Agriculture and Environment, 7 (1982) 223--235 223

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

TOXICITY OF AMMONIA TO PLANTS

L.J.M. VAN DER EERDEN

Research Institute for Plant Protection, Wageningen (The Netherlands)

(Accepted 11 May 1982)

ABSTRACT

Van der Eerden, L.J.M., 1982. Toxicity of ammonia to plants. Agric. Environm., 7: 223--235.

The toxicity of ammonia was evaluated and an estimate is given of (mass) concentra- tion for no adverse effect: 75 ~g/m 3 for a yearly average, 600 ug/m 3 for 24 h and 10 000 ug/m s for 1 h. Ammonia can cause various types of injury, including necrosis, growth reduction, growth stimulation and increased frost sensitivity. Several plant species have been assessed for sensitivity to ammonia. Some conifer species were relatively sensitive to low concentrations in the long term; some cultivars of cauliflower and tomato were relatively sensitive to somewhat higher concentrations for a short term. Plants were more sensitive in the dark than in daylight and better adapted to ammonia in high than in low temperatures. Availability of carbohydrates probably plays an important role: the plant can detoxify ammonia as long as it can convert ammonia into amino acids.

Special at tention has been paid to plant injury around intensively managed livestock. The emission from these sources consists of a large number of components, ammonia prov- ing to be the main toxic component.

INTRODUCTION

Although ammonia (NH3) is not one of the major air pollutants, it is neces- sary to have some knowledge about its toxicity and about the concentration that will give no adverse effect. Research has been done on the contribution of NH3 to the nitrogen supply to the vegetation (Faller, 1972; Hutchinson et al., 1972; Porter et al., 1972; Meyer, 1973; Hutchinson, 1974; Denmead et al., 1979; Farquhar et al., 1979), but this subject is here only of indirect relevance. Other publications on this subject deal with accidents during transport of concentrated NH3 (Temple et al., 1979; De Temmerman, 1980) or release from refrigeration systems (Brennan et al., 1962).

Research has also been done into vegetation injury caused by intensively managed livestock. Kfihne (1966) and Garber and Schtirmann (1971) men- tioned several cases of plant injury around poultry and pig farms within 50 m of the buildings. Hunger (1978) and Tesche and Schmidtchen (1978) how- ever, found injury to a spruce stand up to 400 m from the pollution source, and Ewert (1978) estimated the total area of injury to forest stands in East

0304-1131/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company

224

Germany, caused by the emissions from livestock farms, at 2000--3000 ha. We studied the cause, character and extent of this injury, mainly on ac-

count of complaints about crop injury around animal housings and difficulties with the Dutch law on nuisance when farmers sought permission to build units for intensively managed livestock, for instance near tree nurseries.

In the literature injury to plants near intensive livestock units is mostly at tr ibuted to ammonia (NI-I3), although no literature has come to light that indicates causality. The main phytotoxic components in emissions from in- tensively managed livestock are amines, hydrogen sulphide (H2S), organic acids and NH3.

Van Raay (1974) concluded that amines caused the same kind of effects but were more toxic than NHH3. Probably the sequence in decreasing toxicity is tertiary amines, secondary amines, primary amines and NI-I3. However, a mixture of amines and NH3 in a volume ratio of 0.05 caused the same amount of injury as NH3 alone. In practice the ratio is mostly 0.01 or less. Hence, at the concentrations in air from livestock, amines do not contribute measur- ably to the total effect.

Another component in air from livestock is hydrogen sulphide (H2S). Krause (1979) found a growth reduction of 20% in several plant species with H2S at 200 ttg/m 3 for 14 days. According to Thompson and Kats (1978), a long-term average of 150 ttg/m 3 was toxic for conifers. However, concentra- tions in air inside housing are usually not more than 1 mg/m 3 and mostly less than 50 pg/m 3 (Schaefer et al., 1974; Vetter and Kowalewsky, 1979). Only when slurry was pumped up out of the cellars would H2S concentration increase to very high levels, and then only for a relatively short period. I have found no evidence that this can cause plant injury. Therefore H2S probably pl~ys no role in the toxici ty of air from livestock.

Organic acids are not very toxic. Van Haut and Prinz (1979) saw necrosis

TABLE I

Volume ratios of amines and organic acids with respect to NH3, used to simulate air from l ives tock

Ammonia NH 3 Methylamine CH3NH 2 Dimethylamine (CH3)~NH Ethylamine C2HsNH 2 n-propylamine CH~(CH2)2NH 2 Isopropylamine (CH3)2CHNH 2 n-butylamine CH3(CH2)3NH 2 n-amylamine CH3(CH2)4NH 2 A c e t i c acid CH3COOH Propionic acid CH3CH:COOH n-butyric acid CH3(CH2)2COOH Isobutyric acid (CH3)2CHCOOH n-valeric acid CH3(CH2)3COOH Isovaleric acid (CH~)~CHCH2COOH

1000 2 1 4 1 2 0.2 0.2

35 5

10 5 2 2

225

on the leaves of a sensitive species (Lepidium sativum) after fumigation for 2 days with 870 gg/m 3 acetic acid. This is much higher than the concentra- tion present in air from livestock.

