Long-term ecosystem nitrogen limitation from foliar δ 15 N data and a land surface model.

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.
(© 2021 The Authors. Global Change Biology published by John Wiley & Sons Ltd.)