SUPPLEMENTARY INFORMATION

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1 SUPPLEMENTARY INFORMATION NGS C Contribution of manure and fertiliser nitrogen to atmospheric nitrous oxide since 1860 Supplementary Information Eric A. Davidson 1 1 The Woods Hole Research Center, 149 Woods Hole Road, Falmouth, MA, USA This historical reconstruction of atmospheric N 2 O concentrations and related N inputs is provided in Table S1. Here, I explain how this reconstruction was accomplished. Atmospheric N 2 O concentrations and burden A record of atmospheric concentrations of N 2 O have been complied by Holland et al. (ref. S1), who relied on several published S2,S3 and unpublished datasets. I calculated the annual increase in the atmospheric burden of N 2 O according to the following formula: N 2 O growth (Tg N 2 O-N yr -1 ) = [N 2 O t2 N 2 O t1 ] ppb x 4.81 Tg N 2 O-N ppb -1 (EqS1) where N 2 O t2 and N 2 O t1 are atmospheric concentrations in two successive years and 4.81 is the conversion factor derived from Prather et al. (ref S4). A ten-year running average of this annual growth rate was calculated to smooth some of the noisiness of the atmospheric measurement record. Stratospheric N 2 O sink Natural sources and sinks are thought to have been roughly in equilibrium prior to the industrial revolution, at 10.2 Tg N 2 O-N yr -1. As discussed below in the section on Pre- nature geoscience 1

2 supplementary information 1860 conditions, this equilibrium was not exact, due to some possible anthropogenic sources and to climatic variability affecting natural sources, so there is some uncertainty of the preindustrial sources and sinks. Table S1 shows that atmospheric N 2 O was probably increasing at a rate of about 0.35 Tg N 2 O-N per yr -1 in the mid 1800s, which serves as a reasonable upper-bound estimate of the error term of the pre-industrial stratospheric sink estimate of 10.2 Tg N 2 O-N yr -1. As atmospheric concentrations increased further in the 19 th and 20 th centuries, due to anthropogenic sources, the stratospheric sink also increased to 11.9 Tg N 2 O- N yr -1 at the beginning of the 21st century S4,S5. To account for the temporal development of this additional anthropogenically-induced sink, I scaled the sink to the historic atmospheric concentration according to the following formula: Anthropogenic stratospheric sink (Tg N 2 O-N yr -1 ) = 1.7 x [(N 2 O t -270)/45.7] (EqS2) where N 2 O t is the atmospheric N 2 O concentration (ppb) in year t, 1.7 Tg N 2 O-N yr -1 is the increased stratospheric sink in 2000, 270 ppb is the pre-industrial N 2 O concentration when the natural sources and sinks were in approximate balance, and 45.7 ppb is the increase in atmospheric N 2 O between pre-industrial times and Decline in natural N 2 O source from tropical forest soils Tropical forest soils are the largest natural terrestrial source of N 2 O (ref. S6). This source is likely to have declined as forests have been cleared for cattle ranching and other forms of agriculture and often subsequently abandoned S7,S8. Emissions of N 2 O from native soils of other biomes tend to be lower, and changes in these emissions are less likely to influence the global budget. Moreover, the increased biogenic N 2 O emissions from fertilized croplands implicitly includes the difference between increased emissions from the fertilized cropland and the emissions from the untilled temperate grassland, for example. In contrast, many cattle pastures in tropical regions become abandoned, and the decreased N 2 O emissions from these lands relative to the original forest soil emissions would not be accounted for, 2 nature geoscience

3 supplementary information implicitly or explicitly, in any other aspect of this N 2 O budget. I used published estimates of historical tropical forest cover S9, updated to 2004 by personal communication from the author. A short-lived increase in N 2 O soil emissions has been observed following initial forest clearing S7, but significant declines occur to well below rates of emissions from native forests as pastures age S7,S8. For Amazonian forest-to-pasture conversion, it has been estimated that the net mean annual decrease integrated over a 20-year post-deforestation period is about 1 kg N 2 O-N ha -1 yr -1 (ref. S10). I multiplied this value times the area of net tropical deforestation to obtain annual estimates of the decrease in this natural source. The estimate for the late 1990s (0.9 Tg N 2 O-N ha -1 yr -1 ) agrees well with the independent estimate cited by Crutzen et al. (ref. S5). These estimates of decreased natural sources must be added to the estimate of anthropogenic N 2 O sources in order to balance the annual global N 2 O budget. A sensitivity analysis is discussed below. Industrial adipic acid production as a source of N 2 O Information on the history of nylon production was obtained from a web site by Cook- Hauptman Associates S11. The first commercial production of adipic acid for making nylon started in 1938 and increased to 0.05, 1.0, and 7.5 billion pounds of nylon by 1951, 1960, and 1980, respectively. I converted these values to metric units and then to N 2 O-N assuming the following conversion factors extracted from the literature S12 : N 2 O-N = Y kg nylon x 0.6 kg adipic acid/kg nylon x 191 g N 2 O-N/kg adipic acid (EqS3) Estimates of N 2 O emissions from adipic acid production for 1990, 1995, 2000, and 2005 were taken from an EPA study S13. Values for years between estimates were linearly interpolated. nature geoscience 3

