Environ. Res. Lett. 11 (2016) 085003 doi:10.1088/1748-9326/11/8/085003

LETTER

Biomass burning drives atmospheric nutrient redistribution withinforested peatlands in Borneo

AlexandraGPonette-González1, LisaMCurran2,3, AliceMPittman3, KimberlyMCarlson4, Bethel G Steele1,DessyRatnasari5,Mujiman5 andKathleenCWeathers6

1 Department ofGeography and the Environment, University ofNorth Texas, Denton, TX,USA2 Woods Institute for the Environment, StanfordUniversity, Stanford, CA,USA3 Department of Anthropology, StanfordUniversity, Stanford, CA,USA4 Department ofNatural Resources and EnvironmentalManagement, University ofHawai’i atManoa,Honolulu,HI, USA5 Living Landscapes Indonesia, Pontianak,West Kalimantan, Indonesia6 Cary Institute of Ecosystem Studies,Millbrook,NY,USA

E-mail: alexandra@unt.edu

Keywords:fire, Indonesia, land-use change, nitrogen, oil palm, phosphorus, sulfur

Supplementarymaterial for this article is available online

AbstractBiomass burning plays a critical role not only in atmospheric emissions, but also in the deposition andredistribution of biologically important nutrients within tropical landscapes.We quantified theinfluence offire on biogeochemical fluxes of nitrogen (N), phosphorus (P), and sulfur (S) in a 12 haforested peatland inWest Kalimantan, Indonesia. Total (inorganic+organic)N, -NO3 –N, +NH4 –N, total P, -PO4 3 –P, and -SO4 2 –Sfluxesweremeasured in throughfall and bulk rainfall weekly fromJuly 2013 to September 2014. To identify fire events, we used concentrations of particulatematter(PM10) andMODISActive Fire Product counts within 20 and 100 km radius buffers surrounding thesite. Dominant sources of throughfall nutrient depositionwere explored using cluster and back-trajectory analysis. Ourfindings show that this Bornean peatland receives some of the highest P(7.9 kg -PO4 3 –P ha

−1yr−1) and S (42 kg -SO4 2 –S ha−1yr−1) deposition reported globally, and thatN

deposition (8.7 kg inorganicN ha−1yr−1) exceeds critical load limits suggested for tropical forests. Sixmajor dry periods and associated fire events occurred during the study. Seventy-eight percent offireswithin 20 km and 40%within 100 kmof the site were detectedwithin oil palmplantation leases(industrial agriculture) on peatlands. Thesefires had a disproportionate impact on below-canopynutrient fluxes. Post-fire throughfall events contributed>30%of the total inorganicN ( -NO3 –N+

+NH4 –N) and -PO4 3 –Pflux to peatland soils during the study period. Our results indicate thatbiomass burning associatedwith agricultural peatfires is amajor source ofN, P, and S in throughfalland could rival industrial pollution as an input to these systems duringmajorfire years. Given thesheermagnitude offluxes reported here, fire-related redistribution of nutrientsmay have significantfertilizing or acidifying effects on a diversity of nutrient-limited ecosystems.

1. Introduction

Coupled changes in climate and land use are alteringfire regimes in tropical forest regions (Siegertet al 2001, Cochrane 2003, Bowman et al 2009,Hansenet al 2009, Margono et al 2014), with consequences forglobal, regional, and local biogeochemical cycles(Crutzen andAndreae 1990, Andreae andMerlet 2001,van der Werf et al 2010). For example, over the past

two decades, Indonesia has transitioned from ahistorically long fire-return interval regime (Gold-ammer 2007) to become one of the most importantsources of biomass burning emissions worldwide (vander Werf et al 2010). In 2015, carbon dioxide (CO2),methane (CH4), and nitrous oxide (N2O)CO2-equiva-lent emissions from Indonesian fires were estimated toexceed Japan’s 2013 fossil fuel CO2 emissions (GlobalFire EmissionsDatabase 2015).

