lndicadores de estabilidade da matéria orgânica em terras pretas nos sítios arqueológicos Jabuti e Jacarequara (Pará)

Abstract

Environmental conditions in the Amazon region favor the weathering and decomposition of soil organic matter, making it depleted in nutrients and difficult agricultural use. But in the same region, areas that have been modified by human action preterit, known as Terra Preta Archaeological (TPA), have different properties, among which stands out the high stability of soil organic matter (SOM) that in some research it is attributed to interactions between the MOS and other soil constituents such as pyrogenic carbon and minerals from the soil. In this study were selected two archaeological sites in the state of Pará, the Jabuti, the cemetery housing type, located in the city of Bragança, and the Jacarequara, the sambaqui type, located in Barcarena, in order to assess the stability of organic matter TPA from soil extracted solutions (depths 30 and 80 cm) and own soil (collected during the implementation of extractors in December 2013) in areas of TPA and vicinity. The characterization of soil solutions was conducted in the period between March and June 2013, based on the macroscopic properties and the chemical indicators: dissolved carbon concentrations (organic, inorganic and total), determined by the combustion method; pH, Eh and conductivity. The SOM stability assessments in the solid phases of the TPA and surrounding areas (ADJ) were based on textural verification of soil chemical indicators (pH, organic carbon concentrations and Ca, K, P, Na, and Mg) and biological represented by the microbial biomass, determined by the method of irradiation / extraction and expressed in terms of carbon (Cbm) and nitrogen (Nbm). The results of the soil solutions showed that two sites in the pH values are higher in depth (80 cm), and at the site Jacarequara values were determined for this parameter up to 7.2, while the site Jabuti pH results do not exceed the value 6. The maximum values of Eh (mV), conductivity (μs) and dissolved organic carbon (mg L-1) in place Jacarequara, 30 cm deep were respectively +201 mV, 427 µs e 13 mg L-1 and in the area adjacent to this site, at the same depth the highest values were +128 mV, 72 µs e 23 mg L-1 for the same parameters. At the Jabuti site and its ADJ, in 30 cm deep, the respective maximum values of the same areas were Eh +108 mV and +96 mV; conductivity 138.87µs and 59.85µs, dissolved organic carbon 12 mg L-1 and 21.08 mg L-1. Comparing the areas of TPA and their ADJ, the data Eh and dissolved organic carbon refer to more stable components in soil areas of solutions of TPA, owing to the more oxidizing values and smaller dissolved organic carbon concentrations, the results conductivity, which is an indicator of the concentration of ions is higher in the TPA reporting the increased availability of nutrients. In both sites, soils presented sandy texture, both in the areas of TPA as the ADJ, the latter being more sandy. In the soil Jacarequara site and its ADJ in the range of 20 to 30 cm deep, the following values were obtained respectively: 119.82 g kg-1 and 20.34 g kg-1 for SOM; pHH2O equal to 6.8 and 4.9; 183 mg/dm3 and 5 mg/dm3 P (available); 39 mg/dm3 and 29 mg/dm3 K (exchangeable); 14.8 cmolc/dm3 and 0.7 cmolc/dm3 Ca (exchangeable); 0.1 cmolc/dm3 and 1.7 cmolc/dm3 Al (exchangeable), 181.26 μg g-1 and 88.74 μg g-1 of Cbm and 3.27 mg kg-1 and 1.91 mg kg-1 Nbm. In the soil of Jabuti site, the determined values were: 83.66 g kg-1 of MOS, pHH2O equal to 4.4; 55 mg/dm3 P (available); 59 mg/dm3 K (exchangeable); 0.3 cmolc/dm3 Ca (exchangeable); 4 cmolc/dm3 of Al (exchangeable); 92.56 mg kg-1 of Cbm and 1.41 mg kg-1 Nbm; in the area adjacent to this site, the values were: 13.13 g kg-1 of MOS, pHH2O equal to 4.6; 4 mg/dm3 P (available); 29 mg dm3 K (exchangeable); 0.3 cmolc/dm3 Ca (exchangeable); 1 cmolc/dm3 Al (exchangeable), 27.54 mg kg-1 of Cbm and 0.96 mg kg-1 Nbm. As well as other archaeological sites with TPA, Jacarequara and the Jabuti had significantly higher levels of nutrients compared to surrounding areas, with the exception of Ca element in Jabuti. In the sites, carbonaceous particles were investigated, showing no intrinsic results pyrogenic carbon. In areas of TPA, the results obtained from the soil analysis indicated positive correlation between the microbial biomass, organic matter and nutrients, which can be associated with better quality of the soil in these areas compared their ADJ, consistently with the data highlighted in the soil solutions. Comparing the two sites, the results indicate that the SOM in Jacarequara site shows more stable constituents.