Sesbania rostrata (Bremek and Oberm.) as biological nitrogen fixator for sustainable lowland rice production

Nitrogen plays a crucial role in rice cultivation. It is usually supplied as chemical fertilizer (urea). However, rice monocropping without fallow and loss of urea-N through leaching, often lead to soil fertility decrease, decline in soil organic matter and environmental pollution, which negatively affect rice yield. Climate change is expected to worsen this situation. All this may affect food security especially against the backdrop of increasing population levels, particularly in Sub-Sahara Africa. New and alternative N resources should be explored to sustainably supply rice crop needs and thus reduce the adverse environmental effects of current urea-N use in rice. Biological Nitrogen Fixation (BNF) by legumes is a promising and environment-friendly mechanism that can be used to convert atmospheric di-nitrogen into fertilizers for agriculture. As such, BNF could increase the sustainability of traditional rice production systems. In this respect, Sesbania rostrata, which is native to Africa and frequently used as a green manure in lowland rice production systems, is one of the best N-fixating plant species because its N-fixing bacteria nodulate on both stem and roots, whereas the plant itself grows fast, has high N-fixation rates and tolerates waterlogged soils. The plant, when plowed under at 34-42 days after sowing, can add 90 to 150 kg N ha−1 and 7.4 t ha−1 of biomass to the soil. It has been found that S. rostrata, supplied as a green manure, can double rice yields after one rice cultivation cycle. Findings further reveal a 35% residual effect on yield of the same green manure application on grain yield in a second rice cultivation cycle. It is probably the cheapest alternative N source to smallholder farmers.