Abstract: A study was carried out to explore the composition of root exudates and the content of allelochemicals of sorghum root system in different continuous cropping durations. The sorghum was planted in buckets with the method of soil cultivation. Root exudates of sorghum in different continuous cropping years were extracted using two types of polar solvents, methylene chloride and ethyl acetate, and quantitatively identified with GC-MS, to clarify the composition and content of sorghum root exudates in different continuous cropping years and their relationship with crop yield, providing reference for the mechanism of continuous cropping obstruction and the sustainable development of sorghum planting. The results showed that the sorghum root exudates extracted by the two types of polar solvent contained alkane, alcohol, ester, benzene, ketone, aldehyde, while methylene chloride extracts contained 21 kinds of common root exudates, and the extracts of ethyl acetate contained 35 different types of root exudates. With increasing continuous cropping duration, the type number and relative content of root exudates increased first, peaked at 4 years of continuous cropping and decreased slightly thereafter, while the difference between the 4 years and 12 years of continuous cropping was insignificant. With increasing continuous cropping years, the relative content of hydrocarbon compounds increased first, peaked at 3 years of continuous cropping and decreased thereafter; the relative content of ester compounds increased gradually, with increasing amplitudes of 3.43%-40.13%. The relative contents of benzene, tone, and aldehyde all decreased first and increased thereafter. The quantitative analysis results of allelochemicals showed that, with increasing continuous cropping duration, the content of n-octadecane increased first and then decreased, and the contents of ethylbenzene and butyl acetate increased continuously. Furthermore, crop yield was not impacted by 2 years of continuous cropping, but the growth of sorghum was restrained notably at 3, 4, 12 years of continuous cropping: the ear length, branch number, thousand kernel weight, yield of single plant and the dry matter of single plant all decreased, among which the yield decreased by 2%-41%. Correlation analysis showed that ethylbenzene and butyl acetate contents negatively correlated with ear length, branch number, thousand kernel weight, yield of single plant, and dry matter of single plant, especially for branch number. In conclusion, the continuous cropping sorghum significantly influenced the types and contents of sorghum root exudates, increased the contents of ethylbenzene and butyl acetate (two types of inhibiting allelochemicals), and hence reduced the yield significantly.

Key words: canopy structure, understory light factor., community characteristics, pine-oak mixed forest