NITROGEN-FIXATION RESPONSE OF STEM-NODULATING AND ROOT-NODULATING SESBANIA SPECIES TO FLOODING AND MINERAL NITROGEN

The flood adapted green manure legumes Sesbania cannabina and S. rostrata differ in their nodulation, the former nodulating on root only but the latter on both root and aerial stem. In the early wet season pre-rice niche in lowlands, these legumes are exposed to varying soil aeration and N status with NO3- as the main N source in aerobic and NH4+ in flooded soil. To better use these legumes, the influence of soil aeration status and the resulting differences in soil N supply on N assimilation characteristics and biological nitrogen fixation (BNF) must be understood. We examined the interactions among soil aeration status, supply and form of mineral N, and BNF (estimated by N-15 dilution) by growing S. rostrata and S. cannabina for 40 days in aerobic or flooded potted soil (3 kg) with zero to 3000 mg applied N. Both NH4+ and NO3- were used in aerobic soil but only NH4+ in flooded soil. In aerobic soil, N accumulation potential of S. rostrata and S. cannabina were similar with NO3-, NH4+, or BNF as the major N source. Soil flooding increased N accumulation, but consistently more by S. rostrata than by S. cannabina. The maximum N accumulation of 275 mg plant(-1) by S. rostrata was 20% greater than the maximum N accumulation by S. cannabina, in flooded soil, and 80% greater than maximum N accumulation by both legumes in aerobic soil. Regardless of soil aeration status and nodule location, mineral N increasingly substituted for biologically fixed N until total replacement occurred at non-limiting N supply. Reduction in the amount of BNF due to applied mineral N was greater in aerobic soil (with NO3- as the N source) than in flooded soil (with NH4+ as the N source) for both legumes. Compared to root nodules, stem nodule BNF activity was less sensitive to NH4+ in flooded soils and resulted in more BNF-N in S. rostrata. Higher N requirement, stem-nodulation, and probably an ammoniphyllic character, allow S, rostrata to accumulate more N than S. cannabina in flooded soils.