改良剂连续施用对农田水稻Cd吸收的影响

冉洪珍,郭朝晖,肖细元,史 磊,封文利



改良剂连续施用对农田水稻Cd吸收的影响

冉洪珍,郭朝晖*,肖细元,史 磊,封文利

(中南大学冶金与环境学院,湖南 长沙 410083)

通过连续2a田间试验,研究了在Cd重度污染土壤上施用有机肥、石灰、石灰与有机肥配施1a后,第2a连续施用和不再施用改良剂对稻田土壤有效态Cd含量和水稻Cd吸收的影响.结果表明,有机肥、石灰单施及石灰与有机肥配施均能显著提高稻田土壤pH值,降低土壤中有效态Cd含量和水稻各部位Cd含量,第1a有机肥、石灰、石灰与有机肥配施处理的糙米中Cd含量较对照分别显著降低35.9%、69.2%和65.4%.与对照相比,第2a连续施用有机肥、石灰及石灰与有机肥配施处理下,稻田土壤pH值分别显著升高0.27、0.57和1.05个单位,土壤有效态Cd分别显著降低26.6%、29.7%和59.4%;糙米中Cd含量较对照分别显著降低63.1%、79.5%和83.6%,其中,第2a连续石灰与有机肥配施处理下糙米中Cd含量为0.20mg/kg,达到《食品安全国家标准食品中污染物限量》(GB2762-2017)中糙米限量值.第2a不再施用有机肥、石灰及石灰与有机肥配施处理的糙米中Cd含量较对照分别显著降低49.2%、69.7%和75.4%.双因素方差分析结果表明其值与连续施用改良剂的处理无显著性差异.上述结果表明,石灰与有机肥配施可有效降低污染稻田土壤中有效态Cd含量和水稻Cd含量,施加后一年内可不施或减少改良剂施用量.

土壤改良剂；镉；有效性；水稻；连续施用

我国南方部分地区农田土壤存在Cd超标问题,给作物生产带来严重不利影响[1-3],因此解决农田Cd污染问题,实现粮食安全生产意义重大.化学原位钝化修复技术具有操作简单、见效快、可以实现边修复边生产等优势而被广泛应用[4-6].近年来,选择一些廉价有效的改良剂,如石灰、泥炭、羟基磷石灰、有机肥、沸石等[7-8],进行原位稳定土壤中Cd是实现Cd污染稻田安全生产的有效措施.施用石灰可显著提高土壤pH值和阳离子交换量,增加了土壤微生物对碳源的利用能力,提高了其功能多样性,进而降低土壤中重金属有效态含量[9-10];史磊等通过施用石灰显著降低了稻田土壤中重金属Cd的弱酸可提取态和可还原态比例,增加其残渣态比例,从而降低了稻米中Cd含量[11].但连续施用石灰容易破坏土壤团粒结构,造成土壤板结[9],同时,土壤再次酸化会导致改良剂的效果退化[12].有机肥可以通过影响土壤pH值,土壤团聚体数量和稳定性,土壤吸附性能,土壤微生物丰度和活性等改变土壤中重金属赋存形态,从而降低重金属对植物的毒性[13-14].然而,目前大部分以畜禽粪便为主要原料的市售有机肥重金属含量超标[15].石灰与低重金属含量的菜籽饼有机肥施用能够有效钝化重金属,同时减少土壤结构破坏[16-18]; Zhou等研究石灰和海泡石、腐植酸与沸石复配能显著抑制水稻植株Pb、Cd、Cu和Zn的吸收积累,从而减少重金属的毒性[19].

土壤改良剂对土壤中重金属的影响因改良剂的种类、施用量、土壤自身理化性质的差异而不同[20-21].湖南省部分耕地土壤重金属污染严重,存在石灰施用不合理且利用率低导致农业资源浪费,土壤理化性质被破坏等问题,制约了农业的可持续发展,因此研究改良剂修复Cd污染土壤及后效,对改善改良剂的不合理施用及其利用率低的问题具有重要作用.研究表明在Cd轻度污染土壤上,石灰、石灰与海泡石配施、石灰与甘蔗渣配施处理对抑制水稻各部位Cd的吸收均有一定的后效[22-23],石灰、木炭等处理对铜镉由活性态向非活性态和潜在活性态转化也有一定的后效[24],但缺少与连续施用改良剂处理对比试验,因此不能较好地说明施加改良剂的科学性.同时,研究Cd重度污染土壤上施用改良剂后效的报道相对较少.因此,本试验针对湖南省典型Cd重度污染土壤开展田间试验,通过对比连续施用和单季施用石灰、有机肥及石灰与有机肥配施,分析稻田土壤pH值、土壤有效态Cd含量以及水稻植株各部位对Cd的吸收特征,以期为进一步优化Cd污染稻田土壤治理效果和合理利用农业资源提供科学依据.

