Land carbon-concentration and carbon-climate feedbacks are significantly reduced by nitrogen and phosphorus limitation

Abstract

We quantify the impact of nutrient (nitrogen and phosphorus) limitation on land carbon uptake and the sensitivity of this uptake to atmospheric carbon dioxide (CO2) concentrations (land carbon-concentration feedback) and surface air temperature (land-carbon climate feedback). We analyse simulations of the Australian Earth System Model (ACCESS-ESM1.5) with a prescribed 1% yr−1 CO2 increase in three different configurations: (1) without nutrient limitation, (2) with nitrogen limitation only and (3) with nitrogen and phosphorus limitation combined. Our results suggest a reduction in land carbon uptake and feedback strength of about 30% by including nitrogen limitation only. This reduction agrees well with the ensemble mean of models with and without nitrogen limitation from the sixth Coupled Model Intercomparison Project (CMIP6). By adding phosphorus limitation to our model, the response is even stronger with a further 50% reduction for the carbon-concentration feedback and a further 75% reduction for the carbon-climate feedback. We find that the feedback strength in all three nutrient configurations is mostly determined by vegetation productivity (i.e. Net Primary Production) with little contribution from soil heterotrophic respiration. Our simulations show that nutrient limitation has the largest impact in the northern mid latitudes (around 50∘), reducing land carbon uptake by about 50% when accounting for nitrogen and phosphorus limitation combined. The strong response of land carbon uptake and feedback strength to nutrients in our model simulations underlines the importance of including both nitrogen and phosphorus limitation in other Earth System Models in order to derive better estimates of future land carbon uptake and to assess the effectiveness of land-based carbon mitigation.