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Although fires in Indonesia are anthropogenic––ignited during drier conditions primarily to clear vege-tation for agriculture––these land-clearing fires andassociated wildfires have increased dramatically inSumatra and Kalimantan since the early 1990s as adirect result of governmental land-use policies, parti-cularly logging and forest degradation, economicincentives for industrial agriculture, and government-sponsored transmigration, among other factors (Sie-gert et al 2001, Curran et al 2004, Dennis et al 2005,Langner et al 2007, Murdiyarso and Adiningsih 2007,Langner and Siegert 2009, Page et al 2009). El NiñoSouthern Oscillation (ENSO) conditions exacerbatedry season droughts and result in nonlinear increasesin fire activity (van derWerf et al 2008). Mounting evi-dence suggests that fire regimes in Sumatra and Kali-mantan are again changing; large severe fires can occurfollowing brief dry spells and during non-ENSO years(Pittman et al 2013, Gaveau et al 2014). Moreover, anincreasing proportion of these fires occur within peat-lands (Miettinen et al 2011a,Marlier et al 2015).

Indonesian peatlands (∼200 000 km2) comprise∼5% of global peatland area and store an estimated57 Gt carbon (C), ∼9.4–12% of the global peat C pool(Page et al 2011). These ecosystems are highly sensitiveto reductions in precipitation––especially extendedENSO-associated droughts (Field et al 2009)––thatlower the water table, thereby leading to peat desicca-tion and enhancing peat flammability (Usupet al 2004, Wooster et al 2012). Due to high rates offorest conversion and peatland drainage associatedwith industrial-scale agricultural expansion (e.g., oilpalm), peatlands have become increasingly vulnerableto fire (Hooijer et al 2006, Miettinen et al 2011a,Carlson et al 2013, Margono et al 2014, Turetskyet al 2015).

Peatland fires not only turn peatlands from C sinkto source, but they also release large amounts of othergases and particles to the atmosphere; peat emissionsare often orders of magnitude greater than those fromother land-cover types (Christian et al 2003, Iinumaet al 2007, van der Werf et al 2010, Akagi et al 2011).For example, peat combustion is a significant source ofbiologically important nutrients, including nitrogen(N), phosphorus (P), sulfur (S), and potassium (K)(Andriesse 1988) that are often stored in peat for mil-lennia (Weiss et al 2002). Emitted gases, particles, andchemicals are eventually delivered to downwind eco-systems, either dissolved in precipitation (wet or fogdeposition) or directly in dry form (dry deposition;Weathers and Ponette-González 2011). After hittingthe forest canopy, exchange via uptake and/or leach-ing can occur before nutrients are deposited to soils inthroughfall (Ponette-González et al 2014), resulting inthe redistribution of nutrients across the landscape(Ponette-González et al 2010a).

Ombrogenous peatlands are particularly sensitiveto changes in atmospheric nutrient loading becausethey derive nutrients exclusively from the atmosphere

and are also often nutrient limited (Tipping et al 2014).Research in boreal and temperate peatlands indicatesthat increases in atmospheric deposition can alter eco-system C fluxes through effects on species composi-tion, productivity, decomposition, and CH4 flux(Gauci et al 2004, Bragazza et al 2006, Limpenset al 2008, Frolking et al 2011).

Given the magnitude of peatland fire emissionsand the potential consequences of resultant depositionfor biogeochemical cycling, here we address the fol-lowing question: what is the influence of fires on bio-geochemical fluxes of three biologically importantplant nutrients (N, P, S) to adjacent, nutrient-limitedpeatland ecosystems? Specifically, what is the magni-tude of fire-related N, P, and S fluxes to peat soils andwhat are the most important sources of N, P, and S inthroughfall?

