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.

Figure 1.

Percent change (compared to ambient or controls) in shoot %N in response to elevated CO₂ (eCO₂) (A) or eCO₂ plus warming (B) at different eCO₂ classes (ambient + <300 or ≥300 ppm) and warming (C) or eCO₂ plus warming (D) at different temperature classes (ambient + <1.5, 1.5–5 or >5 °C). Each data point represents the mean ± 95 % CI. Numbers within parentheses represent the number of experimental observations. The dashed vertical line is the reference line at 0 % change. Treatment effects are non-significant at P < 0.05 if CIs overlap the zero line, and differences among treatments are non-significant if CIs overlap.

Figure 2.

Percent change (compared to ambient or controls) in root %N in response to elevated CO₂ (eCO₂) (A) or eCO₂ plus warming (B) at different eCO₂ classes (ambient + <300 or ≥300 ppm) and warming (C) or eCO₂ plus warming (D) at different temperature classes (ambient + <1.5, 1.5–5 or >5 °C). Each data point represents the mean ± 95 % CI. Numbers within parentheses represent the number of experimental observations. The dashed vertical line is the reference line at 0 % change. Treatment effects are non-significant at P < 0.05 if CIs overlap the zero line, and differences among treatments are non-significant if CIs overlap.

Figure 3.

Percent change (compared to ambient or controls) in shoot %N in different growth forms (A–C), functional groups (D–F) and elevated CO₂ (eCO₂) treatment techniques (G–I, OTC = open-top chambers; CTC = closed-top chambers; GH = greenhouses; FACE = free-air CO₂ enrichment; TGT = temperature-gradient tunnels; GC = growth chambers) in response to eCO₂ (A, D, G), warming (B, E, H) and eCO₂ plus warming (C, F, I). Each data point represents the mean ± 95 % CI. Numbers within parentheses represent the number of experimental observations. The dashed vertical line is the reference line at 0 % change. Treatment effects are non-significant at P < 0.05 if CIs overlap the zero line, and differences among treatments are non-significant if CIs overlap.

Figure 4.

Percent change (compared to ambient or controls) in root %N in different growth forms (A–C) and elevated CO₂ (eCO₂) treatment techniques (D–F, CTC = closed-top chambers; GH = greenhouses; FACE = free-air CO₂ enrichment; GC = growth chambers) in response to eCO₂ (A, D), warming (B, E) and eCO₂ plus warming (C, F). Each data point represents the mean ± 95 % CI. Numbers within parentheses represent the number of experimental observations. The dashed vertical line is the reference line at 0 % change. Treatment effects are non-significant at P < 0.05 if CIs overlap the zero line, and differences among treatments are non-significant if CIs overlap.

Figure 5.

Percent change (compared to ambient or controls) in root N-uptake rate in response to elevated CO₂ (A), warming (B) and elevated CO₂ plus warming (C). Each data point represents the mean ± 95 % CI. Numbers within parentheses represent the number of experimental observations. The dashed vertical line is the reference line at 0 % change. Treatment effects are non-significant at P < 0.05 if CIs overlap the zero line.

Figure 6.

Percent change (compared to ambient or controls) in the concentration of total protein in different tissue types in response to elevated CO₂ (A), warming (B) and elevated CO₂ plus warming (C). Each data point represents the mean ± 95 % CI. Numbers within parentheses represent the number of experimental observations. The dashed vertical line is the reference line at 0 % change. Treatment effects are non-significant at P < 0.05 if CIs overlap the zero line, and differences among treatments are non-significant if CIs overlap.