Effect of soil pH on the chemical composition of organicmatter in physically separated soil fractions in twobroadleaf woodland sites at Rothamsted, UK

Although acid soils are common in forest ecosystems, and there is documented evidence of pH influencingtransformations of organic matter in soil, there are surprisingly few studies on the influence of soil pH onthe chemical structure of physically fractionated soil organic matter (SOM). The aim of this study was tocharacterize the influence of pH on the chemical and physical processes involved in SOM stabilization. Forestsoils 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 siltand clay (S + C, <25 μm) were characterized using elemental, isotopic (δ13C), thermogravimetric, differentialthermal, diffuse reflectance infrared Fourier transform spectroscopy and high-resolution magic angle spinning1H nuclear magnetic resonance analyses. The quantitative distribution of carbon (C) between SOM fractionsdiffered between the two soils. Carbon contents in the light fractions from the acid soil were significantlygreater than in those of the alkaline soil. In contrast, in S + C fractions, C content was greater in the alkalinesoil. FLF from the acid soil was characterized by a greater C:N ratio, smaller δ13C and greater content ofthermo-labile compounds compared with FLF from the alkaline soil. In contrast, there was only a weak effectof 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, amideand peptide derivates. This suggested that physical mechanisms, involving the interactions between SOM andmineral surfaces, are of greater importance than the presence of chemically recalcitrant species in protectingSOM associated with the finest soil fractions.