2.Methods

2.1. Study site and climateThis study was conducted in Kubu Raya District, WestKalimantan, Indonesia, ∼35 km from the coast in a12 ha intact forested peatland (mean peat depth3.5±0.1 m (±SE), range 2.6 –5.4 m; 0 °12′55″ S,109 °25′38″ E). Based on vegetation mapping andmonitoring from 2005–2014, the density of woodystems >10 cm dbh is ∼458 stems ha−1, while meanleaf area index (LAI) under closed canopy is3.4±0.69 m2 m−2. Mean tree basal area is15±0.1 m2 ha−1, with aboveground biomass of148±4.0 Mg ha−1. Large-scale oil palm plantations,degraded forest fragments, government-sponsoredtransmigration areas, open burned abandoned areas,and smallholder agricultural fields surrounded thesampling area in 2015 (figure 1(b)).

Climatic data compiled from Supadio Interna-tional Airport in Pontianak (8 km from site) indicatethat mean annual rainfall (1960–2014) is3197±481 mm (±SD), with considerable intra- andinter-annual precipitation variability due to localland-sea breezes, seasonal monsoons, and ENSOevents (Qian et al 2013). Total annual rainfall rangesnearly two-fold, 2329–4912 mm, from strong dry ElNiño to strong wet La Niña years. Monthly rainfall ishighest during the onset of the northeast monsoon(Oct–Dec: 314–359 mmmo−1) and lowest when thesouthwest monsoon prevails (Jun–Sep:186–250 mmmo−1). Fire events mirror these rainfallpatterns with few fires recorded during wet monthsand peak burning during drier periods (Vadrevuet al 2015).

2.2. Fire, land use, and air quality observationsTheMODISActive Fire Product (Giglio et al 2003)wasused to determine fire locations within 20 and 100 kmradius buffers surrounding the forested peatland. Firecounts are likely underestimated with these MODIS

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Environ. Res. Lett. 11 (2016) 085003

data because clouds and smoke as well as forests withhigh LAI obscure fire detection (Roy et al 2008). More-over, smoldering fires typical of peatlands are impos-sible to discern from flaming fires based on MODISdata, and are likely to be undetectable if they occurbelowground (Elvidge et al 2015). Within 100 kmsurrounding this site, we digitized provincial anddistrict agricultural oil palm lease records. Usingpeatland distribution information from RePPProT(1990) and Wetlands International (Wahyunto andSubagjo 2004), we then compiled fire hotspots by: (1)oil palm lease versus non oil palm lease; and, (2) soiltype (i.e., mineral or peatlands; figure 1(c)). Dailyobservations of particulate matter concentration

within each canopy cover class, for 18 total collectors(figure S1). Three bulk rainfall deposition collectors(i.e., collectors that remain open between samplingevents) were established in an adjacent clearing with-out canopy cover. Rainfall volume was recorded everyfiveminutes with a 20 cmdiameter tipping bucket raingauge (Rainwise, Rainew 111) connected to a HOBOevent logger (Onset, UA-003-64).

Throughfall and bulk deposition collectors wereconstructed with a 15 cm diameter funnel set on aPVC tube 1 m aboveground (afterWeathers et al 2006,Ponette-González et al 2010b). Funnels were rinsedwith deionized water and connected to plastic tubing,which drained into a 5 l plastic jug. A polywool filterwas placed inside the funnel to prevent sample con-tamination. Water samples were collected weeklyfrom July 2013 to September 2014. After each sampleweek (i.e., event), water volume was measured and a300 ml aliquot was collected in a plastic bottle, wrap-ped in aluminum foil, and immediately frozen. Frozensamples were shipped to the Indonesian Institute ofSciences Limnology Laboratory in Bogor, Indonesia,where they were analyzed for total N (inorganic+or-ganic N), -NO3 –N, +NH4 –N, total P (inorgani-c+organic P), -PO4 3 –P, -SO4 2 –S, and Cl

−

following standard protocols (APHA 1975, 2005,2012, text S1).