A mixture of NH3, amines and organic acids, in the volume ratio mentioned in Table I, had the same toxici ty as NH3 alone (Van der Eerden, 1978).

Experiments were designed to find which NH3 concentrations had no ad- verse effect, to examine various effects and to find which species are more or less sensitive. Some factors influencing metabolism and effects of NH3 were evaluated.

MATERIAL AND METHODS

Plants were exposed to NH3 in several ways. They were fumigated out of doors (in open-top chambers) and in fumigation chambers within a con- trolled climate. Long-term exposure (about one week and more) was at a temperature of 8--12°C, a relative humidi ty of 60--80% and a (vertical) wind velocity of 0.13 m/s. Short-term exposure was at a temperature of 18--25°C, a relative humidi ty of 50--80% and a wind velocity of 1.2 m/s. Besides these fumigations we evaluated the toxici ty of air containing NH3, amines, organic acids or a mixture of these components and also of air passing over poultry manure. In this air we used and measured NH3 as a tracer. We controlled the concentrat ions in this mixture by adapting the dilution of the air from a small poul t ry unit which we then circulated through the fumigation chamber. In addition, plants were exposed near pig fattening and poul t ry farms, where we measured NH3 concentrations during exposure. In the pre-exposure period plants were cultivated in climatic conditions similar to the conditions during fumigation.

NH3 was measured by ultraviolet (UV) absorption (~ = 210 nm) or by way of impingers with the 'Nessler' reaction (Buck and Stratmann, 1965). Amino acids were measured by liquid chromatography and sugars by change in UV absorption (), = 340 nm) after an enzymic reaction (Boehringer, 1980).

RESULTS

Effects

The major effect we saw was necrosis, mostly of the older leaves or nee- dles. Sometimes the symptoms were specific and indicative for NH3, e.g. black spots on the back of the leaves of cauliflower, or the sharply bordered necrotic tip of the older needles of Taxus baccata. Mostly, however, the symptoms were similar to those caused by other stress factors such as drought, some plant diseases, salt, and other air pollutants. The growth of some species was reduced and that of others was stimulated. Necrosis was not always com- bined with growth reduction (see Table II).

A major indirect effect of NH3 is probably increased sensitivity to cold.

TA

BL

E

II

Gro

wth

res

pon

ses

and

lea

f in

jury

on

seve

ral

crop

s af

ter

fum

igat

ion

w

ith

NH

,

Cro

p

Age

N

um

ber

E

xpos

ure

D

ry w

eigh

t (g

) L

eaf

(day

s)

of

sam

ple

s d

ays

mg/

nP

Fu

mig

ated

C

ontr

ol

Fum

igat

ed

Sign

. in

jury

Mea

n

(S.D

.) M

ean

(S

.D.)

in %

of

dif

f.

con

trol

(P

= 0

.05)

Lol

ium

m

ultif

loru

m

(cv.

O

pti

ma)

P

oa a

nnua

B

rass

ica

oler

acea

va

r.

botr

ytis

(c

v.

le

Cer

f)

Bra

seic

a ol

erac

ea

var.

bo

tryt

is

(cv.

V

erb

. M

ech

.) B

rass

ica

peki

nens

is

(cv.

G

rana

at )

Lac

tuca

sa

tiua

var.

ca

pita

ta

(cv.

A

ttra

ctie

)

60

8 30

0.

6 7.

03

(0.3

2)

5.62

(0

.32

125

yes

no

60

8 30

0.

6 10

.06

(1.0

2)

5.68

(0

.32)

17

7 ye

s no

75

15

16

0.6

7.56

(0

.85)

8.

67

(1.1

4)

87

yes

75

7 16

0.

6 3.

90

(0.2

6)

7.25

(0

.65)

75

7 16

0.

6 0.

76

(0.0

91)

0.82

(0

.072

)

54

93

yes

yes

no

no

35

7 35

7

35

7 35

7

55

5 55

5

2 4.

2 2.

38

(0.5

2)

2.32

(0

.60)

10

3 no

no

6

4.2

1.44

(0

.15)

1.

96

(0.3

6)

73

no

yes

2 2.

0 2.

60

(0.5

5)

2.59

) (0

.51)

10

0 no

no

6

2.0

2.43

(0

.43)

1.

92

(0.5

8)

127

no

yes

3 0.

