Diffuse reflectance Fourier transform infrared (DR-FTIR) spectroscopy was used to investigate the effects of organic and conventional management practices on soil organic matter (SOM) chemistry. We
characterized physically and chemically isolated SOM fractions collected from Rodale’s Farming Systems Trial (FST) experiment and used ratios of reactive (O-containing) and recalcitrant (C, H and/or N) functional group heights to characterize SOM fraction composition. Based on peak ratio comparisons, we determined that humic acid (HA) in the manure-amended organic rotation was more reactive than HA isolated from crop-residue-amended soils. Reactive/recalcitrant (O/R) peak ratios indicated that fulvic acid (FA) and particulate organic matter or light fraction (LF) isolated in April were most reactive in the organic cash-grain-based rotation. By June LF reactivity had changed; O/R ratios of LF and litter (LT) fractions isolated from the conventional and organic cash-grain soils were greater than the manure-amended soil’s ratios. We did not draw any conclusions about management impacts on SOM composition from spectra of fine-clay- (FC) and sand- (S) sized fractions. Using DR-FTIR we were able rank SOM fraction reactivity; average O/R ratios were FA > LT > HA > LF > FC. No consistent link between SOM fraction O/R ratios and reported C/N ratios or biological availability was made. Humic acid O/R ratios were most consistent with other estimates of overall SOM biological activity. More research is needed to clarify the relationship between the composition of specific SOM fractions and their functional significance in mineral soils.