Soil carbon and nitrogen stocks in traditional agricultural and agroforestry systems in the semiarid region of Brazil

José Augusto Amorim Silva Do Sacramento,Ana Clara Rodrigues Cavalcante,Ana Caroline De Moraes Araújo,Maria Eugenia Ortiz Escobar,Teógenes Senna De Oliveira,Francisco Alisson Da Silva Xavier

doi:10.1590/s0100-06832013000300025

Abstract

In the semiarid region of Brazil, inadequate management of cropping systems and low plant biomass production can contribute to reduce soil carbon (C) and nitrogen (N) stocks; therefore, management systems that preserve C and N must be adopted. This study aimed to evaluate the changes in soil C and N stocks that were promoted by agroforestry (agrosilvopastoral and silvopastoral) and traditional agricultural systems (slash-and-burn clearing and cultivation for two and three years) and to compare these systems with the natural Caatinga vegetation after 13 years of cultivation. The experiment was carried out on a typical Ortic Chromic Luvisol in the municipality of Sobral, Ceará, Brazil. Soil samples were collected (layers 0-6, 6-12, 12-20, 20-40 and 40-60 cm) with four replications. The plain, convex and concave landforms in each study situation were analyzed, and the total organic C, total N and densities of the soil samples were assessed. The silvopastoral system promoted the greatest long-term reductions in C and N stocks, while the agrosilvopastoral system promoted the smallest losses and therefore represents a sustainable alternative for soil C and N sequestration in these semiarid conditions. The traditional agricultural system produced reductions of 58.87 and 9.57 Mg ha-1 in the organic C and total N stocks, respectively, which suggests that this system is inadequate for these semiarid conditions. The organic C stocks were largest in the concave landform in the agrosilvopastoral system and in the plain landform in the silvopastoral system, while the total N values were highest in the concave landform in the native, agrosilvopastoral and silvopastoral systems.

Highlights

In northeastern Brazil, in the State of Ceará, two million hectares are planted crop area, second only to the State of Bahia (Indicadores de Desenvolvimento Sustentável, 2010)
The results of this study were compared with data reported by Maia et al (2007) for the same layers and experimental area, in 2002; Total soil organic carbon (TOC) contents of 34.8, 22.3 and 34.4 g kg-1 in 0-6 cm and TOC contents of 17.2, 14.4 and 23.1 in the 6-12 cm layer were found in the SILV, AGP and traditional agricultural (TR) systems, respectively
In the SILV system, lower amounts of organic material were added to the soil because the vegetation is used for animal grazing; no additional residues produced by animals were applied in this area. These results clearly indicate that any tillage, regardless of intensity, promotes reduced soil TOC levels (Figure 2) (Maia et al, 2007, 2008; Veum et al, 2011; Novara et al, 2012); this is most likely due to an increased rate of organic matter decomposition (Novara et al, 2012)

Summary

Introduction

In northeastern Brazil, in the State of Ceará, two million hectares are planted crop area, second only to the State of Bahia (Indicadores de Desenvolvimento Sustentável, 2010). The region is characterized by high temperatures, low rainfall, little weathered soils and low plant biomass production (Araújo Filho, 2002). Under these conditions, the inadequate management of cropping systems and low plant biomass production may effectively contribute to reductions in soil organic matter (SOM) stocks. SOM is involved in the balance between the C and N cycles, and its content is stable in soils under native vegetation (Bortolon et al, 2009) When this balance is disrupted, the organic residue input is smaller relative to the output (Cerri et al, 2008)

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Conclusion