Qualitative differences between day- and night-time rhizodeposition in maize (Zea mays L.) as investigated by pyrolysis-field ionization mass spectrometry

Abstract

Rhizodeposition is an important pathway of atmospheric C-input to soil, however, quantity and quality of plant rhizodeposition are insufficiently known. Therefore, the composition and diurnal dynamics of water-soluble root-derived substances and products of their interaction with sandy soil were investigated in maize plants (Zea mays L.) by pyrolysis-field ionization mass spectrometry (Py-FIMS). In both night- and day-rhizodeposits the C, N and S concentrations were larger by factors ranging from 3.0 to 9.7 than the samples from non-cropped soil. The rhizodeposition was larger during the day than during the night-time and the composition of these deposits was different. The largest differences in the Py-FI mass spectra resulted from signals assigned to amino acids (aspartic acid, asparagine, glutamic acid, leucine, isoleucine, hydroxyproline and phenylalanine) and carbohydrates, in particular pentoses, which were exuded in the photosynthetic period. Marker signals in the Py-FI mass spectra and the curves of their thermal volatilization provided unequivocal evidence for the occurrence of free amino acids in the day-rhizodeposits. Other compounds detected in the Py-FI mass spectra were interpreted as constituents of rhizodeposits (lipids, suberin, fatty acids) or products of the interaction of rhizodeposits and microbial metabolites with stable soil organic matter (lignin dimers and alkylaromatics). It was concluded that the diurnal dynamics in the molecular-chemical composition between day- and night-rhizodeposits resulted from the exudation carbohydrates and amino acids during the photosynthetic period, the deposition of other root-derived compounds such as lipids, suberin and fatty acids, and the microbial metabolism of all available organic compounds in the rhizosphere. Furthermore, applications of the presented approach in C-turnover and phytoremediation research, and for risk assessment of genetically modified crops are proposed.

Introduction

Rhizodeposition is classified into water-soluble exudates (e.g. sugars, amino acids, organic acids), secretions (polymeric carbohydrates, enzymes), plant mucilages (more complex organic compounds originated in root cells or from bacterial degradation), mucigel (a gelatinous layer composed of mucilages and soil particles intermixed), and lysates (e.g. content of sloughed-off cells, cell walls, whole roots) (Brady, 1990). The soluble and colloidal rhizodeposits can be rapidly decomposed as a nutrient source by soil microorganisms, bound by the soil particles, or react with the soil organic matter (SOM). Furthermore, roots release different compounds which interact with the soil matrix or are biologically active and affect the soil microorganisms (De Vries et al., 1999).

The chemical composition of root-derived compounds was examined by different researchers using high performance liquid chromatography (HPLC) (Merbach et al., 1999, Gransee and Wittenmayer, 2000, Hütsch et al., 2002). Three main classes of compounds were quantified: carbohydrates, carboxylic and amino acids. However, rhizodeposits can include further groups of compounds, not detectable by HPLC, which are likely to interact with the soil ecosystem. Fan et al. (2001) offered a combination of multinuclear and 2D nuclear magnetic resonance with gas chromatography/mass spectrometry (GC-MS) to examine the role of exudate metal ion ligands in the acquisition of Cd and transition metals by barley. A new approach to isolate the root exudates and their products of interaction with soil was proposed by Kuzyakov and Siniakina (2001). They have also reported about higher intensity of root exudation during the day-time in perennial ryegrass (Lolium perenne L.). However, the composition of rhizodeposits has not been studied. Thus, the molecular composition of bulk rhizodeposits and products of their interaction with the soil are still unknown. Furthermore, it is not clear if the higher intensity of root exudation and respiration during the day-time (Kuzyakov and Siniakina, 2001) has any influence on the chemical composition of rhizodeposits.

Pyrolysis-field ionization mass spectrometry (Py-FIMS) was successfully applied for the analysis of SOM (Schulten and Leinweber, 1999) and SOM fractions such as dissolved organic matter (DOM) (Schulten et al., 2002). In addition to the compound classes, identifiable by HPLC, such important groups as lipids, loosely attached to humic substances, or free fatty acids, peptides, N-containing aromatic or heterocyclic compounds, phenols and lignin monomers, sterols, etc. can be detected by Py-FIMS (Schulten et al., 1998). Recently, Py-FIMS was used for the analysis of root exudates from maize plants (Kuzyakov et al., 2003). Spectra evaluation with principal component analysis indicated differences between exudates leached from pure quartz sand and non-sterile sandy loam, and collected for the day and night period.

The objectives of the present study were (1) to reproduce the initial findings of Kuzyakov et al. (2003) with a sandy soil; (2) to investigate the chemical composition of leached rhizodeposits in comparison to leachates from non-cropped soil; and (3) to investigate differences in the chemical composition between day- and night-rhizodeposits by Py-FIMS of leached samples and pure reference substances.