The Chemistry and Biochemistry of Organic Components in the Soil Solutions of Wheat Rhizospheres

As very little is known about the chemical identities of organic compounds present in soil solutions, the purpose of this research was to chemically characterize soil solutions obtained from a Brunisol on which wheat (Triticum aestivum) was grown to maturity. Soil solutions were obtained by high-speed centrifugation from the rhizospheres at four growth stages of wheat: four leaves unfolded, flag leaf, grain early milk, and grain-dry seed. Pyrolysis-field ionization mass spectrometry (Py-FIMS) was used to chemically characterize the soil solutions. We identified 346 organic components in each soil solution, which we grouped into the following 11 compound classes: carbohydrates, amino acids, phenols+lignin monomers, lignin dimers, n-alkyl benzenes, flavonoids, alkyl radicals, fatty acids+alkanes, N-heterocyclics+other N-compounds, sterols and steroids, and esters of suberin. The most abundant classes were lipids (fatty acids and alkanes), N-heterocyclics, and phenols+lignin monomers and dimers. The highest content of most organic components (except carbohydrates) in soil solutions occurred at the flag leaf stake of wheat growth. Many of the identified organics in soil solutions are secondary plant metabolites which represent key weapons in the continuous struggle of plant and soil microorganisms for ensuring vital space and access to nutrients and energy resources. Some of the identified compound are also chemical signals for the interactions between plants and soil microorganisms. The chemical components of the four soil solutions from wheat rhizosphere resembled that of humic substances (HSs). Organic components in soil solutions are substrates for the microbial and chemical synthesis of HS.