4 supplementary information Mobile and stationary sources of N 2 O from fossil fuel combustion Estimates of N 2 O emissions from mobile and stationary sources of fossil fuel combustion were taken from an EPA study for 1990, 1995, 2000, and 2005 (ref. S13). Values for years between estimates were linearly interpolated. Prior to 1990, I scaled estimates of NO x emissions from Holland et al. (ref. S1) to derive estimates of N 2 O emissions from fossil fuel combustion. Comparing the NO x emissions from Holland et al to the N 2 O emissions from the EPA study in the 1990s, I derived a mean conversion factor of Tg N 2 O-N/Tg NOx-N. I then estimated N 2 O emissions annually from 1860 to 1989 using this scaling factor. Because the NOx estimates scale, in part, with fossil fuel combustion, the N 2 O emissions from this sector are likely to also scale roughly, but perhaps not exactly, with NO x emissions. This approach introduces potential errors due to unknown changing ratios of N 2 O/NO x emissions as fuels and pollution control technologies changed during the 20 th century. However, the estimated N 2 O emissions from this sector are <0.3 Tg N 2 O-N yr -1 prior to 1980, so the errors resulting from assuming a constant N 2 O/NO x ratio are likely to be small relative to the total global budget. Biomass burning as a source of N 2 O Fire is a natural process in many ecosystems, especially tropical savannas and boreal forests, but humans have increased the incidence of fire in these and other ecosystems, either by deliberately using fire as a management tool or through unintentional escape of fire or climate change S14. Presumably, N 2 O emissions caused by natural and anthropogenic fire before the industrial revolution were sufficiently small that the global N 2 O budget remained approximately balanced. The intent here is to estimate emissions due to increased anthropogenic sources since the industrial revolution. To derive a time-course estimate of historic N 2 O emissions from human-induced biomass burning, I started with the modern estimate of about 0.5 Tg N 2 O-N yr -1 from two literature sources S6,S13. A study by Ito and Penner S14 did not specifically address N 2 O, but 4 nature geoscience

5 supplementary information they reconstructed historic emissions of black carbon, particulate organic matter, and carbon monoxide from burning of open vegetation since Each of these show a fairly consistent pattern of emissions in 1870 at about 40% present values, gradually increasing to 50% by 1950, and then increasing to present values between 1950 and Therefore, I set emissions at 0.2 Tg N 2 O-N yr -1 in 1860, linearly increased them to 0.25 Tg N 2 O-N yr -1 in 1950, and linearly increased them to 0.5 Tg N 2 O-N yr -1 by 1990, which is consistent with the temporal trends shown for the other biomass burning emissions S14. It is possible that the biomass burning emissions of N 2 O prior to the industrial revolution were as high or higher than 0.2 Tg N 2 O-N yr -1, which adds uncertainty to this analysis and which is discussed below in the section on sensitivity analysis. Inputs of newly fixed N nitrogen oxides (NOx) Estimates of NOx emissions from industrial and transportation sources since 1890 have been compiled by Holland et al. (ref. S1) and are used here. Inputs of newly fixed N fertilizer-n Holland et al. (ref. S1) also provide estimates of N fixation by the Haber-Bosch process, relying largely on historical estimates by Vaclav Smil. However, Smil S15 also provides estimates back to 1850 of the use of guano and Chilean nitrate deposits of N as fertilizer. Although these forms of N do not qualify as new N fixation, they do represent new mobilization of previously fixed N that was likely to enhance N 2 O emissions when applied to croplands. I therefore use the estimates of Smil S15 at 5-10 year intervals from 1850 to 2000 of the sum of Haber-Bosch synthesis of fertilizer-n and these other sources of fixed N used as fertilizers, linearly interpolating for years between his estimates. An additional estimate for 2005 was obtained from the International Fertilizer Industry Association S16. nature geoscience 5

