Modelling changes in nitrogen cycling to sustain increases in forest productivity under elevated atmospheric CO₂ and contrasting site conditions

Abstract. If increases in net primary productivity (NPP) caused by rising concentrations of atmospheric CO₂ (C_a) are to be sustained, key N processes such as soil mineralization, biological fixation, root uptake and nutrient conservation must also be increased. Simulating the response of these processes to elevated C_a is therefore vital for models used to project the effects of rising C_a on NPP. In this modelling study, hypotheses are proposed for changes in soil mineralization, biological fixation, root nutrient uptake and plant nutrient conservation with changes in C_a. Algorithms developed from these hypotheses were tested in the ecosystem model ecosys against changes in N and C cycling measured over several years under ambient vs. elevated C_a in Free Air CO₂ Enrichment (FACE) experiments in the USA at the Duke Forest in North Carolina, the Oak Ridge National Laboratory forest in Tennessee, and the USDA research forest in Wisconsin. More rapid soil N mineralization was found to be vital for simulating sustained increases in NPP measured under elevated vs. ambient C_a at all three FACE sites. This simulation was accomplished by priming decomposition of N-rich humus from increases in microbial biomass generated by increased litterfall modelled under elevated C_a. Greater nonsymbiotic N₂ fixation from increased litterfall, root N uptake from increased root growth, and plant N conservation from increased translocation under elevated C_a were found to make smaller contributions to simulated increases in NPP. However greater nutrient conservation enabled larger increases in NPP with C_a to be modelled with coniferous vs. deciduous plant functional types. The effects of these processes on productivity now need to be examined over longer periods under transient rises in C_a and a greater range of site conditions.