Root organic acid exudates and properties of rhizosphere soils of white spruce (Picea glauca) and subalpine fir (Abies lasiocarpa)

Abstract

Rhizosphere processes, such as the production and release of organic acids, contribute to differences in properties between rhizosphere (R) and nonrhizosphere (NR) soils. We compared the organic acid composition/concentrations in the R and NR soil extracts of Picea glauca and Abies lasiocarpa, and in the root exudates of the same tree species. We also evaluated the chemical properties and mineral composition of the soil samples. Higher concentrations of organic acids were observed in R than in NR soils with aliphatic acids predominating in both soil extracts and root exudates. Acetic, formic, and protocatechuic acids were the predominant aliphatic and aromatic organic acids in R and NR soil extracts. Several organic acids in R soil extracts were not detected in NR soil extracts, with the reverse being true as well. Malonic and oxalic acids were predominant in the root exudates of both plant species. The total organic acid concentration was higher in P. glauca than in A. lasiocarpa. Glutaric and isocitric acids were found only in A. lasiocarpa, while gluconic, succinic, and protocatechuic acids were exclusive to P. glauca. Acetic and formic acids were detected in all R soil extracts, but not in root exudates. Glutaric acid (in P. glauca) and gluconic/succinic acids (A. lasiocarpa) were undetectable in root exudates, but were present in the corresponding R soil extracts. Soil pH was lower in the R than NR soil of P. glauca. Cation exchange capacity, and exchangeable Ca2+ and K+ were higher in R than in NR soils except for the A. lasiocarpa LFH horizon. Chlorite, mica, kaolinite, and vermiculite were detected in all soil samples. Smectite was detected more frequently in R than NR soils. Our results suggest that high amounts of organic acids might have played a significant role in the transformation of mica and chlorite into smectite in R soils.Key words: Rhizosphere, root exudates, organic acids