Quantification of biological nitrogen fixation associated with sugar cane using a 15N aided nitrogen balance

Abstract

A nitrogen balance study was performed on four commercial sugar cane cultivars grown in pots containing 64 kg of soil from a sugar cane growing area, with the objective of quantifying possible contributions of biological nitrogen fixation (BNF) to the plants. Distillery (stillage) waste from an alcohol distillery was added to half of the pots, and all treatments were replicated 10 times. The pots were maintained in the field and amended with the equivalent of 80 kg N ha −1 of 15N-labelled urea fertilizer. After 12 months of growth the plants accumulated the equivalent of between 10 and 24% of the total N in the soil and added fertilizer. In the next 9 months of growth, with no more N additions, the plants further accumulated the equivalent of between 8.5 and 19% of the original soil + fertilizer N. The cultivar CB 47–89 accumulated significantly more N than the other cultivars. This difference was greatest in the absence of distillery waste, and in this treatment the 15N enrichment of this cultivar was almost half that of the others, which suggests that there was a contribution of unlabelled N from the air to this cultivar via BNF. After 21 months of growth the N content of the soil and roots was determined on 5 replicates. These analyses revealed that the planted plots lost between 7.5 and 12.5 g of N from soil + fertilizer, but all accumulated more than this in plant tissue. In view of the fact that unplanted plots lost between 10 and 14 g of N during this period, the data suggest that all of the cultivars received some N from associated BNF. In the case of the cultivar CB 47–89, in the absence of distillary waste the plants removed approx. 10 g of N from the soil and fertilizer but accumulated almost 35 g of N. These data constitute the first direct evidence of very significant contributions of plant-associated BNF to a sugar cane cultivar although further experiments must be performed to determine whether such large contributions would be encountered under field conditions.