The Effect of Atmospheric Nitrogen Deposition on Plant and Soil Carbon Sequestration and Nitrous Oxide Emissions for Seven Ecosystems in the United States.

To reduce the accumulation of green house gases (GHGs), such as CO₂ and N₂O, in the atmosphere, ecosystem C storage must be maintained or enhanced while GHG emissions are reduced.  Climate change, rising atmospheric CO₂, and N deposition could favor C sequestration or loss depending on relative response of plant production and heterotrophic respiration.  CO₂ fertilization may increase the rate of photosynthesis in some plants,  though one major uncertainty in predicting the long-term ecosystem C balance is whether the CO₂ fertilization effect can be sustained.  Nutrient and water availability, temperature, and plant physiology will determine if and when the CO₂ fertilization effect will saturate.  Atmospheric N deposition may alleviate some nutrient limitations.  Though any benefit of N-deposition enhanced C-sequestration could be countered by degradation of stream and soil water quality and increased N₂O emissions. Per kilogram N₂O has 296 times the 100-year warming potential of CO₂.
We used DayCent-Chem, a daily timestep process-based ecosystem-hydrochemical model, to examine how expected long-term changes in climate, atmospheric CO₂, and N deposition may affect soil/plant nutrient cycling, C storage, N₂O emissions, and surface water chemistry for seven diverse ecosystems in the U.S.  With steady or increasing N deposition, plant production and heterotrophic respiration increased at each site, as did live biomass C and total system C.  While net ecosystem production (NEP) stayed positive, it decreased over time for some sites and increased for others.   A few sites still showed a net loss of soil C over time.  N₂O flux increased with N deposition.  The contribution of the N₂O (in C equivalents) was similar in magnitude to the change in ecosystem of soil C.  Stream N flux and concentration changed (increased or decreased) with N deposition at some sites, but were also dependent on changes to evapotranspiration, N uptake, and soil mineralization.