COS 208-3 - Plant tissue nutrients over 140 years of rising atmospheric CO₂

Background/Question/Methods

The rise in atmospheric CO₂ caused by human emissions has a direct effect on plants due to the role of CO₂ in photosynthesis. When CO₂ levels rise, the photosynthetic process is stimulated, leading to an increased production of the carbohydrates that make up plant structures. The result is a trend towards increasing plant growth over time. To better understand this phenomenon, my question is, how is plant nutrient concentration impacted by CO₂ levels rising over time? I hypothesize the increased plant growth from CO₂ stimulated photosynthesis will lead to larger plants whose nitrogen concentration has been diluted by carbohydrates. I have designed an observational study that uses herbarium specimens from the last 140 years to obtain carbon and nitrogen concentration data. I destructively sampled a small amount of tissue from herbarium specimens of false indigo bush (Amorpha fruticosa) to be run through an elemental analyzer. I then obtained records from both ice core (pre-1969) and Mauna Loa (post-1969) CO₂ data for the dates of specimen collections. Using this data, I compared plant nutrient concentrations to atmospheric CO₂ concentrations.

Results/Conclusions

My preliminary data has shown an increase in carbon concentration for all plant tissues in correlation with increasing atmospheric CO₂. For nitrogen, the data suggest tissues involved in photosynthesis maintain their nitrogen concentration, while tissues primarily involved in other plant functions decline in nitrogen concentration as CO₂ increases. More specifically, leaves of Amorpha fruticosa maintain a nitrogen concentration of around 2.7% while the flowers decline in nitrogen concentration at a rate of -0.026% per 1 ppm increase of CO₂. While these results are not statistically significant, this limited preliminary dataset shows trends that support my hypothesis. Over the next few months, I will be able to collect much more extensive data from a broader geographic range, multiple plant species, and more plant tissue types. Given more data, the results will contribute to our understanding of how rising atmospheric CO₂ may affect the nutritional quality of primary producers, and therefore the nutritional quality of higher trophic levels in the food chain.