Variations in the 15N natural abundance of plant-available N with soil depth: Their influence on estimates of contributions of biological N2 fixation to sugar cane

• New evidence of significant sugarcane reliance on BNF are presented.
• Changes in natural abundance of 15N in the soil profile were investigated.
• A new method for the quantification of BNF to the plant, based on the natural abundance of 15N, is used.

The objective of this study was to investigate the application of the 15N natural abundance technique to quantify BNF inputs to different varieties of field-grown sugarcane. The experiment was planted on a low-fertility sandy soil and no N fertilizer was added. Cane yields of seven Brazilian commercial varieties at the first harvest after 18 months were similar to the mean national yields. Nitrogen accumulation of most varieties exceeded 100 kg N ha−1 yr−1 even after two ratoons. The 15N abundance values of reference weed plants at the first harvest were significantly higher than leaf sample of the cane indicating inputs of BNF between 80 and 170 kg N ha−1 during the plant crop (18 months). The hypothesis that the lower 15N abundance of the cane varieties was due to the deeper rooting habit of the cane enabling it to access mineral N of significantly lower 15N abundance than present in the rooting zone of weed reference species was discarded, as weed reference species grown in soil samples taken at seven depth intervals to 75 cm showed higher 15N abundance than samples taken from the surface 10 cm. Weeds taken from the plots of the ratoon crops did not differ significantly in 15N abundance from the sugarcane varieties which suggested insignificant contributions of BNF at this time.