1 材料与方法

1.1 区域概况

田间试验选择在湖南某退役老工业基地周边污染稻田进行.试验田区域属湘江下游河谷及丘陵带,成土母质为第四纪红土,属亚热带季风性湿润气候,年平均气温16℃至18℃,年降水量在1500mm左右.供试田块土壤基本理化性质见表1.

表1 供试土壤基本理化性质(mg/kg) Table1 Basic properties of tested soil sample (mg/kg)

1.2 试验材料与设计

2016年6月,在该Cd污染田块开展田间试验,试验田翻耕后设置对照(CK)、施用有机肥(OM)、石灰(L)和石灰与有机肥配施(LOM)共4个处理,每个处理重复6次.每个处理小区面积为5m´6m=30m2,随机排列.试验小区周边设保护行,并用聚乙烯加厚塑料膜对小区田埂进行保护用来消除各区之间的干扰.按照试验设计要求将改良剂施入相应田块小区中(石灰施加量为1500kg/hm2,有机肥施加量为2250kg/hm2,组配改良剂施用前先将两种改良剂混合),随后将改良剂和土壤充分混匀,老化7d.2016年7月15日移栽秧苗,11月8日收获并采集土壤和相应水稻样品(2016T).2017年6月任选每个处理的三个小区继续按照2016年的试验设计施加改良剂(2017T),其余三个小区不再施加改良剂(2017NT),并按照对照处理进行移栽秧苗.整个水稻生长期间均按照当地一般农田的管理模式.

两年水稻种植品种均为天优华占,为籼型三系杂交水稻.田间试验用改良剂均为市售商品,其中石灰购自农资市场,氧化钙质量分数达80%,Cd含量为0.40mg/kg.有机肥购自湖南省湘晖农业技术开发有限公司的菜籽饼有机肥,N+P2O5+K>5%,有机质质量分数>45%,Cd含量为0.43mg/kg.

1.3 样品采集及分析

水稻成熟期采集田间长势均匀的代表性水稻植株样品(每个处理3株),用自来水洗净后,再用去离子水洗净,然后按水稻根、茎叶以及籽粒分开,在105℃杀青30min,60℃烘干至恒重,记录生物量.将样品粉碎后分别放入聚乙烯封口袋中密封备用.采集水稻植株相应的0~20cm表层土壤,经自然风干,除去动植物残体及碎石等杂物后研磨,分别过10目和100目尼龙筛后放入聚乙烯封口袋中备用.

土壤pH值采用1:2.5土水比浸提,用pH计(上海雷磁,PHS-3C)测定;土壤有机质含量采用重铬酸钾容量法测定;碱解氮采用碱解扩散-硫酸滴定法测定;有效磷采用碳酸氢钠提取-钒钼黄比色法测定;速效钾采用醋酸铵提取-火焰光度计法测定[25].土壤中有效态Cd采用DTPA提取法提取[25].土壤样品用HNO3-HF-HClO4法消解,水稻植株和糙米样品采用HNO3-HClO4法消解,浸提液和消解液中Cd含量采用ICP-MS(美国,Thermo Fisher X2)测定.

1.4 数据处理

数据统计采用Microsoft Excel2013进行分析,采用origin 9.0软件作图.单因素方差分析(One-way ANOVA)和相关性分析均采用SPSS 19.0完成,<0.05表示处理间有显著性差异,<0.01表示处理间有极显著差异.

2 结果与分析

2.1 土壤改良剂对土壤pH值,有机质和有效态Cd含量的影响

由图1可知,不同改良剂处理对土壤pH值影响程度不同.但与对照相比,施加有机肥、石灰及石灰和有机肥配施均能处理能显著(<0.05)提高土壤pH值.2016年施用OM、L和LOM的处理比对照土壤pH值显著(<0.05)提高0.46~1.20,施用石灰和有机肥均能在短期内不同程度提高土壤pH值[26-27]. 2017年连续施用OM、L和LOM处理的土壤pH值较对照显著提高0.27~1.05,仅2016年水稻季施用土壤改良剂的土壤pH值分别较对照提高0.19、0.37和0.75.说明连续施加土壤改良剂能够更有效维持土壤pH值.改良剂提高土壤pH值的效果表现为LOM>L>OM,说明对于Cd污染土壤,石灰和有机肥配施能进一步提高土壤pH值[28].有机肥的施用将大量的有机物带入土壤中.一方面,有机肥中有机物腐解过程产生的有机酸对土壤中重金属产生活化效应,增加重金属的生物有效性[29];但另一方面,有机质中大量的官能团吸附重金属[30],腐殖质分解产生的腐植酸与重金属离子形成络合物均能降低土壤重金属的有效性[31].从图1可知,与对照相比,2016年施用OM、L和LOM,2017年连续施用和不施用OM、L和LOM的土壤有机质质量分数均无显著性差异.仅2017年连续施用改良剂的土壤有机质质量分数略高于单季施用改良剂的处理.这可能与土壤有机质本底值较高,该种菜籽饼有机肥最佳肥效期为45d,其后有机质含量下降[32]及有机肥的施用量较少有很大的关系.