Organic Nwas estimated as the difference betweentotal N and dissolved inorganic N (DIN, -NO3 –N+ +NH4 –N) and organic P as the differencebetween total P and -PO4 3 –P. Chloride was used todetermine the seasalt- -SO4 2 ( )- -ss SO4 2 fraction inbulk rainfall using the average - -SO Cl4 2 ratio in sea-water (0.14, Kroopnick 1977). Non-seasalt -SO4 2

(nss– )-SO ,4 2 an indicator of pollution, was calculatedas the difference between -SO4 2 and ss– -SO .4 2

Volume-weighted mean (VWM) bulk rainfall andthroughfall concentrations were computed for eachsample event (i.e., week) or overall:

( ) ( )*= S SVWM conc precip precip 1i i i

where i is the collector, conc is the solute concentra-tion (mg l−1) and precip is the rainfall or throughfallamount (mm). Bulk rainfall deposition and through-fall chemical fluxes for each event were then calcu-lated by multiplying VWM concentrations by watervolume (de Souza et al 2015). To compute theweightedmean and standard deviation of constituentconcentrations across all weeks, as well as post-fireand normal weeks, we used the SDMTools package inR (VanDerWal et al 2014). From weekly VWMs, wecalculated overall mean and standard deviation,weighting by the total volume across all collectorseach week. We applied weighted least squares regres-sion to determine whether concentrations betweenpost-fire and normal weeks were significantlydifferent.

2.4. Chemical signatures and sourcesHierarchical cluster analysis was used to identifygroups of throughfall events with similar chemicalcomposition. Prior to analysis, non-normally distrib-uted data were log-transformed and all data werestandardized by subtracting the mean and dividing bythe standard deviation of each variable (Templet al 2008). Cluster analysis was performed on -NO3 –N, +NH4 –N, organic N, -PO4 3 –P, -SO4 2 –S, andCl− concentrations using Ward’s clustering method.Organic P was correlated with -PO4 3 –P and thusexcluded to minimize redundancy. After clustering,sources of elements in throughfall were explored usingvarious methods. We examined between-cluster dif-ferences in molar ratios (N/P, -N SO ,4 2

- -SO NO ,4 2 3 + -NH NO ,4 3 - -SO PO ,4 2 4 3 and)- -SO Cl4 2 using Kruskal–Wallis tests with post-hoc

Steel Dwaas comparisons and within-cluster Pearsoncorrelation coefficients among variables. Significancewas set at p

(figure 2(b)). Daily PM10 values ranged from194–886 μg m−3, with 15 d> 301 μg m−3.

3.2. Throughfall N, P, and SfluxesFrom July 2013 to September 2014, a total of 44throughfall events (i.e., weeks) were sampled. AverageVWM concentrations of all solutes measured inthroughfall over the study period were significantlygreater than in rainwater (p78% of total P in

throughfall, indicating that -PO4 3 –P is deposited pri-marily in dry form and/or that organic P is convertedto -PO4 3 –P after deposition (Tipping et al 2014).

Total throughfall -NO3 –N and +NH4 –N fluxeswere 117% and 110% higher (table 1), respectively,than bulk rainfall deposition of these ions (table S2).On average, mean DIN flux to soils was0.25 kg ha−1 wk−1. Differences between throughfallflux and bulk deposition were greater for DIN(5.8 kg ha−1/15mos) than for organic N (4.7 kg ha−1/15 mos), indicating higher dry deposition and canopyexchange of DIN than of organic N. However, organicN represented a greater fraction of total N thanDIN inboth bulk rainfall (69%, table S2) and throughfall flux(60%, table 1).

Total -SO4 2 –S flux was two-fold higher than totalN flux. Bulk rain S deposition was 31 kg ha−1/15mos,of which 45% was nss– -SO4 2 –S. An additional22 kg S ha−1/15 mos were delivered in throughfall,resulting in a total flux of 53 kg S ha−1 /15 mos topeatland soils (table 1). Assuming a similar proportionof nss– -SO4 2 –S in throughfall as in rainfall, 24 kgnss– -SO4 2 –S ha

−1/15moswere deposited to this site.

