Abstract
A key part of the uncertainty in terrestrial feedbacks on climate change is related to how and to what extent nitrogen (N) availability constrains the stimulation of terrestrial productivity by elevated CO2 (eCO2 ), and whether or not this constraint will become stronger over time. We explored the ecosystem-scale relationship between responses of plant productivity and N acquisition to eCO2 in free-air CO2 enrichment (FACE) experiments in grassland, cropland and forest ecosystems and found that: (i) in all three ecosystem types, this relationship was positive, linear and strong (r(2) = 0.68), but exhibited a negative intercept such that plant N acquisition was decreased by 10% when eCO2 caused neutral or modest changes in productivity. As the ecosystems were markedly N limited, plants with minimal productivity responses to eCO2 likely acquired less N than ambient CO2 -grown counterparts because access was decreased, and not because demand was lower. (ii) Plant N concentration was lower under eCO2 , and this decrease was independent of the presence or magnitude of eCO2 -induced productivity enhancement, refuting the long-held hypothesis that this effect results from growth dilution. (iii) Effects of eCO2 on productivity and N acquisition did not diminish over time, while the typical eCO2 -induced decrease in plant N concentration did. Our results suggest that, at the decennial timescale covered by FACE studies, N limitation of eCO2 -induced terrestrial productivity enhancement is associated with negative effects of eCO2 on plant N acquisition rather than with growth dilution of plant N or processes leading to progressive N limitation.
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Claims 7
Claim · model-written summary
Productivity and nitrogen acquisition responses to elevated CO2 were positively related.
Not reportedDirection
Verbatim
e0“in all three ecosystem types, this relationship was positive, linear and strong (r(2) = 0.68)”
e1“responses of plant productivity and N acquisition to eCO2”
Claim · model-written summary
Plant nitrogen acquisition decreased by 10% when productivity changed little.
Verbatim
e0“plant N acquisition was decreased by 10% when eCO2 caused neutral or modest changes in productivity”
Claim · model-written summary
Plant nitrogen concentration was lower under elevated CO2.
Verbatim
e0“Plant N concentration was lower under eCO2”
e1“ambient CO2 -grown counterparts”
Claim · model-written summary
The lower nitrogen concentration did not depend on productivity enhancement.
- Taxon
- Plante1
- Treatment
- eCO2e0
- Outcome
- Plant N concentratione1
- Qualifier
- independent of the presence or magnitude of eCO2 -induced productivity enhancemente0
Not reportedDirection
Verbatim
e0“this decrease was independent of the presence or magnitude of eCO2 -induced productivity enhancement”
e1“Plant N concentration was lower under eCO2”
Claim · model-written summary
The productivity effect of elevated CO2 did not diminish over time.
Verbatim
e0“Effects of eCO2 on productivity and N acquisition did not diminish over time”
e1“plant productivity”
Claim · model-written summary
The nitrogen acquisition effect of elevated CO2 did not diminish over time.
Verbatim
e0“Effects of eCO2 on productivity and N acquisition did not diminish over time”
e1“responses of plant productivity and N acquisition to eCO2”
Claim · model-written summary
The elevated CO2 related decrease in plant nitrogen concentration diminished over time.
Verbatim
e0“the typical eCO2 -induced decrease in plant N concentration did”
e1“Effects of eCO2 on productivity and N acquisition did not diminish over time”