6 supplementary information Inputs of newly fixed N leguminous crops Nitrogen fixation by leguminous crops prior to the industrial revolution has been estimated at 15 Tg N yr -1 (ref. S17). Apparently, the N 2 O emissions supported by these new N inputs to the biosphere were sufficiently small so that the global budget remained approximately in balance. The most recent IPCC methodology S18 indicates that N 2 O emissions directly attributable to soybean production are negligible, apparently because modern soybean varieties effectively retain most of their fixed N, although small indirect emissions are calculated from crop residue inputs and human sewage. Hence, I do not include N 2 O emissions from soybean cultivation in this analysis. I do, however, include estimates of new N fixation by leguminous crops in Table S1 in order to be consistent with Crutzen et al. (ref. S5) when applying their emission factor of 4% of newly fixed N to make their estimate of N 2 O. Nitrogen fixation by leguminous crops increased during the 20 th century to about 31.5 Tg N yr -1 in 1995 (ref. S17). I interpolated linearly to estimate N fixation by leguminous crops during this period. The sum of post-industrial anthropogenic new N fixation was calculated as the sum of NO x and fertilizer-n and that part of leguminous crop input in excess of the 15 Tg N yr -1 that occurred before the industrial revolution. Manure Historical estimates of manure production and its N content are taken from Holland et al. (ref. S1). 6 nature geoscience

7 supplementary information Human Sewage Historical estimates of human sewage production and its N content were compiled in order to test the possibility of including this factor as a third independent variable in the regression analysis presented in the main paper. Historical global human population estimates were obtained from the US Census Bureau S19. Trends of protein content of human diet for were obtained from a Food and Agriculture Organization report S20, which ranged from global averages of 64 g protein per person per day in 1968 to 75 g protein per person per day in I assumed that mean global protein consumption in 1860 was 60 g protein per person per day S21. Values between dates were linearly interpolated. Average N content of protein consumed by humans is assumed to be 16% (ref S13). The N in human sewage was then calculated according to the follow equation for each year from 1860 onward: Sewage N = population X per capita protein consumption X 0.16 (Eq. S4) The results (Table S1) are in good agreement with other estimates of modern human waste production S13,S21. According to IPCC tier 1 methodology, mean N 2 O emissions are estimated as 1% of the N content of human sewage 13, which corresponds to 0.2 to 0.3 Tg annual N 2 O-N for contemporary budgets (Table 1 of main manuscript, ref 13). Not surprisingly, historical patterns of N in human sewage follow similar temporal trends as historical patterns of human food production, which, in turn, is related to manure-n production, and fertilizer-n use. The R 2 values of the correlations between estimates of sewage-n from and manure-n, fertilizer-n, and manure+fertilizer-n are 0.91, 0.96, and 0.98, respectively. This temporal covariation, along with the fact that sewage is a relatively small source of N 2 O, results in very little effect of adding human sewage as a third independent variable for predicting biological anthropogenic emissions of N 2 O using Eq. 2 in the methods section of the main paper. The R 2 values of the regression increases from nature geoscience 7

8 supplementary information using only fertilizer-n and manure-n as independent variables to predict biogenic emissions of N 2 O to when human sewage-n is added as a third variable. This result does not mean that human sewage is not a source of N 2 O. The best estimates available indicated that about Tg N 2 O-N is currently linked to introduction of human sewage into the environment 13. The result does mean that this regression analysis does not permit distinguishing human sewage from other indirect sources of N 2 O attributed to fertilizer-n and manure-n or to identify a unique temporal pattern of human sewage as a direct source. Pre-1860 This analysis begins in 1860, because the available data are too sparse prior to then. However, the global N 2 O and N budgets were not exactly at steady state in Increases in anthropogenic N 2 O sources as human populations and agricultural gradually expanded from 1700 to 1860 may have been offset by reductions in natural N 2 O emissions due to deforestation and degradation or to cooling during the little ice age S22. There is no trend of changing atmospheric N 2 O concentrations in the ice record from (ref S3), suggesting that any increases in N 2 O production due to agricultural expansion may have been offset by decreases in natural emissions of N 2 O (refs S3, S22). Atmospheric N 2 O concentrations decreased from 1800 to 1830 (S3), presumably due to global cooling, and then increased about 0.08 ppb per year for the remainder of the 19 th century. This increase could have been due to a rebound of natural sources after the little ice age, further agricultural expansion, or some combination. In this analysis, I assume that the late 19 th century increases in atmospheric N 2 O were due primarily to anthropogenic causes, including agricultural expansion and intensification, which accelerated simultaneously with the onset of the industrial revolution S22. 8 nature geoscience

