Using 15N natural abundance and tracer studies to constrain simulated nitrogen dynamics in forest ecosystems under changing atmospheric CO2 concentrations.

Increasing CO2 concentrations and future climate change will affect the ability of ecosystemsto sequester carbon due to increasing nutrient limitation, in particular nitrogen (N), which isthe key limiting nutrient in many terrestrial ecosystems. However, the N deficit is difficult, ifnot impossible, to quantify simply from the ecosystem monitoring data. One of the mostcommon tools used is the N stable isotope composition, either at natural abundance, wherethe isotopic fractionation of different processes can indicate N limitation, or asadded tracer in manipulative experiments to trace the fate of N in the ecosystem.Despite the wealth of data, terrestrial biosphere models do not often include thecapacity to include 15N in their predictions, making it difficult to use the availableinformation. We have developed a new terrestrial biosphere model QUINCY (QUantifying Interactionsbetween terrestrial Nutrient CYcles and the climate system) which includes fully coupledcarbon, nutrient and water cycles, as well as an improved representation of plant and soilphysiological processes. Most importantly, we include a 15N tracer to all soil and vegetationpools that allows us to evaluate our model against observations directly. Here, wepresent the first results from a land surface model which includes explicit isotopetracers. First, we explore the trends in foliar 15N during the last century at over 400 forest sitesworldwide. The observed trends of foliar 15N show a decline during the last decades implyingincreasing N deficiency in the ecosystems but attributing the driver of these changes is notstraightforward simply from observations. Our model is able to replicate the observed trendand we show that it is caused by the CO2 fertilisation effect, rather than changes in climateand N deposition. Next, we test the model at two manipulative experimental sites, the Free-Air Carbondioxide Enrichment (FACE) experiment at Oak Ridge National Laboratory (ORNL),where measurements of natural abundance 15N allow an in-depth exploration ofN limitation under elevated CO2, and the 15N enrichment experiment at Cornell,where the 15N tracer allows us to explore fate of added N in soil and vegetationpools. These three case studies showcase the utility of a terrestrial biosphere model withintegrated isotope tracers to not only predict the observed state of the ecosystem but alsoexplore the fundamental drivers and internal ecosystem processes. [ABSTRACT FROM AUTHOR]