Buffering of modeled terrestrial extreme water-carbon response from nitrogen cycle
Terrestrial carbon and water cycles are coupled in multiple temporal and spatial scales and are crucial in the feedback to future climate changes. Water related climate extremes, such as drought and intense precipitation, can substantially affect regional and global carbon cycle. Meanwhile, nitrogen/nutrient interactions are widely recognized to be important in predicting terrestrial carbon cycle and can potentially alter the water-carbon responses. To understand the role of nitrogen in the responses of terrestrial carbon cycle to moisture anomalies,the Geophysical Fluid Dynamics Laboratory’s dynamic land surface model LM3V(N) was applied to derive net primary productivity (NPP), heterotrophic respiration (Rh) and net ecosystem productivity (NEP, i.e. the difference between NPP and Rh). Carbon fluxes were related to dry/wet extreme conditions indicated by the standardized precipitation evapotranspiration index (SPEI, positive for wet conditions and negative for dry).
We detected the temporal scale dependent correlations between each of the carbon flux and SPEI, globally, with a strong legacy response from Rh where empirical long term studies are scarce. Consideration of nitrogen dynamics acted to buffer terrestrial carbon cycle in response to water anomalies. The buffering effect constrained the over-growth of vegetation in wet extremes as well as the reduction of NPP in droughts. Extra gain of soil moisture from the downregulation of canopy transpiration by nitrogen limitation, and the shift in relative importance of water and nitrogen limitation during dry and wet extreme events are mechanisms that contribute to the buffering of nitrogen on modeledNPP. The role of nitrogen in NEP response depends on the net effect from both NPP and Rh, while the later (Rh) is complex and spatial-temporal scale dependent, and should have the priority in future studies.