Mechanisms and pathways of N accumulation in the terrestrial biosphere and consequences for its CO2 response

Background/Question/Methods

The response of the land’s vegetation to CO2 is a critical determinant of future climate change. Simulation results from models carrying a prognostic nitrogen (N) cycle suggest that CO2 fertilization (i.e. increase in carbon sinks with rising CO2) is dampened if nutrients are limiting. Since many of these models are built based on more or less rigid stoichiometric constraints, carbon (C) sequestration is directly tied to N buildup. C sequestration thus requires that levels of new N (from N deposition or N fixation) increase or that losses from plant/soil pools (dissolved inorganic/organic nitrogen (DIN/DON), fire volatilization) decrease. Using LM3, the Princeton/GFDL global land model, we explore how the different pathways of N accumulation and retention affect the level and time scale of land C sequestration. We examine the response to an instantaneous doubling of CO2 for 4 model scenarios: a) a static model where N fixation is a prescribed constant and hydrologic N losses occur strictly from the DIN pool. b) a scenario with DON scaling to litter transformation rates. c) a dynamic input - dynamic loss scenario with N fixation responding to N demand and light availability on top of DON losses. d) a C-only scenario, where C accumulation occurs independently of N.

Results/Conclusions

In the C-only simulation, the response to CO2 was fastest among all scenarios, and lead to the largest C uptake due to unlimited N supply. In contrast DON losses act as a negative feedback where production and hydrologic export tend to increase with stock size, leading to sustained N limitation with little response to CO2. Up-regulation of N fixation helps to alleviate the N-limiting effect of DON losses, particularly the presence of N fixing trees in tropical forest may allow for substantial carbon uptake. Finally in the static model, the CO2 response has the potential to be large; however, due to small rates of N supply C accumulation could be delayed by centuries. We conclude that the challenge to understand the global CO2 response of the land's biosphere it is not only to quantify external fluxes by constructing N budgets, but also to understand the nature and mechanism how biogeochemical feedbacks regulate N input and losses.