Abstract
Nitrogen (N) use in agriculture substantially alters global N cycle with the short- and long-term effects on global warming and climate change. It increases emission of nitrous oxide, which contributes 6.2%, while carbon dioxide and methane contribute 76% and 16%, respectively of the global warming. However, N causes cooling due to emission of NOx, which alters concentrations of tropospheric ozone and methane. NOx and NH3 also form aerosols with considerable cooling effects. We studied global temperature change potential (GTP) of N use in agriculture. The GTP due to N2O was 396.67 and 1168.32 Tg CO2e on a 20-year (GTP20) and 439.94 and 1295.78 Tg CO2e on 100-year scale (GTP100) during years 1961 and 2010, respectively. Cooling effects due to N use were 92.14 and 271.39 Tg CO2e (GTP20) and 15.21 and 44.80 Tg CO2e (GTP100) during 1961 and 2010, respectively. Net GTP20 was 369.44 and 1088.15 Tg CO2e and net GTP100 was 429.17 and 1264.06 Tg CO2e during 1961 and 2010, respectively. Thus net GTP20 is lower by 6.9% and GTP100 by 2.4% compared to the GTP considering N2O emission alone. The study shows that both warming and cooling effects should be considered to estimate the GTP of N use.
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Introduction
Nitrogen is the most limiting nutrient controlling the primary production of agricultural systems. Intensively cultivated systems require external application of N to increase and sustain global food production. Consumption of fertilizer N has increased globally from ~12 Tg in 1960 to ~113 Tg in 20101. If current N consumption trends continues, considerably higher amount of fertilizer N will be used in agriculture to provide food for an additional 2 billion people by 20502. The N cycle involves five steps i.e., N fixation (N2 → NH3/
), nitrification (NH3/
→ 
), assimilation (uptake of 
and 
into plant tissues), ammonification (organic N → NH3) and denitrification (
→ N2)3. During the N cycle several reduced (NH3) and oxidised N compounds (NOx, NO, N2O, 
) are emitted to the atmosphere affecting the climate system4.
Climate change due to emission of greenhouse gases (GHGs) viz. carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) contributing 76.0%, 16.0% and 6.2%, respectively is likely to affect agricultural productivity and food security adversely5. Addition of N in agricultural soil alters the fluxes of GHGs6,7,8. The reactive N (Nr) has direct as well as indirect effects on N2O emission from agricultural soil9,10,11,12. Emission of N2O is a major concern because of its long atmospheric lifetime (about 116 years), higher global warming potential (GWP) i.e., 310 times that of CO25 and high global temperature change potential (GTP) of 290 on 100-year basis13.
The GWP is the global mean radiative forcing of 1 kg pulse emissions of a greenhouse gas relative to 1 kg of reference gas i.e., CO214. The GWP is an index of time-integrated radiative forcing. However, it does not give a quantitative information on effect of GHG emission on global temperature13,15,16. The GTP is the global average temperature change at time t due to emission of a GHG relative to CO2 emission13,17. The GTP is directly related to surface temperature changes as a result of GHG emission. Thus GTP has an advantage in quantifying temperature change compared to GWP.
In addition to N2O emission, N use in agriculture results in increased emission of NH3 and NOx contributing to climate change indirectly18. The NOx impacts global warming by (i) formation of ozone (O3), which contributes to warming19 and (ii) removal of CH4 by hydroxyl radical, thus contributing to cooling20. Moreover, CH4 enhances ozone formation in the upper atmosphere over longer time-scales. Thus NOx can also reduce production of O3 and contribute to cooling21. Both NOx and NH3 enhance formation of light-scattering sulphate and organic aerosols. NOx can be oxidised to form nitric acid (HNO3), which forms aerosols of ammonium nitrate (NH4NO3) in presence of NH322. Moreover, use of N usually increases net primary productivity with more CO2 fixation in terrestrial systems23,24,25,26 and enhances carbon sequestration in soil due to more litter production27. The direct and indirect impacts of reactive N (Nr) on global warming and cooling are summarized in Table 1.
The previous reports have evaluated the emission of N2O only due to N use in agriculture for a short period. However, besides global warming due to N2O emission, N use in agriculture has other direct and indirect effects causing warming and cooling. To assess the impacts of N use on climate change, therefore, the warming as well as cooling effects should be considered18. Moreover, such warming and cooling effects need to be assessed for a sufficiently long period as the N use in global agriculture has undergone substantial changes in the last decades. The present study quantified the global warming and cooling potentials of N use in global agriculture during last 50 years (1961–2010).
