1

Nitrogen deposition in tropical forests from deforestation and

savanna fires

Running title: Fire effects on tropical N fluxes

Yang Chen, Department of Earth System Science, University of California, Irvine, CA 92697, USA

James T. Randerson, Department of Earth System Science, University of California, Irvine, CA

92697, USA

Guido R. van der Werf, Faculty of Earth and Life Sciences, VU University, 1081HV Amsterdam,

Netherlands

Douglas C. Morton, NASA Goddard Space Flight Center, Biospheric Sciences Branch, Code 614.4,

Greenbelt, Maryland USA 20771

Prasad S. Kasibhatla, Nicholas School of the Environment, Duke University, Durham, NC 27708,

USA

Kewords: atmospheric transport, pyro-denitrification, nitrogen limitation, Hadley circulation, tropical

biomes, the global carbon cycle, and biomass burning

Category of article: Primary Research Article

Corresponding author

Yang Chen

Email: yang.chen@uci.edu

Phone: 1-949-824-0597

Department of Earth System Science, University of California, Irvine, CA 92697

Manuscript prepared for Global Change Biology

June 11, 2009

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Abstract

Tropical forests account for nearly half of global net primary production (NPP) and may contribute

substantially to contemporary and future land carbon (C) sinks. We used satellite-derived estimates of

global fire emissions and a chemical transport model to estimate atmospheric nitrogen (N) fluxes from

deforestation and savanna fires in tropical ecosystems. N emissions and deposition led to a substantial

net transport of N equatorward, from savannas and areas undergoing deforestation to tropical forests.

On average, N emissions from fires were equivalent to approximately 28% of biological N fixation

(BNF) in savannas (4.8 kg N ha-1 yr-1) and 38% of BNF from ecosystems at the deforestation frontier

(9.1 kg N ha-1 yr-1). N deposition occurred in interior tropical forests at a rate equivalent to 4% of their

BNF (1.1 kg N ha-1 yr-1). This percentage was highest for African tropical forests in the Congo Basin

(16%; 3.7 kg N ha-1 yr-1) owing to equatorward transport from northern and southern savannas. These

results suggest that land use change, including deforestation fires, may be enhancing nutrient

availability and carbon sequestration in nearby tropical forest ecosystems.

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INTRODUCTION

Nitrogen (N) is an essential element in Earth's atmosphere, biosphere, and hydrosphere (Galloway et

al., 2003). The majority of N in the atmosphere is N2, which is biologically unavailable to most

organisms. N2 must be converted to reactive N (Nr), which includes inorganic oxidized forms (e.g.,

NO, NO2, and HNO3), inorganic reduced forms (NH3 and NH4+), and organic forms (e.g., amino acids

and urea), prior to its use by microbes and plants. N2 to Nr conversion (N fixation) occurs during

electrical discharges in lightning and by some bacteria and algae via a process known as biological

nitrogen fixation (BNF) (Cleveland et al., 1999). Humans have more than doubled the flux of N2 to Nr

through fossil fuel use, industrial N fertilizer production, and cultivation of N fixing leguminous crops

(Vitousek et al., 1997; Smil, 1999; Galloway et al., 2004, 2008).

Atmospheric transport and deposition of N emissions substantially changes N availability in terrestrial

ecosystems. Major effects of N deposition have been documented in terrestrial and marine ecosystems

(e.g., Fenn et al., 2003). Since most terrestrial ecosystems are N-limited, N addition to these

ecosystems stimulates net primary production (NPP) (LeBauer & Treseder, 2008) and may enhance

carbon (C) storage by this mechanism (e.g., Holland et al., 1997, Reay et al., 2008). However, chronic

N enrichment may lead to loss of C by increasing decomposition in organic soils (Mack et al., 2004)

or in soil carbon components that have decadal turnover times (Neff et al., 2002). In some ecosystems,

N addition also reduces microbial biomass and decreases soil respiration (Treseder, 2008). N

deposition can also influence ecosystem function by means of acidification, eutrophication (Rabalais,

2002), and loss of species diversity (Wilson and Tilman, 2002; Suding et al., 2005; Phoenix et al.,

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2006).

Much of our knowledge of the N cycle and the consequences of N enrichment are from temperate

ecosystems, where industrial and agricultural modifications of N availability are substantial. Tropical

ecosystems respond in different ways to perturbations in N cycling (Matson et al., 1999; Davidson et

al., 2004) and important uncertainties remain with respect to our understanding of N deposition,

fixation, and loss pathways in these ecosystems. With implementation of stricter air quality standards,

N emission and deposition are likely to slow down in temperate regions, particularly in developed

countries (Lamarque et al., 2005; Dentener et al., 2006). In contrast, N emission and deposition in

tropical and subtropical regions should increase (Galloway et al., 1994). Since tropical ecosystems

account nearly half of global NPP (Field et al., 1998) and may contribute substantially to

contemporary and future land C sinks (Friedlingstein et al., 2006; Stephens et al., 2007), it is

imperative to identify and quantify the consequences of changing levels of N deposition and N cycling

in tropical ecosystems.

