A meta-analysis of the combined effects of elevated carbon dioxide and chronic warming on plant %N, protein content and N-uptake rate

Abstract

Elevated CO₂ (eCO₂) and high temperatures are known to affect plant nitrogen (N) metabolism. Though the combined effects of eCO₂ and chronic warming on plant N relations have been studied in some detail, a comprehensive statistical review on this topic is lacking. This meta-analysis examined the effects of eCO₂ plus warming on shoot and root %N, tissue protein concentration (root, shoot and grain) and N-uptake rate. In the analyses, the eCO₂ treatment was categorized into two classes (<300 or ≥300 ppm above ambient or control), the temperature treatment was categorized into three classes (<1.5, 1.5-5 and >5 °C above ambient or control), plant species were categorized based on growth form and functional group and CO₂ treatment technique was also investigated. Elevated CO₂ alone or in combination with warming reduced shoot %N (more so at ≥300 vs. <300 ppm above ambient CO₂), while root %N was significantly reduced only by eCO₂; warming alone often increased shoot %N, but mostly did not affect root %N. Decreased shoot %N with eCO₂ alone or eCO₂ plus warming was greater for woody and non-woody dicots than for grasses, and for legumes than non-legumes. Though root N-uptake rate was unaffected by eCO₂, eCO₂ plus warming decreased N-uptake rate, while warming alone increased it. Similar to %N, protein concentration decreased with eCO₂ in shoots and grain (but not roots), increased with warming in grain and decreased with eCO₂ and warming in grain. In summary, any benefits of warming to plant N status and root N-uptake rate will generally be offset by negative effects of eCO₂. Hence, concomitant increases in CO₂ and temperature are likely to negate or decrease the nutritional quality of plant tissue consumed as food by decreasing shoot %N and shoot and/or grain protein concentration, caused, at least in part, by decreased root N-uptake rate.

Keywords: Climate change; elevated CO2; heat stress; meta-analysis; nitrogen metabolism; nitrogen translocation; nitrogen-uptake rate; protein; warming.