Interciencia ISSN: 0378-1844 [email protected] Asociación Interciencia Venezuela

Salimon, Cleber I.; Foster Brown, I. Secondary forests in western amazonia: significant sinks for carbon released from deforestation? Interciencia, vol. 25, núm. 4, julio, 2000, pp. 198-202 Asociación Interciencia Caracas, Venezuela

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SECONDARY FORESTS IN WESTERN AMAZONIA: SIGNIFICANT SINKS FOR CARBON RELEASED FROM DEFORESTATION?

Cleber I. Salimon and I. Foster Brown

SUMMARY

Conversion of tropical forests into pastures and years, but demonstrates the decreasing rate of carbon uptake with have serious impacts on carbon allocation in ecosystems and re- time, consistent with the classical pattern of secondary succession leases carbon to the atmosphere, mainly as CO2 and CH4. The where slower growing tree species replace fast growing pioneer abandonment of these land uses typically results in secondary forest species. The time necessary to achieve half of primary forest biom- growth which can serve as a sink for carbon. To determine the ass is about 30 years. Recent estimates of deforestation of relative importance of this sink for the western portion of ’s range between 40,000 and 50,000 ha yr-1. For secondary forests to arc of deforestation in Acre State, we measured the aboveground compensate for the carbon loss from deforestation, 80,000 to biomass of six secondary forests (51 to 136 Mg/ha) with ages rang- 100,000 ha would need to be abandoned each year and left to ac- ing from 6 to 35 years. Based on a negative exponential model, the cumulate carbon for 30 years. Pilot studies indicate that secondary biomass accumulation into secondary forests in Acre is approxi- forests are not increasing but rather decreasing in area in Acre. mately 6.2*e-0.025t Mg ha-1 yr-1 , of which approximately 50 percent is Consequently, secondary forests in Acre are insignificant sinks of carbon. This simple model underestimates rates during the first ten carbon at present.

Introduction portant effects on the atmos- moval of carbon from the at- the landscape into a signficant phere (Houghton, 1993). One mosphere when vegetation of sink of carbon (Foody and The amount of organic car- such change is the conversion high biomass replaces one of Curran, 1994; Lucas et al., bon stored in terrestrial eco- of high biomass forests to low low biomass; such is the case 1998) or not (Salomão et al., systems is about three times biomass pastures and agricul- when secondary forests, also 1996; Fearnside and Guima- greater than that of gaseous tural plots which releases car- known as regenerating forests, rães, 1996; Alves et al., 1997). carbon in the atmosphere. Be- bon mainly in the form of CO2 fallows, or capoeiras, grow on Since these secondary forests cause the carbon exchange be- from the biomass to the atmos- abandoned pastures and agri- show rapid increase in biom- tween the biosphere and at- phere (Woodwell, 1984; IPCC, cultural plots. ass, they rapidly absorb atmos- mosphere is large, changes in 1992). Land use change, how- Opinions diverge whether pheric carbon. On the other terrestrial ecosystems have im- ever, can also result in the re- secondary forests could turn hand, the amount absorbed by

KEY WORDS / Secondary Forests / Western Amazonia / Carbon Sinks / Growth rate /

Cleber Ibraim Salimon. Biological Nuclear na Agricultura/USP, Foster Brown. Senior scientist, University of Acre, Rio Sciences degree, Universidade Piracicaba, SP, Brazil. Address: Hole Research Center, Branco, AC, Brazil. Address: Estadual de , PR, Bra- Centro de Energia Nuclear na Woods Hole, MA, USA. Part- The Woods Hole Research zil. Masters in /Plant Agricultura/USP; Laboratório time professor, Geochemistry Center, PO Box 296, Woods Ecology, Universidade Federal de Ecologia Isotópica - Av. Department, Federal Flumin- Hole, MA 02543 USA; Depto. do Paraná, , PR, Brazil. Centenário, 303, C.P. 96, CEP ense University, Niteroi, RJ, De Geoquímica, Universidade Currently on a PhD pro- 13400-970, Piracicaba, SP, Brazil. Adjunct researcher, Federal Fluminense, Niteroi, gramme, Centro de Energia BR. Zoobotanical Park. Federal RJ, Brazil

