Chemical and spectroscopic composition of humic substances in soil subjected to pig manure applications for ten years

Application of pig manure (PM) in agriculture can influence the amount and composition of soil organic matter (SOM). This study evaluated the changes in contents and stock of C in chemical fractions of SOM and the chemical and spectroscopic composition of humic substances (HS) in a Typic Hapludult ( Argissolo Vermelho-Amarelo) ) after ten years of PM application. Experimental area received 90 and 180 kg ha-1 of N in the form of pig slurry (PS90 and PS180) and pig deep litter (DL90 and DL180), in addition to the control, without application (SA). Soil samples were prepared, and the chemical fractioning of SOM, elemental analysis, and Fourier Transform Infrared Spectroscopy of HS were performed. Applications of PM favored the accumulation of C in soil (up to 53 %), and PS180 increased humic acids (HA) (up to 185 %), while applications of DL favored the increase of hydrophilic substances extracted with HCl and humin (HU) (up to 10 times and 60 %, respectively). Applications of PS180 and DL180 promoted an increase in the aromatic and carboxylic character of the HS, increasing cationic exchange capacity. Therefore, PM applications, especially with DL, contribute positively to C fixation in the soil and the chemical composition of organic material.