Carbon exchange in rainfed and irrigated cropland in the Brazilian Cerrado

Abstract

The Cerrado is one of the most important agricultural production areas in Brazil where soybean and maize have expanded in recent decades through deforestation. The effects of cropland on the carbon (C) balance still needs to be understood at the ecosystem scale to better situate the role of land management in the tropical C cycle. In this study, we measured the C exchange of two fields (irrigated and rainfed) with rotations from different cultures. Results showed that both fields were C sinks over the course of the crops’ development cycles, but this C was mostly removed via harvest when considering the C stored in the grain (as soybean, maize, rice, and bean crops). Maize, intercropped with Brachiaria, had the most positive C balance (as a loss of C from the field), in part due to its longer stay in the field. The rainfed and irrigated fields acted as a net C source due to emissions to the atmosphere from periods of stubble and the Brachiaria intercrop, soil preparation and soybean planting. The irrigated field was a more important C sink than the rainfed field, suggesting that irrigation can reduce C losses resulting from possible drought, while at the same time allowing for a third harvest in the same calendar year. Our results confirm that practices such as no-till farming, crop rotation, intercropping, the reduction of fallow periods and the use of irrigation are key to mitigating C losses from agriculture in the Cerrado, while also helping reduce pressure on remaining natural forests in the region through agricultural intensification.