atmospheric deposition and forest nutrient...

of Florida (1990)LJ. Livingston

Tropical Biota: Ecosystemnd Global Challenges (1990)G. Goldammer

-Cycle Concept of Ecosystems

[. Remmert

leterogeneity (1991)Koiasa and S.T.A. Pickett

Grasses; The NutritionalCquids, and Their Impact onue (1991)in

nd El Nino: Responses toital Stress (1991). Trillmich and K.A. Ono

Multidisciplinary Study

J.C. Kuiper and M. Bos

istry of a SubalpineLoch Vale Watershed (1992)II Baron

Deposition andent Cycling.W. Johnson and

Dale W. JohnsonSteven E. Lindberg

Editors

Atmospheric Deposition andForest Nutrient Cycling

A Synthesis ofthe Integrated Forest Study

With 236 Illustrations

Springer-VerlagNew York Berlin Heidelberg London Paris

Tokyo Hong Kong Barcelona Budapest

Dale W. JohnsonDesert Research InstituteUniversity of Nevada SystemBiological Sciences CenterReno, NV 89506 USAandRange, Wildlife and ForestryCollege of AgricultureUniversity of Nevada, RenoReno, NV 89512 USA

Steven E. LindbergOak Ridge National LaboratoryEnvironmental Sciences DivisionOak Ridge, TN 37831 USA

Library of Congress Cataloging-in-Publication DataAtmospheric deposition and forest nutrient cycling : a synthesis of the integrated forest

study / Dale W. Johnson, Steven E. Lindberg, editors,p. cm. — (Ecological studies)

Includes bibliographical references and index.ISBN 0-387-97632-9 (alk. paper)1. Forest ecology. 2. Acid deposition—Environmental aspects.

3. Mineral cycle (Biogeochemistry) I. Johnson, D.W. (Dale W.),1946- . H. Lindberg, Steven E. HI. Series.QH541.5.F6A85 1992581.5'2642—dc20 91-20122

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Ecological StudiesAnalysis and Synthesis

Edited by

W.D. Billings, Durham (USA) F. Golley, Athens (USA)

O.L. Lange, Wurzburg (FRG) J.S. Olson, Oak Ridge (USA)

H. Remmert, Marburg (FRG)

Volume 91

166 Atmospheric Deposition and Forest Nutrient Cycling

elevation sites ST and WF, which received 1900 and 1100 mol ha ' yr ',respectively, of total N deposition.

The net canopy exchange of both NO3~ and NH4+ was negative (implying

canopy uptake), but the NCE of organic N was always positive. InorganicN in throughfall and stemflow (TF H- SF) was about 60% of the inorganicN deposited to the canopy, suggesting that the canopy retained or trans-formed the other 40%. Total N (organic + inorganic) in TF + SF was about84% of total N deposition. Canopy uptake of N ranged up to 531 mol ha~ l

yr"1 but was generally small compared to the N requirements of the forests.

Experimental Laboratory Measurements of Reactive N GasDeposition to Forest Landscape Surfaces: Biological and

Environmental ControlsP.J. Hanson, G.E. Taylor, Jr., and J. Vose

Introduction

As discussed earlier, natural or anthropogenically produced oxides of N oc-cur in the atmosphere in various forms including nitric oxide (NO), N diox-ide (NO2), and nitric acid vapor (HNO3). Ambient concentrations of theseoxides reflect a balance between natural or anthropogenic emissions, at-mospheric chemical cycling with ozone, and deposition to landscape sur-faces (Russel et al. 1985; Finlayson-Pitts and Pitts 1986). Nitrogen dioxideis typically the most concentrated form of atmospheric N oxides rangingfrom 1 to 2 nl L~l in rural (pristine) areas, 5 to 10 nl L~' in suburban areas,and as much as 50 nl L"1 in highly polluted urban/industrial regions (EPA1982; Bytnerowicz et al. 1987). However, HNO3, with concentrations com-monly in the range from 0.5 to 1 nl L~', is the principal chemical sink forremoval of NOX from the atmosphere (Cadle et al. 1982; Galbally and Roy1983).