Fires in savanna and at the deforestation frontier are a major source of air pollution in the tropics

(Crutzen and Andreae, 1990). Nitrogen oxide (NO) and ammonia (NH3) are the primary Nr gases

emitted from these fires. They are converted to other Nr gases and to particulate species (NO3-, NH4

+,

and organic aerosols) by means of multiple reaction pathways (Crutzen and Andreae, 1990; Atkinson,

2000). The ultimate fate of Nr in the atmosphere is removal by wet and dry deposition. Gas and

aerosol components of dry deposition are important in many regions (Dentener et al., 2006). Regional

variations in emissions and atmospheric transport of Nr have the potential to create strong gradients of

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N deposition across both terrestrial and ocean ecosystems. Emissions of NO and NH3 probably

account for only 40-50% of the N present in fuels prior to combustion, as some is retained within the

ecosystem in combusted residues (approximately 20%) and the remainder (another 30–40%) is

transformed directly into N2 during the combustion processes (Lobert et al., 1990; Kuhlbusch et al.,

1991). The N2 emission represents a net loss of N from the biosphere– contrasting with Nr fluxes that

instead lead to redistribution of Nr across different ecosystems.

In this study, we assessed the contribution of fires to N cycling in tropical ecosystems. We explored

the loss of N by emissions from biomass burning and the gain of Nr by deposition from the

atmosphere. We estimated fire emissions using the Global Fire Emission Database version 2

(GFEDv2, van der Werf et al., 2006), which combines satellite observations of burned area (Giglio et

al., 2006) and a biogeochemical model constrained by other satellite data to estimate fuel loads. We

estimated N2 emissions from biomass burning using data derived from a chamber burning experiment

(Kuhlbusch et al., 1991). The transport and deposition of N were simulated by using the GEOS-Chem

global chemical transport model (CTM) (Bey et al., 2001). We performed 10 years (1997–2006) of

global-scale simulations but focus our analysis and discussion on tropical ecosystems. We validated

some of our modeled fluxes by comparing our results with published observations of N deposition. We

also estimated the net N flux from fire emissions in different tropical ecosystems. These fluxes were

then compared with terrestrial and marine BNF fluxes (Cleveland et al., 1999; Deutsch et al., 2001;

Lee et al., 2002).

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METHODS

Nr emissions from biomass burning

The GFEDv2 product consists of 1°×1° gridded monthly burned area (BA), fuel loads (FL),

combustion completeness (CC), and emissions factors (EMFs) for different gas and aerosol species

(van der Werf et al., 2006). Burned area was derived using Moderate Resolution Imaging

Spectroradiometer (MODIS) active fire and burned area datasets as described by Giglio et al. (2006).

Fuel loads were estimated using the Carnegie-Ames-Stanford-Approach (CASA) biogeochemical

model constrained by additional satellite observations of fractional tree cover and the fraction of

absorbed photosynthetically active radiation by plant canopies (van der Werf et al., 2003, 2006).

Emission rates (E) for each gaseous N species were calculated by applying EMFs to fire-emitted

carbon fluxes from CASA:

E = EMF × BA × FL × CC (1)

Organic dry matter (DM) in the fuel was assumed to be comprised of 45% carbon. Emission factors,

defined as the emission of species (g) per 1 kg burned DM, were obtained from Andreae and Merlet

(2001). Comparison with atmospheric carbon monoxide (CO) observations provides some validation

of carbon emissions. GFEDv2 CO emissions in equatorial Asia, for example, required a small negative

adjustment (from 0-22% depending on atmospheric inversion approach) to match Measurements of

Pollution in the Troposphere (MOPITT) observations during 2000-2006 (van der Werf et al., 2008).

CO anomalies from GFEDv2 in the GEOS-CHEM model had a mean correlation of 0.79 with

observations from NOAA Global Monitoring Division (GMD) stations in the Northern Hemisphere

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and 0.59 with GMD stations in the Southern Hemisphere (Kai et al., 2009). Similarly, the ratio of

modeled to observed standard deviations of CO anomalies from GMD was 0.86 and 1.09,

respectively, for northern and southern hemispheres. These results indicate that while emissions

estimates are still uncertain, they are of sufficient quality to study the impact of fire-emitted species on

atmospheric composition.

Nr can be emitted into the atmosphere in different chemical forms and physical phases. NOx and NH3

account for the majority (greater than 90%) of Nr emissions from biomass burning (Andreae and

Merlet, 2001). In our model simulations and calculations, we used the sum of gaseous NOx and NH3

emissions to represent total Nr emissions from biomass burning. The low bias we expect from

neglecting the remaining Nr emissions (e.g., N2O, HCN, and acetonitrile) is small compared with

uncertainties associated with our estimates of burned area (e.g., Giglio et al., 2006) and total carbon

emissions (e.g., van der Werf et al., 2008).

Nr to N2 conversion during biomass burning

A large gap has been observed in the N balance between its content in the fuel and the sum of

measured N in gaseous emissions and what remains after combustion in the ash (Lobert et al., 1990).

By burning different biomass materials in a stainless steel chamber, Kuhlbusch et al. (1991) showed

that the missing N is mostly molecular N (N2), which accounts for approximately one third of fuel N.

This experiment also showed that N2 is formed primarily during the flaming stage and thus the total

amount of N2 production (as a fraction of fuel N) depends on fire intensity. CO/CO2 emission ratio

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serves as a powerful indicator for the relative extent of flaming combustion and smoldering

combustion. Analysis of the Kuhlbusch et al. (1991) measurements showed that the N2-N to Nr-N ratio

of fire emissions was inversely related to its CO/CO2 ratio (see Fig. S1 in Supporting Information).

