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 fr ont the published literature (76 papers) and from our own results on 311 wh ole soils, physical fractions (25 clay-, 43 silt-, and 52 sand-size fractio ns) 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 i n organic C content from 0.42 to 53.9% would show any universal influence o f management practice on the chemical composition of soil organic matter (S OM). The relative abundance of functional groups was calculated for the fol lowing chemical shift regions: 0-50 ppm (alkyls), 50-110 ppm (O-alkyls), 11 0-160 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, uncultiva ted, forest), climate (from tropical rainforest to tundra), cropping practi ce, fertilizer or manure application, and the different spectrometer charac teristics and experimental conditions used. Functional groups in whole soil s were always in the same abundance order despite the generally wide propor tion 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 final ly carbonyls (mean 10%, decreasing with soil C content). Humic and fulvic a cids contained much smaller proportions of O-alkyls than whole soils (means of 26%). Clay-size fractions were the most different from whole soils, bei ng more aliphatic (+8%). Sand-size fractions generally gave very similar re sults to whole soils.