Characterization of transformations of maize residues into soil organic matter

An awareness of the transformation of plant residues returned to cultivated soils is vital for a better understanding of carbon cycles, the maintenance of soil fertility and the practice of a sustainable agriculture. The transformation of maize (Zea mays L) straw residues into soil organic matter (SOM) in a one year incubation experiment was studied in a soil that had been under long term cultivation with wheat (Triticum aestivum L) for >30years. A novel sequential exhaustive extraction and fractionation procedure isolated a series of fractions of SOM. The samples were characterized by elemental and δ¹³C analyses, by amino acids and neutral sugars analyses, by Fourier transformed infrared (FTIR) spectrometry, and by solid state ¹³C nuclear magnetic resonance (NMR) spectroscopy and with chemical shift anisotropy (CSA) -filter and dipolar dephasing (DD) spectral editing NMR techniques. The δ¹³C data indicated that 59% and 38% of the newly transformed organic carbon was in the humic and fulvic acid fractions, respectively, and in general a greater proportion of the transformed carbon was in the fractions isolated at the higher pH values. Results for SOM fractions from the amended soil indicate dominant contributions from carbohydrate and lignin-like material, and that can be clearly identified by FTIR, CP/TOSS, and spectral editing of CSA-filter and DD. The compositions of the fractions from the amended and non-amended soils fractions can be clearly differentiated using principal component analysis (PCA) for the data collected. The sequential extraction procedure showed that the hydrophilicity of humic fractions increased as the result of the maize amendment, and the aromaticity of the fraction decreased. The data may give some indications of transformations that take place during humification processes.