Abstract: New PhytologistVolume 196, Issue 2 p. 327-328 CommentaryFree Access Climate change increases soil nitrous oxide emissions S. J. Del Grosso, Corresponding Author S. J. Del Grosso Natural Resources Research Center, USDA-ARS-SPNR, 2150 Centre Ave, Fort Collins, CO, 80526-8119 USAAuthor for correspondence: tel +1 970 492 7281; email [email protected]Search for more papers by this authorW. J. Parton, W. J. Parton Natural Resource Ecology Laboratory, Colorado State University, Campus Mail 1499, Fort Collins, CP, 80523-1499 USASearch for more papers by this author S. J. Del Grosso, Corresponding Author S. J. Del Grosso Natural Resources Research Center, USDA-ARS-SPNR, 2150 Centre Ave, Fort Collins, CO, 80526-8119 USAAuthor for correspondence: tel +1 970 492 7281; email [email protected]Search for more papers by this authorW. J. Parton, W. J. Parton Natural Resource Ecology Laboratory, Colorado State University, Campus Mail 1499, Fort Collins, CP, 80523-1499 USASearch for more papers by this author First published: 17 September 2012 https://doi.org/10.1111/j.1469-8137.2012.04334.xCitations: 19AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Human activity has substantially altered nitrogen (N) cycling, primarily by increasing the amount of reactive N put into the biosphere by extensive use of N fertilizers and cultivation of N-fixing crops and forages. One result of this is increased nitrous oxide (N2O) emissions to the atmosphere, mostly from cropped and grazed soils but also from enhanced emissions from nonmanaged systems. Climate change also can affect N2O emissions because the biochemical processes that result in N2O emissions are strongly influenced by water and temperature. In this issue of New Phytologist, Xu-Ri et al. (pp. 472–488) present evidence that climate change enhances N2O emissions from nonmanaged soils, implying a positive feedback because N2O is a greenhouse gas (GHG). 'Although models are simplifications of reality, they have the ability to represent gradual changes in global change forcings.' The concentration of N2O in the atmosphere is much lower than that of CO2, but it is an important GHG because on an equivalent mass basis, N2O has c. 300 times the global warming potential of CO2 (IPCC, 2007). In addition to being a strong GHG, N2O is the primary stratospheric ozone depleting substance (Ravishankara et al., 2009). Soil processes, mainly nitrification and denitrification, are responsible for approximately two-thirds of global N2O emissions (Thomson et al., 2012). Nitrification refers to the oxidation of ammonium (NH4) to nitrate (NO3) (with N2O produced as an intermediate) while denitrification is the reduction of NO3 to NOx, N2O, and N2. Both of these processes are enhanced by N availability in soils. Extensive use of N fertilizers and cultivation of N-fixing crops and forages have substantially increased the amount of reactive N put into the plant–soil system. Recent analyses suggest that c. 3–5% of this new N is converted to N2O annually and this is largely responsible for the increase in N2O mixing ratio from c. 270 ppbv in 1860 to 315 ppbv in 2000 (Smith et al., 2012). In addition to increasing reactive N inputs, anthropogenic climate change has the potential to alter carbon (C) and N cycling rates in nonmanaged systems and thus induce reinforcing (positive) or stabilizing (negative) feedbacks. Large amounts of C and N are cycled between the atmosphere and terrestrial systems and relatively small changes in cycling rates can lead to large changes in the atmospheric pools of CO2, methane (CH4), and N2O, the primary long-lived biogenic GHGs. For example, as soils warm, microbial decomposition and CO2 emissions increase (Bond-Lamberty & Thomson, 2010). Similarly, thawing of permafrost can lead to large CH4 fluxes from previously frozen soils (O'Connor et al., 2010) and contribute to further warming. But it is complicated to calculate the net impact of climate change on GHG flux rates because both input and output rates are affected. In the case of N2O, climate change impacts the supply of N in the soil by affecting N cycling rates as well as plant N demand by influencing growth rates. Xu-Ri et al. used a process-based global vegetation model (DyN-LPJ) to investigate four aspects of global change: increased atmospheric CO2 concentration, increased temperature, changes in precipitation, and increased N deposition to nonmanaged soils. The impacts of these factors have been addressed in field studies that manipulate them as single factors and in combination. However, experimental manipulations have a major limitation: the global change factors are implemented all at once whereas in reality these changes occur very gradually. Although models are simplifications of reality, they have the ability to represent gradual changes in global change forcings. Other advantages of models include complete spatial coverage and different global change scenarios can readily be simulated. Models are also necessary to project regional and larger scale impacts of climate and land-use change. In addition to plant growth, the DyN-LPJ model includes C and water cycling, N uptake, N allocation and turnover in plants, and soil N transformations (mineralization, biological