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  农业环境科学学报  2021, Vol. 40 Issue (4): 693-699  DOI: 10.11654/jaes.2021-0278
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引用本文  

赵方杰, 赵星宇, 陶祎敏, 等. 秸秆移除对降低土壤镉含量的效果有限[J]. 农业环境科学学报, 2021, 40(4): 693-699.
ZHAO Fang-jie, ZHAO Xing-yu, TAO Yi-min, et al. Straw removal has a limited effect on decreasing cadmium concentration in soil[J]. Journal of Agro-Environment Science, 2021, 40(4): 693-699.

基金项目

国家自然科学基金项目(21661132001)

Project supported

The National Natural Science Foundation of China(21661132001)

作者简介

赵方杰 ??教授,博士生导师,国家特聘专家。研究方向为植物吸收重金属及必需微量元素的分子机理、污染元素生物地球化学、污染土壤修复。1992年获英国Newcastle大学博士学位。1992—2012年任职英国洛桑研究所(Rothamsted Research),先后任研究员(Principal Scientist)、资深研究员(Senior Principal Scientist)。2012年起任南京农业大学全职特聘教授。主要从事环境生物学与植物营养学研究,在植物重金属吸收与解毒的分子机制、土壤污染成因与防控、作物硫素和微量元素营养等方面研究取得重要成果。迄今已发表SCI论文300余篇,在Web of Science被引用3万余次,H指数95,入选2017—2020年科睿唯安全球高被引用学者,爱思唯尔中国高被引学者

文章历史

收稿日期: 2021-03-09
录用日期: 2021-03-17
秸秆移除对降低土壤镉含量的效果有限
赵方杰 , 赵星宇 , 陶祎敏 , 王汉卿 , 汪鹏     
南京农业大学资源与环境科学学院, 南京 210095
摘要:秸秆移除是重金属中低污染农田安全生产广泛采用的措施之一,其目的是通过移除积累在秸秆中的重金属,逐步降低土壤重金属含量。本文通过对文献报道的数据进行模拟计算,以评估水稻秸秆移除对降低土壤镉(Cd)含量的效果。基于田间试验数据,水稻秸秆Cd含量与土壤Cd含量呈显著的线性关系。根据回归方程和秸秆平均生物量估算,在土壤总Cd含量为0.5~5.0 mg·kg-1范围内,单季水稻秸秆可移除的Cd占耕层土壤Cd总量的0.23%~0.37%(平均值为0.27%),秸秆和籽粒Cd总移除率的平均值大约为0.30%。研究表明,水稻秸秆移除对降低耕层土壤Cd含量效果非常有限。
关键词秸秆移除        田间试验数据    移除率评估    
Straw removal has a limited effect on decreasing cadmium concentration in soil
ZHAO Fang-jie , ZHAO Xing-yu , TAO Yi-min , WANG Han-qing , WANG Peng     
College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, China
Abstract: Straw removal is a widely used practice in low-moderately contaminated agricultural soils with the aim to remove heavy metals from the soils. However, there are few reports on the effectiveness of this practice. Here, we used field-based data reported in the literature to evaluate the potential of rice straw removal for reducing cadmium(Cd) concentration in soil. Straw Cd concentration correlated linearly and significantly with soil Cd concentration. The regression equation obtained and average straw biomass were used to estimate the amounts of Cd removal in straw from soils with 0.5~5 mg Cd·kg-1. Straw Cd removal represents 0.23%~0.37%(mean 0.27%) of the total Cd in the plow layer. Including Cd removal in rice grain increases the removal rate to approximately 0.30%. The results suggest that rice straw removal has a limited effect on decreasing cadmium concentration in soil.
Keywords: straw removal    cadmium    field-based data    removal rate evaluation    