与对照相比,连续或单季施用OM、L和LOM处理均能显著(<0.05)降低土壤有效态Cd含量,2016年水稻收获后施用土壤改良剂的处理比对照土壤有效态Cd含量分别显著(<0.05)降低25.1%、20.8%和55.8%.与对照相比,2017年水稻季连续施用土壤改良剂土壤有效态Cd分别显著降低26.6%、29.7%和59.4%;不继续施加土壤改良剂的土壤有效态Cd含量也显著(<0.05)降低.土壤有效态Cd含量与土壤pH值的变化趋势一致,相比于石灰和有机肥单施,石灰和有机肥配施处理能进一步显著(<0.05)降低土壤有效态Cd含量.施用石灰能显著提高土壤pH值,增加土壤胶体表面的负电荷,同时促进土壤中铁锰氧化物的形成,进而增加土壤重金属的吸附位点和增强重金属的吸附能力,减少土壤重金属有效态[10,33].由于有机肥自身较高的pH值以及分解过程中释放的少量NH3,添加有机肥能一定程度提高土壤pH值,同时有机肥所含的有机质含有较大的比表面积和大量的官能团,使得土壤有效态Cd向其他形态转化[34-36].从图1中可以看出,连续施用土壤改良剂并不会进一步降低土壤有效态Cd含量,说明土壤改良剂的施用能有效降低土壤有效态Cd含量,单季施加的土壤改良剂有较强的后效[37].可见有机、无机改良剂配施能有效降低土壤有效态Cd含量,施加改良剂后的一定年限内可不施或适当减少改良剂的施用量以便降低农业生产投入成本和避免改良剂施用不合理导致的利用效率低的问题.

2.2 土壤改良剂对水稻各部位Cd吸收的影响

水稻植株中Cd含量的变化如图2所示.与对照相比,连续或单季施用OM、L和LOM处理均能显著(<0.05)降低水稻植株各部位Cd含量.水稻植株各部位Cd含量差异明显,大小顺序为根＞茎叶＞糙米,这与植物各部位代谢程度有关[38].石灰和有机肥通过控制土壤中有效态Cd含量减少Cd向水稻植株各部位迁移[13,19,39-40].2016年施用OM、L和LOM处理比对照处理下水稻根中Cd含量显著(<0.05)降低44.9%~85.8%.2017年连续施用OM、L和LOM处理的水稻根中Cd含量较对照分别显著(<0.05)下降60.4%、77.3%和86.2%,略低于单季施加OM、L和LOM处理下水稻根中Cd含量,其下降幅度分别为56.0%、73.5%和87.1%(<0.05).与对照相比,2016年施用OM、L和LOM处理茎叶中Cd含量均显著(<0.05)下降,尤其是LOM处理下水稻茎叶中Cd含量降低至0.58mg/kg.2017年连续施用OM、L和LOM处理水稻茎叶中Cd含量略低于单季施加土壤改良剂处理,与对照相比,水稻茎叶中Cd含量分别显著(<0.05)下降68.8%~88.7%和54.7%~ 86.1%.与水稻根和茎叶中Cd含量变化趋势相一致,与对照相比,单季或连续施用OM、L和LOM处理均能显著(<0.05)降低糙米中Cd含量.2016年施用OM、L和LOM处理糙米中Cd含量比对照处理显著(<0.05)降低35.9%、69.2%和65.4%.2017年连续施用OM、L和LOM处理的水稻糙米中Cd含量分别比对照显著(<0.05)降低63.1%、79.5%和83.6%,其中石灰与有机肥配施处理下糙米中Cd含量降低至0.20mg/kg,达到《食品安全国家标准食品中污染物限量》(GB2762-2017)中Cd限量值.与对照相比,单季施加土壤改良剂处理的糙米中Cd含量分别显著(<0.05)降低49.2%、69.7%和75.4%,双因素方差分析结果表明(表2)不连续与连续施用土壤改良剂对糙米中Cd含量无显著性影响.仅钝化剂种类对水稻地上部Cd含量产生极显著(<0.01)影响;就衡量因素作用大小的平方和(SS)来看,钝化剂种类因素的影响最明显,即水稻对Cd的吸收特征主要受钝化剂种类的控制.上述结果表明,施用土壤改良剂对减少水稻Cd吸收有较强的后效.

