Identification of autotoxins in rhizosphere soils under the continuous cropping of cowpea

We studied phytotoxicity and chemical composition of three rhizosphere soils used for continuous cropping of cowpea for 0, 4 and 8 years, respectively. The concentration and phytotoxicity of four organic acids (i.e., phenylacetic acid, cinnamic acid, 4-hydroxybenzoic acid, and phthalic acid) from the continuous cropping soils were examined and they were also identified from the cowpea root extracts. Compared with the distilled water and extract of soils from the continuous cropping of cowpea for 0 years, the extracts of rhizosphere soils from continuous cropping for 4 and 8 years significantly inhibited the seed germination rates and shoot growth of cowpea. Using GC-MS, we detected 25, 27 and 28 principal chromatographic peaks in the soil extracts of continuous cropping cowpea for 0, 4, and 8 years, respectively. Twenty-one compounds (including organic acids, esters, ketones and hydrocarbons) were found in all three extracts. With increase in the years of cropping, the phenylacetic acid, cinnamic acid, 4-hydroxybenzoic acid contents were increased, whereas that of squalene decreased. HPLC analysis showed that phenylacetic acid, cinnamic acid, 4-hydroxybenzoic acid and the phthalic acid were accumulated to the inhibitory concentrations in these soils. Although these organic acids partially came from the root extracts of cowpea, but most of the organic acids present in the root extracts were not found in cowpea soils. In conclusion, the existence of autotoxicity in soils under continuous cowpea cropping was verified due to the accumulation of the above four organic acids.