Estoque de carbono e nitrogênio sob diferentes usos da terra em floresta tropical sazonalmente seca

Abstract

The Caatinga biome is a significant carbon deposit that is still poorly known and exploited, considering its peculiars characteristics of vegetation and climate. Based on this, the hypothesis has suggested that the soils of Seasonally Dry Tropical Forest (caatinga) present different carbon and nitrogen stocks due to land use. The general objective of the present study was to analyze the effect of changes in land use under Total Organic Carbon Stocks (EST.COT) and Total Nitrogen (EST.NT) in Seasonally Dry Tropical Forest Soils (FTSS) in the Brazilian semiarid. The study was conducted in three sites located in the municipalities of Pentecoste, Piquet Carneiro and Quixadá, in the two most representative soil classes of the studied region: Entisols and Alfisols in the backwoods of Ceará, Brazil. Soil samples were collected in trenches measuring 70 x 70 cm at the following horizons: 0-10, 10-20, 20-30, 30-40, 40-60 and 60-80 cm or until the bedrock was reached (in the event that it was less than 80 cm deep), in four uses of soils: dense caatinga (DC), open caatinga (OC), agriculture (AG) and pasture (PA). The following soil attributes were evaluated: soil bulk density, total organic carbon (TOC), total nitrogen (TN), C:N ratio, EST.TOC and EST.TN. The data were submitted to the mean test analysis and compared by the Mann-Whitney test (p<0.05). To understand the behavior of the evaluated attributes among land uses, the technique of multivariate analysis, Hierarchical Cluster Analysis (HCA) was used. The use of (HCA) resulted in the formation of 3 distinct groups in both classes of soils investigated. The highest values of EST.TOC and EST.TN in the class of Alfisols and Entisols lithic were recorded in group 3, from the combination of land use and horizon (CD 0-10 and CA 0-10cm) and (CD 0-10, CA 0-10 and PA 0-10cm), respectively. The values of EST. TOC and EST. TN were 11.29 Mg ha-1 and 3.36 Mg ha-1 for the Alfisols and 8.68 Mg ha-1 and 0.68 Mg ha-1) for Entisols. The largest stocks of TOC and NT are associated with deep and well drained soils, being the largest values of EST.TOC and EST.TN were found in the Afilsols class when compared to the Entisols class. The changes in land use in the FTSS showed effects under EST.TOC and EST.TN. Therefore, it is necessary to adopt strategies and strengthen conservation practices in the areas under agriculture and pasture uses, and reduce the degradation process and promote the recovery process of these areas. Such actions will reduce C and N losses and increase the TOC and TN contents and stocks.