Trends in (13)C/ (12)C ratios and C isotope discrimination of wheat since 1845

Archived wheat grain and straw samples from the unfertilised plot of the Broadbalk Continuous Wheat Experiment at Rothamsted, England, were used to investigate the trends in stable C isotope ratios since 1845. Grain δ(13)C was higher than straw δ(13)C. Both grain and straw δ(13)C have decreased by approximately 2.5-2.8‰ over the last 153 years, and since the 1960s the decrease has been more rapid than the decrease of the δ(13)C of atmospheric CO2. The C isotope discrimination (Δ), and hence the ratio of CO2 concentration in leaf intercellular space to that in the ambient air (c i /c a), remained relatively stable or decreased slightly between 1845 and the mid-1960s, then increased considerably. The period with increasing Δ and c i /c a corresponds to the introduction of the modern short-straw cultivars of wheat. When grown on the unfertilised plot in the same seasons, a modern wheat cultivar had slightly higher Δ values than an old cultivar. The c i derived from both grain and straw Δ have increased by 33-37% over the last 153 years, with the increase being more rapid after than before the mid-1960s. However, there was no clear trend in the yields of straw and grain, primarily because of the limitations of N and other nutrients, and also because of large year-to-year variations.