198 0378-1844/00/04/198-05 $ 3.00/0 JUL 2000, VOL. 25 Nº 4 RESUMEN

La conversión de bosques tropicales en pastizales y para la años, pero demuestra la tasa decreciente de captación de carbón agricultura causa serios impactos en la distribución de carbón en en el tiempo, consistente con el patrón clásico de sucesión secunda- los ecosistemas y libera carbón a la atmósfera, principalmente ria en que las especies de árboles de crecimiento más lento reem- como CO2 y CH4. El abandono de estos usos de la tierra típica- plazan a las especies pioneras de rápido crecimiento. El tiempo mente resulta en crecimiento de bosque secundario que puede ser- necesario para alcanzar la mitad de la biomasa primaria es de vir como sumidero de carbón. Para determinar la importancia re- alrededor de 30 años. Estimaciones recientes de la desforestación lativa de este sumidero para la parte occidental del arco de de Acre van de 40.000 a 50.000 ha año-1. Para que los bosques desforestación del Estado de Acre, Brasil, medimos la biomasa secundarios compensen el carbón perdido por la desforestación aérea de seis bosques secundarios (51 a 136 Mg/ha) con edades sería necesario abandonar cada año 80.000 a 100.000 ha y dejar entre 6 y 35 años. Con base en un modelo exponencial negativo, la que acumulen carbón por 30 años. Estudios pilotos indican que los acumulación de biomasa en bosques secundarios en Acre es bosques secundarios no están aumentando, sino que su área dismi- aproximadamente 6,2*e-0.025t Mg ha-1 año-1, de lo cual 50% es car- nuye en Acre. En consecuencia, los bosques secundarios son sumi- bón. Este modelo sencillo subestima tasas durante los primeros 19 deros de carbón insignificantes en el presente en Acre.

RESUMO

A conversão de bosques tropicais em pastos e para a demonstra a taxa decrescente de captação de carvão no tempo, agricultura causa sérios impactos na distribuição de carvão nos consistente com o patrão clássico de sucessão secundária em que ecossistemas e libera carvão à atmosfera, principalmente como as espécies de árvores de crescimento mais lento substitui às

CO2 y CH4. O abandono destes usos da terra tipicamente resulta espécies pioneiras de rápido crescimento. O tempo necessário para em crescimento de bosque secundário que pode servir como alcançar a metade da biomassa primária é de ao redor de 30 sumidouro de carvão. Para determinar a importância relativa deste anos. Estimações recentes da devastação do Acre vão de 40.000 a sumidouro para a parte ocidental do arco de devastação do Estado 50.000 hectares ano-1. Para que os bosques secundários do Acre, Brasil, medimos a biomassa aérea de seis bosques compensem o carvão perdido pela devastação seria necessário secundários (51 a 136 Mg/ha) com idades entre 6 e 35 anos. Com abandonar cada ano 80.000 a 100.000 hectares e deixar que base em um modelo exponencial negativo, a acumulação de acumulem carvão por 30 anos. Estudos pilotos indicam que os biomassa em bosques secundários no Acre é aproximadamente bosques secundários não estão aumentando, senão que sua área 6,2*e-0.025t Mg hectares-1 ano-1, do qual 50% é carvão Este modelo diminui em Acre. Em conseqüência, os bosques secundários são simple subestima taxas durante os primeiros 19 anos, mas sumidouros de carvão insignificantes neste momento no Acre.