The N2-N to Nr-N ratio derived from this relationship was used to estimate global N2 emissions from

biomass burning. In this study, we estimated the CO/CO2 ratio for three different fire types: savanna

and grassland fires, tropical forest fires, and extratropical forest fires using the emission factors of CO

and CO2 listed in Table 1 of Andreae and Merlet (2001). A 2°(latitude) × 2.5°(longitude) vegetation

map of these three types (Andreae and Merlet, 2001) was used to derive the global gridded N2-N to

Nr-N ratio. For each grid cell, the GFEDv2 Nr emission rate was scaled by this ratio to approximate

the fire emission rate of N2.

Model simulations of N transport and deposition

We used the GEOS-Chem model to calculate the transport, transformation, and deposition of biomass

burning emitted N species. GEOS-Chem is a global 3-D CTM (Bey et al., 2001) driven by assimilated

meteorological observations from the Goddard Earth Observing System (GEOS) of the NASA Global

Modeling and Assimilation Office (GMAO). In this study, we used version 08-01-01 of the model

with 2°×2.5° horizontal resolution and 30 vertical layers between the surface and 0.01 hPa.

The GEOS-Chem model includes a detailed description of tropospheric O3-NOx-hydrocarbon

chemistry (Wang et al., 1998). Emission sources of N include NOx from lightning, soil and fertilizer,

aircraft, biofuel, fossil fuel, biomass burning, and NH3 from biofuel, fossil fuel, biomass burning, and

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natural sources (e.g., soils, vegetation, and wild animals). N tracers are present in gas phase (e.g.,

NOx, peroxyacyl nitrate (PAN), HNO3, N2O5, and NH3) or aerosol phase (NH4+, N on sulfate, and N

on sea salt). These tracers are transported by meteorology-driven advection and convection processes,

and finally removed by the action of precipitation (wet deposition) or by contact with surface or

vegetation (dry deposition).

We performed GEOS-Chem full chemistry simulations over a 10-year period (1997-2006) using the

GFEDv2 inventory, which resolves the interannual variability of biomass burning emissions. N

emissions from other sources were obtained from climatological inventories which represent

contemporary (circa 2000) emissions. After a three-month spin-up period with the same initial

conditions, two sets of simulations were conducted: one with biomass burning emissions prescribed

from GFEDv2, and another without these emissions. The difference between these two simulations

represents the effect of biomass burning emissions on N transport and deposition. We recorded

monthly mean deposition rates for each N species through dry and wet deposition. Wet deposition

included convective and large-scale components (Liu et al., 2001). Total deposition rate of N from

biomass burning was then calculated for each 2°×2.5° model grid cell.

Biological N fixation

Cleveland et al. (1999) reviewed published studies and generated three linear regressions

(conservative, central, and upper-bound) between BNF and evapotranspiration (ET) in terrestrial

ecosystems. Galloway et al. (2004) argued that the true N fixation rate lies at the lower end of this

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range, due to the inherent biases noted in plot-scale studies. In this study, we therefore used the mean

of central and conservative regressions between BNF and ET from Cleveland et al. (1999) to estimate

N fixation rates over land. The global ET map we used was developed using satellite data and a bio-

meteorological approach by Fisher et al. (2008).

Estimates of global ocean BNF range from 60 to 200 Tg N yr-1 (Duce et al., 2008). Large uncertainty

remains in the estimation of regional marine N fixation (Mahaffey et al., 2005). In this study, we used

reported estimates of BNF rates over tropical Atlantic (Lee et al., 2002) and Pacific (Deutsch et al.,

2001). Due to a dearth of data for other regions, we assumed that the mean of these two rates

represented the N fixation rate over other tropical oceans.

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RESULTS

Validation of wet and dry deposition

Relative to Northern Hemisphere industrial regions, fewer N deposition measurements exist for

tropical land and ocean regions. Here we compiled wet and dry N deposition measurements in tropical

regions published in recent years. We only considered observations from 1996 to 2006 to ensure

overlap with the period of our satellite-derived fire emissions time series. We differentiated urban and

non-urban observations according to the description of the site in the original literature or the

proximity of the measurement station to an urban center. Based on the original data, we calculated wet

and dry deposition rates for total oxidized N (NO3-N) and reduced N (NH4-N) with units of kg N ha-1

yr-1. Altogether, we reported wet and dry deposition rates from 26 non-urban stations. The location

and time period of these measurements, as well observed and modeled deposition rates, are

summarized in Table S1. We compared these observations with modeled deposition fluxes, which

were sampled in grid cells corresponding to observations during the month when observations were

available. For locations that had measurements only during 1996, we compared the deposition rates

with our 10-year (1997–2006) mean model results.

Overall, the model simulation corresponded reasonably well to wet and dry deposition observations at

non-urban sites (Fig. 1). About 75% of the wet deposition rates and 80% of the dry deposition rates

agreed within ±50% with the observations. This agreement was similar to previous comparisons of N

deposition between models and observations (Lamarque et al., 2005; Dentener et al., 2006). The

model somewhat underestimated NO3-N wet deposition and overestimated NH4-N dry deposition.

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Simulations were more strongly correlated with measurements for dry deposition than for wet

deposition, as indicated by the r values shown in Fig. 1.

In contrast to non-urban sites, the model was generally biased low relative to the high urban data,

which were likely affected by nearby fossil fuel emissions (Fig. 1). Urban data were generally not

representative of the 2°×2.5° grid cells in the model, so they were not included in our linear

regressions. We note most of these urban measurement sites were located in Southeast Asia.

