Structural investigation of organic matter in physical soil fractions from two broadleaf forests by High Resolution Magic Angle Spinning NMR

1H HR MAS NMR is a powerful technique that allows to examine the chemical and physical properties of heterogeneous and polydisperse macromolecular systems such as soil organic matter (SOM). SOM plays a major role in the fate, transport, and bioavailability of organic and inorganic components and its dynamics depends on its chemical composition as well as by physical protection mechanisms with the soil fractions. Soil pH seems to be a variable that affects the SOM decomposition progression by influencing microbial activity, hydrolysis and protonation processes. Protonation regulates many soil processes such as solubilisation and complexation which controls sorption and desorption of organic C on mineral surfaces. Despite its documented importance as factor influencing SOM transformation and despite the high frequency of acid soil in forest ecosystems the effect of pH on SOM dynamics has been studied rather little. In the present study 1H HR/MAS NMR is applied to SOM extracted from two forest soils differing mainly for soil pH: the acid site, called Gesscroft, and the calcareous site, called Broadbalk. The two soils were physically fractionated according to Sohi’s procedure. Only the SOM of three fractions for each site was investigated: free light fraction (FLF), light fraction (IALF) and fine silt and clay (S+C); in fact they represented together the 70-80% of the total SOM in our soils and the organic C content was above the threshold value of the 3%, needed to obtain good signal/noise ratios in NMR spectra. 2-D 1H-1H homo-correlation spectroscopy (COSY), 1H-13C heteronuclear single quantum coherence (HSQC) and 1-D experiments were assessed for both a qualitative and a quantitative analysis of the soil matter. For both sites the S/N (signal to noise) ratio is higher for FLF-SOM and IALF-SOM with respect to S+C-SOM fractions in agreement with the decrease in carbon content. Amino acid residues, peptides and carbohydrates are the prevalent species and 6 typical chemical shift regions can be identified for each sample. Although S+C-SOM fractions of both sites show a finely decomposed matter, which is confirmed by the presence of sharp peaks, in particular the decrease in aromatic content of Gesscroft extracts with respect to Broadbalk ones suggests that the effect of acid pH seems to be relevant for degradation processes.