Using 15N Tracer Experiments with Land Surface Model to Assess Nitrogen Cycle Modelling

The nitrogen cycle plays a pivotal role in determining the magnitude of the land carbon sink. The manifold interactions of the carbon and nitrogen cycle, and their dependence on many complex processes of the nitrogen cycle pose a significant challenge to observe and quantify the likely effect of nitrogen cycling on terrestrial carbon storage. One promising method for model evaluation is using isotopic 15N tracer experiments both in modelling and field experiments, in which nitrogen flows can be tracked and evaluated without a major perturbation of the ecosystems biogeochemical cycles. In this work we use tracer experiment conducted in 2007 in a temperate mixed forest located in NY, USA. The fate of the added 15N tracer was measured at the end of 2007, the following year and five to six years later. We use a recently developed land surface model QUINCY (QUantifying Interactions between terrestrial Nutrient CYcles and the climate system). Overall the QUINCY model was successful in capturing the dynamics and magnitude of the 15N tracer experiment. The simulated tracer recovery rate after six years was somewhat higher, but of comparable magnitude to the observation. This may be either due to a better closure of mass-balance in the model, or to an underestimation of nitrogen loss pathways by the model. An analysis of the temporal development of the partitioning of 15N between vegetation and soils reveals the importance of nitrate assimilation into soil organic material. Different to the observations, which suggest slow transport of the tracer to the above-ground biomass compartments, in the model the uptake of nitrogen tracer from roots can very quickly result in tracer recovery in foliar tissues, suggesting that the usage of the two- nitrogen storage pool concept of the QUINCY model is not sufficient to simulate the travel-time time needed for a root signal to reach the leaves. In the model the tracer is within a couple of years transferred to the soil litter pool through leaf litterfall. In the observations this change occurs slower, partly caused by the lower leaf pool size of the model compared to observed. In the model the tracer stays in the litter pool for a long time, suggesting that the turnover rate of the model's litter pool might be too low.