9 supplementary information Balancing the budget in 1950 To demonstrate the implausibility of using annual N fixation as the basis for estimating N 2 O production during the first half of the 20 th century, consider the budget for 1950, which is just before the subsequent dramatic increase in use of synthetic N fertilizers. The estimate of total new post-industrial anthropogenic N fixation in 1950 is 19.1 Tg N, which includes 8.5 Tg additional leguminous crop N fixation beyond the 15 Tg estimated for the pre-industrial period, plus 5.6 Tg from NO x production and 4.8 Tg from fertilizers (Table S1). The annual biogenic anthropogenic N 2 O source for that year is estimated at 1.91 Tg N, which includes 1.15 Tg as the 10-yr running average of atmospheric accumulation of N 2 O, plus 0.70 Tg as an anthropogenic atmospheric sink, plus 0.38 Tg as compensation for a decreased tropical forest soil source, minus 0.25 Tg for the biomass burning source, and minus 0.07 for the transportation sector source (Table S1). Hence, the fraction of total new anthropogenic N fixation that was emitted as N 2 O is 10% (1.91/19.1), which is an unrealistically high emission factor. A more likely explanation is that a significant fraction of the N 2 O emitted that year and during the previous 90+ years was due to the effects of that era s expanding agriculture on mining of soil N that had been fixed in previous decades and centuries. Sensitivity analyses The temporal patterns of N 2 O emissions from nylon production and from transportation are very similar to that of fertilizer-n, increasing sharply after Therefore, errors in these terms would primarily affect the regression coefficient of the fertilizer-n term (Eq. 2 of main paper). For each 0.1 Tg N 2 O-N yr -1 that these estimates are in error in 2000, the regression coefficient changes about 0.001, which means that the inferred fraction of fertilizer-n that is released as N 2 O changes 0.1%. The EPA estimates S13 appear to be based nature geoscience 9

10 supplementary information on the most recent and most robust datasets, so I used those in this analysis, but they are on the lower end of the ranges reported in other studies (Table 1 of main paper). If the industrial emissions are doubled, which would put them at the upper end of the range of published estimates, then the fitted manure-n emission factor would decrease from 2.0% to about 1.9% and the fitted fertilizer-n emission factor would decrease from 2.5% to 1.7%. In other words, larger industrial sources of N 2 O since 1960 mean that a somewhat smaller fraction of fertilizer-n needs to be converted to N 2 O to balance the global budget, but the fraction remains close to 2%. Estimates of N 2 O emissions from biomass burning are highly uncertain, although they are relatively consistent among studies. Probably the greatest uncertainty with respect to this analysis is estimating the fractions of current and historical biomass burning emissions that may have been part of a quasi-balanced global budget before the industrial revolution. However, as noted above, the effects of anthropogenic emissions may be confounded with changes in natural emissions during the little ice age. If we assume that the estimated 0.2 Tg N 2 O-N yr -1 source from biomass burning in 1860 also applied to pre-industrial times and was part of a quasi-balanced global N 2 O budget, then running the regression analysis with only the increased biomass burning N 2 O estimate since 1860 causes the fitted manure emission factor to increase from 2.0% to about 2.3% and the fertilizer-n emission factor to decrease from 2.5% to 2.0%. Hence, reducing by about 40% the amount of biomass burning N 2 O attributable to post-industrial anthropogenic activities requires that the overall biogenic emissions needed to balance the global N 2 O budget increase slightly, and requires that the make-up of this source shift slightly from fertilizer-n to manure-n as the source. Estimates of manure production used in this analysis were taken from the data compilation by Holland et al. (ref S1). Smil S23 argued that these and similar estimates for the 1990s were too high because many livestock in the developing world have below average mass and poor diets. However, the most recent FAO review of this topic S24, while 10 nature geoscience

