Increased influence of nitrogen limitation on CO2 emissions from future land use and land-use change

Abstract:

In the latest projections of future greenhouse gas emissions for the Intergovernmental Panel on Climate Change (IPCC), few Earth System Models included the effect of nitrogen limitation, a key process limiting forest regrowth. We estimate the impacts of nitrogen limitation on the CO₂ emissions from land use and land-use change (LULUC), including wood harvest, for the period 1900-2100. We use a land-surface model that includes a fully coupled carbon and nitrogen cycle, and accounts for forest regrowth processes following agricultural abandonment and wood harvest. Future projections are based on the four Representation Concentration Pathways used in the IPCC Fifth Assessment Report, and we account for uncertainty in future climate for each scenario based on ensembles of climate model outputs. Results show that excluding nitrogen limitation will underestimate global LULUC emissions by 34-52 PgC (20-30%) during the 20^th century (range across three different historical LULUC reconstructions) and by 128-187 PgC (90-150%) during the 21^st century (range across the four IPCC scenarios). The full range for estimated LULUC emissions during the 21^st century including climate model uncertainty is 91 to 227 PgC (with nitrogen limitation included). The underestimation increases with time because: (1) Projected annual wood harvest rates from forests summed over the 21^st century are 380-1080% higher compared to those of the 20^th century, resulting in more regrowing secondary forests, (2) Nitrogen limitation reduces the CO₂ fertilization effect on net primary production of regrowing secondary forests following wood harvest and agricultural abandonment, and (3) Nitrogen limitation effect is aggravated by the gradual loss of soil nitrogen from LULUC disturbance. Our study implies that: (1) Nitrogen limitation of CO₂ uptake is substantial and sensitive to nitrogen inputs, (2) If LULUC emissions are larger than previously estimated in studies without nitrogen limitation, then meeting the same climate mitigation target would require an equivalent additional reduction of fossil fuel emissions, and (3) The effectiveness of land-based mitigation strategies will critically depend on the interactions between nutrient limitations and secondary forests resulting from LULUC.