Oxides of N can be harmful to plants, depending on a variety of biologicaland environmental factors. However, the threshold concentrations of NO2

known to directly impede plant physiological responses are seldom attainedunder ambient conditions (Hill and Bennett 1970; Furukawa and Totsuka

NO2 and HNO3, have also been hypothesized to contribute to forest declinein several ways: disrupting the normal development of winter hardiness(Nihlgard 1985; Waring 1987), creating nutrient imbalances (Schulze 1989),and increasing shoot:root ratios (McLaughlin 1983). Conversely, as a sourceof N for plant growth, atmospheric oxides of N have been shown to con-tribute N for amino acid production within leaves (Rogers et al. 1979a; Kajiet al. 1980), to enhance the nutrition of plants growing under conditions oflow fertility (Yoneyama and Sasakawa 1979), and to increase the activityof N-assimilating enzyme systems (Norby et al. 1989).

rient Cycling

and 1100 mol ha"1 y r ~ l ,

, was negative (implyingIways positive. Inorganicout 60% of the inorganic:anopy retained or trans-ic) in TF + SF was aboutmged up to 531 mol ha~'quirements of the forests.

of Reactive N Gases: Biological andIsj J. Vose

produced oxides of N oc-itric oxide (NO), N diox-t concentrations of theseropogenic emissions, at-osition to landscape sur-1986). Nitrogen dioxide

phenc N oxides rangingnl L ~ ' in suburban areas,/industrial regions (EPAwith concentrations com-mcipal chemical sink for1982: Galbally and Roy

on a variety of biologicald concentrations of NO3

mses are seldom attained; Furukawa and Totsuka: sources of N, includingntnbute to forest declinelent of winter hardinessbalances (Schulze 1989),. Conversely, as a sourceave been shown to con-R.ogers et al. 1979a; Kajiving under conditions ofI to increase the activity>89).

6. Nitrogen Cycles 167

Deposition of NO2 and HNO3 to forest landscape surfaces may representa potentially significant addition of N to the biogeochemical cycle of forestecosystems, as discussed earlier in Chapter 6. Unfortunately, quantitativedata describing rates and locations of NO2 and HNO3 deposition to woodyplant tissues are not widely available. Deposition measurements are neededas input data for models of atmospheric chemistry, biogeochemical cycling,and studies of pollutant effects on plants (Hosker and Lindberg 1982). Foliarsites of N deposition (internal versus external) must be determined to predictthe fate of dry-deposited gases. In addition, because bark or forest floorsurfaces account for between 20% and 40% of the total landscape area avail-able for deposition (100% for broadleaf forests in winter; Halldin 1985),deposition to these surfaces must also be quantified. Laboratory studies wereconducted to provide direct deposition data for use in evaluating assumptionsof the IPS stand-level deposition model. The objectives of the laboratorystudies were to characterize deposition of NO2 and HNO3 to a variety oflandscape surfaces, to evaluate the significance of the leaf surface and theleaf interior as sites for NO2 or HNO3 deposition, and to contrast the de-position characteristics of NO2 and HNO3. Because the N budgets sum-marized previously in Chapter 6 do not include estimates of NO2 deposition,we assessed the amount of additional N deposition that may have resultedfrom NO2 inputs. Laboratory measurements of foliar conductances to NO2

were used together with available information on ambient NO2 concentra-tions to estimate the probable contribution of NO2 to N deposition in theIPS research sites.

Direct Measurements and Controlling Factors: Laboratory MassBalance Techniques

Bare-root seedlings of the following species were obtained from commercialnurseries and grown under glasshouse conditions before measurements: redmaple (Acer rubrum L.), white ash (Fraxinus americana L.), tulip poplar(Liriodendron tulipifera L.), white oak (Quercus alba L.), sycamore (Pla-tanus occidentalis L.), red spruce (Picea rubens L.), white pine (Pinus stro-bus L.), and loblolly pine (Pinus taeda L.). Bark samples were obtainedfrom branches or boles of four mature tree species, including shagbark hick-ory [Carya ovata (Mill.) K. Koch], tulip poplar (Liriodendron tulipifera),loblolly pine, and southern red oak (Quercus falcata Michx. var. falcata).Forest floor samples from loblolly pine or mixed hardwood forest standsnear Oak Ridge, Tennessee, were collected as undisturbed cylindrical coresapproximately 19 cm in diameter. Hanson et al. (1989) provided completedetails of the growing conditions and measurement procedures for these rep-resentative forest surfaces.

Mass balance measurements of HNO3 and NO2 deposition to elementsrepresentative of a forest landscape (e.g., foliage, bark, soil) were conductedin an open gas-exchange system. The system simultaneously monitored the


exchange of CO2, H2O, and either NO2 or HNO3 under controlled conditionsof temperature, light, vapor pressure, and soil water availability. Techniquesfor HNO3 deposition measurements were similar, but employed a techniquebased on thermal decomposition (Burkhardt et al. 1988) for measurementsof HNO3 concentration. Deposition rates (nmol m~2 s~') were calculated asthe product of flow rate and the inlet-outlet concentration differential nor-malized for surface area and corrected for losses to chamber walls. Mea-surements of HNO3 and NO2 deposition to foliage shoots were conductedunder light and dark conditions to establish patterns of diurnal variabilityassociated with stomatal conductance. Shoot conductance (K,) to a reactiveN gas, a leaf-level measurement analogous to the deposition velocity (Vd),was determined by dividing the rate of deposition by the ambient concen-tration of the gas being measured.

