Carbon stocks in oxisols under agriculture and forest in the southern Amazon of Brazil

We evaluated the initial effects of different agricultural management systems on carbon stocks in Oxisols in the southern Amazon and compared them to the adjacent native forest. Four management systems in a commercial area of grain production (soybeans in monoculture, summer soybeans with corn and Panicum milaceum as second crop, summer soybeans with corn and Urochloa brizantha as second crop, and summer soybeans with U. brizantha + Crotalaria juncea) were evaluated in a completely randomized design with five repetitions. It was found with removal of native vegetation and introduction of annual crops that there was a reduction of 30% (-8.1 Mg ha(-1)) in soil C stocks with the traditional management system of soybean monoculture/second crop corn. However, it is possible to maintain C stocks at levels comparable to the native vegetation with soy cultivation in summer and cover crops U. brizantha + C. juncea as second crop. With this strategy, there was storage of 0.2 and 0.12 Mg ha(-1) C-CO2 per year at depths of 0-0.1 m and 0.1-0.2 m, respectively. In soybean monoculture/ corn there was C-CO2 emission of 6.45 and 2.09 Mg ha(-1) yr(-1) at depths of 0-0.1 m and 0.1-0.2 m, respectively. These results suggest that the practices traditionally used in commercial agricultural areas may not be sufficient to maintain C stocks in Amazon soil compared to the native vegetation. Therefore, large-scale management for low green-house gas emission (C-friendly agriculture) in the Amazon must be rethought and alternative strategies, such as rotation with pastures (e. g. integrated crop-livestock systems), should be adopted in order to preserve and/or increase C stocks, reducing the impact of CO2 emissions into the atmosphere.