Impact of nitrogen limitation on terrestrial carbon cycle responses to climate variations and atmosphere CO2

Tuesday, 16 December 2014
Qian Zhang, Duoying Ji and Yong Jiu Dai, Beijing Normal University, Beijing, China
The responses of the terrestrial carbon cycle to its natural and anthropogenic driving factors are considered to be altered substantially by nitrogen dynamics. In this study, we use a land surface model coupling the carbon (C) and nitrogen (N) cycles to quantify the effect of nitrogen cycle on the sensitivity of terrestrial carbon cycle to atmosphere CO2 and concurrent climatic change. The model is Common Land Model (CoLM) updated by adopting the plant and soil C and N scheme from the Dynamic Nitrogen Scheme (DyN). We forced the model with reconstructed historical climate fields of CRUNCEP data and observed rising atmospheric CO2 concentration from 1900 to 2012. The simulated sensitivity of carbon fluxes by our carbon only (CoLM-C) and carbon nitrogen cycles model (CoLM-CN) to climate variability and atmospheric CO2 trends are compared with other independent studies. Global-scale results of CoLM-CN show that the model produces realistic estimates of current period C and N stocks, despite some regional biases. In response to rising atmospheric CO2 concentration, the simulated Gross Primary Production (GPP) and Net Primary Production (NPP) increases are suppressed by N limitations by 30% and 20%, respectively. The relative response of NPP to CO2 (12% per 100 ppm) when N is accounted for compares well with the sensitivity derived from Free-Air CO2 Enrichment (FACE) experiments (13% per 100 ppm). For the last 30 years, N limitation decreases the Net Biosphere Production (NBP) sensitivity to atmosphere CO2 by 16%. In response to the climatic changes, our results show that the interannual variability of C fluxes (GPP, NPP, NBP) is more closed controlled by precipitation in tropical and temperate ecosystems, while temperature is more important in boreal ecosystems. Including N cycle did not change the phase but reduce the magnitude of interannual variability of these fluxes. Globally, the model simulated a positive correlation between NBP and precipitation (2.2±1.5 Pg C per 100 ppm of interannual precipitation change by CoLM-C, and 1.6±1.2 PgC per 100 ppm of interannual precipitation change by CoLM-CN). Our results highlight the importance of including N cycle to reduce the uncertainties of climate sensitivity of carbon fluxes and understand the responses of terrestrial carbon cycle to climate system.