3.3. Contribution offires to nutrient loadingVWM concentrations and fluxes were calculatedseparately for the six post-fire throughfall events (14%of all 44 events) and normal events (table 1). Concen-trations of inorganic N and P in throughfall were most

Figure 2.Throughfall N, P, and Sfluxes (kg ha−1 wk−1) to a tropical forested peatland inWest Kalimantan, Indonesia from July 2013to September 2014. (a)Weekly throughfall waterflux (mm); (b)MODIS-detected fires within 20, 50, and 100 km radius bufferssurrounding the 12 ha forested peatland andmaximumweekly PM10 concentrations (grey line); (c) localfires (20 km from the site)andweekly -NO3 –N fluxes; (d) localfires andweekly -PO4 3 –Pfluxes; and, (e) localfires andweekly -SO4 2 –Sfluxes.Hatched barsindicate post-fire throughfall events (n=6; table S1). Horizontal dashed lines indicatemean fluxes over the sample period and blackarrows illustrate rain-driven enhancements in chemicalfluxes.

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Table 1.Volume-weightedmean (VWM) throughfall concentrations (mg l−1) and throughfall fluxes to soils for post-fire throughfall events (n=6, precedingweeks10fires within 20 kmbuffer followed by rain), normal events (n=38),and all events sampled from July 2013 to September 2014 in a rainfed forested peatland inWest Kalimantan, Indonesia. Contribution of post-fire and normal events to the total throughfallflux is shown over the entire 15 month samplingperiod.Means±SD are shown forVWM.Asterisk (*) indicates significant differences inVWMbetween post-fire throughfall and normal events (p

elevated after fire, whereas organic N concentrationswere similar between post-fire throughfall and allother events. Only -SO4 2 –S concentration was lower,on average, in post-fire throughfall compared to theother events.

Nineteen to 34% of the total throughfall chemicalflux over the 15 month periodwas accounted for by sixpost-fire events (table 1). During these events, over athird of ‘biologically-available’ nutrient depositionoccurred: -PO4 3 –P (31%), -NO3 –N (34%), and

+NH4 –N (29%). Post-fire events contributed 19% oftotal throughfall S and an estimated 19% of

- -nss SO4 2 –S flux.We also compared total N, total P, and S fluxes

sampled in throughfall during two water years (Octo-ber 2012–September 2013, October 2013–September

2014; text S2)with similar rainfall but a three-fold dif-ference in total annual fire counts 20 km from thesite (figure 3). Total P and -SO4 2 –S in throughfallwere two-fold higher, while total N was only slightlygreater, during the high-fire compared to the low-fireyear (figure 3). The slight increase in total N likelyreflects the prevalence of organic N in throughfall,whichwas little affected by fire (table 1).

3.4. Chemical signatures and sourcesCluster analysis discerned five unique throughfallevent clusters (table 2). Cluster 1 had the lowest

+ -NH NO4 3 ratio and the highest - -SO Cl4 2 ratio.Throughfall -SO4 2 and organic P concentrations weresignificantly and positively correlated (r=0.62,p=0.043), suggesting a biomass source. Cluster 1

Figure 3.Total N, total P, and -SO4 2 –Sfluxes in throughfall sampled during a high-fire (October 2012–September 2013) and low-fire(October 2013–September 2014) year in a forested peatland inWest Kalimantan, Indonesia. (a)Total throughfall waterflux(mm yr−1); (b) totalfire detections (20 kmof the site); (c) totalN flux (kg ha−1 yr−1); (d) total Pflux (kg ha−1yr−1); and, (e) total

-SO4 2 –S flux (kg ha−1yr−1).

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contained three of six post-fire throughfall events,with most samples collected during peak biomassburning months in Indonesia and northern Australia(July–September; figure 4). Backward trajectories con-firmed prevailing southeasterly winds that originatedin northern Australia and then passed over southernKalimantan before arriving at the focal site.