these forests slows down as of its capital, Rio Branco, and two study areas of the Univer- (RADAMBRASIL, 1976; Es- succession proceeds (Fearn- there is also a dynamic shift- sidade Federal do Acre. The tação Meteorológica da side, 1996), and the time ing and/or abandonment of first is the Parque Zoobotânico- UFAC, personal information, needed for a secondary forest pastures. PZ (09o57’S; 67o52’W), a 1998): is classified as to achieve the biomass of an To help understand the former rubbertapper area that Am (Köpen); mean tempera- intact forest could be more process of carbon storage in was turned into small farms for ture in 1997 was 26oC; rainfall than 190 years (Saldarriaga et the biomass of secondary for- pasture and that ranges between 1750 and al.,1988). In some regions the ests in Western Amazonia, the were abandoned over the last 2000 mm with a well defined secondary forests are cut down present study addresses the decades. The area is now occu- dry season of three months for small cultivation, impeding following questions: (1) What pied by secondary forests of with less than 60 days with no their accumulation of carbon is the standing alive above- different ages. The second rain; and relative humidity of (Fearnside & Guimarães, ground biomass in secondary area is the Fazenda Experi- 85% (annual mean). The soils 1996; Salomão et al., 1996). forests of different ages in the mental Catuaba (10o04’S; are classified as dystrophic The State of Acre lies at vicinity of Rio Branco, Acre? 67o37’W), a test area for eco- and eutrophic Ultisols with oc- the western end of the defor- (2) What rates best describe logical research that is located casional patches of Oxisols. estation arc where most of the their annual biomass accumu- about 25 km from Rio land conversion has occurred lation? (3) Is the landscape a Branco. The dominant vegeta- Methods in Brazilian Amazonia. While net sink of carbon because of tion of Catuaba is Open Om- the area deforested in Acre is these secondary forests? brophilous Forest with Bam- Former and current occu- relatively small at present, it boo (Veloso et al., 1991) pants were interviewed about is a site of current develop- Study Area with patches of Dense Om- the ages of each site. They ment and if trends continue brophilous Forest. Within this can usually relate the aban- as in Rondonia and Mato Rio Branco, the capital of primary forest matrix a few donment of the site with Grosso - where the rates of the state of Acre, Brazil (Fig- areas are used as small pas- some historic events, –death deforestation in 1995-96 were ure 1), was chosen for the tures and plantations which and birth of relatives and so 2,400 and 6,500 km2 year-1, present study because the land- are abandoned usually after 1 on– which is sufficient to de- respectively (INPE, 1998) — cover/land-use changes in the to 2 years of cultivation. De- fine chronosequences that can significant areas will be de- surrounding areas have oc- scription of each study site is elucidate the successional pro- forested. There is already a curred in patterns similar to given in Table I. cesses and the accumulation considerable landcover and those in Rondonia and Mato In both areas the following of biomass. We estimate the landuse change in the vicinity Grosso. Data were collected in climate conditions prevail uncertainty in ages is ±1 year

