Continual input of reactive nitrogen (N) is required
to support the natural turnover of N in terrestrial ecosystems.
This “N demand” can be satisfied in various ways, including
biological N fixation (BNF) (the dominant pathway under
natural conditions), lightning-induced abiotic N fixation, N
uptake from sedimentary substrates, and N deposition from
natural and anthropogenic sources. We estimated the global
new N fixation demand (NNF), i.e. the total new N input
required to sustain net primary production (NPP) in nonagricultural
terrestrial ecosystems regardless of its origin,
using a N-enabled global dynamic vegetation model (DyNLPJ).
DyN-LPJ does not explicitly simulate BNF; rather, it
estimates total NNF using a mass balance criterion and assumes
that this demand is met from one source or another.
The model was run in steady state and then in transient mode
driven by recent changes in CO2 concentration and climate.
A range of values for key stoichiometric parameters was considered,
based on recently published analyses. Modelled NPP
and C : N ratios of litter and soil organic matter were consistent
with independent estimates. Modelled geographic patterns
of ecosystem NNF were similar to other analyses, but
actual estimated values exceeded recent estimates of global
BNF. The results were sensitive to a few key parameters: the
fraction of litter carbon respired to CO2 during decomposition
and plant-type-specific C : N ratios of litter and soil. The
modelled annual NNF increased by about 15 % during the
course of the transient run, mainly due to increasing CO2
concentration. The model did not overestimate recent terrestrial
carbon uptake, suggesting that the increase in NNF demand
has so far been met. Rising CO2 is further increasing
the NNF demand, while the future capacity of N sources to
support this is unknown.