Author : Manguiat, Ireneo J.; Calo, Marilene U.; Rocamora, Patrick M.; Singleton, Paul W.; Perez, Alicia S.

Summary : Field and screenhouse experiments were conducted with the following objectives: 1) to compare the performance of promising local isolates (Srs 1 and Srs 4) of stem-nodulating rhizobia with a standard strain (ORS 571) in influencing nodulation, N2 fixation, and biomass production of S. rostrata and (2) to examine the effect of applied N and inoculation site on the biomass production and N2 fixation of S. rostrata at different growth stages. Results from the field experiment indicated that when inoculated on either stem or root+stem, ORS 571 and Srs 1 produced more stem nodules than when inoculation was done on the root alone. In terms of total nodule dry weight, Srs 1 was comparable in performance with ORS 571. The stem nodule dry weight due to Srs 1 and ORS 571 inoculated on the stem contributed significantly to the total nodule dry weight of S. rostrata at 7 WAE ( weeks after emergence) by about 97 and 92%, respectively. When inoculation was done on the stem, all rhizobial isolates significantly increased the stem acetylene reduction activity (ARA). Regardless of rhizobial isolate, stem inoculation alone was equally effective in producing fresh biomass as the root+stem inoculation in the field. Under screenhousoe conditions, root+stem inoculation alone at 5 WAE. at 9 WAE, root+stem inoculation showed a consistently better performance than stem inoculation alone based on root dry biomass production, root nodule dry weight. N. application level significantly reduced the proportion of N derived from the athmosphere (%Ndfa) at all growth stages. Younger plants fixed less atmospheric N2 than the older plants. At WAE (usual age of S. rostrata used for green manuring), about 80% of the N accumulated by S. rostrata must have come from athmospheric N2 fixation if it was grown in a soil with low level of available N. The relationship between % N derived from fertilizer and N level was almost linear implying that under field conditions, if the available soil N level is high,