Studies of the organic matter components in Irish grassland soils and their drainage waters

The overall objective of this project was to provide fundamental information about the nature and the associations of the organic matter (OM) in two representative Irish grassland soils; Clonakilty, a well drained Brown podzolic soil (WDS) and Rathangan, a poorly drained Gley, (PDS) that have been installed in a lysimeter system at the Teagasc Environmental Research Centre, Wexford, Ireland. NaOH (0.1 M) was utilised to extract humic fractions from the WDS and XAD-8 and XAD-4 resins were used to fractionate the extract into humic (HA) and fulvic acid (FA) fractions. Drainage water (DW) was collected from the soil in the lysimeter system and dissolved organic matter (DOM) was isolated from it using XAD-8 and XAD-4 resins which fractionated the DOM into hydrophobic (Ho) and hydrophilic (Hi) acids. Using solid state nuclear magnetic resonance spectroscopy (NMR) and Electrospray ionisation (ESI) coupled to ultrahigh resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry, it was seen that there were definite similarities between the highly oxidised organic components isolated from WDS at pH 7.0 and the DOM isolated from the DW. The influence of management practices on DOM exports from grassland soils was also examined. Hydrophobic acids (Ho) were recovered from the DWs from the WDS and the PDS Irish grassland soils in lysimeters (control, no amendment) and amended with fertilizer and with bovine urine (U) and were studied using 1D and 2D solution state NMR spectroscopy. The Ho acids were found to consist of lignin residues, carbohydrates, protein/peptides, and aliphatic components derived from plant waxes/cuticular materials and from microbial lipids. The fertiliser amendments increased the concentrations of Ho in the DW and increased the contributions of the lignin-derived components. Applications of fertilizer + urine decreased the losses of Ho, (compared to the fertiliser amendments alone) and very significantly decreased those of the lignin derived materials, indicating that enhanced microbial activity from urine gave rise to enhanced metabolism of the Ho components. In contrast the less biodegradable aliphatic components containing cuticular materials increased as the result of applications of fertiliser + urine. This study helps our understanding of how management practices influence the movement of C between terrestrial and aquatic environments. A comprehensive sequential exhaustive extraction procedure of 0.1 M NaOH; 0.1 M NaOH and 6 M Urea, followed by a mixture of DMSO + 6% H2SO4 (v/v) was utilised to isolate HAs, FAs and humin fractions from the WDS and PDS soil. The highly oxidized components were concentrated in the humic fractions isolated at pH 7, and lignin-derived components were concentrated in fractions isolated at pH 12.6. Through the use of 6 M urea combined with 0.1 M NaOH, further HS were released from what was previously humin in the classical definition. The humic fractions isolated in base/ urea solvent system resembled the humic fractions isolated at pH 12.6 more closely that the residual material following base/urea extraction. These are in the range of humin materials in the conventional definitions, but are likely to be part of humic acids and fulvic acids trapped or selectively preserved in the soil humin matrix. Differences can be seen also at different pH values and at different depths, even within narrow ranges in the surface soils in both soils. The humic fractions isolated from the lower depths in both soils were more highly humified that the corresponding fractions from the surface soil, possible due to the input of fresh organic substrate in the surface layers. The exhaustive extraction and fractionation procedures revealed significant differences in the composition of the HS isolated from different grassland soils with different drainage regimes. A humin sample was isolated from the WDS following exhaustive extraction with a 0.1 M NaOH, followed by 0.1 M NaOH and 6 M Urea urea solvent sequence system. Using solid state NMR and higher resolution magic angle spinning NMR spectrsoscopy, the humin was found to contain fatty acids, and cuticular materials, carbohydrate, peptide, and with only minor contributions from lignin components. A comprehensive sequential, exhaustive extraction procedure was used to isolate humin samples from the PDS soil at the surface layer, 0-10cm and at a lower depth of 90-100 cm a 94% DMSO + 6% conc H2SO4 solvent system. The 2D solution-state NMR analysis indicated that humin contained contributions predominantly from protein, lipids/cuticlar waxes, carbohydrates and with minor contributions from lignin. Humin from the surface soil (0-10cm) shows evidence for more plant derived components, while the 90-100 cm humin has greater microbial inputs. Observations of humification of plant residues in soil are fundamental for defining the carbon cycle in natural and cultivated environments. The transformation of maize residues into relatively stable soil organic matter during the periods of 12 and 24 months of laboratory incubation experiments were observed. This work contributes to our knowledge and understanding of the transformation of organic materials to humic substances by allowing the humification process to be followed over time.