For Cluster 2, the mean - -SO NO4 2 3 ratio was asmuch as 14-fold higher compared to the other clusters(table 2), and nss– -SO4 2 comprised 79% of -SO4 2 inrainwater. Volcanic eruptions occurred in SoutheastAsia during five of seven throughfall events included inthis cluster (Global Volcanism Program 2013).

However, SO2mass over Java and western Borneo waselevated during only two of these events, which fol-lowed the eruption of Sangeang Api in May 2014,coinciding with southeasterly airflow (NASA’s OzoneMonitoring Instrument image database http://so2.gsfc.nasa.gov/). Cluster 2 also had the highest

+ -NH NO4 3 ratio, an indicator of an agricultural Nsource. These results suggest that volcanic and anthro-pogenic pollution as well as agriculture were the domi-nant sources inCluster 2 throughfall.

The - -SO Cl4 2 ratio in Cluster 3 was most simi-lar, albeit depleted, to that in rainwater, and -SO4 2

and Cl− concentrations showed a strong positive

Table 2.Meanmolar ratios in throughfall for five clusters of throughfall events (n) and bulk precipitation sampled from July2013 to September 2014 inWest Kalimantan, Indonesia. Letters indicate significant differences in ratios among clus-ters (p

correlation (r=0.74, p=0.024). Most events in thiscluster occurred during the wet season when north-easterly winds prevail. Regardless of wind direction,backward trajectories show that air masses lingeredover the ocean before their arrival at the focal site(figure 4), indicative of a dominant marine source inthroughfall.

Similar to Cluster 2, Cluster 4 had a mean+ -NH NO4 3 ratio>1 (table 2) and, as with Cluster 1,

throughfall -SO4 2 concentrations were strongly posi-tively correlated with organic P (r=0.72,p=0.0086). Sulfate concentrations were also corre-lated with -PO4 3 (r=0.75, p=0.0053). Cluster 4throughfall events were primarily influenced by south-easterly winds and otherwise variable winds in Jan-uary, April, and May (figure 4). All events weretemporally subsequent to two major fire pulses, sug-gesting a mixture of burning (Cluster 1), pollution,and agricultural sources (Cluster 2).

Cluster 5 contained two of six post-fire throughfallevents, including the largest post-fire event in March2014, and was characterized by a low - -SO Cl4 2 ratio.Throughfall events were affected by northeasterly andnorthwesterly winds, whereby air masses lingered overthe ocean. This cluster had the lowest - -SO PO4 2 4 3

ratio and distances traveled by the air masses werenotably less than the other clusters, suggesting thatthroughfall chemistry was influenced mainly by localbiomass burning rather than by long-range sources.

4.Discussion

4.1. Forested peatlands receive highN, P, and Sdeposition loadsAt our peatland site, the magnitude of N, P, and Sfluxes was remarkably high compared to other tropicalsystems where these fluxes have been measured(Ponette-González et al in press, Das et al 2011). Ofmajor significance, -PO4 3 –P flux in throughfall was7.9 kg ha−1yr−1. Such high fluxes rarely have beendocumented, and only within lowland tropical forest(e.g., 11 kg ha−1yr−1, Venezuela; Jordan et al 1980).However, bulk rainfall -PO4 3 –P deposition levels fellwithin the range of values for open field sites aroundthe world (e.g., 0.02–0.65 kg ha−1yr−1; Tippinget al 2014). Comparison of bulk rainfall(0.24 kg ha−1yr−1) and throughfall flux(7.9 kg ha−1 yr−1) indicates that the vast majority(97%) of inorganic P deposited to peatland soils wasthe result of dry deposition, a finding similar toobservations from mature Neotropical dry forests(Das et al 2011).