Field tsN Exposures

To quantify internal deposition of HNO3, foliage in the upper canopy ofmature eastern white pine trees at the Coweeta Hydrologic Lab was exposedto H13NO3 in branch cuvettes (9.0-L volume) constructed from Teflon film.H15NO3 was generated from calibrated permeation tubes (KIN-TEK Labo-ratories, Texas City, Texas) and mixed with HNO3-free air created by pull-ing ambient air through a nylon filter (Nylasorb, Gelman) at a flow rate of9.0 L min"1. Three treatments were imposed to represent a range of ex-posure conditions: (1) 10 ppb for 30 h; (2) 50 ppb for 12 h; and (3) 100ppb for 30 h. After exposures, foliage was immediately rinsed with 1000ml deionized water to remove surface-deposited H15NO3 (Marshall and Ca-dle 1989), separated by age-class (current year and 1 year), dried at 60°Cfor 48 h; and ground to 100 ju,m using a ball mill. Tissue subsamples (n = 2)were analyzed for total N and atom 15N% using mass spectrometry (EuropaScientific Instruments, ISO-TEC Labs, Miamisburg, Ohio). Excess 15N wasdetermined by subtracting total 15N of exposed foliage from total 15N ofunexposed foliage. Additional details of the H15NO3 exposure methodologyhave been provided by Vose et al. (1989).

Deposition of NO2 to Plant Foliage

Under daylight conditions and a mean concentration of 33 nl L"1, NO2 de-position to foliage of forest tree species varied by more than an order ofmagnitude, ranging from 0.35 (loblolly pine) to 5.75 nmol rrT2 s"1 (syca-more), and the flux to most broadleaf species was greater than depositionto conifers (Table 6.2 and Figure 6.11). For this comparison all surfaces ofleaves containing stomata were used as the reference area for calculations(i.e., one side for the broadleaf species and all sides for the conifers). Thebroadleaf species exhibiting the highest rates of NO2 deposition had greatershoot conductance to water vapor (Figure 6.11). Interspecific variation inshoot conductance to water vapor reflects variation in stomatal frequencies

Nutrient Cycling

>_, under controlled conditions/ater availability. Techniquesir, but employed a techniqueal. 1988) for measurements

I m~ : s~ ' ) were calculated asjncentration differential nor-ses to chamber walls. Mea-liage shoots were conductedatterns of diurnal variabilityinductance (K,) to a reactivethe deposition velocity (Vd),:ion by the ambient concen-

age in the upper canopy ofiydrologic Lab was exposed>nstructed from Teflon film.:ion ;ubes (KIN-TEK Labo-•4O:,-free air created by pull-). Gelman) at a flow rate ofto represent a range of ex-ppb for 12 h; and (3) 100

mediately rinsed with 1000: H'5NO3 '(Marshall and Ca-and 1 year), dried at 60°C

. Tissue subsamples (n = 2)mass spectrometry (Europa

•urg, Ohio). Excess 15N was1 foliage from total 15N ofN'O3 exposure methodology

Foliage

ition of 33 nl IT1, NO, de-by more than an order of

> 5.75 nmol rrT2 s~ ' (syca-vas greater than depositioncomparison all surfaces of

rence area for calculationssides for the conifers). TheNO: deposition had greater. Interspecific variation in.on in stomatal frequencies


Table 6.2. Conductance of Various Terrestrial Surfaces to NO2 Deposition

Conductance to NO2"Surface (cm s~')

Distilled waterBark

DryWet

Plant shoots*DeciduousConiferous

Forest floorDeciduousConiferous

0.021 ± 0.02

0.047 ± 0.0010.093 ± 0.023

0.093 ± 0.0420.049 ± 0.015

0.47 ± 0.260.48 ± 0.12

" Data for bark and plant shoots are expressed on total area basis and on planar area for intactforest floor samples.•' Data for plant shoots correspond to conditions of maximal stomatal conductance.

6 -

oa0)a

3 -

2 -

1 -

-1

a Sycamore+ Tulip Poplar° White Oaka Red Maplex White Ash

0.04 0.08 0.12 0.16 0.2

SHOOT CONDUCTANCE TO H2O (mol nrV]

0.24 0.28

Figure 6.11. Linear relationship between NO2 deposition and shoot conductance towater vapor. Symbols represent individual measurements for five broadleaf (maingraph) and three conifer species (inset). Data for broadleaf species are expressed onprojected leaf area basis; those for conifers are on total area basis. Mean NO2 con-centration was 34 nl L~'. (Hanson et al. 1989. Reprinted with permission fromAtmospheric Environment, vol. 23, NO2 Deposition to Elements Representative ofa Forest Landscape, Copyright 1989 Pergamon Press PLC.)


