Using seed nitrogen concentration to estimate crop N use-efficiency in high-yielding irrigated cotton

Improving the efficiency of nitrogen (N) fertiliser use is one means of reducing green-house gas emissions, particularly in irrigated crops such as cotton (Gossypium hirsutum L.). Over six cropping seasons (2004–2009), lint yield, crop N uptake and cotton seed N concentration were measured within two N fertiliser rate experiments that covered a wide range of N fertility. The economic optimal N fertiliser rates and the internal crop N use efficiencies (iNUE) were determined. When the economic optimal N fertiliser rate was applied, crop iNUE averaged 12.5 ± 2.0 kg lint/kg crop N uptake (previously published) and cotton seed averaged 3.53 ± 0.21%N. Higher cotton seed N concentrations were correlated with high crop N uptake and above-optimal N fertiliser application. In 2010, cotton seed N concentrations were determined in 449 commercial cotton modules (∼15 t seed cotton) sampled from 61 fields in three cotton-growing regions. The cotton seed N concentrations were above optimal in 203 modules (45%), optimal in 201 modules (45%) and below optimal in 45 modules (10%). Averaged over all modules, about 25 kg N/ha was applied in excess of the economic optimum, as cotton seed N% averaged 3.65. For the 203 modules that indicated high cotton seed N concentration (average 3.88%N), about 83 kg N/ha N had been applied in excess of the optimum. This study demonstrated that cotton seed N concentration can be used to assess crop N use-efficiency and indicate where N fertiliser management can be improved.

► Cotton seed N concentration was closely related to internal crop N use-efficiency, crop N uptake and excess N fertiliser application.
► Cotton seed N concentration of 3.53%N indicated optimal N fertiliser application.
► A survey indicated that 45% of 447 samples of harvested seed cotton came from fields that had received excess N fertiliser.