CONTRIBUTION OF NITROGEN-FIXATION TO SUGAR-CANE - N-15 AND NITROGEN-BALANCE ESTIMATES

Continuous sugarcane (Saccharum spp.) culture in Brazil, with low N inputs and almost total removal of plant biomass at each harvest, has not depleted soil N reserves. This, and high numbers of N2-fixing bacteria associated with the plants, suggests that the crop may be obtaining considerable N from biological nitrogen fixation (BNF). This 3-yr study assessed the importance of such contributions to three sugarcane species (S. officinarum L., S. barberi Jesw., and S. spontaneum L.), and seven commercial Brazilian hybrids. The plants were grown in a concrete tank containing N-15-labeled soil in order to use N-15 isotope dilution to estimate the BNF contributions. A grass, Brachiaria arrecta (cv. IRI 442), was included as a non-N2-fixing control. All aerial tissue was harvested annually, and the roots and stem bases were removed at the end of the experiment. For all 3 yr, the commercial hybrids and the S. spontaneum cultivar (Krakatau) accumulated more N at significantly lower N-15 enrichments than the control. These data suggest that the plants obtained considerable BNF contributions, but interpretation of the N-15 data was prejudiced by (i) the steadily declining N-15 enrichment of the soil mineral N, (ii) carry-over of N from one harvest to the next in the roots and stem bases, and (iii) shading of the control crop by the tall cane plants. Several of the sugarcane cultivars had significantly positive N balances, however, and there was good agreement between the estimates of BNF contributions derived from N balance and isotope dilution. Krakatau and the commercial hybrids CB 45-3 and SP 70-1143 obtained the largest contributions from BNF, but methodological problems did not allow exact determinations.