Gross N emissions from fires

Emissions of Nr and N2 by fires were 10.4 Tg N yr-1 (6.5 kg N ha-1 yr-1) in savannas and 5.3 Tg N yr-1

(3.8 kg N ha-1 yr-1) in tropical forests, with these two biomes accounting for 67% of global fire N

emissions (Fig. 2a and Table 1). High emission rates occurred in African savannas, as well as in

deforestation regions (See Fig. S2) in South America and Southeast Asia. Losses directly to N2

accounted for about half of the total N emissions flux. This fraction was slightly higher for savannas

than for tropical forests because of more complete combustion (and lower CO/CO2 ratios in savannas).

N deposition rates from biomass burning

Fire emissions represented a considerable fraction of N deposition in the tropics (See Fig. S3).

Deposition rates were highest near source regions - with maxima on the perimeter of the Congo basin

in Africa (up to 9.9 kg N ha-1 yr-1), in southern Borneo in equatorial Asia (up to 9.7 kg N ha-1 yr-1), and

across the Brazilian state of Mato Grosso, eastern Bolivia, and Paraguay in South America (up to 5.3

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kg N ha-1 yr-1) (Fig. 2b). These results imply that undisturbed forest and savanna ecosystems in these

regions (e.g. parks and other protected areas) experience relatively high N deposition from nearby

deforestation and savanna fires.

Seasonal variations in N deposition were closely linked with the timing of fire emissions (Fig. 3).

Total wet deposition was comparable to total dry deposition across the tropics, although regionally the

relative importance of these two pathways varied. Wet deposition accounted for a larger fraction of

total deposition in Southeast Asia (57%) and Northern Hemisphere (NH) Africa (55%) than in South

America (46%) or Southern Hemisphere (SH) Africa (39%). About 2/3 of the total N wet deposition

occurred in the gas phase (NH3 and HNO3). The relative importance of wet deposition as an

atmospheric loss pathway increased towards the end of the fire season on all three continents (Fig. 3),

probably as a consequence of increasing precipitation levels. Model estimates of the total deposition

rate for oxidized N and reduced N were similar. Separation of N deposition by its oxidation form is

important for assessing N deposition effects on ecosystem function. Reduced N, for example, can

reduce base cation uptake by plants (de Graaf et al., 1998), causing changes in species composition in

ecosystems such as heathlands (van der Berg et al., 2008) and peatlands (Paulissen et al., 2004).

Changes in N surface budgets caused by fire emissions

The emission and deposition of N from fires led to a net loss of N from the terrestrial biosphere and a

redistribution of N among ecosystems. Savanna ecosystems in Africa and South America and tropical

deforestation frontiers in all three regions experienced a net loss of N (Figs. 2c, S2). Net gains at a

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regional scale occurred in interior tropical forests and over tropical oceans.

The net loss of N (Nr and N2) from savanna ecosystems was largest in Africa (Table 1), where fire

return times were lower than on other continents (Giglio et al., 2006; Van der Werf et al., 2008). Net

fire losses were equal to approximately 39% of BNF in savanna ecosystems in Africa, corresponding

to 6.19 Tg N yr-1 or 6.7 kg N ha-1 yr-1 in savanna ecosystems (Table S2). Of the directly emitted Nr

from African savanna fires, only 53% was returned to savannas via deposition. Most of the remainder

was deposited in downwind tropical forests (e.g., in the Congo basin) and over the tropical Atlantic

Ocean (Fig. 2b). For savannas across all three continents, net fire N losses were equal to 28% of BNF

(4.8 kg N ha-1 yr-1), with 55% of Nr emissions remaining within these ecosystems via deposition in

nearby areas.

Tropical forests received large N deposition inputs from savanna fires but also lost N through

deforestation fires. Overall, tropical forest fires had a net N loss equal to approximately 10% of BNF

(2.83 Tg N yr-1 or 2.0 kg N ha-1 yr-1). In Southeast Asia, net losses were higher (35%), because a larger

proportion of fire emissions were associated with deforestation in tropical forests. In interior tropical

forests that were removed from the deforestation frontier (areas with fire emissions below our 500 kg

C ha-1 yr-1 threshold), the net effect of remote tropical fires was an increase atmospheric deposition.

For interior forests in Africa, for example, the net atmospheric flux from fires was 0.46 Tg N yr-1 (3.7

kg N ha-1 yr-1), corresponding to approximately 16% of BNF. In contrast, forests at the deforestation

frontier in South America and Southeast Asia had a large net N loss at a regional scale from fire

emissions. Expressed as a percentage of BNF, net N losses at the deforestation frontier (38%) were

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even higher than in savanna ecosystems.

Tropical oceans were a large recipient of N from fire emissions. Approximately 37% (2.85 Tg N yr-1)

of fire-emitted N was deposited over oceans. The tropical Atlantic, in particular, received high N

deposition fluxes – up to 3.8 kg N ha-1 yr-1 (Fig. 2), owing to high levels of fire emissions in Africa

and the prevalent westerly wind during the fire season (see Fig. S4). A small amount of the N from

African fire emissions was transported across the Atlantic and deposited over South America. This

pattern is consistent with analysis of lidar measurements from Manaus that show long range transport

of smoke and dust from Africa (Ansmann et al., 2009). The southern part of South China Sea also

received substantial N deposition (up to 2.7 kg N ha-1 yr-1) from fires in Indonesia and Peninsular

Malaysia.

