Chemical and Spectroscopic Characteristics of Humic Acids Under No-Tillage and Forest Systems

Abstract

ABSTRACT This study aims to evaluate the no-tillage system (NT) with different implantation times and vegetation cover on changes in the chemical and structural compositions of soil humified organic matter. Five areas with different NT management histories were studied; for 6 (NT6), 14 (NT14), 22 (NT22) years with succession of soybean/corn crops; for 12 years and 4 more years with corn and Brachiaria (NT+B), pasture for 39 years and native forest. Soil samples were collected at the depths of 0.0–0.05 and 0.05–0.10 m. The changes in the quality of humic acid (HA) were evaluated by several chemical techniques (total organic carbon (TOC), fulvic acid fraction (FA), humic acid fraction (HA) and humin fraction (HU), ionizable functional groups, elemental composition) and spectroscopic techniques (UV–Vis absorption, fluorescence, and Fourier transform mid-infrared photoacoustic spectroscopy, called FTIR-PAS). On the surface, the TOC, carbon of the HA and HU fractions, total acidity, phenolic acidity, Carbon, Nitrogen, Sulfur, Hydrogen and Hydrogen/Carbon ratio were higher in the areas of forest, pasture and NT22. On the subsurface, the responses are the same as on the surface. However, the NT22 system presents higher FA, HA, carboxylic acidity and humification degree (HD). UV–Vis absorption spectroscopy, fluorescence spectroscopy and FTIR-PAS can discriminate soils under different managements and under natural conditions. Agronomic practices have strongly affected the soil organic matter, in which the NT promotes the formation of HA with characteristics closer to those of the forest, as the implantation time increases, such as 22-year-old NT and pasture. This indicates that care must be taken in managing the no-till system to maximize its potential to promote stabilization of organic matter, thereby contributing to carbon sequestration.