Abstract
A comprehensive model of terrestrial N dynamics has been developed and coupled
with the geographically explicit terrestrial C cycle component of the Integrated Science
Assessment Model (ISAM). The coupled C-N cycle model represents all the major
processes in the N cycle and all major interactions between C and N that affect plant
productivity and soil and litter decomposition. Observations from the LIDET data set
were compiled for calibration and evaluation of the decomposition submodel within
ISAM. For aboveground decomposition, the calibration is accomplished by optimizing
parameters related to four processes: the partitioning of leaf litter between metabolic and
structural material, the effect of lignin on decomposition, the climate control on
decomposition and N mineralization and immobilization. For belowground decomposition,
the calibrated processes include the partitioning of root litter between decomposable and
resistant material as a function of litter quality, N mineralization and immobilization.
The calibrated model successfully captured both the C and N dynamics during
decomposition for all major biomes and a wide range of climate conditions. Model results
show that net N immobilization and mineralization during litter decomposition are
dominantly controlled by initial N concentration of litter and the mass remaining during
decomposition. The highest and lowest soil organicNstorage are in tundra (1.24 KgNm2)
and desert soil (0.06 Kg N m2). The vegetation N storage is highest in tropical forests
(0.5 Kg N m2), and lowest in tundra and desert (<0.03 Kg N m2). N uptake by
vegetation is highest in warm and moist regions, and lowest in cold and dry regions.
Higher rates of N leaching are found in tropical regions and subtropical regions where soil
moisture is higher. The global patterns of vegetation and soil N, N uptake and N leaching
estimated with ISAM are consistent with measurements and previous modeling studies.
This gives us confidence that ISAM framework can predict plant N availability and
subsequent plant productivity at regional and global scales and furthermore how they can
be affected by factors that alter the rate of decomposition, such as increasing atmospheric
[CO2], climate changes, litter quality, soil microbial activity and/or increased N.