ISOTOPE N-15 ENRICHMENT OF SOIL-AMMONIUM N AS A REFERENCE TO ESTIMATE N-2 FIXATION BY STEM AND ROOT-INOCULATED S-ROSTRATA

A pot experiment in the greenhouse was conducted to compare the contribution of N derived from the atmosphere or from biological N2 fixation by Sesbania rostrata inoculated with Azorhizobium caulinodans, applied either to roots or to roots and stems (single or multiple stem inoculation). Two subsequent crops were grown for 50 days under flooded conditions. N derived from air was estimated by N-15 dilution using N-15 enrichment of soil NH4+-N and of Echinochloa crus-galli as the non-N2-fixing reference datum and compared with estimates obtained by the N-difference method. The first crop was grown to stabilize the N-15 into the soil organic N fraction. The N-15 enrichment of soil NH4+-N in the second crop declined slowly. The extractability ratio (N-15 enrichment of extractable soil N to N-15 enrichment of total soil N) decreased from 4.8 to 4.1 50 days after planting. The enrichment of soil NH4+-N was comparable to that of E. crus-galli, resulting in similar estimates of N derived from air when either soil NH4+-N or enrichment of E. crus-galli was used as a non-fixing reference. The N-difference method did not always provide reliable estimates of N derived from air; percentages ranged from 75 to more than 80 by 50 days after planting in both crops and did not differ among treatments. The study demonstrates the potential of using N-15 enrichment of soil NH4+-N as a non-N2-fixing reference for reliable BNF estimates of crops in lowland puddled soil.