In Brazil N fertilization of sugarcane (Saccharum spp.) is low compared to most other countries. N-15-aided studies and the occurrence of many N-2-fixing bacteria associated with cane plants suggest significant contributions from biological N-2 fixation (BNF). The objective of this study was to evaluate BNF contributions to nine cane varieties under field conditions using N balance and N-15 natural abundance techniques. The field experiment was planted near Rio de Janeiro in 1989, replanted in 1999 and harvested 13 times until 2004. Soil total N was evaluated at planting and again in 2004. Samples of cane leaves and weeds for the evaluation of N-15 natural abundance were taken in 2000, 2003 and 2004. N accumulation of the commercial cane varieties and a variety of Saccharum spontaneum were persistently high and N balances (60 to 107 kg N ha(-1) yr(-1)) significantly (p < 0.05) positive. The delta N-15 of leaf samples were lower than any of the weed reference plants and data obtained from a greenhouse study indicated that this was not due to the cane plants tapping into soil of lower N-15 abundance at greater depth. The results indicate that the Brazilian varieties of sugarcane were able to obtain at least 40 kg N ha(-1) yr(-1) from BNF.