and stomatal apertures. Stomatal frequencies of Quercus species (540 sto-mata mm""2) are greater than that of Acer and Fraxinus species (372 and210 stomata mm"2; Kramer and Kozlowski 1979). The average rate of NO2

deposition to these species in the light followed the same trend: Quercusalba (2.05 ± 1.44 nmol irT2 s~') > Acer rubrum (0.78 ± 0.11) > Fraxinusamericana (0.57 ± 0.21). Similarly, lower NO2 deposition to conifers withrespect to broadleaf species is consistent with their lower conductance towater vapor (Komer et al. 1979). Okano et al. (1988) reported a positivecorrelation between NO2 uptake and stomatal conductance for eight differentcrop species that followed a trend associated with stomatal densities of thefoliage, and Grennfelt et al. (1983) found a similar correlation for Pinussylvestris.

Measurements of NO2 deposition in the dark provided information toevaluate surface versus internal deposition of NO2. The contrasting light:darkmeasurements of NO2 deposition indicated that the principal foliar site ofNO2 deposition was the leaf interior, constituting typically more than 90%of total deposition to individual leaves (Hanson et al. 1989). These dataconfirm previous hypotheses of stomatal control over NO2 and other tracegas deposition (Rogers et al. 1979b; Weseley et al. 1982; Saxe 1986b; Tay-lor et al. 1988). Accordingly, NO2 deposition is strongly influenced by sto-matal conductance, which is governed, in turn, by the plant's physiologicalstate as well as a host of environmental factors (e.g., light, vapor pressure,water availability).

Because the atmosphere-leaf exchange of NO2 is strongly controlled bystomatal physiology, conventional modeling approaches of gas exchange op-erating at the level of individual leaves or within whole canopies (and basedon analogy to water vapor) should be appropriate for characterizing NO2

deposition to vegetation surfaces. Although interspecific variation in NO2

deposition was high (see Figure 6.11), a linear regression of the combineddata set for deposition to broadleaf shoots versus shoot conductance to watervapor accounted for 85% of the variation, indicating that a single relation-ship might be useful for predicting deposition to broadleaf forests. A linearregression of the combined data on NO2 deposition to conifer shoots againstshoot conductance to water vapor explained 66% of the variation.

Deposition ofHNO3 to Plant Foliage

For similar ambient concentrations and low shoot conductances to water va-por, surface deposition of HNO3 vapor to plant shoots exceeded that for NO2

(Figure 6.12). The HNO3 deposition measurements were necessarily limitedto conditions of low humidity leading to plants with low shoot conductanceto water vapor. Therefore, comparisons of deposition between HNO3 andNO2 were restricted to the NO2 measurements corresponding to lea-ves hav-ing closed stomata (low water vapor conductances). This limitation meansthat the data for foliar surfaces in Figure 6.12 should be viewed as a com-

lutrient Cycling

: Qitercus species (540 sto-Fraxinus species (372 and

3). The average rate of NO2

:d the same trend: Quercusi (0.78 ± 0.11) > Fraxinus, deposition to conifers withtheir lower conductance to(1988) reported a positive

iductance for eight differentith stomatal densities of themilar correlation for Pinus

rk provided information tos. The contrasting light:dark

the principal foliar site ofig typically more than 90%n et al. 1989). These data1 over NO2 and other traceal. 1982; Saxe 1986b; Tay-strongly influenced by sto-

by the plant's physiologicale.g., light, vapor pressure,

3-. is strongly controlled by-caches of gas exchange op-whoie canopies (and basedate for characterizing NO2

srspecific variation in NO:

regression of the combinedshoot conductance to wateratmg that a single relation-broadleaf forests. A linear

on to conifer shoots against7o of the variation.

Foliage

t conductances to water va-loots exceeded that for NO2

its were necessarily limitedvith low shoot conductanceDsition between HNO3 andjrresponding to leaves nav-ies). This limitation meanslould be viewed as a com-


u.o

0.7

0.6

"Vo 0.5S

CO

ND

UC

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NC

E

(c13

o

o

o

o-*

o '-

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i cj

j*.

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-

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• N02

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-

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TEFLON WATER BARK R. SPRUCE R. MAPLE

DEPOSITION SURFACE

W. OAK SYCAMORE

Figure 6.12. Comparison of conductance of NO2 and HNO3 to water, bark, andplant cuticular surfaces. Because dry conditions needed to measure HNO3 exchangeshoots resulted in reduced stomatal conductance, data are compared only with NO2

data from Figure 6.1 that correspond to very low conductances. N, negligible.

parison of the deposition of HNO3 and NO2 to external cuticular leaf sur-faces. Independent of these constraints on the laboratory data, the HNO3

and NO2 data are consistent with field observations that employed micro-meteorological techniques (Wesely et al. 1982; Meyers et al. 1989).

