Nitrogen deposition and greenhouse gas emissions from grasslands: uncertainties and future directions

Authors: GOMEZ-CASANOVAS, NURIA - University Of Illinois; HUDIBURG, TARA - University Of Illinois; Bernacchi, Carl; PARTON, WILLIAM - Colorado State University; DELUCIA, EVAN - University Of Illinois

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2015
Publication Date: 4/1/2016
Citation: Gomez-Casanovas, N., Hudiburg, T., Bernacchi, C.J., Parton, W., DeLucia, E. 2016. Nitrogen deposition and greenhouse gas emissions from grasslands: uncertainties and future directions. Global Change Biology. 22:1348-1360.

Interpretive Summary: Greenhouse gases, including carbon dioxide, methane, and nitrous oxide, are primarily responsible for increasing global temperatures. Grasslands play an important role in determining the concentration of greenhouse gases in the atmosphere. Changes in the rate of nitrogen, a fertilizer, addition to grasslands can influence the emissions of greenhouse gases. Many experiments have been conducted to determine the relationship between nitrogen deposition and greenhouse gas emissions, and we have compiled a large number of these studies and determined general patterns from these previous experiments. The vast majority of nitrogen additions in these experiments were higher than levels these ecosystems will experience in the future, and represented treatments well into the N saturation-decline stage. This means that the experimental treatments were too high, even for future conditions. The results of this work will help to identify realistic nitrogen addition rates, and will help to constrain estimates of future greenhouse gas emissions from grasslands.

Technical Abstract: Increases in atmospheric nitrogen deposition (Ndep) can strongly affect the greenhouse gas (GHG; CO2, CH4 and N2O) sink capacity of terrestrial ecosystems. Grasslands play an important role in determining the concentration of GHGs in the atmosphere. Robust predictions of the net GHG sink strength of grasslands depend on how experimental N loads compare to projected Ndep rates, and how accurately the GHG-N dose-response relationship is characterized. We surveyed the available literature of GHG fluxes of grasslands and show that the vast majority of experimental nitrogen loads were higher than levels these ecosystems will experience in the future, and represented treatments well into the N saturation-decline stage. Using a process-based biogeochemical model, we predicted that at low levels Ndep enhanced or reduced the net GHG sink strength of most grasslands, but as it continued to increase, grasslands transitioned to a N saturation-decline stage in which the sensitivity of the sink or source strength to Ndep declined. Our results indicate that the responses of GHG fluxes to N are nonlinear and that the N saturation thresholds for GHGs varied greatly with grassland and with fire management. We showed that during the 21st century some grasslands will be at the N limitation stage whereas others will transition into the N saturation-decline stage. Our results suggest that linear approaches commonly employed for estimating global net GHG sink strength can either over- or underestimate predictions of the net GHG sink strength of grasslands depending on their N baseline status, and that next generation of global change experiments should be designed at multiple N loads consistent with future Ndep rates to improve our empirical understanding and predictive ability.