11 supplementary information acknowledging this debate, provides a new estimate, based on regionally specific analyses, that is only 5% lower than the estimates of Holland et al. (the 2004 estimates are 135 Mt and Mt manure-n production in 2004 for refs S24 and S1, respectively). I take this most recent estimate as the most reliable one available and the difference between these two estimates to reflect uncertainty. The regression coefficients are sensitive to the magnitude of the manure-n and fertilizer-n inputs. Assuming that the Holland manure-n estimate is systematically 5% too high throughout the study period, then the manure emission factor would increase to 2.1%. The manure estimate is also subject to the same pre-1860 question as discussed above for biomass burning. It is possible that some of the human and livestock manure produced before the industrial revolution supported N 2 O emissions that were part of a quasi-balanced global N 2 O budget. Based on changes in human populations, Kroeze et al. (ref S22) estimated that livestock manure production in 1500 may have been about 13% of 1970 values, which would be about 14 Tg N yr -1. Substracting this amount from the annual manure production estimate from 1860 onward (from ref S1) and re-running the regression analysis, the fitted manure emission factor increases to about 2.7% and the fertilizer-n emission factor decreases to about 1.9%. Hence, assuming that some manure production was part of a pre-industrial quasi-balance, then the estimated fraction of increased manure production since 1860 that is emitted as N 2 O must be larger to match the atmospheric record. This emission factor of 2.7% represents the high end of range in this sensitivity analysis. The temporal pattern of the decline in the natural tropical forest soil N 2 O source has aspects of the temporal patterns of both increasing manure and fertilizer production i.e., a steady decrease from 1860 to 1960 and then a sharper decrease thereafter. Estimates of the reduced emissions from tropical forest soils due to deforestation are highly uncertain. The estimate of a net average annual reduction of 1 kg N 2 O-N ha -1 yr -1 when humid tropical forest is converted to cattle pasture is based on a few studies in the Amazon Basin, and may not nature geoscience 11

12 supplementary information apply to tropical deforestation globally. Moreover, the areal estimates of tropical deforestation span a range of climatic zones from forest-savanna woodlands to dense rainforests. The N cycle and N 2 O emissions recover during forest regrowth, although this process takes many decades S25. It is difficult to estimate either the direction or the magnitude of likely errors in these estimates. If natural sources of N 2 O have not changed at all, which seems unlikely, and the entire estimated modern decrease of 0.9 Tg N 2 O-N yr -1 is ignored in this analysis, then the fitted manure-n emission factor would decrease from 2.0% to 1.6% and the fitted fertilizer-n emission factor would decrease from 2.5% to 2.1%. In summary, in the unlikely event that industrial N 2 O emissions are underestimated by a factor of two, then the fertilizer emission factor would drop as low as 1.7%. None of the sensitivity analyses of input data result in an increase in the fertilizer emission factor, so the upper bound of the range is set by the 95% confidence interval of the regression coefficient, which is an emission factor of 2.7%. For the manure emission factor, a lower bound of 1.6% is set by the extreme assumption that there has been no change in the biogenic source of N 2 O due to tropical deforestation. The upper bound for the manure emission factor is 2.7%, which is derived by assuming that pre-industrial manure production was already part of the preindustrial quasi-steady state of the global N 2 O budget. Hence, the probable emission factor ranges established by this sensitivity analysis are % for fertilizer-n and % for manure-n. Additive errors in the same direction could further expand these ranges somewhat, but errors in opposite directions are similarly plausible, which would cancel each other and would narrow the ranges. These ranges are not meant to apply to every field situation, where more extreme values may be observed, but rather reflect the global average for the fractions of fertilizer-n and manure-n that are released as N 2 O, including both direct and indirect emissions. 12 nature geoscience

13 supplementary information References S1. Holland, E. A., Lee-Taylor, J., Nevison, C., & Sulzman, J. Global N cycle: fluxes and N 2 O mixing ratios originating from human activity. Data set available on-line [ from Oak Ridge National Laboratory Distributed Active Archive Center, Oak Ridge, Tennessee, U.S.A. (2005). S2. Battle, M., et al. Atmospheric gas concentrations over the past century measured in air from firn at the South Pole. Nature 383, (1996). S3. Machida, T., Nakazawa, T., Fujii, Y., Aoki, S., & Watanabe, O. Increase in the atmospheric nitrous-oxide concentration during the last 250 years. Geophys Res Lett. 22, (1995). S4. Prather M., et al. Atmospheric Chemistry and Greenhouse Gases. In Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change (eds. Houghton, J., Ding, Y. et al.), pp Cambridge University Press, New York (2001). S5. Crutzen, P. J., Mosier, A. R., Smith, K. A., & Winiwarter, W. N 2 O release from agrobiofuel production negated global warming reduction by replacing fossil fuels. Atmos. Chem. Phys. 8, (2008). S6. Mosier, A., et al. Closing the global N 2 O budget: nitrous oxide emissions through the agricultural nitrogen cycle. Nutr. Cycl. Agroecosys. 52, (1998). S7. Melillo, J.M. et al. Nitrous oxide emissions from forests and pastures of various ages in the Brazilian Amazon. J. Geophys. Res. 106, 34,179-34,188 (2001). S8. Verchot, L.V. et al. Land use change and biogeochemical controls of nitrogen oxide emissions from soils in eastern Amazonia. Global Biogeochem. Cy. 13, (1999). S9. Houghton R. A. The annual net flux of carbon to the atmosphere from changes in land use Tellus, 51B, (1999). nature geoscience 13