fixation, nitrification, denitrification, ammonia (NH3) volatilization, and NO3 leaching). Xu-Ri et al. first demonstrate that the model can reliably represent N2O emissions from nonmanaged systems by comparing model outputs with observations from over 60 field experiments around the world. Included in this model validation were sensitivity analyses in which step changes for warming, increased precipitation, and increased N deposition were simulated to show that model outputs agreed with results from global change experiments. Lastly, global scale simulations were conducted at 0.5° resolution for the 20th century during which CO2, temperature, precipitation, and N deposition were gradually varied as single factors and in combination based on the observed trends in these global change factors. Model results suggest that independent of land-use change, N2O emissions from nonmanaged soils increased by c. 18% during the 20th century, and that increased temperature and N deposition were primarily responsible. The main reason for this is that both increased N deposition and temperature increase N availability in soil, the former directly and the latter through increased mineralization rates. Given that precipitation events control short-term patterns of N2O emissions, it is perhaps surprising that changes in precipitation did not significantly alter the long-term trend in N2O emissions at the global scale. One reason for this is that precipitation only increased by 3% at the global scale during the 20th century. Another is that although increased precipitation is expected to enhance nitrification and denitrification rates, it also increases plant growth, and hence N uptake, thus limiting the supply of substrate for the microbial processes that result in N2O emissions. The impact of CO2 was essentially neutral as a single factor, but became positive as an interaction. The positive interaction is likely related to soil–water relations. Under elevated CO2, plant stomatal conductance and transpiration tend to decease and compensate for the tendency of warming to dry soils (Morgan et al., 2011). The DyN-LPJ model thus has the ability to represent synergistic responses because in some cases the simulated interactions are greater than the sum of simulations of individual factors. This result provides confidence that process-based models can represent the interactions that characterize real-world systems. How does the 'extra' N2O from nonmanaged soils due to global change compare with the more directly human influenced emissions from agricultural soils? Recent estimates of global emissions from agricultural soils are c. 3.8 Tg N (Del Grosso et al., 2008). The simulations conducted by Xu-Ri et al. suggest that global change (increased N deposition, climate change, elevated CO2) increased emissions from nonmanaged soils by c. 1.8 Tg N, which is almost half of the emissions from agricultural soils. Increased temperature and N deposition were the most important factors, accounting for c. 0.8 Tg N increase each. When comparing different sources of anthropogenic N2O emissions, it is important to consider indirect emissions. Indirect emissions are from N that left the farm or pasture in a form other than N2O and was converted to N2O offsite and are typically counted as agricultural emissions. Consequently, it is useful to isolate the extra N2O from nonmanaged systems independent of increased N deposition which is already counted as agricultural N2O. The extra N2O defined as such is c. 1 Tg N, which is > ¼ the direct emissions from agricultural soils. This implies that it is important to account for the impacts of climate change on emissions from nonmanaged systems when calculating the overall human impact on N2O emissions. In addition, the C and N cycles are interlinked and increased plant growth and associated C storage in nonmanaged systems due to increased N inputs partially compensate for the climate forcing of anthropogenic N2O emissions (Zaehle et al., 2011). There are various limitations to global modeling studies. The resolution of the simulations, 0.5°, is rather coarse. There is also a fair amount of uncertainty in modeled N2O emissions. The measurements of soil gas fluxes, which are used to validate model results, are themselves often somewhat uncertain. Phosphorous (P) cycling is not included in the current version of the DyN-LPJ model. Not accounting for P limitation is a potential problem in tropical systems, where P is often a limiting nutrient for plant growth. This is important because plants compete with microbes for N, which is the substrate for N2O emissions. Finally, the impacts of land-use change on emissions were not considered. Nonetheless, the Xu-Ri et al. analysis is an important contribution because it includes one of the first global scale estimates of the sensitivity of N2O emissions from nonmanaged lands to climate change using a process based model. References Bond-Lamberty B, Thomson AM. 2010. Temperature-associated increases in the global soil respiration record. Nature 464: 579– 582. Del Grosso SJ, Wirth T, Ogle SM, Parton WJ. 2008. Estimating agricultural nitrous oxide emissions. EOS 89: 529– 530. Intergovernmental Panel on Climate Change (IPCC). 2007. 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