我国部分地区农田土壤重金属镉(Cd)污染问题比较突出,造成农产品Cd含量超标,不仅影响农产品质量安全,还可能对人体健康产生不良影响[1-3]。按照《土壤环境质量农用地土壤污染风险管控标准(试行)》(GB 15618—2018)划分,土壤Cd含量高于风险筛选值,但低于风险管制值的农用地为“安全利用类”,在这类农用地进行农业生产必须采取措施以确保农产品达标。土壤Cd的风险筛选值和风险管制值因土壤pH不同而异,前者为0.3~ 0.6 mg·kg-1,后者为1.5~4.0 mg·kg-1。“安全利用类”属于中低污染,占污染农用地的绝大部分。我国2016年颁布的《土壤污染防治行动计划》要求,到2020年和2030年,受污染耕地安全利用率要分别达到90% 左右和95% 以上。因此,全国各地都在开展污染耕地阻抗农产品重金属积累的措施研究。一个广泛采用的措施是秸秆移除,其目的是通过移除积累在秸秆中的重金属,逐步降低土壤重金属含量。这个措施的效果如何,很少见到报道。本文通过对文献报道的数据进行模拟计算,评估水稻秸秆移除对降低土壤Cd含量的效果。

1 材料与方法

通过对发表于1998年至2020年的文献进行检索和筛查,从中获取相关数据进行分析与计算。英文文献检索自Web of Science与Google Scholar,以“Rice”“China”“Cadmium”;“Rice”“har? vest index”等组合的关键词进行检索。中文文献检索自《中国知网》,检索关键词为“水稻”“镉”;“水稻”“收获指数”。首先对检索出的论文进行筛选与剔除,剔除标准为:(1)非田间试验数据;(2)没有重复或没有明确标明重复次数的数据;(3)试验场地非中国地区的数据。对筛选后的文献进行数据收集和汇总,包括试验年份、地点、土壤Cd含量、水稻品种、秸秆生物量、秸秆Cd浓度、稻谷产量、稻米Cd浓度、收获指数等。

2 结果与讨论

通过文献检索,共查到基于大田试验并同时提供土壤Cd全量和水稻成熟期秸秆Cd含量数据的文献22篇(表 1),包括了44个试验点,试验点分布在我国12个省份(图 1a)。有些试验点包括多个水稻品种,本研究取其不同品种秸秆Cd含量的平均值。将秸秆Cd浓度与土壤Cd浓度作图(图 1b)可见,两者具有显著的线性相关关系,回归分析得到的方程如下:

(1)
表 1 秸秆Cd浓度与土壤Cd浓度线性关系数据 Table 1 Data of the linear relationship between straw Cd concentration and soil Cd concentration

图 1 试验点分布及秸秆Cd浓度与土壤Cd浓度的关系 Figure 1 Distribution of field trial sites and the relationship between straw Cd concentration and soil Cd concentration

式中:y代表水稻秸秆Cd浓度,mg·kg-1x代表土壤Cd总浓度,mg·kg-1

对22篇文献报道的籽粒和秸秆Cd浓度(表 2)作图可见,两者呈现显著的线性关系(图 2),回归方程的斜率为0.11,该斜率与Duan等[25]基于两试验点466份水稻品种的结果(斜率为0.12)相似。秸秆Cd浓度与籽粒Cd浓度的比率变幅为2.8~43.8,中值为8.0。

表 2 籽粒Cd浓度与秸秆Cd浓度线性关系数据 Table 2 Data of the linear relationship between grain Cd concentration and straw Cd concentration

图 2 水稻籽粒Cd浓度与秸秆Cd浓度的关系 Figure 2 The relationship between rice grain Cd concentration and straw Cd concentration

为了计算水稻秸秆积累Cd量,首先需要知道秸秆的生物量。绝大多数文献未报道秸秆生物量,但是根据稻谷产量和收获指数可以计算秸秆生物量。从32篇我国基于田间试验文献中,得到水稻稻谷的生物量(平均值±SD)为8.01±1.9 t·hm-2n=168),收获指数为0.50±0.06(n=139),即秸秆与稻谷生物量相等。

