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 C-13/C-12 ratio or delta C-13, expressed as Delta C-13 discrimination in photosynthesis and delta N-15 natural abundance by the wheat crop. To assess whether Delta C-13 and delta N-15 natural abundance can be used to evaluate the tillage and N use effects on wheat crop yield, N uptake, and nitrogen use efficiency (NUB), we examined the delta N-15 and Delta C-13 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 x tillage, year x N, tillage x N interactive effects on grain yields. Anthesis dry matter delta N-15 values and wheat grain delta N-15 values were closely correlated. Grain delta N-15 differed significantly among years. Wheat under NT practice had lower delta N-15 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 delta N-15 values of the wheat crop can be used to distinguish total N uptake from applied N fertiliser and soil derived N. Also, the NUE x WUE interactions can be discerned by Delta C-13 values of the grain in bread wheat in a semi-arid rainfed environment. (c) 2012 Elsevier B.V. All rights reserved.