外源有机酸对土壤pH值、酶活性和Cd迁移转化的影响

为了筛选植物修复土壤Cd污染适宜的外源有机酸,采用盆栽试验,在温室内以油菜为试验材料,在模拟重度Cd污染的土壤(原土Cd含量为0.838 mg·kg~(-1),人工喷洒CdCl_2水溶液,制备成Cd含量为4.838 mg·kg~(-1)的试验土)中加入5种有机酸:乙酸、草酸、柠檬酸、苹果酸和酒石酸,设置6个浓度:1、2、3、4、5、6 mmol·kg~(-1),以不加有机酸为对照(CK),测定了油菜收获时的土壤pH值、酶活性和油菜干物质量以及Cd累积量,并分析了土壤理化指标与土壤Cd形态之间的关系。结果表明:1、3、4、5、6 mmol·kg~(-1)乙酸处理,4、5、6 mmol·kg~(-1)柠檬酸处理,3 mmol·kg~(-1)苹果酸处理,3、6 mmol·kg~(-1)酒石酸处理可显著增大土壤pH值(P0.05),草酸处理pH值与CK差异不显著;但施加有机酸对土壤酶活性的影响不明显。1、4、6 mmol·kg~(-1)乙酸处理显著提高了油菜地上部干物质量,1、6 mmol·kg~(-1)乙酸处理根干物质量较CK增加了1倍以上,差异显著(P0.05),4、6 mmol·kg~(-1)苹果酸处理根干物质量较CK显著增加了77.13%和88.30%(P0.05),其余处理与CK差异不显著。1 mmol·kg~(-1)乙酸处理地上部Cd累积量较CK增加了51.52%,2mmol·kg~(-1)草酸处理根系Cd累积量较CK增加了1.58倍,1 mmol·kg~(-1)柠檬酸处理地上部+根系Cd吸收总量高于CK,差异均显著(P0.05);增加苹果酸量有利于提高根系Cd吸收总量,1、2 mmol·kg~(-1)酒石酸处理也提高了根系Cd累积量,但与CK差异均不显著(P0.05)。施加乙酸时,土壤pH值与铁锰氧化物结合态Cd和土壤总Cd显著负相关,施加柠檬酸时,土壤pH值与碳酸盐结合态Cd和试验结束时土壤中的总Cd量显著正相关,施加苹果酸时土壤pH值与可交换态Cd显著正相关,其余处理土壤pH值与Cd形态相关性不显著。在碱性土上种植油菜,施加5种有机酸均会增大收获时土壤的pH值,且不同有机酸施加量对土壤Cd形态的影响不同。5种有机酸中乙酸最有利于提高油菜干物质量和油菜Cd累积量。

Effects of exogenous organic acids on soil pH, enzyme activity, and cadmium migration and transformation

To screen suitable exogenous organic acids for phytoremediation of soil Cd pollution, a pot experiment was conducted in a greenhouse with rapeseed as the test material. Five organic acids(acetic acid, oxalic acid, citric acid, malic acid, and tartaric acid)were added to heavily Cd-contaminated soil(the Cd content of original soil was 0.838 mg·kg^-1; by spraying CdCl₂ solution, we created a test soil with a Cd content of 4.838 mg·kg^-1), with no organic acid treatment as the control. Six organic acid gradients were set at 1, 2, 3, 4, 5 mmol·kg^-1, and 6 mmol·kg^-1. Soil pH value, enzyme activity, dry matter content, and Cd absorption of rapeseed were determined when the rapeseed was harvested. The relationship between physical and chemical indexes and soil Cd morphology was analyzed. The results showed that:Compared to the CK, 1, 3, 4, 5, 6 mmol·kg^-1 acetic acid, 4, 5, 6 mmol·kg^-1 citric acid, 3 mmol·kg^-1 malic acid, and 3, 6 mmol·kg^-1 tartaric acid could significantly increase soil pH, and there was no significant difference between the oxalic acid treatment and that of the CK. However, the effect of organic acid application on soil enzyme activity was not obvious. The application of 1, 4 mmol·kg^-1, and 6 mmol·kg^-1 acetic acid significantly improved dry matter content of rapeseed shoots, 1 mmol·kg^-1 and 6 mmol·kg^-1 acetic acid significantly increase the dry matter content of rapeseed roots, 4 mmol·kg^-1 and 6 mmol·kg^-1 malic acid significantly increased the dry matter content of rapeseed roots by 77.13% and 88.30%, respectively, and there was no significant difference between other treatments with that of the CK. Cd uptake in shoots in the 1 mmol·kg^-1 acetic acid treatment was 51.52% higher than that of the CK, Cd uptake in roots of the 2 mmol·kg^-1 oxalic acid treatment was 1.58 times higher than that of the CK, and the total uptake in shoots and roots of 1 mmol·kg^-1 citric acid treatment was significantly higher than that of the CK. Increasing malic acid application was beneficial in increasing the total uptake of Cd in roots, and applying 1 mmol·kg^-1 and 2 mmol·kg^-1 tartaric acid was also beneficial in increasing root Cd uptake, but they exhibited no significant difference with that of the CK. Under the acetic acid condition, soil pH was negatively correlated with Fe-Mn oxide Cd and total Cd, whereas under the citric acid condition, soil pH was positively correlated with carbonate Cd and total Cd. When malic acid was applied, there was a significant positive correlation between soil pH and exchangeable Cd, but there was no significant correlation between pH and Cd form under other treatments. When rapeseed was planted on alkaline soil, the application of the five organic acids increased the pH value of the soil at harvest, and the effect of different organic acids on the form of Cd in the soil was different. Among the five organic acids, acetic acid was the most beneficial in improving the quality of dry matter content and Cd accumulation of rape.