有效控制水稻糙米中重金屬的含量.

表3 土壤pH值,有機質、DTPA-Cd含量與水稻各器官Cd的相關系數 Table 3 The correlation analysis between Cd contents in rice and soil pH, organic matter and DTPA-extractable Cd content in soil

注:表中數據為相關性系數(=3);* 表示顯著相關性水平0.05;**表示極顯著相關性水平0.01.SOM表示土壤有機質.

3 結論

3.1 有機肥、石灰單施及石灰與有機肥配施對提高稻田土壤pH值、降低土壤中有效態Cd含量和水稻各部位Cd含量均有顯著效果,其中石灰與有機肥配施效果更顯著.在4.70mg/kg Cd污染稻田土壤中,石灰與有機肥連續配施的糙米Cd含量降低至0.2mg/kg,較未施用處理顯著降低83.6%,達到《食品安全國家標準食品中污染物限量》(GB2762-2017)中糙米限量值.

3.2 石灰、有機肥和石灰與有機肥配施處理對抑制水稻各部位Cd的吸收有較強的后效,尤其是石灰與有機肥配施處理,因此在Cd污染稻田土壤上施加改良劑后一年內可不施或少施改良劑.

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Effects of continuous application of soil amendments on cadmium availability in paddy soil and uptake by rice.

RAN Hong-zhen, GUO Zhao-hui*, XIAO Xi-yuan, SHI Lei, FENG Wen-li

(School of Metallurgy and Environment, Central South University, Changsha 410083, China)., 2019,39(3)：1117~1123

A two-year field experiment was conducted to study the effects of organic fertilizer and lime on remediation of Cd-contaminated paddy soil. The application strategies of the soil amendments including continuous application and application only in the first year were compared. The results showed that the individual or combined application of organic fertilizer and lime both increased significantly pH and available Cd contents in the soil as well as Cd contents in different parts of the planted rice. The Cd contents of brown rice in the first year were reduced significantly by 35.9%, 69.2%, and 65.4% compared with the control, respectively. Under the continuous application strategy, the soil pH significantly increased by 0.27, 0.57 and 1.05 units respectively, the contents of available Cd in the soils decreased by 26.6%, 29.7% and 59.4% respectively and the rice Cd concentrations decreased significantly by 63.1%, 79.5% and 83.6%, respectively. Especially under continuous application of both organic fertilizer and lime, the rice Cd contents was 0.20mg/kg which had reached the National Standard of Pollutant in Food of China (GB 2762~2017). Under the strategy of application only in the first year, the Cd contents in the brown rice reduced significantly by 49.2%, 69.7% and 75.4%, respectively. The two-way ANOVA suggested no significant difference between the two strategies of continuous application and application only in the first year. The results indicated that the application of organic fertilizer and lime can effectively reduce the contents of available Cd in soil and Cd in rice, and the remediation effects can lasts two years without significant reduction after the first application.

soil amendment；cadmium；effectiveness；rice；continuous application

X53

A

1000-6923(2019)03-1117-07

冉洪珍(1994-),女,重慶巫溪人,中南大學冶金與環境學院碩士研究生,主要從事重金屬污染土壤修復研究.發表論文5篇.

2018-07-26

國家科技支撐計劃課題(2015BAD05B02);重金屬污染耕地修復機理及技術模式優化集成項目(農業部財政部農辦財函〔2016〕6號)

* 責任作者, 教授, zhguo@csu.edu.cn