Fulvic acid composition in degraded fenlands

Abstract

Fulvic acids (FAs) from topsoil and ground water solutions were investigated to discover effects of land use and peat degradation on their molecular chemical composition and thermal properties. The FAs were extracted from three Gleysols under arable land, intensive and extensive grassland, and from three Histosols under alder forest, extensive grassland, and a natural succession in a long-term (> 200 years) cultivated fen area. Functional groups and molecular subunits of the FAs were investigated by 13C Nuclear Magnetic Resonance (13C NMR) spectroscopy. Thermal properties and structural molecular subunits were investigated by off-line pyrolysis, and Pyrolysis-Field Ionization Mass Spectrometry (Py-FIMS). The 13C NMR spectra showed that the FAs from topsoil solutions had smaller proportions of alkyl C (mean: -8 %) and more aromatic C (mean: + 6 %) than FAs from ground water. This clear differentiation of dissolved FAs in the soil profiles is consistent with Py-FIMS data which have shown enrichments of lipids in ground water FAs. Furthermore, Py-FIMS revealed that the FAs from topsoils were richer in phenols + lignin monomers, carbohydrates, as well as mostly aromatic N-containing compounds. These molecular subunits of FAs, relatively enriched in topsoil, were also the main indicators of land use and peat degradation. For topsoil solutions, the proportions of phenols + lignin monomers and carbohydrates increased stepwise with peat degradation in Gleysols and Histosols. Correspondingly, the thermal properties indicated the incorporation of these compounds into FAs by chemical bonds of larger thermal stability. Statistical evaluation by principal component analysis of Py-FIMS clearly supported the differentiation of FAs according to the origin from topsoils and ground water, different soil types, and land use and peat degradation. Hence, it is concluded that water soluble FAs can be utilized as objective ecological indicators for soil effects on adjacent ground and surface waters.