Cynthia D. Nevison

Closing the global N2O budget: nitrous oxide emissions through the agricultural nitrogen cycle

In 1995 a working group was assembled at the request of OECD/IPCC/IEA to revise the methodology for N2O from agriculture for the National Greenhouse Gas Inventories Methodology. The basics of the methodology developed to calculate annual country level nitrous oxide (N2O) emissions from agricultural soils is presented herein. Three sources of N2O are distinguished in the new methodology: (i) direct emissions from agricultural soils, (ii) emissions from animal production, and (iii) N2O emissions indirectly induced by agricultural activities. The methodology is a simple approach which requires only input data that are available from FAO databases. The methodology attempts to relate N2O emissions to the agricultural nitrogen (N) cycle and to systems into which N is transported once it leaves agricultural systems. These estimates are made with the realization that increased utilization of crop nutrients, including N, will be required to meet rapidly growing needs for food and fiber production in our immediate future. Anthropogenic N input into agricultural systems include N from synthetic fertilizer, animal wastes, increased biological N-fixation, cultivation of mineral and organic soils through enhanced organic matter mineralization, and mineralization of crop residue returned to the field. Nitrous oxide may be emitted directly to the atmosphere in agricultural fields, animal confinements or pastoral systems or be transported from agricultural systems into ground and surface waters through surface runoff. Nitrate leaching and runoff and food consumption by humans and introduction into sewage systems transport the N ultimately into surface water (rivers and oceans) where additional N2O is produced. Ammonia and oxides of N (NOx) are also emitted from agricultural systems and may be transported off-site and serve to fertilize other systems which leads to enhanced production of N2O. Eventually, all N that moves through the soil system will be either terminally sequestered in buried sediments or denitrified in aquatic systems. We estimated global N2O–N emissions for the year 1989, using midpoint emission factors from our methodology and the FAO data for 1989. Direct emissions from agricultural soils totaled 2.1 Tg N, direct emissions from animal production totaled 2.1 Tg N and indirect emissions resulting from agricultural N input into the atmosphere and aquatic systems totaled 2.1 Tg N2O–N for an annual total of 6.3 Tg N2O–N. The N2O input to the atmosphere from agricultural production as a whole has apparently been previously underestimated. These new estimates suggest that the missing N2O sources discussed in earlier IPCC reports is likely a biogenic (agricultural) one.

Global oceanic emissions of nitrous oxide

The global N2O flux from the ocean to the atmosphere is calculated based on more than 60,000 expedition measurements of the N2O anomaly in surface water. The expedition data are extrapolated globally and coupled to daily air-sea gas transfer coefficients modeled at 2.8°×2.8° resolution to estimate a global ocean source of about 4 (1.2–6.8) Tg N yr−1. The wide range of uncertainty in the source estimate arises mainly from uncertainties in the air-sea gas transfer coefficients and in the global extrapolation of the summertime-biased surface N2O data set. The strongest source is predicted from the 40–60°S latitude band. Strong emissions also are predicted from the northern Pacific Ocean, the equatorial upwelling zone, and coastal upwelling zones occurring predominantly in the tropical northern hemisphere. High apparent oxygen utilization (AOU) at 100 m below the mixed layer is found to be correlated positively both to N2O production at depth and to the surface N2O anomaly. On the basis of these correlations, the expedition data are partitioned into two subsets associated with high and low AOU at depth. The zonally averaged monthly means in each subset are extrapolated to produce two latitude-by-month matrices in which monthly surface N2O is expressed as the deviation from the annual mean. Both matrices contain large uncertainties. The low-AOU matrix, which mainly includes surface N2O data from the North Atlantic and the subtropical gyres, suggests many regions with positive summer deviations and negative winter deviations, consistent with a seasonal cycle predominantly driven by seasonal heating and cooling of the surface ocean. The high-AOU subset, which includes the regions most important to the global N2O ocean source, suggests some regions with positive winter deviations and negative summer deviations, consistent with a seasonal cycle predominantly driven by wintertime mixing of surface water with N2O-rich deep water. Coupled seasonal changes in gas transfer coefficients and surface N2O in these important source regions could strongly influence the global ocean source.

