Soil organic matter composition of spodic horizons in Podzols of the Northwest German Lower Plain

Abstract

Podzols and podzolized soils with a spodic subsoil horizon are widespread in the temperate and boreal climate regions. In this paper the soil organic matter (SOM) of several spodic horizons in the Northwest German Lower Plain was examined by wet chemistry, cross polarization magic angle spinning carbon-13 nuclear magnetic resonance spectroscopy (CPMAS 13C-NMR) and pyrolysis-field ionization mass spectrometry (Py-FIMS) in order to (a) elucidate possible effects of soil properties on SOM composition and formation in spodic horizons and (b) to discuss in the light of the results and prior literature the probable ecological significance of these soil horizons with respect to contaminants. The recent SOM data show no statistical correlation with other soil properties. However, rooting intensity determines the protein content in the SOM. In the spodic horizons the litter compounds reflect only a small part of the total SOM pattern, whereas the alkali-extractable humic matter ranges between 55 and 82%. According to the CPMAS 13C-NMR spectroscopy the mean composition of total SOM comprises 30% alkylics, 40% O-alkylics, 20% aromatics and 10% carboxylics. The significant appearance of the polyalkylic lipids has been confirmed with Py-FIMS data. NMR data suggests that within the wet chemical humic fractions (fulvic acids, humic acids, humins) chemistry is not unique and differs from one soil to another, whereas NMR cannot distinguish between available fractions and protected substances due to physical bonding as a result of flocculation and/or precipitation. Under Calluna vulgaris, wet chemical and 13C-NMR data indicate an extraordinarily high amount of alkylic compounds in the SOM and under Picea abies and Pinus silvestris, fulvic acids migrate through the Bh and precipitate in the Bhs Bs probably due to the increase in pH and iron supply. In the spodic horizons of buried Podzols and Podzols under cultivation, the SOM pattern is not significantly different from those under forest and heath. In contrast to the A horizons of arable and forest soils, the humic matter in the spodic soil layers shows a high percentage of aromatics and alkylics. This specific SOM composition suggests a great binding capacity for xenobiotica and heavy metals.