Redistribution of N between ecosystems in Africa

Fires caused a net transport of N from savannas to interior forests in tropical Africa (Fig. 2 and Table

1). Evergreen broadleaf forests were distributed mostly between 5°N and 5°S and were bordered to the

north and south by savannas (Fig. 4a). BNF was highest near the equator and decreased to the north

and south with decreasing ET (Fig. 4b). The highest levels of Nr and N2 fire emissions occurred at

~7°N in the northern hemisphere and ~10°S in the southern hemisphere. The N deposition pattern,

especially that for dry deposition, was similar to that of fire emissions but with a small equatorward

shift in peak deposition in the NH. A portion of Nr emitted from savanna fires was deposited back in

savannas. However, a substantial amount of fire-emitted Nr was transported equatorward and

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deposited in tropical forests. In NH winter, which corresponded to the NH fire season, prevailing

winds were to the south (Fig. 4c). Convective lifting of air masses near the equator increased

precipitation and, subsequently, wet deposition. The transport was reversed in NH summer, when

winds to the north carried emissions from SH savannas to interior tropical forests. The Sahara and

Kalahari deserts, which were located upwind of the African savanna regions during the fire seasons in

the northern and southern hemispheres, respectively, received relatively small N deposition fluxes

from fire emissions.

The combined effects of biomass burning emissions, atmospheric transport, and deposition resulted in

a net transport of N from savannas to forests in Africa (Fig. 4f and Fig. 2c). This equatorward

transport pattern was not unique to Africa. Prevailing winds in South America and Southeast Asia also

caused transport of fire emissions toward interior (and relatively undisturbed) areas of the Amazon

and the Indonesian archipelago (Fig. S4).

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DISCUSSION AND CONCLUSIONS

We estimated emissions and deposition of N from tropical fires. Our results indicated a redistribution

of N between different types of tropical ecosystems. Specifically, we found an equatorward transport

of N from fires in savannas and at active deforestation frontiers to interior, intact tropical forests. On

average, fire N losses were equivalent to 28% of BNF in savannas and 38% of BNF for ecosystems at

the deforestation frontier. Interior tropical forests gained N equivalent to 4% of their BNF.

This atmospheric transport of N was closely related to the seasonality of fire emissions and

meteorological patterns in tropics. Most fires occurred in savannas and in areas that were undergoing

active deforestation. The Hadley circulation carried emissions from these fires equatorward, toward

the Intertropical Convergence Zone (ITCZ) (See Fig. S4). A significant amount of N from this zonal

transport was removed by wet and dry deposition over relatively undisturbed tropical forests. In

tropical Africa, the prevailing winds carried fire emissions from savannas to tropical forest during both

NH and SH fire seasons. As a consequence of widespread savanna burning in Africa, the net transport

of N from savannas to forests in Africa was larger than that observed in South America or Asia.

The cross biome transport of N described here strengthens the case for increases in nutrient deposition

serving as a key driver (e.g., Lewis et al., 2004) of long-term increases in carbon storage observed

within Amazonian (Phillips et al., 2009) and African (Lewis et al., 2009) forests. Although modeling

studies suggest tropical forests have great potential to increase net primary production in response to

elevated levels of carbon dioxide (Friedlingstein et al., 2006, Ciais et al., 2008), a key uncertainty is

whether nutrient availability can keep pace with photosynthesis to allow for long-term increases in

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biomass (e.g., Hungate et al., 2003; Luo et al., 2004). Indeed, free air carbon dioxide enrichment

studies from mid-latitude forests show that rates of carbon accumulation in response to elevated levels

of carbon dioxide increase when nutrient availability is enhanced (Oren et al., 2001). Our analysis

shows how concentrated fire use for deforestation, management of deforested areas, and land use in

neighboring savanna regions may be enhancing nutrient availability in nearby tropical forest

ecosystems– and thus increasing the sensitivity of these ecosystems to changing levels of atmospheric

CO2. Satellite-based fire detections suggest that fire use for pasture and cropland management sustains

high fire activity following deforestation (Schroeder et al., 2005); the net effect of the advancing

deforestation frontier is therefore to expand the area under fire management and to increase the

proximity of savanna fires to highly productive interior forests.

In parallel to the patterns shown here for N, fire-emitted black carbon aerosols have been shown to

contribute in important ways to phosphorous deposition in tropical forests within the Amazon

(Mahowald et al., 2005). Other land use-related changes in nutrient cycling, including the use of N

fertilizer in agriculture after forest clearing and increases in dust, may additionally increase the

nutrient loading to forests near the deforestation frontier. The impacts of these deposition fluxes for

carbon accumulation may be largest in younger secondary forests where nitrogen and phosphorous

limitation are most pronounced (Davidson et al., 2004, Davidson et al., 2007).

In terms of assessing the net impact of land use change on global levels of atmospheric CO2, increased

nutrient loading by deposition of fire emissions in intact and secondary tropical forest ecosystems may

partially offset some of the carbon losses associated with tropical deforestation. In this context, it is

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important to further refine our understanding of the fire-emitted component of deforestation carbon

losses (e.g., DeFries et al., 2008). In terms of atmospheric CO2 levels, decomposition and fire carbon

loss pathways from deforestation may not lead to substantially different outcomes. However, for

redistribution of nutrients, changes in aerosol optical depth and diffuse light (Oliveira et al., 2007;

Mercado et al., 2009; Doughty et al., 2009), ozone, methane, and human health impacts, these loss

pathways are fundamentally different.