Even though large variability and reduced stomatal opening limited rig-orous quantification of internal versus external sites of HNO3 deposition,our data indicated that part of HNO3 deposition to the leaf interior throughleaf stomata is likely to occur coincident with surface deposition (data notshown). More recent experiments employing 15N-labeled HNO3 have con-firmed this dual pathway for HNO3 deposition (P.J. Hanson, personal com-munication).

Field exposures of eastern white pine branches to 15N-labeled HNO3 yieldedinternal foliar deposition^ate^LjangmgJjmn-JLto^3-nmol^^6.13). These deposition rates correspond to a conductance of HNO3 to whitepine foliage of 0.0045 cm s~', which is far lower than the values obtainedusing the mass balance approach that measures total deposition. The dis-crepancy results from efficient removal of surface-deposited HNO3 duringthe postexposure rinse of the needles (see methods described previously).Marshall and Cadle (1989) found that 97% and 59% of foliar-deposited HNO3

was recovered in aqueous leaf washings after 2 and 16 hours, respectively.


f 4

ICLCC

m<

1000 2000 3000 4000

H15N03 CONCENTRATION (nmol m"3)

Figure 6.13. 15N absorption rate as function of HNO3 concentration for white pinefoliage of two age-classes. Squares and circles are 1- and 2- year foliage, respec-tively. Data are normalized to needle dry weight.

Garten and Hanson (1990) have also shown that 70% to 90% of the nitrateions in solution deposited to foliar surfaces remain available for subsequentremoval by aqueous solutions after a 48-hour period. Because HNO3 is likelyto dissociate into H+ and NO3~ on contact with foliar surfaces, a similartendency for removal in an aqueous rinse is plausible, and the 15N-HNO3

data in Figure 6.13 are probably representative of significant deposition tointernal foliar surfaces. Longer residence times for NO3~ on foliar surfacesbetween rain events (>48 h) might allow for more internal deposition throughthe cuticle than was suggested by the work of Garten and Hanson (1990).

Deposition to Nonfoliar Landscape Elements

Deposition of NO2 varied among forest elements measured. Foliar, bark,and forest floor surfaces typically showed greater conductance to NO2 thandistilled water alone, and forest floor surfaces showed a disproportionatelyhigh conductance when compared to bark or foliage (see Table 6.2). Theaverage conductance to NO2 of the materials measured ranged from -0.0045to 0.48 cm s~'. The deposition of NO2 to distilled water was relatively lowby comparison to bark and foliage, but the measured conductance of NO2

from the atmosphere to water (0.021 cm s~') was similar to the 0.010 cms~' value reported by van Aalst (1982). Conductance to NO2 of dry bark

Nutrient Cycling

_L

3000 4000

)N (nmol m )

Oi concentration for white pine!- and 2- year foliage, respec-

it 70% to 90% of the nitratelain available for subsequentriod. Because HNO3 is likelyith foliar surfaces, a similar'lausible, and the 15N-HNO3

: of significant deposition tofor NO3~ on foliar surfaces

re internal deposition throughGarten and Hanson (1990).

ipe Elements

nts measured. Foliar, bark,rer conductance to NO: thanshowed a disproportionatelyoliage (see Table 6.2). Theasured ranged from -0.0045led water was relatively low:asured conductance of NO2

vas similar to the 0.010 cmuctance to NO2 of dry bark


was similar to that for conifer shoots (0.047 and 0.049 cm s ', respectively),and conductance to NO2 of wet bark was similar to values for broadleafshoots (0.093 and 0.093 cm s~', respectively). Conductance to NO2 was notinfluenced by species of bark (data not shown); HNO3 conductance to barksurfaces was 15 times greater than NO2 conductance to the same surface(see Figure 6.12).

Conductance to NO2 of the forest floor, based on a ground area basis,was six- to sevenfold greater than conductance to foliar surfaces or dry bark(see Table 6.2). This high conductance to NO2 is, at least in part, a resultof unaccounted-for convolutions in the samples (i.e., high surface area). Thetrue area of the forest floor samples might easily have been two- or threefoldtimes greater than the actual ground area and would have resulted in de-position values closer to those of leaves and bark surfaces. Judeikis andWren (1978) observed similar or higher conductance of NO2 to both a sandyloam and an adobe clay soil (average velocity of 0.68 cm s"1), which com-pares favorably with our conductance values of 0.47 and 0.48 cm s~ l forthe broadleaf and conifer forest floor samples, respectively. Measured con-ductance to NO2 for autoclaved or oven-dried soils (data not shown) weresimilar to values for unsterilized soil, indicating that soil microorganismswere not responsible for the high conductances. Abeles et al. (1971) andGhiorse and Alexander (1976) also observed no effect of microorganismson NO2 deposition.

