Effects on the composition and structural properties of the humified organic matter of soil in sugarcane strawburning: A chronosequence study in the Brazilian Cerrado of Goiás State

Abstract

The heating of soil organic matter after burning events can significantly change the composition and structure of humified organic matter. The impact of burning on the amount and quality of SOM depends on fire intensity, recurrence, the type of vegetation, soil texture and the local climate. The present study identified the composition and structural properties of humic acids (HA) extracted from a sandy loam textured Latossolo Vermelho (Oxisol) managed with the pre-harvest burning of sugarcane straw for chronosequences of 1, 5, 10 and 20 years in Rio Verde, Goiás (GO). Soil samples were collected at depths of 0–0.05, 0.05–0.10, 0.10–0.20 and 0.20–0.30m. To obtain a reference of natural conditions, soil was also sampled in adjacent areas covered with native vegetation and Pasture grass. The HA samples from soil under different burning conditions and areas (Cerrado and Pasture grass) were analyzed using chemical-spectroscopic characterization and the chemiometric technique. The greatest modifications in HA composition after burning occurred in the most superficial soil layers. At a depth of 0.0–0.05m, the main changes observed were a decrease in mean C and N levels and enrichment in O, irrespective of the length of time that burning practices had been adopted. Additionally, in the most superficial layer, the ultraviolet-visible spectroscopy assay showed that burning enhanced the aromatic properties (<E4/E6) of HA, and FTIR spectroscopy coupled with PCA confirmed that the top soil layer shows higher variations in the functional groups as a function of burning. The PCA analysis of 13C NMR spectra also showed that stronger aromatic properties were found in the most superficial layer, and the PCA of FTIR spectra reinforced the result by grouping HAs in these layers and thereby indicated that the most intense changes occurred in the 0.0–0.05m layer. The most intense disturbances produced by sugarcane burning on the composition and structure of humified organic matter occurred in the most superficial layer. The combined use of the methods adopted was efficient and, along with the chemiometric techniques, consisted of useful tools that can support the development of procedures to study phenomena such as burning.