Print PageThe nitrogen (N) cycle has only recently been implemented in some global land surface models (LSMs) and associated climate models, but attention has largely focused on the development of soil processes, leaving major plant N uptake pathways largely absent. Generally, these models calculate a plant N demand, and then instantaneously remove any available soil N, placing this N directly into the plant tissues. It is well known in the ecophysiology/biology fields that plants expend carbon (C) to acquire N (not accounting passive uptake through transpiration) through active uptake (i.e., exudates), retranslocation, and biological N fixation (for some species). By missing these plant N acquisition processes, LSMs encounter major problems in representing plant growth as well as the overall impact of the N cycle on the global C cycle.
We developed a global model of plant N uptake, retranslocation, and fixation called FUN (Fixation & Uptake of Nitrogen) (Fisher et al. 2010: Global Biogeochemical Cycles) implemented into the UK land surface model (JULES), which is part of the UKMO Hadley Centre’s global climate model. FUN specifies and calculates C allocated to N acquisition as well as remaining C for plant growth (or, conversely, N-limitation to growth), while varying the C costs to N acquisition from the various source pathways.
Results/Conclusions
The model was tested against data from a wide range of sites, including a fertilization experiment in the tropics, the free air CO2 enrichment (FACE) studies, and agro-forestry: observed vs. predicted N uptake r2 was 0.89, and RMSE was 0.003 kg N m-2y-1. Four model tests were performed: (1) fixers versus nonfixers under primary succession; (2) response to N fertilization; (3) response to CO2 fertilization; and, (4) changes in vegetation C from potential soil N trajectories for five dynamic global vegetation models (TRIFFID, ORCHIDEE, LPJ, SDGVM, and HYLAND).
Links with the measurement community point to combining measurements of C and N flow with the modeling framework under different gradients of temperature, fertility, and successional stage. Mycorrhizal dynamics need further development. The fusion of these measurements with this model has the potential to directly answer many of the outstanding questions in plant C–N dynamics and patterns. Currently, a number of modelers are putting FUN into their LSMs.
