Soil Carbon Characterization 10 to 15 Years After Organic Residual Application: Carbon (1s) K-Edge Near-Edge X-Ray Absorption Fine-Structure Spectroscopy Study

Application of organic residuals (e. g., biosolids and composts) to soil may provide an effective method for sequestering carbon (C), but the long-term stability of such C is not well known. Two field sites were investigated to characterize soil C status 10 to 15 years after amendment with biosolids and composts. Particulate organic matter (POM) was extracted from soil samples by physical separation methods. Near-edge X-ray absorption fine-structure spectroscopy was used to characterize the organic C in the POM fraction. Near-edge X-ray absorption fine-structure spectra revealed, on average, the presence of O-alkyl C (43%), aromatic C (17%), alkyl C (17%), carboxylic C (11%), and phenolic C (11%) in the POM. The alkyl C/O-alkyl C ratio was within the range of 0.19 to 0.69. The addition of organic residuals maintained higher proportions of alkyl C and aromatic C and lower O-alkyl C in the POM fraction than the control, thus revealing a higher degree of organic C decomposition. The results suggest that the application of organic residuals can increase soil C decomposition and thus improves long-term soil C stability.