Data collated from around the world indicate that, for every tonne of shoot dry matter produced by crop legumes, the symbiotic relationship with rhizobia is responsible for fixing, on average on a whole plant basis (shoots and nodulated roots), the equivalent of 30-40 kg of nitrogen (N). Consequently, factors that directly influence legume growth (e.g. water and nutrient availability, disease incidence and pests) tend to be the main determinants of the amounts of N 2 fixed. However, practices that either limit the presence of effective rhizobia in the soil (no inoculation, poor inoculant quality), increase soil concentrations of nitrate (excessive tillage, extended fallows, fertilizer N), or enhance competition for soil mineral N (intercropping legumes with cereals) can also be critical. Much of the N 2 fixed by the legume is usually removed at harvest in high-protein seed so that the net residual contributions of fixed N to agricultural soils after the harvest of legume grain may be relatively small. Nonetheless, the inclusion of legumes in a cropping sequence generally improves the productivity of following crops. While some of these rotational effects may be associated with improvements in availability ofN in soils, factors unrelated to N also play an important role. Recent results suggest that one such non-N benefit may be due to the impact on soil biology of hydrogen emitted from nodules as a by-product of'N, fixation.

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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 15N‐labeled soil in order to use 15N 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 15N enrichments than the control. These data suggest that the plants obtained considerable BNF contributions, but interpretation of the 15N data was prejudiced by (i) the steadily declining 15N 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.

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