Banca de DEFESA: MELANIA MERLO ZIVIANI

Stocks of C and N and Humic Substances in Soils with High Organic Matter Content in the Upper Part of Itatiaia National Park. 

Histic horizons, PTF, soil density, humic fractions, labile carbon, SiBCS.

In the upper part of Itatiaia National Park (PNI), soils with surface horizons rich in organic matter are found, many of which are classified as Histosols and others with histic horizons. Considering the greater fragility of these soils in the face of climate change and anthropogenic actions, this study was divided into two chapters. In the first chapter, various classes of soils with high organic matter content in the surface horizon were evaluated with the objectives: i) to validate pedotransfer functions (PTFs) constructed by Beutler et al. (2017) to estimate soil density (SD) of surface horizons; and ii) to assess the carbon (C) and nitrogen (N) stocks in soils. In the second chapter, profiles of Histosols (16) were evaluated aiming: i) to quantify the contents of labile organic carbon in the organic horizons; ii) to quantify the contents of C and N of humic substances (HS) from organic matter; iii) to assess the relationships between C and N of HS based on the stoichiometric ratio (C/N); and iv) to propose soil classification criteria for the lower categorical levels of Folistic and Histic Histosols.

The results obtained in Chapter I showed C content between 5 to 15 g kg-1 and N content above 1.0 g kg-1 in soils. For SD, an increase in value with depth was observed, with values close to 0.50 Mg m-3 in the surface horizons and greater than 0.75 Mg m-3 from 30 cm depth. Considering the estimated area of soils with histic horizons in the upper part of PNI, of 16,403.7 hectares, C stocks reach about 316 Gg of C and 20 Gg of N. Of this total, 10,254.44 ha correspond to the area of Cambisols, which store about 205 Gg of C and 13 Gg of N. Histosols occupy an area of 4,148.38 ha, with stocks of 90.7 Gg of C and 5.6 Gg of N. The area occupied by Neosols is 1,347.74 ha, and the stocks are 19.7 Gg of C and 1.3 Gg of N. For Cambisols, average C and N stock values were observed to be lower than those quantified in Histosols. However, considering the area occupied by these soils, the total C stock is more than twice that quantified in Histosols.

The results obtained in Chapter II showed little variation in labile organic carbon content among Histosol profiles, but a wide variation in total carbon and nitrogen content, especially for the profile classified as Hemic Lithic Folistic Histosol. The indices of the relationships between C and N of the humic acid (HA) and fulvic acid (FA) fractions (mean AHC/FAC = 1.61 and AHN/FACN = 1.05), C and N of the alkaline extract and humin (HUM) (mean AEC/HUMC = 0.71 and AEN/HUMN = 0.38), C and N of the alkaline extract and total C and total N (mean AEC/TC = 0.28 and AEN/TN = 0.19) allowed the assessment of the degree of humification of the profiles. Thus, it is proposed that the attributes of the ratio between the C of the alkaline extract of HS (C of the FA fraction + C of the HA fraction) and the total soil C, as well as the ratio between C and N of the HUM fraction, be adopted to define lower categorical levels of Histosols in the Brazilian Soil Classification System. Similarly, these indicators are recommended to assess the degree of organic matter mineralization in these soils, contributing to the monitoring of alteration processes in the face of climate change or anthropogenic actions and for the preservation of these soils.