以8 t·hm-2的秸秆生物量,再根据回归方程(1)计算得到的秸秆Cd浓度,可以计算秸秆Cd的积累量(表 3)。以耕层深度为0.2 m、土壤容重为1.3 g·cm-3计算不同Cd浓度(0.5~5.0 mg·kg-1)土壤Cd总量(表 3)。单季水稻秸秆可移除的Cd占耕层土壤Cd总量仅为0.23%~0.37%(平均值为0.27%;表 3),移除率随着土壤Cd浓度增加呈下降趋势。总体而言,水稻秸秆移除对降低土壤Cd的效果非常有限。考虑到水稻籽粒Cd含量仅为秸秆Cd含量的1/8左右,水稻秸秆和籽粒对土壤Cd的总移除率为0.30%左右,按此计算,种植100季水稻可移除30%的耕层土壤Cd。

表 3 水稻秸秆对土壤镉移除率 Table 3 Removal rate of soil cadmium by rice straw

假设植物吸收的Cd来自于土壤中的有效态Cd,那么,秸秆移除可能使得土壤中有效态Cd的下降幅度大于土壤总Cd的下降幅度。土壤中不同形态Cd处于动态平衡之中,用同位素交换的方法测定的Cd E值,反映了土壤Cd的活性库(Labile pool)。不同土壤Cd E值占Cd总量的百分比变幅在10%~90% 之间,平均值为60%[27-28]。按此平均值计算,单季水稻秸秆和籽粒移除的Cd占土壤Cd活性库的0.5%。

以上计算结果代表我国不同土壤类型、不同水稻品种和栽培管理方式下的平均值。土壤性质中,pH值和土壤氧化还原状况对Cd的有效性影响很大。酸性和好氧条件下Cd有效性提高[2-3, 29-31],水稻对Cd的吸收增加,移除率也会相应增加。在一些有利于提高土壤Cd有效性的特定条件下,Cd移除率可比平均水平高3~5倍,即单季移除率可达土壤Cd总量的0.9%~1.5%。即使在这些情况下,水稻秸秆和籽粒对土壤Cd的移除效果仍然很有限。

根据已有的报道,我国不同地区大气沉降的Cd通量变化幅度为0.4~25 g·hm-2·a-1 [31-34]。与表 3比较可知,水稻秸秆Cd积累量与大气Cd沉降量基本相等。除了大气沉降外,灌溉水以及有些化肥和有机肥的施用也会向农田土壤输入Cd[2, 33]。因此,考虑到Cd输入,秸秆移除很可能并不会降低土壤Cd含量。

与其他重金属或类金属(如Pb、As、Hg)相比,Cd从土壤向植物的移动性较强,秸秆与土壤Cd的浓度比值(即富集系数)远大于其他重金属[35]。因此,秸秆移除对降低土壤中其他重金属或类金属含量的效果比Cd更低。

秸秆移除减少了有机质向土壤的输入。此外,水稻秸秆富含多种矿质元素,如Si的平均含量高达5%,每季秸秆Si的积累量平均值达400 kg·hm-2[36]。Si对水稻抵抗生物和非生物胁迫起重要作用[37-38],长期秸秆移除可使土壤有效Si含量下降,导致水稻减产[39]。此外,Si对水稻吸收Cd和As有抑制作用[40-41],长期秸秆移除可能削弱这种抑制作用。

3 结论

本研究基于文献报道的数据建立了水稻秸秆Cd浓度与土壤Cd浓度的线性回归方程,根据该方程计算了水稻秸秆对Cd的积累量,发现单季秸秆移除可降低耕层土壤Cd含量平均值为0.27%,降镉效果有限。考虑到秸秆移除可能带来的副作用,秸秆移除不宜作为中低污染农田安全生产广泛推广的措施。

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