Detailed analysis of the effect of atmospheric N deposition on marine ecosystems is complex for

several reasons. First, currently there are not enough observations to derive a global map of ocean N

fixation (Capone et al., 2005). Second, increases in atmospheric N deposition by anthropogenic

activity may be partially offset by lower rates of N fixation (Krishnamurthy et al., 2007).

Nevertheless, N is generally considered a primary limiting nutrient for phytoplankton biomass

accumulation in marine ecosystems, particularly in coastal systems (Rabalais, 2002). A large portion

(~37%) of N from savanna and forest fires is carried to and deposited over tropical oceans (Table 1).

Averaged over the whole tropical ocean, this deposition is relatively small (2%) compared to BNF and

also with respect to N transport to surface waters by upwelling (Capone et al., 2005). However, the

ratio is much higher in coastal regions in Africa and Southeast Asia (Table 1). Further work is needed

to identify how nutrient loading from fires affects marine ecosystem function in these highly impacted

areas, including phytoplankton primary production and the health of coral reefs. In equatorial Asia

after the 1997/1998 El Nino, for example, coral reef death was attributed to extraordinary red tide

caused by deposition of iron from fire emissions (Abram et al., 2003).

20

Here we show that N emissions from savanna and deforestation fires lead to deposition in tropical

forest ecosystems as a consequence of atmospheric transport associated with the ITCZ. A quantitative

understanding of this transport pathway is currently limited by several different types of observations,

including N deposition measurements near deforestation regions and improved estimates of fire

emissions associated with deforestation. More detailed measurements of N losses to N2 during fires

also are needed to reduce uncertainties associated with pyrodenitrification. Although considerable

effort has gone into improving our understanding of the deposition of chemical species in tropics (e.g.,

the Deposition of Biogeochemically Important Trace Species (DEBITS) network,

http://www.igac.noaa.gov/DEBITS.php), measurements of tropical N deposition are still sparse

compared to the density of available observations in temperate regions. Better measurements are

required to improve the model simulation of N transport and deposition. Finally, an improved

understanding of the fate of N lost during deforestation and N cycling during post-deforestation land

use may enable us to better quantify human impacts on N cycling in tropical ecosystems.

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ACKNOWLEDGEMENTS

This work was funded by NASA NNX08AF64G. Kasibhatla P.S. was partially supported by the grant

NNX08AL03G from Duke University. The GEOS-Chem model is managed by the Atmospheric

Chemistry Modeling group at Harvard University with support from the NASA Atmospheric

Chemistry Modeling and Analysis Program.

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Table 1. N fluxes from fires over tropical regions1. The values are 10 year (1997-2006) averaged.

Continent Region Area

Biological Nitrogen Fixation2

Fire emissions

of Nr

Fire emissions of

N2

Deposition of fire-emitted N

Net fire N flux

Mha Tg N yr-1 Tg N yr-1 Tg N yr-1 Tg N yr-1 % of BNF Tg N yr-1 % of BNF

Africa

Tropical forest 288 5.94 0.67 0.69 1.19 20 -0.17 -3

Interior3 123 2.90 0.04 0.04 0.54 19 0.46 16

Frontier4 5 0.11 0.01 0.01 0.01 13 -0.01 -5

Savanna 921 15.83 4.00 4.34 2.15 14 -6.19 -39

Ocean 2966 20.29 0 0 1.21 6 1.21 6

South America

Tropical forest 693 15.69 0.64 0.66 0.61 4 -0.70 -4

Interior 390 9.31 0.07 0.07 0.21 2 0.07 1

Frontier 64 1.51 0.21 0.22 0.12 8 -0.31 -21

Savanna 397 8.01 0.58 0.63 0.39 5 -0.81 -10

Ocean 1494 10.22 0 0 0.28 3 0.28 3

Southeast Asia

Tropical forest 304 4.67 1.16 1.05 0.59 13 -1.63 -35

Interior 16 0.34 0.01 0.01 0.04 13 0.03 8

Frontier 56 1.36 0.52 0.54 0.24 18 -0.82 -60

Savanna 112 1.46 0.32 0.32 0.13 9 -0.51 -35

Ocean 2523 13.80 0 0 0.87 6 0.87 6

Total tropics

Tropical forest 1409 28.52 2.70 2.62 2.49 9 -2.83 -10

Interior 529 12.55 0.12 0.12 0.80 6 0.56 4

Frontier 145 3.44 0.86 0.88 0.42 12 -1.32 -38

Savanna 1610 27.66 5.03 5.41 2.75 10 -7.70 -28

Ocean 19530 120.11 0 0 2.85 2 2.85 2

1 The domains of tropical regions in Africa, South America, and Southeast Asia are shown in Fig. 2. ‘Total tropics’ refers to regions

within 30°S-30°N.

2 Estimated using the mean of central and conservative regressions between BNF and ET as given in Cleveland et al. (1999).

3 ‘Interior’ refers to tropical forest region that is rarely affected by human activities. In this study, it is defined as 2°×2.5° grid cell with

fraction of evergreen broad leaf forest (from MODIS 500 m land cover product) over 70% and the 10-year average fire emission

rate (from GFEDv2) smaller than 500 kg C ha-1 yr-1.

4 ‘Frontier’ refers to tropical forest region undergoing substantial anthropogenic deforestation. Here it is defined as region with forest

cover greater than 10% and the fraction of forest loss during 2000-2005 (Hansen et al., 2008) greater than 1%.

