Relationship between water use and nitrogen use efficiency discerned by 13C discrimination and 15N isotope ratio in bread wheat grown under no-till

Tillage practices affect soil water and nitrogen supply to bread wheat grown under rainfed conditions. It may, therefore, affect grain yields and 13C/12C ratio or δ13C, expressed as Δ13C discrimination in photosynthesis and δ15N natural abundance by the wheat crop. To assess whether Δ13C and δ15N natural abundance can be used to evaluate the tillage and N use effects on wheat crop yield, N uptake, and nitrogen use efficiency (NUE), we examined the δ15N and Δ13C values of wheat dry matter at anthesis and grain sampled at harvest from conventional till (CT) and no-till (NT) treatments, with 0 and 75 kg N ha−1 applied annually over 5 years to account for seasonal rainfall variability. grain yields were higher under the NT practice than the CT practice and with N application, and showing significant year × tillage, year × N, tillage × N interactive effects on grain yields. Anthesis dry matter δ15N values and wheat grain δ15N values were closely correlated. Grain δ15N differed significantly among years. Wheat under NT practice had lower δ15N than the CT practice, indicating increased N utilisation by the crop under the NT practice. Grain NUE varied from 0 to >66% over the 5-year period and tended overall to be higher under NT (34%) than the CT practice (28%). Grain NUE was closely correlated with water use efficiency (WUE) and crop water use by bread wheat. It was estimated that by increasing grain WUE from 8 kg ha−1 mm−1 to 12 kg ha−1 mm−1, the grain NUE increased from 21% to 47%. The δ15N values of the wheat crop can be used to distinguish total N uptake from applied N fertiliser and soil derived N. Also, the NUE × WUE interactions can be discerned by Δ13C values of the grain in bread wheat in a semi-arid rainfed environment.