Calculated NO2 and HNO3 Deposition to Forest Stands

To demonstrate the use of leaf-level data for making estimates of stand-levelNO2 deposition, laboratory observations were extrapolated to 10 of the co-nifer and hardwood forest canopies in the Integrated Forest Study (see Chap-ter 2, this volume) and to the forest canopy of the Walker Branch Watershed(WB). The Walker Branch Watershed represents a oak-hickory forest lo-cated in a suburban area near Knoxville, Tennessee (Johnson 1989) that hashigher levels of gaseous pollutants (Table 6.3). Surface conductances for Noxides (K,) measured in the laboratory were used to approximate the anal-ogous measure for forest canopies (i.e, the deposition velocity, Vd) usingthe following equation:

Vd = K, * LAI [6.1]

where LAI is the leaf area index (Jarvis 1971; O'Dell et al. 1977; Hicks etal. 1987). Rates of total growing season N deposition contributed by NO2

for each forest site were then approximated from the product of the standVd, the mean daytime concentration of NO2 for those stands, and the effec-tive hours of daytime deposition (Table 6.3). A more detailed discussion ofthis calculation has been provided by Hanson et al. (1989). Calculated grow-ing season rates of HNO3 deposition were also determined, based on the

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£ £ £ •= •?? ' H s ^&z ? ? ^ - s -=-7 . 3 ^|« g ^ gs -3 • J J | S Sl =o c ^ ^ 2 o 2 o= o 2 ^-i - g ^ i -5|3 |S8 |E.-S|> 5 U , - U 10 ** >i •— Q«-3 a £ . u E n •- C-r-. au •= S 25 M '£ ° 2 S1"0

• ^ V 5 M S Q - r t i i - ,• « 7 Z 5 « i i " . 2 o S ~ Z^1 A - s S ^ S ^ 0 ^ -i fe-§ l§«I-33S|

ii!!!-JJ!i§ .| o 1-s §£ ° | 8^|•Ê,- 3 Sadl^ s J3|- u K - s C D u L 2 1 2 2 "° '3lp§l=IÎ§j j=§,= c^';OcL i - : : i-3

' S S ' o . a S c - 2 2 = Z^lii-3|=.sf §;sli^gl2?!^

Nutrient Cycling 6. Nitrogen Cycles 175

< <z z

232 a

o

S

assumption that deposition of HNO3 during the night was to external canopysurfaces and during the day to external surfaces and to internal leaf sites viaopen stomata. The "growing season" HNO3 deposition rates included in Ta-ble 6.3 and used in Figure 6.14 are provided for direct comparison to therates estimated for NO2. The growing season estimates for HNO3 deposition

•ago i* |fz.S =~s 1•5 u c •£ -o u.0 o sj •— c ^9 2 -o 5 » SIr§ ar !*rs if iC ir ?; .^ O co o 2 c c o

S 3 I2. 1 S c I!- lzH. f S I §

1 ^ . „ >^ f ) 4 1 '•" «*> nj " 55 i; '"*^ 2 O — G rt w

U'|| || i §•Is ^ 8-5 3 o= 3°. = X ^ z

r 1 w > "*• 2? M °i/? rn EiT "S " O — -Jj yIN ro Q U. -j^ a" "S . 3

§ s 'i s ^ " '12 U 5 — •= -, .= uS g S r i O - 2 - oU S ^ |I 2 5 - 2

5-5-5 | § f s =^ J|| z\.r S^ 1| o

S = fe "io CD I o' f sf : - |ij^ Ri* ?li 2-li?i § U a." a J S o. = " rl

t/^ t^l 1 ' " O U ^T^ ~ *M M ("

W < n -^ 1^1 ,°l3ii? ill in•/i " 2 i. r* U "t3 >i ; u ] C •*! X r- !-15x

t f ^ i / ^ S - O ^ - J D ' C 3 ^ ; 2_2 o o r a 2 p % o

•2|c3 |2g|l.-a|

l.z.^llllf 115

are not equivalent to the more ngorous estimates obtanied from the iri>model, which uses additional meteorological and stand structure character-istics to predict gaseous deposition for an entire year (see pp. 152-166;Lindberg et al. 1989).

Estimates of growing season NO2-N deposition to the different forest can-opy types varied 20 fold from a minimum of 10 for the New York redspruce-fir site (WF) to a maximum of 200 mol NO2-N ha" yr~ ' for the oak-hickory site in Oak Ridge (WB). The mean ± SE among all forest sites was49 ± 17 mol NO2-N ha~' yr~'. The estimates for the sites with the lowestambient concentrations of NO2 (FS, NS, DF) averaged 19 mol NO2-N ha"1

yr~', whereas the two most polluted sites exhibited NO2 deposition rates as

900

™mom 500IIJ0.