JUL 2000, VOL. 25 Nº 4 199 for forests less than 15 years TABLE I old and ±5 years for forests AREA AND TYPES OF DATA COLLECTED IN EACH SITE STUDIED IN PRESERVED AREAS more than 20 years old. OF THE UNIVERSIDADE FEDERAL DO ACRE, RIO BRANCO, ACRE In all sites tree height and DBH (stem diameter at breast height) data were collected. Site Location Area Minimum DBH Method Tree height was measured (ha) (cm) with a 6 m rod used as a C6 Fazenda Experimental Catuaba 0.4 2.5 10 20 x 20m plots ruler; DBH was measured C71 Parque Zoobotânico 0.12 5 24 10 x 5m plots with a caliper or a diameter tape; see Table I for details. C122 Parque Zoobotânico 0.145 3 29 10 x 5m plots Standing alive aboveground C303 Parque Zoobotânico 0.25 5 50 10 x 5m plots biomass (SAAB) was calcu- lated for each tree using the C354 Parque Zoobotânico 0.3 5 60 10 x 5m plots allometric equations developed by Uhl et al. (1988), one spe- 1,3-RODRIGUES (1996); 2- FRANÇA (1994); 4- MORAES (1994); cific for the genus Cecropia experiments made by college students during internship. and another one for all the other secondary forest species. The result is given in dry duct some of the experiments TABLE II weight and does not include at the time, they were not in- DENSITY, STANDING ALIVE ABOVEGROUND belowground biomass. The cluded in the present study. BIOMASS - SAAB, AND AVERAGE LINEAR GROWTH unit used here is Mg (mega- RATE FOR THE 5 SITES STUDIED grams), which is equivalent to Results and Discussion metric tons. SITE–AGE DENSITY SAAB Mg*ha-1 AVERAGE Nelson et al. (in press) dis- The biomass increased as (years) individuals.ha-1 GROWTH RATE cusses the influence of differ- expected with the time of Mg*ha-1.year-1 ent allometric equations on abandonment, with the excep- the variability of results, also tion of the last two dates C6 1767 51 8.6 showing that when data to be (Table II). Site C30 had the C7 792 64 9.1 processed are beyond the highest value – 136 Mg*ha-1, C12 1565 75 6.2 range of values used to create higher than the older site C30 1572 136 4.5 the equation, the error be- C35. This may be due to (1) C35 1543 121 3.4 comes greater. Based on these inclusion of a few large re- assumptions and on data by maining individuals of the old Brown et al., 1995, we as- intact forest in C30, (2) sam- would be one that has high stant in years-1. The SAAB of sume the uncertainty of these pling variability (~20%), or initial growth rates and then intact ombrophilous open for- measurements is probably in (3) differing previous land- decreases to zero (or close to est in Acre varies between the range of 20%. use: more intensively used it) as the total forest biomass 250 and 320 Mg*ha-1 (Brown Two other fractions of the land will have a slower soil is reached. One of the sim- et al., 1992; Brown, 1997, carbon stock in an ecosystem recovery and thus slower plest of such models and the unpublished data). Using a are belowground biomass and growth rate (Buschbacher et most mathematically tractable value of 250 Mg*ha-1 for ma- soil organic matter. Although al.,1988; Salomão et al., is equation (1): ture forest SAAB, the only not measured in the present 1996). parameter remaining to esti- study, underground biomass Secondary succession typi- B(t)=B(t large) *(1- e-at), (1) is α. Curvilinear regres- plays an important role in the cally occurs in waves where sion of our data gave a best- carbon cycle, mainly where pioneer species quickly colo- where B(t) is the fit value of 0.025 year-1 for α. watertable is in great depth. nize a site and then are SAAB*ha-1 at year t, B(t Equation (1) now becomes Soil organic matter, although gradually replaced by more large) is mature forest SAAB a significant reservoir of car- shade tolerant, slower grow- ha-1, and α is the rate con- B(t)=250*(1-e-0.025t) in Mg*ha-1 bon, has a low C:N ratio ing tree species. The net ef- (2) compared to woody tissues, fect of this succession pattern and a great part of this pool is a non-linear response for TABLE III is fast cycling (Townsend et forest biomass as a function COMPARISON OF LINEAR RATES BASED ON EXTRAPO- al., 1996); besides, differences of time. The average growth LATING TO THE PAST WITH INSTANTANEOUS RATES are observed in soil organic rates reported in Table II are BASED ON EQUATION (3) matter content after land deceiving because they do not cover and land use changes represent the instantaneous (Camargo et al., 1999). growth rate, nor can they be Years of Linear rate, Instantaneous rate, The analysis of these two extrapolated forward in time. abandonment Mg*ha-1 yr-1 Mg*ha-1 yr-1 compartments is of paramount They are calculated from age importance for the under- zero when biomass is zero to 6 8.6 5.4 standing of the carbon cycle the site age, assuming a linear 7 9.1 5.2 in a changing environment. growth rate. A more realistic 12 6.2 4.6 Although, due to the lack of model for biomass accumula- 30 4.5 3.0 data and infrastructure to con- tion by secondary growth 35 3.4 2.6

200 JUL 2000, VOL. 25 Nº 4 The inverse of α is the characteristic time of the ma- ture forest, which for this case is 40 years. The use of higher mature forest SAAB results in longer characteristic times, such as nearly 65 years for 320 Mg*ha-1. For the case of 250 Mg*ha-1, the time nec- essary for the forest to reach half of its mature biomass is nearly 30 years (= 0.69 x characteristic time, Figure 2). 50 km To determine the instanta- Radius neous rates of biomass accu- º mulation from equation (2) it requires a simple differentia- tion with respect to time: dB(t)/dt=250*0.025*e-0.025t = 6.2*e-0.025t in Mg*ha-1 yr-1 (3) º

This equation can then be used to compare instantaneous rates with those estimated by linear interpolation in Table III. Figure 1 – Rio Branco, AC and the 50 km radius where deforestation has occurred intensely in the last 2 decades. For biomass accumulations that follow a negative expo- tant to take into account the 250 50% mature forest biomass nential pattern, the linear rates “ups and downs” of economy. are typically greater than in- For instance, during the early 200 stantaneous rates. This means years of the “Plano Real” –the

