Quantifying carbon-nitrogen feedbacks in the Community Land Model (CLM4)

Recent studies indicate that nitrogen biogeochemistry affects the carbon cycle feedback in climate simulations. We use the Community Land Model version 4 (CLM4) with carbon-only and carbon-nitrogen biogeochemistry to assess the influence of nitrogen on the land carbon budget for 1973-2004. Carbon-only simulations show that the carbon gain from increasing atmospheric CO₂ (the concentration-carbon feedback) is four times greater than the warming-induced carbon loss (the climate-carbon feedback) over the period 1973-2004. Nitrogen reduces both feedbacks compared with carbon-only biogeochemistry. The decrease in the concentration-carbon feedback is three times greater than the effect on the climate-carbon feedback. Thus, the influence of nitrogen on the CLM4 concentration-carbon feedback is of greater importance for near-term climate change simulations than its effect on the climate-carbon feedback. Furthermore, the land use carbon flux greatly exceeds these carbon-nitrogen biogeochemical feedbacks.