Transient nature of rhizosphere carbon elucidated by supercritical freon-22 extraction and NMR analysis

Abstract

Abstract The region immediately adjacent to established roots of mature trees has been termed the “reoccurring rhizosphere” and it has been hypothesized that organic matter input from fine root turnover, root exudates and sloughing may result in a build up of the soil carbon in this region. The “reoccurring rhizosphere” for first-, second- and third-order roots of select loblolly pines (Pinus taeda L.) were examined on sandy, loamy sand and sandy loam soils. A significant carbon build up next to the root orders was confirmed for the sandy and loamy sand soils. The carbon build up was substantial (55% increase) next to the first-order roots of the sandy soil. However, the sandy loam soil did not display a significant amount of carbon build up next to the root orders. Extraction of the soil samples with supercritical freon-22 showed that the additional carbon in the “reoccurring rhizosphere” was highly soluble. Approximately 60% of the total soil carbon was extracted from the sandy and loamy sand soils, while approximately 40% was extracted from the sandy loam soil. A qualitative comparison of the extracts by liquid state 13 C nuclear magnetic resonance showed that the “reoccurring rhizosphere” region had a higher relative proportion of labile materials (i.e. carbohydrates, proteins, etc.) than the bulk soil. This information coupled with the high solubility in supercritical freon-22 suggests that the carbon build up in the “reoccurring rhizosphere” region of loblolly pines may be transient in nature.