32

FIGURE CAPTIONS

Figure 1: Comparison of observed and modeled N wet and dry deposition rates (kg N ha-1 yr-1) in

tropical regions. Red and blue points represent deposition rates for oxidized N (NO3-N) and reduced N

(NH4-N), respectively. Gray points are observation stations near urban areas. Filled diamonds, circles,

and triangles represent data for Africa, South America, and in Southeast Asia, respectively. Dashed

lines indicate 1:1, 1:2, and 2:1 relationships. Red and blue solid lines show linear regression fits for

NO3-N and NH4-N deposition rates.

Figure 2: Simulated 10-year average (a) N emissions from fires (the sum of Nr and N2), (b) N

deposition from biomass burning, and (c) net fire N flux with respect to the surface. The net fire N

flux includes both losses from Nr and N2 emissions and gains from N deposition. The values are in kg

N ha-1 yr-1. Blue and red colors represent N addition to and loss from terrestrial ecosystems. Solid pink

boxes define the domains of tropical Africa (AFR), tropical South America (SAM), and tropical

Southeast Asia (SEA) used in this paper.

Figure 3: Seasonal variability of total dry and wet deposition (Tg N yr-1) of N from biomass burning

over NH Africa, SH Africa, South America, and Southeast Asia (the domains are defined in Fig. 2).

The values are the 10 year (1997-2006) mean from the GEOS-Chem simulation.

Figure 4: Latitudinal distributions of mean (a) fraction of forest and savanna cover, (b) biological N

fixation (kg N ha-1 yr-1) estimated from Cleveland et al. (1999) using a global map of ET from Fisher

et al. (2008), (c) meridional wind speed (m/s) at 850hPa, with northward wind denoted as positive,

(d) N emissions from fires, (e) N deposition from biomass burning, and (f) net fire N and Nr fluxes (kg

33

N ha-1 yr-1) for tropical African region (30°S-30°N; 10°E-30°E). The net fire Nr flux includes losses

from Nr emissions and gains from N deposition. This domain covers, from north to south, the Sahara

desert, NH savanna, tropical forest, SH savanna, and the Karahari desert in Southwest Africa. The

values are 10 year (1997-2006) averaged. The meridional wind speed, derived from GEOS-4

reanalysis data, is averaged over February (representing fire season in NH) and August (representing

fire season in NH).

34

Figure 1

35

Figure 2

36

Figure 3

37

Figure 4

38

SUPPORTING INFORMATION

The following Supporting Information is available for this article:

Figure S1 N2-N/Nr-N ratio vs. CO/CO2 ratio

Figure S2 Forest cover loss due to deforestation and tropical ecosystem masks

Figure S3 Biomass emission and wind vectors

Table S1 Summary of N deposition observations

Table S2 N fluxes from fires in tropical regions– expressed in units per unit of land area

39

Figure S1. N2-N/Nr-N ratio as a function of CO/CO2 ratio. Data are based on results from a chamber

experiment (Kuhlbusch et al., 1991), excluding two pine needles samples (gray points) because of the

limited abundance of conifer plant functional types in tropical regions. We also excluded three wood

points below the N2 detection limit. The dash-dotted, dashed, and dotted vertical lines represent the

CO/CO2 values for savanna fires, tropical forest fires, and extratropical forest fires, respectively.

40

Figure S2. (a) Deforestation within the humid tropics during 2000-2005, shown as the percent loss

within each 0.5º×0.5º cell. The original data were compiled by Hansen et al. (2008). (b) Tropical

forest cover fraction (%). (c) Tropical savanna cover fraction (%). (d) Tropical ecosystem masks in

2º×2.5º resolution. The tropical ecosystem cover fractions and masks were derived from MODIS 500

m land cover product.

41

Figure S3. Latitudinal distribution of Nr emission and deposition rates (kg N ha-1 yr-1) from fire and

other sources. Inserted plots show the mean values and fire contribution fractions for three tropical

regions as defined in Fig. 2. The values are 10 year (1997-2006) averaged.

42

Figure S4. The GEOS-4 wind vectors at 850hPa in tropical regions for February and August, 2004.

The full length of an arrow represents a wind speed of ~20m/s. Red shaded areas depict the Nr

emissions (kg N ha-1 yr-1) from biomass burning for these months (10-year average).

43

Table S1. List of wet and dry N deposition observations and corresponding simulated values at non-urban sites in tropical regions. The shaded rows represent dry

deposition measurements and other rows represent wet deposition measurements.

Region Location Latitude Longitude Time NO3-N (obs)

(kg N ha-1 yr-1)

NO3-N (mod)

(kg N ha-1 yr-1)

NH4-N (obs)

(kg N ha-1 yr-1)

NH4-N (mod)

(kg N ha-1 yr-1) Reference

Africa

Zoétélé, Cameroon 3º10' N 11º58' E 09/1996 – 04/2000 1.93 1.83 3.00 4.18 Sigha-nkamdjou et al., 2003

Banizoumbou, Niger 13º20' N 2º25' E 06/1996 – 10/1996 0.84 1.25 0.88 1.71 Galy-Lacaux et al., 1998

Louis Trichardt, RSA 23º00' S 30º02' E 1986 - 1999 0.84 0.53 1.02 0.83 Mphepya et al., 2004. Updated in Galy-Lacaux et al., 2009

Lamto, Côte d'Ivoire 6º N 5º W 1995 - 2002 1.29 1.52 2.97 2.52 Yoboué et al., 2005

Skukuza, RSA 25º S 31º38' E 07/1999-06/2002 2.56 0.60 2.83 0.80 Mphepya et al., 2006

