Organic geochemical studies of soils from the Rothamsted Classical Experiments - IV. Preliminary results from a study of the effect of soil pH on organic matter decay

Total lipid extracts and solvent insoluble organic matter in soils from the Park Grass Experiment at Rothamsted Experimental Station, Harpenden, U.K. were studied to determine the effect of pH on the preservation/degradation of plant derived biomolecules. Analyses involved high temperature-gas chrom atography (HT-GC), HT-GC-mass spectrometry (HT-GC-MS), GC combustion-isotop e ratio MS (GCC-IRMS) and flash pyrolysis-GC (Py-GC) and Py-GC-MS. The plot s selected for study have pH values ranging from 3.7 to 7.3, with acidic so ils exhibiting two distinct horizons (i.e. humic rich top layer and mineral soil). The total lipid extracts of the soil samples with low pH exhibited higher relative abundances of long-chain (>C-20) organic acids believed to be derived largely from oxidation of plant lipids. The vegetation signature in the low molecular weight fraction is only retained in the humic rich to p layer. The signal in the mineral layer is believed to derive primarily fr om previous vegetation. Compound specific stable carbon isotope (delta(13)C ) measurements of long-chain n-alkanols are considered to reflect differenc es in the rate of incorporation of plant lipids into the humic top layer re lated to the grass species dominating the standing vegetation. In the soil samples of low pH, lignin contributes to the high molecular weight fraction of the humic layer. In contrast, the mineral layer of the same soil shows little evidence of intact lignin, but is instead dominated by amino acid py rolysis products, probably deriving from (degraded) polypeptides. The pyrol ysates of the mineral soils of high pH yield a distribution of products sim ilar to that found in the deeper layer of the low pH samples but with evide nce of lignin derived moieties. Overall, soil pH was found to have a signif icant effect on the preservation of higher plant derived biomolecules inclu ding ligno-cellulose. (C) 1998 Elsevier Science Ltd. All rights reserved.