Impacto do cultivo da cana-de-açúcar sobre a dinâmica do carbono em solo do semiárido brasileiro

Abstract

The change in land use promotes changes in carbon stocks (C) of the soil, reflecting the emission of carbon dioxide (CO2), aggravating the greenhouse effect. Sugarcane (Saccharum spp), because of its importance in Brazilian agribusiness and large cultivated area, plays an important role in soil C dynamics. The impact of sugarcane cultivation on C dynamics in soils where climatic conditions favor the rapid degradation of soil organic matter (SOM), such as Brazilian semi-arid areas, is little known. The objective of this work was to evaluate the dynamics of carbon in Brazilian semi-arid soils cultivated with irrigated sugar cane after the removal of the caatinga. The specific objectives were: 1) to determine the chemical and physical characteristics of the soil in areas of caatinga and in areas with different times of cultivation of sugarcane; 2) quantify the total carbon and nitrogen stocks of the soils of the different areas; 3) to determine the origin (native vegetation or sugar cane) of the C stocked along the soil profiles of these areas and, 4) to determine the average residence time (TRM) of the organic carbon originating from the caatinga in the soil cultivated with sugar cane. The areas are located in the Farm AGROVALE, in the municipality of Juazeiro, Bahia, Brazil, in the following situations: 1) native vegetation (caatinga) virgin, 2) areas with implantation of the sugarcane cultivation 10 years ago; 3) areas with implantation of the sugarcane cultivation 20 years ago and 4) areas with implantation of sugarcane cultivation for 30 years. All areas have Vertisols and similar farms. Three trenches were opened in each area, to the depth of 100 cm or until reaching the rock. Deformed and undisturbed samples were collected in the 0-10, 10-20, 20-40, 40-60, 60-80 and 80-100 cm layers for the chemical, physical and isotopic characterization. Plants and litter were also collected for isotopic characterization. The cultivation of sugarcane did not promote changes in the physical properties of the soil, however, changes in some chemical properties were observed, such as sodium and electrical conductivity reduction. Cane cultivation for 30 years promoted a significant increase in the amount of C in these soils, with an increase of approximately 84.4 Mg ha-1 when only the depth of 0-60 cm was considered. The average residence time of C in these areas is relatively low, reaching 18.9 years in the 0-10 layer and 82 years in the 40-60 cm layer. This rapid degradation of C becomes clear when the contribution of the two different C sources to the stock of cultivated areas is observed. There was enrichment of 13C in the cultivated areas, resulting in more than 90% of C coming from plants with C4 photosynthetic cycle in the superficial layers. In addition, it was also verified that land use change promoted soil enrichment in 15N and total nitrogen (N). Therefore, conversion of caatinga to irrigated cane cultivation promotes the increase of C and N stocks and enrichment of 13C and 15N.