Insights into mechanisms governing forest carbon response to nitrogen deposition: a model–data comparison using observed responses to nitrogen addition

摘要

In many forest ecosystems, nitrogen (N) deposition enhances plant uptake ofcarbon dioxide, thus reducing climate warming from fossil fuel emissions.Therefore, accurately modeling how forest carbon (C) sequestration respondsto N deposition is critical for understanding how future changes in Navailability will influence climate. Here, we use observations of forest Cresponse to N inputs along N deposition gradients and at five temperateforest sites with fertilization experiments to test and improve a globalbiogeochemical model (CLM-CN 4.0). We show that the CLM-CN plant C growthresponse to N deposition was smaller than observed and the modeled responseto N fertilization was larger than observed. A set of modifications to theCLM-CN improved the correspondence between model predictions andobservational data (1) by increasing the aboveground C storage in responseto historical N deposition (1850–2004) from 14 to 34 kg C per additional kg N added through deposition and (2) by decreasing the aboveground net primaryproductivity response to N fertilization experiments from 91 to 57 g C m?2 yr?1. Modeled growth response to N deposition was mostsensitive to altering the processes that control plant N uptake and thepathways of N loss. The response to N deposition also increased with a moreclosed N cycle (reduced N fixation and N gas loss) and decreased whenprioritizing microbial over plant uptake of soil inorganic N. The net effectof all the modifications to the CLM-CN resulted in greater retention of Ndeposition and a greater role of synergy between N deposition and risingatmospheric CO₂ as a mechanism governing increases in temperate forestprimary production over the 20th century. Overall, testing models with boththe response to gradual increases in N inputs over decades (N deposition)and N pulse additions of N over multiple years (N fertilization) allows forgreater understanding of the mechanisms governing C–N coupling.