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 examination of 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 physical protection mechanisms within soil fractions. Soil pH seems to be a variable that affects the rate and processes of SOM decomposition by influencing microbial activity, hydrolysis and protonation. 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 in pH. The acid site (Geescroft) and the calcareous site (Broadbalk) are both temperate deciduous woodlands at Rothamsted Research, UK. The two soils were physically fractionated according to the procedure of Sohi et al (2001). SOM in three fractions for each site was investigated: free light fraction (FLF), intra-aggreagte light fraction (IALF) and fine silt and clay (S+C). Together these fractions represented 70-80% of the total SOM in the soils and the organic C content was above the threshold value of 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 SOM in the the three fractions. For both sites the S/N (signal to noise) ratio is higher for FLF-SOM and IALF-SOM than for S+C-SOM fractions, in agreement with the lower carbon concentration in S+C-SOM. Amino acid residues, peptides and carbohydrates are the prevalent species and 6 typical chemical shift regions can be identified in each sample. The S+C-SOM fractions of both sites show strongly decomposed material compared to FLF and IALF, confirmed by the presence of sharp peaks at specific positions. The smaller content of aromatic groups in Geescroft extracts compared to Broadbalk suggests that the effect of acid pH influences degradation processes.