Review of the IPCC methodology for estimating nitrous oxide emissions associated with agricultural leaching and runoff

Abstract Context abstract : The constraint imposed by the observed atmospheric N 2 O increase suggests that the IPCC may overestimate the anthropogenic N 2 O source. The 1996 Revised IPCC methodology, which will be used by Parties to the Kyoto Protocol, predicts that N 2 O accounts for 10% or more of national aggregate greenhouse gas emissions in many countries. This percentage contribution is comparable to or greater than the overall emissions reductions required by the Protocol. N 2 O emissions associated with agricultural leaching and runoff contribute a significant share of the IPCC N 2 O source. The current methodology may significantly overestimate these emissions, with implications for the total IPCC anthropogenic N 2 O source and for national greenhouse gas inventories. Main abstract : N 2 O emissions associated with leaching and runoff play an important role in determining both the magnitude and the uncertainty of the agricultural N 2 O source, as estimated by the 1996 revised IPCC methodology. According to the methodology, leaching/runoff emissions account for over 1/4 of the total agricultural N 2 O source and nearly 1/2 of the range of uncertainty in the total source. Notably, the observed atmospheric N 2 O increase of 3.9 Tg N/yr, which provides an important and well documented constraint on the anthropogenic N 2 O source, is significantly lower than the IPCC total agricultural source of 6.3 Tg N/yr. Several areas of uncertainty in the IPCC estimate of leaching and runoff-related N 2 O emissions are identified in this review. First, in the current methodology, a default-leaching fraction for fertilizer and animal waste of 30% is recommended for all countries, despite large variations within individual watersheds and agricultural systems. Second, the N 2 O emission factor associated with groundwater may be overestimated by an order of magnitude. Currently, groundwater accounts for 60% of leaching-related N 2 O emissions, with the remainder assumed to occur from rivers and estuaries. Finally, leaching fractions and associated N 2 O emission factors may not be defined in a conceptually consistent manner.

A reexamination of the impact of anthropogenically fixed nitrogen on atmospheric N2O and the stratospheric O3 layer

The impact of anthropogenic nitrogen fixation on atmospheric N2O is estimated using the approach of the 1970s, which assumed that some fraction β of anthropogenically fixed nitrogen is rapidly denitrified back to the atmosphere, with a significant fraction α of the end product as N2O. Appropriate values for β and α are discussed and applied to current anthropogenic nitrogen fixation rates, which are dominated by synthetic fertilizer and crop production. These calculations yield an N2O source of about 3.5 Tg N/yr associated with anthropogenic nitrogen fixation, which accounts for most of the observed atmospheric N2O increase of 3–5 Tg N/yr. This simple nitrogen cycle-based approach toward estimating anthropogenic N2O sources provides a useful check on the more complex approaches employed today, in which emissions from a large number of small, independent sources are estimated by extrapolating measured emissions coefficients. Such approaches can be inconsistent with considerations of the global nitrogen cycle and likely have underestimated the fertilizer N2O source and double counted other sources. A box model of atmospheric N2O which assumes an anthropogenic N2O source proportional to past and projected future rates of anthropogenic nitrogen fixation can reproduce much of the historic growth in N2O. Continued growth in the rate of anthropogenic nitrogen fixation could increase atmospheric N2O to 400–500 ppbv by the year 2100. Two-dimensional model calculations suggest that the corresponding increase in stratospheric NOx would cause a small loss of O3, which would be superimposed upon a larger recovery due to the phaseout of anthropogenic halocarbons. An increase in N2O could put more NOx into the middle and upper stratosphere than supersonic aircraft, although the relevant time scale is considerably longer. To better understand the impact of anthropogenic nitrogen on atmospheric N2O and the stratospheric O3 layer, more information is needed about future anthropogenic nitrogen fixation rates, the N2O yields of denitrification and nitrification, net storage/loss of naturally and anthropogenically fixed nitrogen, and NOx chemistry in the stratosphere.

