Nitrogen budgets for Boro rice (Oryza sativa L.) fields in Bangladesh

Nitrogen (N) budgets are a valuable tool for improving N efficiency because they assess the size and interactions of various N pools, as well as their gains from the atmosphere and losses to the environment. To understand the impact of changes in management practice upon a farming system, it is necessary to increase the complexity of the N budgets to include N flows. Therefore, a project was undertaken in lowland irrigated systems of Bangladesh to study the N budgets of Boro rice grown under ecological and conventional farming systems in four locations (Dhamrai, Daulatpur, Gabtali and Shibgonj) in Bangladesh in 2007 and 2008. The N budget focuses on the total-N inputs and losses of the entire system. The budgets were negative for both farming systems in both years. Overall, ecological farming system produced a less negative balance in both years (−6 to −36 kg N ha−1 in 2007 and −76 to −160 kg N ha−1 in 2008) than the conventional farming system (−28 to −80 kg N ha−1 in 2007 and −91 to −157 kg N ha−1 in 2008). Nitrogen balance studies highlighted losses of mineral N (26–53 kg N ha−1) which accumulated prior to irrigation and also losses due to N removal (13–28 kg N ha−1) by weeds. Beneficial impacts of ecological farming on N balances were observed due to the elimination of fertiliser N loss (30–133 kg N ha−1). The difference between conventional and ecological management reflects the high losses of fertiliser N under conventional management. These fertiliser N losses reflect the low agronomic efficiency of N fertiliser. An understanding of various N losses and their consequences is important to provide a basis for developing efficient N management strategies in boro rice. These N budgets can be used to improve or design new technologies that tackle soil fertility management problems and also can help improve the financial performance of the farmers. Soil N budgets will continue to challenge agricultural scientists by slowly revealing fundamental principles. By understanding these principles and the factors influencing them, basic and applied scientists will have a stronger foundation for improving N use efficiency and concurrently reducing N losses to the environment.