Abstract
Abstract Understanding carbon dynamics of switchgrass ecosystems is crucial as switchgrass ( Panicum virgatum L.) acreage is expanding for cellulosic biofuels. We used eddy covariance system and examined seasonal changes in net ecosystem CO 2 exchange ( NEE ) and its components – gross ecosystem photosynthesis ( GEP ) and ecosystem respiration ( ER ) – in response to controlling factors during the second (2011) and third (2012) years of stand establishment in the southern Great Plains of the United States (Chickasha, OK). Larger vapor pressure deficit ( VPD > 3 kPa) limited photosynthesis and caused asymmetrical diurnal NEE cycles (substantially higher NEE in the morning hours than in the afternoon at equal light levels). Consequently, rectangular hyperbolic light–response curve ( NEE partitioning algorithm) consistently failed to provide good fits at high VPD . Modified rectangular hyperbolic light– VPD response model accounted for the limitation of VPD on photosynthesis and improved the model performance significantly. The maximum monthly average NEE reached up to −33.02 ± 1.96 μmol CO 2 m −2 s −1 and the highest daily integrated NEE was −35.89 g CO 2 m −2 during peak growth. Although large differences in cumulative seasonal GEP and ER were observed between two seasons, total seasonal ER accounted for about 75% of GEP regardless of the growing season lengths and differences in aboveground biomass production. It suggests that net ecosystem carbon uptake increases with increasing GEP . The ecosystem was a net sink of CO 2 during 5–6 months and total seasonal uptakes were −1128 ± 130 and −1796 ± 217 g CO 2 m −2 in 2011 and 2012, respectively. In conclusion, our findings suggest that the annual carbon status of a switchgrass ecosystem can be a small sink to small source in this region if carbon loss from biomass harvesting is considered. However, year‐round measurements over several years are required to assess a long‐term source‐sink status of the ecosystem.
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Claims 12
Claim · model-written summary
Larger vapor pressure deficit limited switchgrass photosynthesis.
- Taxon
- Switchgrass
Panicum virgatumt0 - Treatment
- vapor pressure deficite0
- Outcome
- photosynthesise0
- Direction
- limitede0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“over a young Switchgrass stand”
e0“Larger vapor pressure deficit ( VPD > 3 kPa) limited photosynthesis and caused asymmetrical diurnal NEE cycles”
Claim · model-written summary
NEE in the morning hours was higher than in the afternoon at equal light levels.
- Taxon
- SwitchgrassPanicum virgatumt0
- Outcome
- NEE in the morning hourse1
- Direction
- highere1
- Comparator
- in the afternoone1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“over a young Switchgrass stand”
e1“substantially higher NEE in the morning hours than in the afternoon at equal light levels”
Claim · model-written summary
The modified rectangular hyperbolic light–VPD model significantly improved model performance compared with the rectangular hyperbolic light–response curve.
- Taxon
- SwitchgrassPanicum virgatumt0
- Outcome
- model performancee3
- Direction
- improvede3
- Comparator
- rectangular hyperbolic light–response curvee2
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“over a young Switchgrass stand”
e2“Consequently, rectangular hyperbolic light–response curve ( NEE partitioning algorithm) consistently failed to provide good fits at high VPD”
e3“Modified rectangular hyperbolic light– VPD response model accounted for the limitation of VPD on photosynthesis and improved the model performance significantly”
Claim · model-written summary
The rectangular hyperbolic light–response curve failed to give good fits at high VPD.
- Taxon
- SwitchgrassPanicum virgatumt0
- Outcome
- good fitse2
- Direction
- failede2
- Qualifier
- at high VPDe2
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“over a young Switchgrass stand”
e2“Consequently, rectangular hyperbolic light–response curve ( NEE partitioning algorithm) consistently failed to provide good fits at high VPD”
Claim · model-written summary
The maximum monthly average NEE was −33.02 ± 1.96 μmol CO2 m−2 s−1.
- Taxon
- SwitchgrassPanicum virgatumt0
- Outcome
- maximum monthly average NEEe4
- Value
- −33.02 ± 1.96 μmol CO 2 m −2 s −1e4
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“over a young Switchgrass stand”
e4“The maximum monthly average NEE reached up to −33.02 ± 1.96 μmol CO 2 m −2 s −1”
Claim · model-written summary
The highest daily integrated NEE was −35.89 g CO2 m−2 during peak growth.
- Taxon
- SwitchgrassPanicum virgatumt0
- Outcome
- highest daily integrated NEEe5
- Value
- −35.89 g CO 2 m −2e5
- Qualifier
- during peak growthe5
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“over a young Switchgrass stand”
e5“the highest daily integrated NEE was −35.89 g CO 2 m −2 during peak growth”
Claim · model-written summary
Total seasonal ecosystem respiration was about 75% of gross ecosystem photosynthesis regardless of season length or biomass.
- Taxon
- SwitchgrassPanicum virgatumt0
- Outcome
- total seasonal ERe6
- Value
- about 75%e6
- Comparator
- GEPe6
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“over a young Switchgrass stand”
e6“total seasonal ER accounted for about 75% of GEP regardless of the growing season lengths and differences in aboveground biomass production”
Claim · model-written summary
Cumulative seasonal GEP and ER showed large differences between the two seasons.
- Taxon
- SwitchgrassPanicum virgatumt0
- Outcome
- cumulative seasonal GEP and ERe6
- Qualifier
- between two seasonse6
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“over a young Switchgrass stand”
e6“Although large differences in cumulative seasonal GEP and ER were observed between two seasons, total seasonal ER accounted for about 75% of GEP regardless of the growing season lengths and differences in aboveground biomass production”
Claim · model-written summary
Net ecosystem carbon uptake increases with increasing GEP.
- Taxon
- SwitchgrassPanicum virgatumt0
- Treatment
- increasing GEPe7
- Outcome
- net ecosystem carbon uptakee7
- Direction
- increasese7
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“over a young Switchgrass stand”
e7“It suggests that net ecosystem carbon uptake increases with increasing GEP”
Claim · model-written summary
Total seasonal uptakes were −1128 ± 130 and −1796 ± 217 g CO2 m−2 in 2011 and 2012 in Chickasha, OK.
- Taxon
- SwitchgrassPanicum virgatumt0
- Outcome
- total seasonal uptakese1
- Value
- −1128 ± 130 and −1796 ± 217 g CO 2 m −2e1
- Qualifier
- in 2011 and 2012e1
- Setting
- Chickasha, OKe2
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“over a young Switchgrass stand”
e1“total seasonal uptakes were −1128 ± 130 and −1796 ± 217 g CO 2 m −2 in 2011 and 2012”
e2“in the southern Great Plains of the United States (Chickasha, OK)”
Claim · model-written summary
The ecosystem was a net sink of CO2 during 5–6 months.
- Taxon
- SwitchgrassPanicum virgatumt0
- Outcome
- net sink of CO 2e1
- Value
- 5–6 monthse1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“over a young Switchgrass stand”
e1“The ecosystem was a net sink of CO 2 during 5–6 months”
Claim · model-written summary
The annual carbon status of switchgrass can be a small sink to small source if carbon loss from biomass harvesting is considered.
- Taxon
- SwitchgrassPanicum virgatumt0
- Outcome
- annual carbon statuse1
- Qualifier
- if carbon loss from biomass harvesting is considerede1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“over a young Switchgrass stand”
e1“the annual carbon status of a switchgrass ecosystem can be a small sink to small source in this region if carbon loss from biomass harvesting is considered”