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

Abstract Tillage practices affect soil water and nitrogen supply to bread wheat grown under rainfed conditions. It may, therefore, affect grain yields and 13 C/ 12 C ratio or δ 13 C, expressed as Δ 13 C discrimination in photosynthesis and δ 15 N natural abundance by the wheat crop. To assess whether Δ 13 C and δ 15 N 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 δ 15 N and Δ 13 C 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. Wheat 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 δ 15 N values and wheat grain δ 15 N values were closely correlated. Grain δ 15 N differed significantly among years. Wheat under NT practice had lower δ 15 N 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 δ 15 N 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 Δ 13 C values of the grain in bread wheat in a semi-arid rainfed environment.