A global model of changing N2O emissions from natural and perturbed soils

A high resolution global model of the terrestrial biosphere is developed to estimate changes in nitrous oxide (N2O) emissions from 1860–1990. The model is driven by four anthropogenic perturbations, including land use change and nitrogen inputs from fertilizer, livestock manure, and atmospheric deposition of fossil fuel NOx. Global soil nitrogen mineralization, volatilization, and leaching fluxes are estimated by the model and converted to N2O emissions based on broad assumptions about their associated N2O yields. From 1860–1990, global N2O emissions associated with soil nitrogen mineralization are estimated to have decreased slightly from 5.9 to 5.7 Tg N/yr, due mainly to land clearing, while N2O emissions associated with volatilization and leaching of excess mineral nitrogen are estimated to have increased sharply from 0.45 to 3.3 Tg N/yr, due to all four anthropogenic perturbations. Taking into account the impact of each perturbation on soil nitrogen mineralization and on volatilization and leaching of excess mineral nitrogen, global 1990 N2O emissions of 1.4, 0.7, 0.4 and 0.08 Tg N/yr are attributed to fertilizer, livestock manure, land clearing and atmospheric deposition of fossil fuel NOx, respectively. Consideration of both the short and long-term fates of fertilizer nitrogen indicates that the N2O/fertilizer-N yield may be 2% or more.

Contribution of ocean, fossil fuel, land biosphere, and biomass burning carbon fluxes to seasonal and interannual variability in atmospheric CO2

Seasonal and interannual variability in atmospheric carbon dioxide (CO2) concentrations was simulated using fluxes from fossil fuel, ocean and terrestrial biogeochemical models, and a tracer transport model with time-varying winds. The atmospheric CO2 variability resulting from these surface fluxes was compared to observations from 89 GLOBALVIEW monitoring stations. At northern hemisphere stations, the model simulations captured most of the observed seasonal cycle in atmospheric CO2, with the land tracer accounting for the majority of the signal. The ocean tracer was 3–6 months out of phase with the observed cycle at these stations and had a seasonal amplitude only ∼10% on average of observed. Model and observed interannual CO2 growth anomalies were only moderately well correlated in the northern hemisphere (R ∼ 0.4–0.8), and more poorly correlated in the southern hemisphere (R < 0.6). Land dominated the interannual variability (IAV) in the northern hemisphere, and biomass burning in particular accounted for much of the strong positive CO2 growth anomaly observed during the 1997–1998 El Nino event. The signals in atmospheric CO2 from the terrestrial biosphere extended throughout the southern hemisphere, but oceanic fluxes also exerted a strong influence there, accounting for roughly half of the IAV at many extratropical stations. However, the modeled ocean tracer was generally uncorrelated with observations in either hemisphere from 1979–2004, except during the weak El Nino/post-Pinatubo period of the early 1990s. During that time, model results suggested that the ocean may have accounted for 20–25% of the observed slowdown in the atmospheric CO2 growth rate

Measurements of the NO y ‐N2O correlation in the lower stratosphere: Latitudinal and seasonal changes and model comparisons

The tracer species nitrous oxide, N2O, and the reactive nitrogen reservoir, NOy, were measured in situ using instrumentation carried aboard the NASA ER-2 high altitude aircraft as part of the NASA Airborne Southern Hemisphere Ozone Expedition/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA) and Stratospheric Tracers of Atmospheric Transport (STRAT) missions. Measurements were made throughout the latitude range of 70°S to 60°N over the time period of March to October 1994 and October 1995 to January 1996, which includes the period when the Antarctic polar vortex is most intense. The correlation plots of NOy with N2O reveal compact, near-linear curves throughout data obtained in the lower stratosphere (50 mbar to 200 mbar). The average slope of the correlation, ΔNOy/ΔN2O, in the southern hemisphere (SH) exhibited a much larger seasonal variation during this time period than was observed in the northern hemisphere (NH). Between March and October in the potential temperature range of 400 K to 525 K, the correlation slope in the SH midlatitudes increased by 28%. A smaller but still positive increase in the correlation slope was observed for higher-latitude data obtained within or near the edge of the SH polar vortex. At NH midlatitudes the correlation slope did not significantly change between March and October, while between October and January the slope increased by +7%. The larger SH midlatitude increase is consistent with ongoing descent throughout the winter and spring and also suggests that denitrification, the irreversible loss of HNO3 through sedimentation of cloud particles, is not a significant term (<10–15%) in the budget of NOy at SH midlatitudes during the wintertime. A secular increase in the correlation slope is ruled out by comparison with SH data obtained during the 1987 Airborne Antarctic Ozone Expedition (AAOE) aircraft campaign. These results suggest that a seasonal cycle exists in the correlation slope for both hemispheres, with the SH correlation slope returning to the April value during the SH spring and summer. Changes in stratospheric circulation also probably play a role in both the SH and the NH correlation slope seasonal cycles. Comparisons with two-dimensional model results suggest that the slope decreases when the denitrified Antarctic vortex is diluted into midlatitudes upon vortex breakup in the spring and that through the descent of stratospheric air, the slope recovers during the following fall/winter period.

