Climate extremes, including drought and above average rates of precipitation have a significant impact on plant productivity and thus terrestrial carbon sequestration. The representation of how the terrestrial carbon cycle responds to these extremes remains a challenge for Earth system models. Here, we investigate, how the inclusion of nitrogen dynamics affects the response of a land-surface model to anomalies in water availability to plants. We specifically evaluate the response of three central carbon fluxes, net primary productivity (NPP), heterotrophic respiration (Rh), and net ecosystem productivity (NEP, the difference between NPP and Rh) to drought on water surpluses in the LM3V-N model. Water anomalies are characterized using the standardized precipitation and moisture index (SPEI).
Results/Conclusions
We find that globally, the relationship between each of the carbon flux and SPEI depends on the timescale, and in particular on Rh showed a strong legacy effect in response to these climate extremes. The consideration of nitrogen constraints carbon flux anomalies in in response to extreme dry/wet events. Nitrogen essentially acted as a buffer constraining the growth of plants under wet extremes and allowed for enhanced growth during droughts relative to model simulations without nitrogen constraints. Extra gain of soil moisture from the downregulation of canopy transpiration by nitrogen limitation and shifts in the relative demand for water and nitrogen limitation during dry/wet extreme events are possible mechanisms contributing to this buffering of NPP and NEP. Given the efforts to include phosphorus as further constraint in Earth system model, alternation between water and nutrient limitation need critical attention as they impact interannual variability and also possibly long-term trajectory of the terrestrial carbon cycle.