300

100

0

—

—

—

-

"-•

,-^—^ i — i i i i i i i i ii — i i — i i — i I I 1 1 1 1 1 1 1 I I 1

ST NS DF CP WF MS FS HF DL

IPS SITES

LP

_xr

"

~

"~

-

~

WB UC UD

culated HNO3-N. Horizontal line at 100% indicates equivalent molar amounts ofHNO3 and NO2 deposition. Forest site abbreviations are as follows: [EPRI sites] CP,white pine, North Carolina; DF, Douglas fir, Washington; DL, loblolly pine, NorthCarolina; FS, slash pine, Florida; HF, northern hardwoods, New York; LP, loblollypine, Tennessee; MS, red spruce, Maine; NS, Norway spruce, Norway; RA, redalder, Washington; ST, red spruce, North Carolina; WF, red spruce, New York;[Non-EPRI sites] WB, oak hickory, Tennessee; UB and UC, hypothetical urbandeciduous and coniferous forests.


much as 10 fold higher, of 62 (LP) and 200 mol NO2-N ha~' yr~' (WB).Higher deposition to the WB and LP site can be accounted for principallyby higher NO2 concentrations and K,. The high predicted deposition of NO2

to the CP site is a result of its large LAI. Extrapolated measurements ofI5N-HNO3 deposition from Figure 6.13 to the-stand level indicate that of thetotal HNO3 deposited perhaps only 10% to 20% is absorbed immediately bythe forest canopy. If sufficiently short intervals were to occur between pre-cipitation events, this observation would indicate that most dry-depositedHNO3 is removed to the forest floor in throughfall. These data corroboratethe earlier conclusions in this chapter that NO3~ losses from the IPS canopiesin throughfall were predicted to result from the accumulation of dry-depos-ited NO3~ from HNO3 between rain events.

The dry deposition of NO2 is not included in the N budgets presented inthe preceding portion of Chapter 6. However, growing season NO2 depo-sition was less than 10% of annual total N deposition for the IPS sites. Onlythe non-IFS WB site with mean NO2 concentrations of 15.1 ,ug m~3 andhigh foliar conductances to NO2 was predicted to have a substantially higherpercentage (21%) of N deposition resulting from NO2 (see Table 6.3). Forestcanopies for which estimated NO2 deposition contributed more than 5% oftotal N deposition included pristine (DF), intermediate (CP and HP), andpolluted sites (DL, LP, and WB).

If we contrast the predicted rates of NO2 and HNO3 deposition obtainedfrom the extrapolated laboratory measurements (see Figure 6.14), we findthat NO2 deposition will only exceed HNO3 deposition to forest canopiesunder conditions of high ambient NO2 concentrations [i.e., the WB, urbandeciduous (UD), and urban coniferous (UC) forest sites]. In or near urbanenvironments, NO2 concentrations often reach concentrations as high as 60nl L~' and are routinely greater than 30 nl L~' throughout the day (Lefohnand Tingey 1984; Bytnerowicz et al. 1987; Laxen and Noordally 1987). Toestimate NO2 deposition to deciduous or coniferous "urban forests" for com-parison to the IPS and WB sites, we used calculations similar to those de-scribed for the 11 IPS forest canopies (see Table 6.3). Estimated NO2-Ndeposition to urban forest canopies ranged from 330 to 1260 mol ha"1 yr~'for coniferous and deciduous trees, respectively. These hypothetical urban

JSfQjJ^deposiuon^stimates-are^n^rder^f-magnitaof deposition calculated for "natural" forest canopies, suggesting that drydeposition of NO2 must be considered in or near urban areas.

The estimates of NO2 deposition to forest canopies presented in Table 6.3are first approximations that assume maximum canopy conductance,throughout the growing season. They do not account for the impact of droughtconditions on stomatal conductance, which leads to an overestimate of drydeposition. However, they also do not include estimates of deposition tobark and forest floor surfaces, resulting in an underestimate that would par-tially offset such an overestimate. Independent of the shortcomings of these

Nutrient Cycling

mol NO2-N ha~ ' yr~ ' (WB).be accounted for principally

i predicted deposition of NO2

xtrapolated measurements of;tand level indicate that of the7c is absorbed immediately bys were to occur between pre-cate that most dry-depositedhfall . These data corroboratelosses from the IPS canopies

: accumulation of dry-depos-

n the N budgets presented ingrowing season NO: depo-

sition for the IPS sites. Onlyrations of 15.1 /j.g m~3 and0 have a substantially higher1 NO: (see Table 6.3). Forestontributed more than 5% of[mediate (CP and HP), and

