Key reference | | Wang *et al*. (2010, 2011) | Thornton & Zimmermann (2007), Thornton *et al*. (2007) | Parton *et al*. (2010) | Wang *et al*. (2001) |

Time step | | 30 min | 30 min | 1 d | 30 min |

Plant C acquisition | Assimilation (GPP) | Farquhar *et al*. (1980) | Collatz *et al*. (1991) | 2 × NPP_{act} | Farquhar *et al*. (1980) |

N dependence of gross photosynthesis | *f*(leaf N) | NPP_{act}/NPP_{pot} | None | *f*(leaf N) |

Autotrophic respiration | *f*(tissue N, *T*) + *f*(growth rate) | *f*(tissue C, *T*) + *f*(growth rate) | 0.5 × GPP | *f*(tissue C, *T*) + *i*(growth rate) |

N dependence of whole-plant growth (if not GPP – *R*_{a}) | None | Potential growth (NPP_{pot}) limited by stoichiometric N requirement for new tissue growth | Potential growth (*f*(PAR, *T*, moisture, CO_{2})) limited by stoichiometric N requirement for new tissue growth | None |

Plant N acquisition | Nitrogen fixation | Prescribed based on Wang & Houlton (2009) | *f*(NPP) | Plant-associated N fixation: *f*(N : P, plant N demand); soil N fixation: *f*(AET) | None |

Nitrogen uptake | *f*(plant N demand, soil N availability) | *f*(relative strength of plant and microbial N demand, inorganic N pool size) | *f*(root biomass, plant demand, soil N availability) | Competition of soil mineral N between plant and microbial |

Plant growth | Allocation principlea | Fixed allocation fractions, which vary according to phenological state | Fixed allocation fractions, derived from observations at the sites | Hierarchical allocation factors, in which fine roots have priority over leaves and over wood, with prescribed maximum pool sizes | Fixed allocation fractions, which vary according to phenological state |

Maximum leaf areaa | Prescribed (LAI = 8; excess C is allocated to wood and roots) | Predicted | Predicted | Prescribed from observations at the site |

N effect on allocationa | None | None | Nitrogen stress increases root allocation | None |

Plant tissue C : N stoichiometry | Flexible within 10% of the prescribed mean C : N | Fixed | Flexible within prescribed bounds | Flexible within prescribed bounds |

Plant N turnover | N effect on turnover/mortality | None | Indirect via changes in NPP | Leaf turnover increases linearly with leaf N concentration | None |

N retention on leaf and root shedding | 50% of leaf N, 10% of root N | Litter has a fixed C : N (PFT specific) | 50% of leaf N | Retaining ratio depends on current tissue C : N ratio |

Soil N turnover | SOM decay (other than dependent on soil *T* and moisture) | 3 litter pools (metabolic, structural, coarse woody debris), 3 SOM pools with different turnover times, 1st order decay | 3 litter pools, 4 SOM pools, all with different turnover times, 1st order decay | 3 litter pools (above and below ground combined), 4 SOM pools, all with different turnover times, 1st order decay | 3 litter pools; 4 SOM pools with different turnover rates, 1st order decay |

N effect on decomposition | Lignin : N ratio affects microbial efficiency and decomposition rate. Available soil mineral N constrains immobilization | Litter decomposition constrained by available soil N | Lignin : N ratio affects microbial efficiency and decomposition rate. Available soil mineral N constrains immobilization | Litter decomposition constrained by available soil N |

Soil C : N stoichiometry | Fixed for each pool | Fixed for each pool | *f*(mineral N concentration, within bounds) | *f*(mineral N concentration, within bounds) |

Ecosystem N losses | N leaching | Proportional to mineral N pool | *f*(soil water N concentration, drainage) | DON + N leaching = *f*(precipitation, NO_{3} pool size) | *f*(mineral N concentration, drainage and surface runoff) |

gaseous N loss | Proportional to net N mineralization rate | Proportional to gross N mineralization + 10% of mineral N remaining in the soil | NO_{x}, N_{2}O, N_{2} fluxes, as a function of soil N pool size, temperature, water | None |