Abstract:Plant may release volatile monoterpenes into the environment under natural conditions. These volatile monoterpenes play an important role in ecosystems, including allelopathy and chemical interactions among the host plant, herbivore, and parasitoid or predator in tri-trophic systems. Allelopathy of volatile monoterpenes released from various plants species could be achieved by both air and soil media. In this study, chemical constituents of the volatiles released from Ambrosia trifida L. and their effects on the germination of crop seeds and microbial population in soil were investigated. As a result, the monoterpenes are major components (84.2%) of the volatiles from A. trifida. Furthermore, saturated aqueous solution of the volatile monoterpenes from A. trifida affected seed germination and microbial populations in soil, indicating that allelopathic potential of the monoterpenes can be mediated by soil. GC and GC/MS are the effective instruments to determine the chemical constituents of volatiles. However, some scientists who are not chemists often determine the volatiles components by means of GC/MS only, not both GC and GC/MS. And this resulted in incorrect identification of volatile chemical constituents. Actually, the combined analysis of GC retention indexes (Kováts indexes) and GC-MS data with the aid of the mass chromatograms of the characteristic fragment ions reported in the NIST and WILEY databases is essential for the reliable identification of volatile constituents. This study clarified some mistakes in experimental methods, especially in determination of chemical constituents of plant volatiles by means of GC and GC-MS. These methods and results of this study would be helpful to our further understanding of volatile allelochemicals released from plant species to the soils in ecosystems.