Carbon and Nitrogen Stocks and Humic Fractions in Brazilian Organosols

Gustavo Souza Valladares,Lúcia Helena Cunha Dos Anjos,Roni Fernandes Guareschi,Marcos Gervasio Pereira,Adierson Gilvani Ebeling,Vinícius De Melo Benites

doi:10.1590/18069657rbcs20151317

Abstract

Despite limited geographic expression of Organosols in Brazil, their high carbon storage capacity and natural environmental vulnerability justifies further studies on C and N stocks in these soils and their relationship to the nature of organic matter. Evaluation of physical and chemical properties of organic soils and their ability to store C is important so as to develop sustainable management practices for their preservation. The objectives of the study were to measure the total organic carbon stock (OCst), total nitrogen stock (Nst), and humic fractions in Organosols from different environments and regions of Brazil, and to correlate the data with soil chemical (pH, P, K, Ca2+, Mg2+, Al3+, H+Al, CEC, V) and physical properties (soil bulk density, Bd; organic matter density, OMd; total pore space, TPS; minimum residue, MinR; and proportion of mineral matter, MM), and degree of organic matter decomposition (rubbed fiber content; pyrophosphate index, PyI; and von Post index). For that purpose, 18 Organosol profiles, in a total of 49 horizons, were sampled under different land usage and plant coverage conditions. The profiles were located in the following Brazilian states - Alagoas, Bahia, Distrito Federal, Espirito Santo, Mato Grosso do Sul, Minas Gerais, Parana, Rio de Janeiro, Rio Grande do Sul, Santa Catarina, and Sao Paulo. The OCst and Nst varied significantly among horizons and profiles. The Organosols exhibited, on average, 203.59 Mg ha-1 OCst and 8.30 Mg ha-1 Nst, and the highest values were found in profiles with pasture usage. The content of the humic fraction (humin, HUM; fulvic acid, FAF; and humic acid, HAF) and C storage varied in the soil horizons and profiles according to the degree of decomposition and other factors of soil formation. The OCst, Nst, OMd and the C stocks in the humic fractions were positively correlated. The values of acidity were lower in the soils with higher contents of mineral material, and low pH values were related to a high C/N ratio. The OCst and Nst were correlated with different soil properties, the most important being the degree of soil organic matter decomposition, which was inversely correlated.

Highlights

According to the Brazilian Soil Classification System (SiBCS), the soil order Organossolos is defined by an organic carbon (OC) content greater than or equal to 80 g kg-1, in addition to the criteria of the thickness of organic horizons (Santos et al, 2013)
Soil bulk density (Bd), C and N contents in the humic fractions (FAF, HAF. and HUM), and the respective stocks (OCst, nitrogen stock (Nst)) in the horizons studied are shown in table 4
Agricultural usage is reported as a common reason for an increase in Bd in Organosols (Ewing and Vepraskas, 2006; Kechavarzi et al, 2010; Leifeld et al, 2011), and the values of Bd in the literature for Organosols are similar to the ones observed in this study

Summary

Introduction

According to the Brazilian Soil Classification System (SiBCS), the soil order Organossolos is defined by an organic carbon (OC) content greater than or equal to 80 g kg-1, in addition to the criteria of the thickness of organic horizons (Santos et al, 2013) This requirement of OC content is lower than the value used in the World Reference Base for Soil Resources (WRB), where organic soils (equivalent to Histosols) should contain OC greater than 200 g kg-1 (IUSS, 2014), and there are differences in the methods used for determination of OC. The natural vegetation includes mangrove, typhaceae, wetland forests, and highland forests; the last category mentioned occupies small areas in mountainous well-drained environments with high vulnerability to degradation These soils represent an important asset for agriculture, since approximately 50 % of the Brazilian Organosols, mainly in the flat river basin areas, are used for farming or as pasture lands (Valladares, 2003)

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Conclusion