14 supplementary information S10. Davidson, E. A., & Artaxo, P. Globally significant changes in biological processes of the Amazon Basin: results of the Large-scale Biosphere-Atmosphere Experiment. Glob Change Biol. 10, (2004). S11. Cook, J. History of Du Pont s nylon fibers. Unpublished manuscript available at: S12. Thiemens, M. H. & Trogler, W. C. Nylon production: an unknown source of atmospheric nitrous oxide. Nature 251, S13. EPA. Global Anthropogenic Non-CO 2 Greenhouse Gas Emissions: (2006). S14. Ito, A., & J. E. Penner. Historical emissions of carbonaceous aerosols from biomass and fossil fuel burning for the period Global Biogeochem. Cy. 19, GB2028, doi /2004gb (2005). S15. Smil, V. Enriching the Earth, MIT Press, Cambridge, MA, USA (2001). S16. IFA. Sustainable Management of the Nitrogen Cycle in Agriculture and Mitigation of Reactive Nitrogen Side Effects. International Fertilizer Industry Association, Paris, France, (2007). S17. Galloway, J. N., et al. Nitrogen cycles: past, present and future. Biogeochemistry 70, (2004). S18. Klein, C., et al. N 2 O emissions from managed soils, and CO 2 emissions from lime and urea application. In 2006 IPCC Guidelines for National Greenhouse Gas Inventories (eds. Eggleston, H. S., Buendia, L., Miwa, K., Ngara, T. and Tanabe, K.), pp Institute for Global Environmental Strategies, Japan (2006). S19. US Census Bureau. International Data Base. Dataset available on-line, and (2008). 14 nature geoscience

15 supplementary information S20. Kennedy, G. Global trends in dietary energy supply from 1961to1999. Food and Agriculture Organization, (2002). S21. McElroy, M. B., & Wang, Y. X. Human and animal wastes: implications for atmospheric N 2 O and NO x. Glob. Biogeochem Cy. 19, GB2008, doi /2004GB (2005). S22. Kroeze, C., Mosier, A., & Bouwman, L., Closing the global N 2 O budget: A retrospective analysis Glob Biogeochem Cycl. 13, 1-8 (1999). S23. Smil, V. Nitrogen in crop production: an account of global flows. Global Biogeochem. Cy. 13, (1999). S24. Steinfeld, H., Gerber, P., Wassenaar, T., Castel, V., Roslaes, M., & De Haan, C. Livestock's Long Shadow. Environmental Issues and Options. FAO report, Rome, Italy (2006). S25. Davidson, E. A., et al. Recuperation of nitrogen cycling in Amazonian forests following agricultural abandonment. Nature 447, (2007). nature geoscience 15

16 supplementary information Table S1. Annual estimates ( ) of atmospheric N2O concentrations, anthropogenic sources of N2O, and inputs of anthropogenic N fixation into the biosphere. Year Atmos. N2O conc. Atmos. N2O increase Atmos. N2O increase Atmos. N2O increase; 10- yr running mean (A) Anthro. Stratospheric N2O Sink (B) Tropical Forest Area Decreased Forest Soil N2O Source (C) Nylon N2O (D) Fossil Fuel Combustion N2O (E) Biomass burning N2O (F) Anthro. Biological N2O Source (A+B+C-D- E-F) NOx Fertilizer N ppb ppb/yr Tg N/yr Tg N/yr Tg N/yr 10^6 ha Tg N/yr Tg N/yr Tg N/yr Tg N/yr Tg N/yr Tg N/yr Tg N/yr Tg N/yr Tg N/yr Tg N/yr Tg N/yr Legume crop N fixation Postindustrial anthro. N fixation Manure N Human Sewage N 16 nature geoscience

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