Although our measured throughfall flux of 8.7 kgDIN ha−1 yr−1 agrees well with modeled estimates oftotal (wet+fog+dry)DIN deposition for regions ofSoutheast Asia (4–10 kg N ha−1yr−1; Vetet al 2014a, 2014b), the proposed critical N load limitfor tropical forests is 5–10 kg N ha−1yr−1. Negative

effects on species diversity, soil N cycling, and stream-water are expected beyond this limit (Pardo et al 2011),especially on highly acidic peatland soils.

Total annual S flux to this peatland was also extra-ordinarily high, totaling 42 kg S ha−1 yr−1, of whichwe estimated 19 kg ha−1 yr−1 was nss– -SO4 2 –S.Assuming -SO4 2 in throughfall is a robust index oftotal (wet+fog+dry) atmospheric deposition inthis peatland site (Weathers et al 2006, Ponette-Gon-zález et al 2010a), our sample site ranks among theregions with the highest predicted total S depositionglobally (i.e., East Asia 20–50 kg S ha−1 yr−1; Vetet al 2014a, 2014b). The S flux reported here is com-parable to estimates for forests located in SouthernChina’s ‘urban acid islands’, areas

rainwater and aerosol P (Maenhaut et al 2002,Mahowald et al 2008). During major Indonesian bio-mass burning periods in 1997 and 2006, rainwatersampled in Singapore contained elevated concentra-tions of -NO ,3 +NH ,4 and -PO4 3 compared withnon-biomass burning periods (Balasubramanianet al 1999, Sundarambal et al 2010). Hsu et al (2014)collected aerosols downwind of the June 2013 peatfires in Riau Province; concentrations of total andsoluble P in these samples were four-fold higher thanin aerosol samples influenced by East Asian dust andpollution outflows.

While we cannot evaluate the relative contributionof biomass burning versus dust to our measured Pfluxes, we note that soil dust may contribute to P load-ing during fire events. Three potential sources include:(1) dust emissions from biomass burning (Andreaeet al 1998); (2) long-range dust transport from arid andsemi-arid regions in East Asia (Lin et al 2007); and, (3)long-range dust transport from Australia (McGowanand Clark 2008). We are unaware of any study that hasquantified the contribution of fires to atmosphericmineral dust emissions in Indonesia. Dust outflowsfrom East Asia are most prevalent during the north-easterly monsoon (December–March), when only oneof the fire events in this study occurred, and theirinfluence on the southern South China Sea is thoughtto be minimal (Lin et al 2007). However, dust trans-port from Lake Eyre, Australia, to Borneo (June–August) is coincident with the peak biomass burningperiod in Southeast Asia (McGowan and Clark 2008).Therefore, it is possible that some of the P we mea-sured in post-fire throughfall was derived fromdust.

Because several studies report elevated -SO4 2 inrainwater and aerosol during peat fires (Ikegamiet al 2001, See et al 2007), we were surprised to findthat S concentrations were not enhanced in post-firethroughfall events. Two potential explanationsinclude high local or regional S emissions from non-peat sources, and variability in fire emissions chem-istry. First, fossil fuel and volcanic S emissions mayhave masked the influence of peat burning on atmo-spheric S fluxes. In Pontianak, the provincial capital ofWest Kalimantan (23 km from site), oil combustionwas found to be the most important source influen-cing aerosol chemistry (Maenhaut et al 2002). Second,atmospheric emissions from flaming and smolderingfires differ in their chemical composition (Lobertet al 1990). In this study, we used MODIS Active Firedata that cannot distinguish between flaming andsmoldering fires (Elvidge et al 2015). Smoldering firesare the fire type associated with high S emissions.Therefore, our correlations between fire detectionsand throughfall S concentrations possibly reflect thechemistry offlaming rather than smoldering fires.

