Effects of nitrogen deposition and ozone exposure on terrestrial productivity and GHG balance in US

Background/Question/Methods

The altered atmospheric composition (e.g. increasing reactive nitrogen (N) and tropospheric ozone (O₃) concentration) plays a critical role in shaping terrestrial ecosystem productivity and greenhouse gases (GHGs) balance. Together with climate change, atmospheric N input and surface O₃ exposure are expected to further modulate the GHG exchange and thus the potential terrestrial feedback to climate system in the future. Upon this point, multiple airborne pollutants have received increasing attention. However, there is a lack of integrated assessment on impacts of multiple airborne pollutants on terrestrial ecosystem productivity and GHG balance in the nation. In this study, we used a highly-integrated process-based land ecosystem model DLEM (Dynamic Land Ecosystem Model) in conjunction with newly refined regional database, to investigate how terrestrial ecosystem productivity and the balance of three major GHGs (CO₂, CH₄ and N₂O) were and will be affected by N deposition and O₃ pollution over the continental US during the period 1951-2050.

Results/Conclusions

Our simulation results indicate that CO₂ uptake induced by N enrichment was largely offset by its concurrent emissions of CH₄ and N₂O. It implies that warming mitigation of terrestrial ecosystems in the continental US due to N fertilization effect might be lower than what we expected. In some high-N loading areas, the net global warming potential (GWP) of these three GHGs was anticipated to be close to zero or become positive if N input kept rising. Tropospheric O₃ was found to be one of the major environmental stresses reducing productivity and N-induced carbon sinks in some regions. This reduction would be highly pronounced when drought events coincidently took place. Under different emission scenarios, the combined effects of increasing N deposition, tropospheric O₃ pollution and climate change on the dynamics of GHGs were projected to vary with a large range over time and space. Some regions should be recognized as endangered areas where ecological consequences of air pollutants in terms of GWP are far more apparent than other regions and also should be highlighted as prioritized regions to advance Clean Air Act. Our study suggests an insightful implication for policy makers and stakeholders to better manage air as an important Earth’s life support system.