Banizoumbou, Niger 13º20' N 2º25' E 06/1994-09/2005 0.80 0.63 1.26 0.89 Galy-Lacaux et al., 2009

Katibougou, Mali 12º30' N 8º5' W 1997-2005 1.04 0.86 1.88 1.24 Galy-Lacaux et al., 2009

South

America

Rondônia, Brazil 10º04' S 61º56' W Sep-Nov, 2002 0.60 0.29 0.67 0.60 Trebs et al., 2006

Andean montane forest, Ecuador 4º S 79º05' W 05/1998-04/2003 0.62 0.28 1.95 1.07 Boy et al., 2008

Araraquara, Brazil 21º48' S 48º11' W 04/1999-02/2001 2.08 0.81 2.26 2.73 Da Rocha et al., 2005

Petit Sault, French Guyana 5º21' S 53º41' W 1999 2.44 0.63 1.92 1.07 Yoboué et al., 2005

Piracicaba River Basin, Brazil 22º54' S 47º03' W 08/1997-07/1998 2.58 1.09 2.31 3.75 Lara et al., 2007; Averaged over 4 sites

Andean montane forest, Ecuador 4º S 79º05' W Aug-Mar, 1998-2003 1.03 0.78 0.57 0.86 Boy et al., 2008

Andean montane forest, Ecuador 4º S 79º05' W Apr-Jul, 1998-2003 0.23 0.67 0.42 0.90 Boy et al., 2008

Rondônia, Brazil 10º04' S 61º56' W 09/2002 0.20 0.28 0.20 0.22 Trebs et al., 2006, value in kg N ha -1 month-1

Rondônia, Brazil 10º04' S 61º56' W 10/2002 0.10 0.13 0.10 0.09 Trebs et al., 2006, value in kg N ha -1 month-1

Rondônia, Brazil 10º04' S 61º56' W 11/2002 0.06 0.08 0.05 0.06 Trebs et al., 2006, value in kg N ha -1 month-1

Southeast

Asia

Tanah Rata, Malaysia 4º28' N 101º22' E 1996 1.96 1.97 2.38 2.83 Ayers et al., 2002

Serpong, Indonesia 6º15' S 106º34' E 2001 - 2005 8.99 1.26 11.73 4.02 Acid deposition Monitoring Network in East Asia

(EANET), Data available online: http://www.eanet.cc

Kototabang, Indonesia 0º12' S 100º19' E 2000 – 2005 4.56 2.00 1.57 4.14 EANET

44

Tanah Rata, Malaysia 4º28' N 101º22' E 2000 – 2005 2.08 1.66 5.93 2.48 EANET

Patumthani, Thailand 14º02' N 100º46' E 2000 – 2005 3.23 1.31 5.55 3.58 EANET

Khao Lam, Thailand 14º4' N 101º37' E 2000 – 2001 1.07 1.62 1.59 4.53 EANET

Vachiralongkorn Dam, Thailand 14º46' N 98º35' E 2002 – 2005 1.16 0.93 2.31 2.01 EANET

Mae Hia, Thailand 18º46' N 98º56' E 2001 – 2005 1.08 0.93 2.95 2.74 EANET

Hoa Binh, Vietnam 20º49' N 105º20' E 2000 - 2005 3.71 3.01 6.55 10.69 EANET

45

Table S2. Same as Table 1 but expressed in units per unit of ecosystem area (kg N ha-1 yr-1).

Continent Region

Area Biological Nitrogen Fixation

Fire emissions

of Nr

Fire emissions of

N2

Deposition of fire-emitted N

Net fire N flux

Mha kg N

ha-1 yr-1 kg N

ha-1 yr-1 kg N

ha-1 yr-1 kg N

ha-1 yr-1 % of BNF

kg N ha-1 yr-1

% of BNF

Africa

Tropical forest 288 25.4 2.3 2.4 4.1 20 -0.6 -3

Interior 123 25.4 0.3 0.3 4.4 19 3.7 16

Frontier 5 25.4 2.1 2.2 3.0 13 -1.2 -5

Savanna 921 30.2 4.3 4.7 2.3 14 -6.7 -39

Ocean 2966 6.8 0 0 0.4 6 0.4 6

South America

Tropical forest 693 25.4 0.9 1.0 0.9 4 -1.0 -4

Interior 390 25.4 0.2 0.2 0.5 2 0.2 1

Frontier 64 25.4 3.3 3.5 1.9 8 -4.9 -21

Savanna 397 30.2 1.5 1.6 1.0 5 -2.0 -10

Ocean 1494 6.8 0 0 0.2 3 0.2 3

Southeast Asia

Tropical forest 304 25.4 3.8 3.4 1.9 13 -5.3 -35

Interior 16 25.4 0.4 0.5 2.7 13 1.8 8

Frontier 56 25.4 9.2 9.5 4.2 18 -14.5 -60

Savanna 112 30.2 2.8 2.8 1.1 9 -4.5 -35

Ocean 2523 5.5 0 0 0.3 6 0.3 6

Total tropics

Tropical forest 1409 25.4 1.9 1.9 1.8 9 -2.0 -10

Interior 529 25.4 0.2 0.2 1.5 6 1.1 4

Frontier 145 25.4 5.9 6.1 2.9 12 -9.1 -38

Savanna 1610 30.2 3.1 3.4 1.7 10 -4.8 -28

Ocean 19530 6.2 0 0 0.1 2 0.1 2