Soil organic matter stabilization pathways in clay sub-fractions from a time series of fertilizer deprivation

Abstract

We have employed pyrolysis field ionization mass spectrometry (Py-FIMS) to elucidate potential differences in soil organic matter stabilization pathways among clay size fractions taken from a time series corresponding to 18, 44 and 98 years of fertilizer deprivation at Bad Lauchstädt, Germany. Fine clay fractions contained greater quantities of carbohydrates and more peptides but lower quantities of lignin dimers and lipid than coarse clay fractions. Upon prolonged fertilizer deprivation, the organic matter composition remained more or less constant in the fine clay fractions. After 98 years of deprivation, the organic matter in the coarse clay fractions seemed to approach the compound class distribution in the fine clay fractions. The thermal energy required to volatilize organic matter was higher for the fine clay fractions. The thermal stability of organic matter associated with fine clay was constant over time, while that of carbon associated with coarse clay increased with prolonged fertilizer deprivation in the range 10–50 K depending on the compound class. The organic matter associated with the coarse clay fraction responded to fertilizer deprivation by way of both a compositional change approaching the compound class distribution in the fine clay and an increase in thermal stability approaching the conditions encountered in the fine clay. The compound class distribution in the fine clay fraction related more closely to the NaF-reactive hydroxyl groups than that in the coarse clay fractions. Oxalate – extractable Fe (Fe o) normalised to the surface area related to the compound class abundance in the coarse clay, but not in the fine clay. We interpret this as an indication of a relationship between mineral surface reactivity and organic matter composition.