1 HNO3 deposition obtained(see Figure 6.14), we find

^position to forest canopiesations [i.e., the WB, urbanrest sites]. In or near urbanoncentrations as high as 60throughout the day (Lefohn:n and Noordally 1987). Tous "urban forests" forcom-lations similar to those de-)le 6.3). Estimated NO2-N330 to 1260 mol ha~' yr~'. These hypothetical urbanlitude higher than the rateslopies, suggesting that dry• urban areas.pies presented in Table 6.3urn canopy conductance,nt for the impact of droughtto an overestimate of dry

estimates of deposition toerestimate that would par-the shortcomings of these


first-approximation estimates, we believe that the extrapolations from thelaboratory measurements indicate that current concentrations of NO2 in con-junction with other forms of atmospheric N (wet and dry) provide physio-logically significant inputs of N to forest systems. The estimated inputs ofNO2-N and HNO3-N listed in Table 6.3, ranging from 10 to 213 mol N ha"1

yr~ l , are similar to inputs expected for temperate forest systems from non-symbiotic N-fixing bacteria (0-214 mol N ha"1 yr"1; Waring and Schles-inger 1985).

Summary

Laboratory measurements have shown that comprehensive estimates of at-mospheric inputs N to forest stands should consider inputs of NO2 and HNO3

along pathways leading to foliage, bark, and forest floor surfaces. For mostof the IPS sites, contributions of N from dry-deposited NO2 would remainsmall because of low atmospheric NO2 concentrations. However, where theconcentration of NO2 is significant (near polluted urban areas), the followingpoints need to be considered: NO2 deposition to foliage of forest tree speciesvaried by more than an order of magnitude, and deposition of NO2 to mostbroadleaf species was greater than deposition to conifers. Measurements ofNO2 deposition under light and dark conditions indicated the principal foliarsite of NO2 deposition to be the leaf interior and supported previous obser-vations of stomatal control over NO2 deposition. Vegetation surfaces typi-cally showed greater conductance to NO2 uptake than distilled water alone,and forest floor surfaces showed a disproportionately high conductance toNO2 when compared to bark or foliage surfaces.

For a similar atmospheric molar concentration, the rate of deposition ofHNO3 will always exceed that for NO2, but the relative contribution fromeach reactive N gas to dry deposition is largely controlled by existing am-bient air concentrations. Unlike NO2, HNO3 vapor exhibits significant de-position to leaf cuticular surfaces, but a finite amount of internal depositionalso occurs. Models for HNO3 uptake might be enhanced by recognizingtwo pathways of uptake: a cuticular path for surface deposition and a sto-matal pathway leading to internal surfaces. A portion of surface-depositedHNO3 is likely to remain available for subsequent removal from foliar sur-faces during rain events, in contrast to internal uptake of HNO3 and NO2,which are likely assimilated into organic N forms.

Rates of growing seasoTfNO^rafia HNt>3~dry deyasitionrro~nonTirbaTrior^ests are similar to inputs expected from nonsymbiotic N-fixing bacteria intemperate forest ecosystems (0-214 mol N ha"1 yr"1). Dry deposition ofNO2 to the IPS sites was generally less than 10% of the total N depositedfrom other forms (wet, particle, HNO3), but NO2 should not be ignored asa source of dry N deposition when calculating total N loading to forest standsnear urban areas.


in increase inibution of theexchangeablever base satu-e where most:es in nutrientrge (Comptonm. N additionns, albeit over

d sites, whichiration affectsmual N return; tree require-Df tree growth,•ground N mi-ys. Two high-1 N saturationmd the spruce-nfluence of N

ns and micro-il. This is ex-tirough miner-otential. Non-

expressed byiger and foliaraobilized, andN availability

tion chemistryoductivity andorests) has notthis topic. Asanges inducedidications of asase in nitrifi-rally require audy it was notponse to a de-: other factorsill also greatly

:stimates of N:rnal N fluxes,

particularly those that are derived from other (measured or estimated) Nfluxes. Uncertainties in N input-N output budgets arise from the fact thata combination of field measurements and model outputs is needed to esti-mate both N deposition and NO3~ leaching losses. Other fluxes, such as netcanopy exchange, are calculated as a difference between two or more Nfluxes, each estimated with their own level of uncertainty. For example, anerror analysis in one of the high-elevation spruce sites in the Smoky Moun-tains using statistical methods (where possible) and logical constraints (e.g.,soil water flux <90% of precipitation) led to the conclusion that only dif-ferences between inputs and outputs greater than 50% can be consideredstatistically significant and meaningful (Johnson et al. 1991). Despite thisuncertainty regarding absolute values of N deposition and NO3~ leachinglosses and the magnitude of various internal N fluxes, however, the IPSnetwork has offered a unique opportunity to compare biogeochemical cy-cling of N in forest ecosystems spanning a wide range of N deposition re-gimes and at various degrees of N saturation.

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