Trends in 13C/12C 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 delta C-13 was higher than straw delta C-13. Both grain and straw delta C-13 have decreased by approximately 2.5-2.8 parts per thousand over the last 153 years, and since the 1960s the decrease has been more rapid than the decrease of the delta C-13 of atmospheric CO2. The C isotope discrimination (Delta), 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 Delta and c(i)/c(a) corresponds to the introduction of the modem short-straw cultivars of wheat. When grown on the unfertilised plot in the same seasons, a modem wheat cultivar had slightly higher Delta values than an old cultivar. The c(i) derived from both grain and straw Delta 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.