Statistical analysis of published carbon-13 CPMAS NMR spectra of soil organic matter

We have collected solid-state C-13 nuclear magnetic resonance (NMR) data front the published literature (76 papers) and from our own results on 311 whole soils, physical fractions (25 clay-, 43 silt-, and 52 sand-size fractions) and chemical extracts (208 humic and 66 fulvic acids). Our purpose was to see whether a comprehensive analysis of data on >300 soils that ranged in organic C content from 0.42 to 53.9% would show any universal influence of management practice on the chemical composition of soil organic matter (SOM). The relative abundance of functional groups was calculated for the following chemical shift regions: 0-50 ppm (alkyls), 50-110 ppm (O-alkyls), 110-160 ppm (aromatics), and 160-200 ppm (carbonyls). There was a remarkable similarity between all soils with respect to the distribution of different forms of C despite the wide range of land use (arable, grassland, uncultivated, forest), climate (from tropical rainforest to tundra), cropping practice, fertilizer or manure application, and the different spectrometer characteristics and experimental conditions used. Functional groups in whole soils were always in the same abundance order despite the generally wide proportion range: O-alkyls (a mean of 45% of the spectrum, increasing with soil C content), followe(d) by alkyls (mean 25%), aromatics (mean 20%), and finally carbonyls (mean 10%, decreasing with soil C content). Humic and fulvic acids contained much smaller proportions of O-alkyls than whole soils (means of 26%). Clay-size fractions were the most different from whole soils, being more aliphatic (+8%). Sand-size fractions generally gave very similar results to whole soils.