Southern Ocean ventilation inferred from seasonal cycles of atmospheric N 2 O and O 2 /N 2 at Cape Grim, Tasmania

The seasonal cycle of atmospheric N2O is derived from a 10-yr observational record at Cape Grim, Tasmania (41◦S, 145◦E). After correcting for thermal and stratospheric influences, the observed atmospheric seasonal cycle is consistent with the seasonal outgassing of microbially produced N2O from the Southern Ocean, as predicted by an ocean biogeochemistry model coupled to an atmospheric transport model (ATM). The model–observation comparison suggests a Southern Ocean N2O source of ∼0.9 Tg N yr−1 and is the first study to reproduce observed atmospheric seasonal cycles in N2O using specified surface sources in forward ATM runs. However, these results are sensitive to the thermal and stratospheric corrections applied to the atmospheric N2O data. The correlation in subsurface waters between apparent oxygen utilization (AOU) and N2O production (approximated as the concentration in excess of atmospheric equilibrium N2O) is exploited to infer the atmospheric seasonal cycle in O2/N2 due to ventilation of O2-depleted subsurface waters. Subtracting this cycle from the observed, thermally corrected seasonal cycle in atmospheric O2/N2 allows the residual O2/N2 signal from surface net community production to be inferred. Because N2O is only produced in subsurface ocean waters, where it is correlated to O2 consumption, atmospheric N2O observations provide a methodology for distinguishing the surface production and subsurface ventilation signals in atmospheric O2/N2, which have previously been inseparable.

In situ observations of NOy, O3, and the NOy/O3 ratio in the lower stratosphere

Extensive in situ measurements of reactive nitrogen (NO y ) and ozone (O 3 ) were made in the lower stratosphere over a broad latitude range (60°N-70°S) during two different seasons (March and October) in 1994. Both NO y and O 3 mixing ratios show a strong latitude dependence, with values increasing toward the poles. The NO y /O 3 ratio reveals a high-gradient region near the tropics that is not well-represented in standard 2-D photochemical transport models. Improving the representation by changing the horizontal eddy-diffusion coefficients near the tropics has important implications for the predicted impacts of aircraft emissions on stratospheric O 3 .

Quantifying the impact of anthropogenic nitrogen deposition on oceanic nitrous oxide

Anthropogenically induced increases in nitrogen deposition to the ocean can stimulate marine productivity and oceanic emission of nitrous oxide. We present the first global ocean model assessment of the impact on marine N2O of increases in nitrogen deposition from the preindustrial era to the present. We find significant regional increases in marine N2O production downwind of continental outflow, in coastal and inland seas (15–30%),and nitrogen limited regions of the North Atlantic and North Pacific (5–20%). The largest changes occur in the northern Indian Ocean (up to 50%) resulting from a combination of high deposition fluxes and enhanced N2O production pathways in local hypoxic zones. Oceanic regions relatively unaffected by anthropogenic nitrogen deposition indicate much smaller changes (<2%). The estimated change in oceanic N2O source on a global scale is modest (0.08–0.34 Tg N yr-1, ~3–4% of the total ocean source), and consistent with the estimated impact on global export production (~4%).