Impact of a long-term cultivation on low molecular weight organic acids in greenhouse soil and their influence on vegetable uptake heavy metals

Abstract

ABSTRACT Low-molecular-weight organic acids (LMWOAs), a type of small organic molecules with one or more carboxyl groups present in soil, play an important role in the soil ecosystem. Greenhouse soil could be as a key example to understand LMWOA distribution and its ecological effects on heavy metals. In this study, 54 soil samples and 24 vegetable samples were collected from Wuqing, Tianjin, China, to investigate the distribution of LMWOAs in greenhouse soil and its effect on the transfer of heavy metals from soil to vegetables. The results showed that the mean concentrations of lactic acid, acetic acid, formic acid, propionic acid, butyric acid, malic acid, and oxalic acid detected in the greenhouse soil were 5.00, 20.57, 0.62, 23.38, 0.64, 4.67, and 29.03 mg/kg. Except for formic acid, the concentrations of LMWOAs in greenhouse soils were higher than LMWOAs in field soils. The content of oxalic acid, acetic acid, lactic acid, malic acid, propionic acid, and butyric acid in greenhouse soils is 2.3 times, 2.25 times, 1.82 times, 2.38 times, 1.38 times, 1.93 times that of LMWOAs in field soil. LMWOAs in the soils had obvious surface cohesion characteristics. The correlation between LMWOAs indicates that propionic acid has the same origin and/or similar variation characteristics as malic acid, oxalic acid, acetic acid, butyric acid, and lactic acid. Oxalic acid, formic acid, acetic acid, and lactic acid have a significant contribution to soil acidification. Accumulation of available phosphorus (AP) and available potassium (AK) in the soils may result in increased acetic acid, butyric acid, oxalic acid, and lactic acid. The above results indicate that LMTOA has different effects on the transfer of heavy metals from soil to vegetable. In addition, the accumulation of LMWOAs could stimulate the transfer of Cd, Zn, and Cu from soil to vegetable, but inhibit the absorption of Pb by vegetables.