Effect of soil pH on the chemical composition of organic matter in physically separated soil fractions in two broadleaf woodland sites at Rothamsted, UK

Although acid soils are common in forest ecosystems, and there is documented evidence of pH influencing transformations of organic matter in soil, there are surprisingly few studies on the influence of soil pH on the chemical structure of physically fractionated soil organic matter (SOM). The aim of this study was to characterize the influence of pH on the chemical and physical processes involved in SOM stabilization. Forest soils of different pH (4.4 and 7.8) sampled from two long-term experiments at Rothamsted Research (UK) were physically fractionated. The free light fraction (FLF), the intra-aggregate light fraction and the fine silt and clay (S + C, <25 µm) were characterized using elemental, isotopic (δ13C), thermogravimetric, differential thermal, diffuse reflectance infrared Fourier transform spectroscopy and high-resolution magic angle spinning 1H nuclear magnetic resonance analyses. The quantitative distribution of carbon (C) between SOM fractions differed between the two soils. Carbon contents in the light fractions from the acid soil were significantly greater than in those of the alkaline soil. In contrast, in S + C fractions, C content was greater in the alkaline soil. FLF from the acid soil was characterized by a greater C:N ratio, smaller δ13C and greater content of thermo-labile compounds compared with FLF from the alkaline soil. In contrast, there was only a weak effect of soil pH on the chemical composition of the organic matter in S + C fractions. Irrespective of soil pH, these latter fractions contained mainly aliphatic compounds such as carbohydrates, carboxylic acid, amide and peptide derivates. This suggested that physical mechanisms, involving the interactions between SOM and mineral surfaces, are of greater importance than the presence of chemically recalcitrant species in protecting SOM associated with the finest soil fractions.