Rhizosphere microbial diversity as influenced by humic substance amendments and chemical composition of rhizodeposits

The influence of humic substances (HS) on crops and soil microbial communities still lacks a complete understanding. The influence of HS on chemical composition of maize rhizodeposits and diversity of rhizosphere microbial communities was studied here in rhizobox systems. Eleven different HS were characterized by elemental analyses and C-13-CPMAS NMR, and applied to the upper compartment of rhizobox systems cultivated with maize. Once a full rhizoplane was formed, the lower compartment of the rhizoboxes was sampled with a slicing device, in order to obtain rhizopshere (0-2 mm) and bulk (> 10 mm) soil samples, which were subjected to chemical analyses of organic acids and sugars (pentoses and hexoses), bioassays with a luminescent biosensor for the detection and quantification of bioavailable C forms, and analyzed for bacterial diversity by means of PCR-DGGE fingerprint. Multivariate techniques were applied to assess correlations between the parameters involved and to assess the most relevant chemical drivers of rhizosphere microbial diversity. Results showed that the hydrophilic and poorly aromatic HS changed the pattern of sugars and organic acids released in the rhizosphere by maize roots, and this, in turn, affected the composition of rhizosphere microbial communities. Differences in chemical and microbiological composition were also found, but to a lesser extent, among bulk soil samples of different treatments, thus indicating that rhizodeposits can influence soil composition at several mm distance from the root plane. Multiple regression analyses suggested a prominent role of succinic and malic acid in shaping the diversity of microbial communities. The approach and results described here proved to be useful to test bioactivity of HS towards an agricultural crop, and identify correlations between chemical and microbiological constituents at the tri-phase soil-roots-microbes system. (c) 2012 Elsevier B.V. All rights reserved.