Nitrogen acquisition and 15N-fertiliser recovery efficiency of sugarcane cultivar RB92579 inoculated with five diazotrophs

We aimed to evaluate N acquisition and 15N-fertiliser recovery efficiency of sugarcane as a function of N fertilisation and inoculation with plant growth-promoting diazotrophic bacteria (PGPDB) during the first crop season. A field experiment was performed with a randomized complete block design consisting of four replications and four treatments: control without N and without inoculation; control without N but with inoculation of a mixture of five strains of diazotrophic bacteria; N treatment (50 kg ha−1 N-urea) without inoculation; and N treatment with inoculation. Over seven harvests, we evaluated crop growth, biomass accumulation, N content, and N recovery using 15N-urea (excess of 1.5% atoms 15N). In addition, the contribution of biological N fixation (BNF) was measured using the 15N natural abundance technique in the plots without N fertiliser application. Inoculation with the mixture of five PGPDB strains increased the yield by 15% and N content in the shoots by 18% over the cycle. The average contribution of N fertiliser in plant N nutrition was 13%; other sources accounted for a large part of N in the shoots. Plants received N via BNF in 31% (average) of the treatments during the plant cycle; however, inoculation did not interfere with this process. The recovery efficiency of 15N-fertiliser was 50% in this first cycle with no differences among inoculation treatments. Nitrogen fertiliser applied to sugarcane resulted in a low impact on N accumulation, and a single application of an inoculant improved plant growth by mechanisms other than BNF.