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

Abstract:

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.