综上所述,石灰比有机肥能更有效地抑制水稻各部位对Cd的吸收.与石灰和有机肥单施相比较,石灰与有机肥配施效果最佳.石灰、有机肥和石灰与有机肥配施对抑制水稻各部位Cd的吸收均有较强的后效.因此从农业经济成本和改良剂对土壤环境质量影响的角度考虑,在轻Cd污染土壤上施加改良剂后一年内不施改良剂或在重Cd污染土壤上一定年限内适当减少改良剂使用量也可达到显著减少水稻植物体内重金属含量的效果.

表2 钝化剂种类与不同处理方式交互作用对水稻各部位的双因素方差分析 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:表中数据自由度值=3;<0.05表示该变异来源因素对水稻部位中Cd含量影响显著;CdG,CdS,CdR分别表示水稻糙米、茎叶和根中Cd含量(下同).

2.3 相关性分析

相关性分析结果(表3)表明,土壤DTPA提取态Cd含量、水稻根、茎叶和糙米中Cd含量与土壤pH值均呈极显著负相关(0.01).土壤pH值与土壤DTPA-Cd含量的相关系数为0.834,与水稻根、茎叶和糙米中Cd含量的相关系数分别为0.790,0.773, 0.711,说明稻田土壤pH值升高首先会引起土壤有效态Cd含量降低,从而进一步抑制水稻根对Cd的吸收及其向水稻地上部迁移.土壤DTPA-Cd含量、水稻根、茎叶和糙米中Cd含量均与土壤有机质含量呈负相关.在中性偏碱性的土壤中,土壤有机物随着pH值增加溶解度增大,络合能力增强,故大量重金属被络合[41],土壤DTPA-Cd含量减少.表3结果表明,土壤DTPA-Cd含量与土壤有机质含量相关性不显著,说明石灰和有机肥等碱性改良剂主要通过提高土壤pH值来降低土壤有效态重金属含量.李忠义等[42]和Zeng等[43]的研究中,土壤有效态Cd、Cu、Pb和Zn等与土壤pH值呈极显著负相关(<0.01),而与土壤有机质含量呈显著正相关,这可能是由于土壤中有效态重金属含量在短期内受外源添加物的影响较大.糙米中Cd含量与土壤DTPA-Cd含量、水稻根、茎叶中Cd含量均呈极显著正相关(<0.01),相关系数分别为0.666,0.932和0.914,该结果说明Cd首先通过从土壤中向根部转移,再经过木质部的蒸腾作用和功能叶中韧皮部随着同化物一起从输送到籽粒[44].因此,通过降低土壤中有效态Cd的含量,减少土壤Cd向水稻根和茎叶中转移能有效控制水稻糙米中重金属的含量.

表3 土壤pH值,有机质、DTPA-Cd含量与水稻各器官Cd的相关系数 Table 3 The correlation analysis between Cd contents in rice and soil pH, organic matter and DTPA-extractable Cd content in soil

注:表中数据为相关性系数(=3);* 表示显著相关性水平0.05;**表示极显著相关性水平0.01.SOM表示土壤有机质.

3 结论

3.1 有机肥、石灰单施及石灰与有机肥配施对提高稻田土壤pH值、降低土壤中有效态Cd含量和水稻各部位Cd含量均有显著效果,其中石灰与有机肥配施效果更显著.在4.70mg/kg Cd污染稻田土壤中,石灰与有机肥连续配施的糙米Cd含量降低至0.2mg/kg,较未施用处理显著降低83.6%,达到《食品安全国家标准食品中污染物限量》(GB2762-2017)中糙米限量值.

3.2 石灰、有机肥和石灰与有机肥配施处理对抑制水稻各部位Cd的吸收有较强的后效,尤其是石灰与有机肥配施处理,因此在Cd污染稻田土壤上施加改良剂后一年内可不施或少施改良剂.

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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,重庆巫溪人,中南大学冶金与环境学院硕士研究生,主要从事重金属污染土壤修复研究.发表论文5篇.

2018-07-26

国家科技支撑计划课题(2015BAD05B02);重金属污染耕地修复机理及技术模式优化集成项目(农业部财政部农办财函〔2016〕6号)

* 责任作者, 教授, zhguo@csu.edu.cn