that using linear rates for ex- macro-economical plan for sta- trapolating into the future bilizing Brazilian economy– 150 greatly overestimates carbon Brazil experienced an increase sequestration of secondary for- in Amazonian deforestation, 100 ests. For example, extrapolat- mainly in 1993 to 1995 ing the linear rate of the 6- (INPE, 1998). After that pe- year old secondary forest sug- riod, there was a decrease in 50 gests that biomass of a mature deforestation, corresponding to SAAB, Mg/ ha forest is reached in less than a greater uncertainty in the na- 0 30 years (250 Mg*ha-1 /8.6 tional economy – which still 0 15 30 45 60 75 90 Mg*ha-1 yr-1), a pattern ne- prevails in this country today. gated by the data of Table II, Nevertheless, economic pro- Years since abandonment which shows a tendency for gression always leads to re- Figure 2. Standing Alive Aboveground Biomass (SAAB) of secondary for- biomass to reach only half of sources exploitation (Rees, ests as a function of years since abandonment. Error bars show estimated that value. 1999), and even if the defores- 20 percent uncertainty in SAAB measurements. Curved line is best fit for Figure 2 also shows that the tation rates are lower than a the equation SAAB = 250*(1-exp(-0.025 t)) in Mg*ha-1. The independent regression made tends to un- few years ago, they still have variable, t, is in years. Dotted line at 125 Mg*ha-1 indicates that half the derestimate the biomass of an important impact on natural mature forest biomass is reached in less than 30 years. The study sites are secondary forests younger than ecosystems. near Rio Branco, Acre, Brazil. Full circles with error bars: data from ~15 years old, because the Bearing the former assump- present study; triangles: data extracted from Fearnside and Guimarães (1996); crosses: data from Alves et al. (1997). data for secondary forests until tions in mind, we can use the this age are all plotted above data presented for these sec- the curve-data from present ondary forests in the Rio 1996 it was 433 km2.year-1. tons of aboveground live bio- study, Fearnside and Gui- Branco region to determine if By August 1996 a total of mass will be burned or begin marães (1996) and Alves et al. they could significantly reduce 13,742 km2 had been defor- to decay each year – which

(1997). net fluxes of CO2 to the at- ested in Acre, about 9% of the can be classified as commit- mosphere. According to INPE total area of the state. ted net emission (see Fearn- Role of Secondary (1998), the deforested area in Let us consider the smallest side, 1996)). About 5.4 mil- Forests as a Sink the state of Acre was 2,500 values for our calculations, lion tons of carbon (~50% of of Carbon km2 in 1978. In 1994 the de- 250 Mg*ha-1 the biomass of the total biomass is made out forested area expanded to an intact forest and a defor- of carbon [Carvalho et al., When analyzing trends in 12,064 km2. The deforesting esting rate of 433 km2.year-1, 1995]) will be released to the deforestation and the role of rate between 1994-95 was for an optimistic scenario. atmosphere mainly as CO2 secondary forests, it is impor- 1,208 km2.year-1 and for 1995- This means that 10.8 million and CH4. The decay and

