Effect of manure and crop residues continuous application on humic C in a cultivated soil.

The chemical characteristics of soil organic matter (SOM) can be influenced by management and amendment practices which effects can be measured only after long-term experiment. In this long-term study of over 30 years, with a rotation wheat-corn, we compared the effects of adding cattle manure (CM) and crop residues (CR) wheat straw or corn-stalks after each crop, on humic substances (HS). Potentiometric titration, thermal analysis (TG-DTA), and spectroscopic methods such as diffuse reflectance infrared Fourier (DRIFT) and liquid nuclear magnetic resonance (13C NMR) spectroscopies were used in order to investigate humic acid (HA) structure. The amendment practices clearly influenced the humic C and the COOH groups content that only increased in CM treatment. The quality of this humic fraction was affected by the different agricultural practices, so that when the soil did not receive any amendment, the aromatic and carboxylic C decreased, whereas the aliphatic C increased as an effect of the crop rotation. With the amendments, in contrast, the aromatic C generally increased, this increase was mainly due to the incorporation of aromatic groups in the structure of HA, arising from the phenolic groups present in the lignin of the crop residues.