Results and Discussion
Total N input in global agriculture
Total N input in global agriculture increased by 2.95 times during 1961 to 2010 (Fig. 1 and Table 2). In 1961, total N input from different sources was 74.93 Tg N. Animal manure accounted the highest amount (32.30%), followed by biological N fixation (BNF, 29.33%), crop residues (18.75%), fertilizer N (15.47%) and atmospheric deposition (4.16%) (Table 2). In 2010, total N input was 270.70 Tg N (Fig. 1 and Table 2) and fertilizer N was the largest source (51.38%) followed by animal manure (15.41%), crop residue (14.40%), BNF (12.31%) and atmospheric deposition (6.49%) (Table 2).
Total nitrogen consumption, global temperature change potential (GTP) due to N2O emission alone and net GTP of N use in global agriculture (A) 20-year and (B) 100-year time-scales.
GTP of N2O emission
Total N2O emission from agriculture increased from 1.44 Tg to 4.25 Tg during 1961 to 2010 (Fig. 2A). The GTP of total N2O emission, thus increased from 396.67 to 1168.32 TgCO2e in a 20-year time-scale (GTP20) (Fig. 3A) and from 439.94 to 1295.78 Tg CO2e in 100-year time-scale (GTP100) (Fig. 3B) during 1961 to 2010.
Emissions of total (direct + indirect) N2O (A), N2O from nitrate leaching (B), NH3 (C), NOX (D), CH4 (E) and CO2 (F) from global N use in agriculture during 1961–2010.
Warming or global temperature change potential (GTP) due to total (direct + indirect) N2O emission (A,B), direct N2O emission including atmospheric deposition (C,D), N2O emission from nitrate leaching (E,F), N fertilizer and CH4 flux (G,H) and net GTP (I,J) of global N use in agriculture on 20-year (left) and 100-year (right) times-scales.
GTP of NH3 and NOx emissions
Emission of NH3 from global agriculture was 9.10 and 26.80 Tg during 1961 and 2010, respectively (Fig. 2C). Emission of NOx was 0.37 and 1.10 Tg during 1961 and 2010, respectively (Fig. 2D). Cooling impacts due to these emissions of NOx and NH3 were 77.58 and 228.50 Tg CO2e in GTP20 and 0.65 and 1.91 Tg CO2e in GTP100 during 1961 and 2010, respectively (Fig. 4). Aerosol formation from NH3 contributed 69% of the cooling effect, followed by ozone and CH4 alternation due to NOx (22%) and aerosol formation from NOx (9%) (Fig. 4A,C,E). However, on GTP100 (Fig. 4B,D,F) these cooling impacts of NH3 and NOx were smaller compared to GTP20 indicating that as the time horizon becomes longer, short-lived compounds have less effects on GTP18.
Cooling or Global temperature change potential (GTP) due to NH3 aerosol (A,B), NOx aerosol (C,D), NOx-O3-CH4 (E,F), CO2 with N fertilizer (G,H) and total cooling (I,J) of global N use in agriculture on 20-year (left) and 100-year (right) times-scales.
GTP due to altered CH4 and CO2 fluxes
The CH4 is produced in soil during microbial decomposition of organic matter under anaerobic conditions. Soils submerged under water, rice fields for example, are the potential sources of CH4. Addition of N increases CH4 emission by inhibiting CH4 oxidation and reducing CH4 uptake in aerobic soils due to increased concentration of ammonium (
)28 and nitrate (
)29,30 in soil. This increase in CH4 flux due to N use in agriculture ranged from 1.14 Tg in 1961 to 3.35 Tg in 2010 (Fig. 2E) contributing to 42.14 and 124.12 Tg CO2e in GTP20 (Fig. 3G) and 4.44 and 13.80 Tg CO2e in GTP100 (Fig. 3H) in 1961 and 2010, respectively. Fluxes of CO2 decreased by14.56 Tg to 42.89 Tg during the same period (Fig. 2F) due to increased uptake of CO2 as a result of N application (Fig. 4G,H).
Net impact of N use in agriculture on GTP
Net GTP of N use in agriculture was 369.44 and 1088.55 Tg CO2e on GTP20 (Fig. 1A) and 429.17 and 1264.06 Tg CO2e on GTP100 (Fig. 1B) in 1961 and 2010, respectively. The net GTP20 was lower by 6.9% and GTP100 by 2.4% compared to the respective GTPs when N2O emission alone was considered.