JUL 2000, VOL. 25 Nº 4 201 reburning paths would not be would need to be abandoned. Buschbacher R, Uhl C, Serrão EAS Lucas RM, Honzák M, Amaral I, instantaneous and is currently Considering that the human (1988) Abandoned pastures in Curran P, Foody GM, Amaral Eastern Amazonia. II- Nutrient S (1998) Avaliação da compo- being studied under several population is growing in Acre stocks in the soil and vegeta- sição florística, biomassa e es- aspects, such as CO2 ex- State, which would tend to in- tion. Journal of Ecology 76: trutura de florestas tropicais change between soil and at- crease use of converted lands, 682-699. em regeneração: a contribuição mosphere and carbon stocks we conclude that these sec- Camargo PB, Trumbore SE, do sensoriamento remoto. In: in soils (Davidson, et al., ondary forests around Rio Martinelli LA, Davidson EA, Gascon C, Moutinho P (eds.) Floresta Amazônica: dinâmica, 1995; Trumbore et al., 1995; Branco, Acre, and other simi- Nepstad DC, Victoria RL (1999) Soil carbon dynamics regeneração e manejo. MCT/ Fearnside and Barbosa, 1998). lar areas of the western Ama- in regrowing forest of eastern INPA, – AM. Therefore, if we wished to zonian frontier, cannot act as Amazonia. Global Change Bi- Moraes RRM (1994) Levantamento nullify the atmospheric alter- effective carbon sinks on a ology 5: 693-702. florístico e fitossociológico em ations caused by each 433 landscape level. This conclu- Carvalho Jr, Santos Ja JN, Santos cronossequências no Parque km2 deforested per year sion is corroborated by Alves MM, Leitão M, Higuchi N Zoobotânico da Universidade (1995) A tropical rain forest Federal do Acre. Relatório fi- through secondary forests re- et al. (1997) and by Fearnside nal de bolsa PIBIC-UFAC/ 2 clearing experiment by biom- generation, about 800 km of and Guimarães (1996) for Bra- ass burning in a Manaus re- CNPq. Universidade Federal anthropogenic land should be zilian Amazonia. gion. Atmospheric Environment do Acre, Rio Branco, Acre. abandoned for 30 years, which Although they are not effec- 29(17): 2301-2309. RADAMBRASIL (1976) Folha is the time necessary for the tive carbon sinks, secondary Fearnside PM (1996) Amazonian SC.19 Rio Branco. Ministério forest to reach half of its ma- forests are extremely important deforestation and global warm- das Minas e Energia, Departa- ing: carbon stocks in vegeta- mento Nacional da Produção ture biomass. tools for conservation in Ama- Mineral, Projeto RADAM 2 tion replacing Brazil´s Amazon If this rate of 433 km of zonian landscapes, serving to forest. Forest Ecology and BRASIL, , RJ. annual deforestation lasted for restore riparian forests, animal Management 80: 21-34. Rees W E (1999) Consuming the another ten years from 1997, refuges, corridors for gene Fearnside PM, Barbosa RI (1998) earth: the biophysics of sus- this would imply an abandon- flow among isolated fragments Soil carbon changes from con- tainability. Ecological Econom- ment of 8,000 km2 of anthro- of intact forests (although still version of forest to pasture in ics 29: 23-27. pogenic areas supposedly pro- a controversial issue), regional the Brazilian Amazon. Forest Rodrigues AR, Silveira M, Brown Ecology and Management 108: IF (1996) Acúmulo de bio- ductive for 30 years- which hydrological cycles, and al- 147-166. massa em florestas secundárias would be 45% of the total de- tered biogeochemical cycles. Fearnside PM, Guimarães WM no Acre. Relatório de bolsa forested area by 2007. Add (1996) Carbon uptake by sec- PIBIC-UFAC/CNPq. Universi- another 10 years of the same ACKNOWLEDGEMENTS ondary forests in Brazilian dade Federal do Acre, Rio annual deforestation rate and Amazonia. Forest Ecology and Branco, Acre. the area of secondary forests This research was supported Management 80: 35-46. Saldarriaga JS, West DC, Tharp by CNPq, Woods Hole Re- Foody Gile M, Curran PJ (1994) ML, Uhl C (1988) Long-term needed would be 70% of the chronosequence of forest suc- search Center and Universida- Estimation of tropical forest total deforested area by that extent and regenerative stage cession in the upper Rio Negro time. de Federal do Acre. We would using remotely sensed data. of Colombia and Venezuela. Other studies suggest that also like to thank Marcos Journal of Biogeography 21: Journal of Ecology 76: 938- the area of abandoned land Silveira, Átila Oliveira, 223-244. 958. seems to increase over time, Vanilce França, Shirlene França VM (1994) Levantamento Salomão RP, Nepstad DC, Vieira Rodrigues and Nara R. M. de florístico e fitossociológico em ICG (1996) Como a biomassa probably due to land degrada- de florestas tropicais influi no Moraes, who participated in cronosseqüências no Parque tion and social and economical Zoobotânico da Universidade efeito estufa? Ciência Hoje factors (i.e. Skole et al., 1994, the field work; and E. Federal do Acre. Relatório 21(123): 38-47. Mausel et al., 1993). Never- Davidson for comments on the bolsa PIBIC-UFAC/CNPq. Townsend AR, Braswell BH, Hol- theless, preliminary time series paper. Universidade Federal do Acre, land EA, Penner JE (1996) analysis of Landsat images Rio Branco, Acre. Spatial and temporal patterns Davidson E, Ågren G, Daniel O, in terrestrial carbon storage shows that secondary forests due to deposition of fossil fuel REFERENCES Emeis C K-C, Largeau C, Lee near Rio Branco are relatively C, Mopper K, Oades JM, nitrogen. 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