Around our study site, land use––particularlyindustrial-scale agriculture––was a dominant driver ofpeatland burning. Fires were concentrated within oilpalm leases on peatlands within 20 km (78% of fires

versus 49% of land area) and 100 km (40% of fires ver-sus 19% of land area) of the focal site. Yet, our datasuggest that long-range transport of fire emissionsfrom southern Kalimantan and northern Australiaalso influenced deposition at this site (figure 4). Landclearing and burning for oil palm plantation develop-ment are the dominant fire source not only in ourstudy region, but also across Kalimantan and Sumatra(Curran et al 2004, Miettinen et al 2011b, Carlsonet al 2012, 2013, Gaveau et al 2014, Marlier et al 2015).Atmospheric emissions from these activities are pro-jected to increase over the next decade (Carlsonet al 2012, 2013,Miettinen et al 2012).

The enhanced N, P, and S fluxes we documentedin associationwith agricultural peatfires during a non-ENSO year indicate that emissions from larger andmore severe fires typical of strong ENSO years (figureS2) could result in dramatically increased depositionto adjacent ecosystems. Our results showed that athree-fold increase in detected fires resulted in a two-fold increase in annual total P and S flux to peatlandsoils (figure 3). We also captured the influence of asevere fire pulse from January toMarch 2014 (196 fires20 km from the site) on throughfall fluxes (table S1).Following this single fire event, 13% of DIN, 16% of

-PO4 3 –P, and 8% of -SO4 2 –S measured over thestudy period was delivered to peatland soils asthroughfall flux (figure 2).

5. Conclusions

In this study, we show that biomass burning in tropicalpeatlands plays a largely unrecognized yet critical rolein the redistribution ofmajor limiting nutrients withintropical landscapes. Our estimates indicate that asmuch as 30% or more of the annual load of inorganicN and P to Bornean peatlands occurred following fireevents and that fire-emission-deposition of S couldrival that of industrial pollution during high-firestrong ENSO years. As such, biomass fires may havesignificant fertilizing or polluting effects on recipientecosystems, thereby altering the contribution of peat-lands to climate change (Frolking et al 2011). Whilethe ecosystem effects of biomass burning depositionremain uncertain in tropical regions, the sheer magni-tude of the fluxes measured here indicate that fire-driven nutrient redistribution may have unforeseenconsequences across a diversity of ecosystems, includ-ing forested peatlands as well as agroecosystems,wetlands and streams, oligotrophic ocean waters, andcoral reefs.

Acknowledgments

We thank the Indonesian Institute of Sciences Limnol-ogy Laboratory in Bogor for their assistance withchemical analysis and Badan Lingkungan HidupPontianak for providing PM10 data. The authors also

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gratefully acknowledge the NOAA Air ResourcesLaboratory (ARL) for the provision of the HYSPLITtransport and dispersion model and/or READYwebsite (http://www.ready.noaa.gov) used in thispublication. We appreciate D Astiani and her studentsin the Department of Forestry & Agricultural Sciences,University of Tanjungpura, Pontianak, West Kali-mantan, for previous fieldwork and soil and vegetationanalyses. N Lisnawati, Y Purwanto, Ruspita, Supardi,WI Suci, ARohman, RHartono, UAbdurrahman, andD Firnata provided invaluable support. LMC andAPGgratefully acknowledge financial support fromtheNASA Land Cover/Land-Use Change Program (#NNX11AF08G). Previous NASA support to LMC andteams (NNG05GB51G, NNX07AK37H) providedresources for the establishment andmonitoring of thispeatland site with surrounding land use. LMC receivedcritical support from the John D and Catherine TMacArthur Foundation, Santa Fe Institute, StanfordUniversity, and YaleUniversity.

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1. Introduction2. Methods2.1. Study site and climate2.2. Fire, land use, and air quality observations2.3. Throughfall chemical fluxes2.4. Chemical signatures and sources

3. Results3.1. Dry periods and fire3.2. Throughfall N, P, and S fluxes3.3. Contribution of fires to nutrient loading3.4. Chemical signatures and sources

4. Discussion4.1. Forested peatlands receive high N, P, and S deposition loads4.2. Fire drives the redistribution of N, P, and S

5. ConclusionsAcknowledgmentsReferences