Total GTP during 1961–2010
Total warming due to N use in global agriculture during 50 years was 45041.92 Tg CO2e in GTP20 and 43362.98 Tg CO2e in GTP100 (Fig. 5). Emission of N2O due to N use in agriculture contributed 86% and 99% of this warming in GTP20 and GTP100, whereas CH4 contributed 14% and 1% in GTP20 and GTP100, respectively. Total cooling was 8991.28 and 1484.19 Tg CO2e in GTP20 and GTP100, respectively (Fig. 5). The major cooling was due to NH3 aerosol formation (57.8%) followed by NOx induced O3 and CH4 alteration (18.7%), N fertilizer-induced C sequestration (15.8%) and NOx aerosol (7.7%). However, on GTP100 N fertilizer-induced C sequestration contributed the maximum (95.74%) and others were marginal.
Total Global temperature change potentials of global N use in agriculture in 50 year on 20-year (left) and 100-year (right) times-scales.
The net GTP20 was 36050.64 Tg CO2e i.e., 6.84% lower and GTP100 was 41878.79 Tg CO2e i.e., 2.45% lower compared to the respective GTPs when warming due to N2O emission alone was considered.
Methods
Total N use in global agriculture
Total N input in global agriculture (NT) was calculated using the equation (1).
Where, NSN, NAM, NCR, NAD and NBNF are amounts of N added (Tg) to soil annually through fertilizer, animal manure, crop residue, atmospheric deposition, and biological nitrogen fixation (BNF), respectively. Data on NSN, NAM, NCR were obtained from FAOSTAT1. The NAD and NBNF were calculated as per the equations (2) and (3) respectively.
Data on area under global agricultural and pulse crops were obtained from FAOSTAT1 whereas data on deposition factor were calculated from Liu et al.31 and Liu et al.32 and BNF were calculated from Liu et al.31.
Emission/uptake factors
Emission and uptake factors (EF) used in the study are mentioned in Table 3. Factor for direct N2O emission was taken as 0.0133 and N2O from 
leaching was 0.007533. Emission factor for 
leaching, NH3 and NOx emissions were 0.333, 0.1033 and 0.00534 kg kg−1 N applied, respectively. Emissions of CH4 from anaerobic and aerobic fields were taken as 0.008 and −0.012 kg CH4-C ha−1 yr−1 kg−1 N aplied24. The factor for C sequestration was 0.053 kg CO2-C ha−1 yr−1 kg−1 N24.
Emission/uptake fluxes
Total flux (FT) of N2O, 
leaching, NOX, NH3, CH4 and CO2 were calculated using the equation (4).
Where NT, is total amount of N (Tg) added to agricultural land and EFn is the respective emission/uptake factor.
N2O flux from 
leaching was calculated using the equation (5).
GTP of N2O, NOX, NH3, CH4 and CO2 fluxes
The GTP of N2O, NOx and NH3 fluxes were calculated using the equation (6).
Where GTPNt is GTP at ‘t’ time-scale i.e., 20 or 100 years; FT is flux of NOx, NH3 and N2O emission (kg yr−1), GTPtxi is GTP for ‘i’ kg of ‘x’ compound (N2O, NOx, NH3) at time-scale ‘t’. GTP20 and GTP100 used in the study are mentioned in Table 4.
The following equation (7) was used to calculate GTP of CH4 and CO2 emission/uptake (GTPCt).
Where GTPtxi is GTP for ‘i’ kg of ‘x’ compound (CH4 and CO2) at time-scale ‘t’.
Finally, the net GTP (GTPT) of N addition to global agriculture was calculated using the equation (8).
Summary
Globally, nitrogen is the most widely used nutrient in agriculture. Nitrogen fertilizer acts as a source of global warming as it contributes to N2O emission. However, it also contributes to global cooling with emissions of NH3 and NOx. Therefore, while assessing global temperature change potential (GTP), both the warming and cooling effects of N use in agriculture should be considered. Our estimates showed that net GTP in 20-year time-scale is 6.9% lower and in 100-year time-scale 2.4% lower when warming as well as cooling effects of N use in agriculture were considered compared to considering warming due to N2O emission alone.
Additional Information
How to cite this article: Fagodiya, R. K. et al. Global temperature change potential of nitrogen use in agriculture: A 50-year assessment. Sci. Rep. 7, 44928; doi: 10.1038/srep44928 (2017).
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Acknowledgements
We are grateful to ICAR-Indian Agricultural Research Institute, New Delhi and National Innovations on Climate Resilient Agriculture (NICRA) project for providing necessary support for the study.
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H. Pathak designed and conceptualized the study. R. K. carried out the data analysis and prepared the first draft. All the authors provided critical inputs and contributed to the writing of the paper.
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Fagodiya, R., Pathak, H., Kumar, A. et al. Global temperature change potential of nitrogen use in agriculture: A 50-year assessment. Sci Rep 7, 44928 (2017). https://doi.org/10.1038/srep44928
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DOI: https://doi.org/10.1038/srep44928
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