Abstract
Abstract The effect of nutrient availability on plant growth and the terrestrial carbon sink under climate change and elevated CO 2 remains one of the main uncertainties of the terrestrial carbon cycle. This is partially due to the difficulty of assessing nutrient limitation at large scales over long periods of time. Consistent declines in leaf nitrogen (N) content and leaf δ 15 N have been used to suggest that nitrogen limitation has increased in recent decades, most likely due to the concurrent increase in atmospheric CO 2 . However, such data sets are often not straightforward to interpret due to the complex factors that contribute to the spatial and temporal variation in leaf N and isotope concentration. We use the land surface model (LSM) QUINCY, which has the unique capacity to represent N isotopic processes, in conjunction with two large data sets of foliar N and N isotope content. We run the model with different scenarios to test whether foliar δ 15 N isotopic data can be used to infer large‐scale N limitation and if the observed trends are caused by increasing atmospheric CO 2 , changes in climate or changes in sources and magnitude of anthropogenic N deposition. We show that while the model can capture the observed change in leaf N content and predict widespread increases in N limitation, it does not capture the pronounced, but very spatially heterogeneous, decrease in foliar δ 15 N observed in the data across the globe. The addition of an observation‐based temporal trend in isotopic composition of N deposition leads to a more pronounced decrease in simulated leaf δ 15 N. Our results show that leaf δ 15 N observations cannot, on their own, be used to assess global‐scale N limitation and that using such a data set in conjunction with an LSM can reveal the drivers behind the observed patterns.
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Claims 6
Claim · model-written summary
QUINCY captured the observed change in leaf nitrogen content.
Not reportedTaxon
Verbatim
e0“the model can capture the observed change in leaf N content”
e1“land surface model (LSM) QUINCY”
Claim · model-written summary
QUINCY predicted widespread increases in nitrogen limitation.
Not reportedTaxon
Verbatim
e0“predict widespread increases in N limitation”
e1“leaf nitrogen (N) content”
Claim · model-written summary
QUINCY did not capture the globally observed, spatially heterogeneous decrease in foliar δ 15 N.
Verbatim
e0“it does not capture the pronounced, but very spatially heterogeneous, decrease in foliar δ 15 N observed in the data across the globe”
Claim · model-written summary
Adding the observed temporal trend in nitrogen-deposition isotope composition produced a more pronounced decline in simulated foliar δ 15 N.
- Treatment
- addition of an observation‐based temporal trend in isotopic composition of N depositione0
- Outcome
- simulated leaf δ 15 Ne0
- Direction
- more pronounced decreasee0
Not reportedTaxonComparator
Verbatim
e0“The addition of an observation‐based temporal trend in isotopic composition of N deposition leads to a more pronounced decrease in simulated leaf δ 15 N.”
Claim · model-written summary
Foliar δ 15 N observations alone cannot assess global nitrogen limitation.
Verbatim
e0“leaf δ 15 N observations cannot, on their own, be used to assess global‐scale N limitation”
Claim · model-written summary
Combining foliar δ 15 N data with a land surface model can reveal drivers of observed patterns.
Verbatim
e0“using such a data set in conjunction with an LSM can reveal the drivers behind the observed patterns”