Previous research into the nitrogen (N) nutrition of cotton has primarily focused on the mineral N pool, particularly nitrate, due to its association with synthetic fertilisers. However, it is known that between 40% and 70% of a cotton plant’s N uptake is sourced from the soil pool, which is mostly organic in nature. Current understanding is that nitrate and ammonium are taken-up by the cotton plant following ammonification and nitrification of the soil organic N pool. However, recent advances made by CSIRO towards improving the understanding of the biogeochemistry and dynamics in the soil-plant system of the organic N pool, coupled with a growing recognition of the ability of plants to directly access some forms of organic N, highlighted that the organic N pool may play a significant role in the nutrition of cotton. An implication of this is that soil N is currently only managed from a mineral N perspective whereas with the correct tools and analytical techniques it may be possible to achieve significant improvements in N use efficiency through clearer understanding of the contribution of the organic N pool to cotton N nutrition.

The work conducted as part of this project aimed to address current gaps in knowledge and understanding of the role of organic N in the nutrition of cotton by focusing upon the following objectives and research questions:

1. To determine the importance of the organic N pool relative to nitrate and ammonium for cotton nutrition,
2. To determine the impact of soil type on uptake of organic N relative to nitrate and for cotton nutrition,
3. In high-performance soils, does the organic N pool influence fertiliser N use efficiency?
4. Is uptake of organic N a general phenomenon in cotton or is the relative uptake of nitrate, ammonium, organic N variety-specific, and  
5. To determine the influence of soil testing protocols on observed soil N forms.

It was further hypothesised that:

1. both inorganic and organic N forms would be accessed by all cotton varieties on the two soil types investigated, but that a preference for nitrate over both organic N and ammonium would be more greatly expressed in plants grown in clay-richer and thus sorbent soil, and 
2. modern cotton varieties would also express greater preference for nitrate over organic N compounds than older varieties, as their breeding and genetic engineering will have targeted inorganic nitrogen-rich systems. 

A series of interconnected experiments were conducted over three years at the field, glasshouse and laboratory-scales that included soils with varying clay content, three cotton genotypes (a pre-Bt variety, and two Bollgard® 3 lines), and labelled carbon (13C) and nitrogen (15N) techniques, coupled with novel extraction and analytical approaches.