6 7.

68

(1.0

3)

6.73

(0

.78)

11

4 6

0.6

7.37

(0

.65)

7.

76

(0.5

6)

95

no

no

no

yes

Lyc

oper

sico

n es

cule

ntum

(c

v.

Mon

ey

mak

er)

TABLE III

Differences in sensitivity of conifers to NH 3

227

Exposure Season Place Conc. NH 3 (mg/m 3) Degree of

(days) 98% median injury

50 Winter Open-top chambers 0.42 0.25 Heavy 50 Winter Open-top chambers 0.18 0.07 None 53 Spring Open-top chambers 1.50 0.54 None 53 Spring Open-top chambers 0.25 0.10 None 60 Winter Near piggery 0.25 0.06 Moderate to

heavy 53 Spring Near piggery 0.25 0.06 None

100 Controlled environment (8--12°C) 0.92 0.69 Very slight

Explanation of the degrees of injury: None -- no clear effects caused by NH 3. Very slight - - very few necrotic tips. Moderate -- some drop of needles, most needles have a necrotic tip. Heavy -- marked needle drop, nearly all needles have a necrotic tip.

TABLE IV

Sensitivity of conifers to ammonia (the concentrations are an indication of the no adverse effect level with an exposure time of 2 months)

Very sensitive Moderately sensitive Insensitive

< 150 pg/m 3 150--300 pg/m 3 > 300 pg/m 3

Cupressocyparis leylandii Pinus stro bus Picea sitchensis

Picea abies Taxus baccata

Pinus nigra var. maritima Picea omorika Chamaecyparis columnaris var. glauca Taxus baccata 'Fastigiata' Pseudo-tsuga mensiesii Chamaecyparis iawsoniana

Tacus media 'Hicksii' Pinus nigra var. nigra Pinus sylvestris

Thuja occidentalis Pinus mugo vat. mughus Tsuga canadensis 'Nana Compacta'

For instance, NH3 can increase the frost sensitivity of cabbage and leek (in- dependently of direct injury to the tissues by NH3): after fumigation in an open-top chamber with NHs at 500 #g/m 3 for 10 days the leaves of cauli- flower and Brussels sprouts had the black spots specific for NH3 injury. More- over, in the group of fumigated crops, cauliflower and white cabbage were rather heavily injured by frost. In the control group no frost injury was detected. This phenomenon may also contribute to the NH3 effects on coni- fers. Table III indicates that in spite of exposure to higher concentrations for longer (in open-top chambers, near a piggery and in closed fumigation cham- bers), less injury occurs in spring than in winter in the open air.

Tables IV and V list sensitivity of various species to leaf or needle in-

228

jury. 'Conifers' are distinguished from 'vegetables and miscellaneous', because the first group has always been fumigated for 50 days and longer and the second group mostly for 10 hours to 14 days. To define the sensitivity classes, a rough estimate is given of the concentrations for no adverse effect in every class.

TABLE V

Sensitivity of vegetables and miscellaneous to ammonia (the concentrations are an indica- tion of the no adverse effect level with an exposure time of 7 days)

Very sensitive Moderately sensitive Insensitive

< 500 ug/m 3 Brassica oleracea (cauliflower, 3 cvs)

Solanum lycopersicum Cucumis sativus

500--1500 ~g/m 3 Brassica oleracea (Brussels sprouts, cabbage and savoy) Brassica sinensis Capsicum annuum Lactuca sativa

Chenopodium alba

Nicotiana tabacum 'Bel W 3' Brassica juncea Fagopyrum esculentum Prunus laurocerasus 'Ot to Luycken' Phaseolus vulgaris Melilotus alba

> 1500 ag/m 3 Brassica oleracea (curly, red cabbage)

Valerianella olitora Raphanus sativus Pyrus comm. sat. 'DoyennSe du com.' Pyrus malus 'Golden Delicious' Poa annua

Lolium multiflorum Rhododendron (2 cvs) Nepata cataria

Populus euramericana

Metabolism

Metabolism appeared to play an important role in NH3 toxicity. For ex- ample, young tomato plants were injured much more by 24 h fumigation with NH3 at 2.0 mg/m 3 in the dark than in normal light (Fig. 1).

Degree o f in ju ry Carbohydrates Ammonia Glutamine

heavy

moderate

light

none

Light Dark Light Dark Light Dark Light Dark (mg/g)

I 0 0 2 , 5

| |-.-=

K.:~;~;'_~ fumigated I ] control

Fig. 1. Degree of injury and contents in dry matter of carbohydrates (glucose + fructose + sucrose + starch), ammonia (NH 3 + NH4*) and glutamine of young tomato plants fumigated with NH 3 at 2 mg/m ~ for 24 h in daylight and in the dark.

229

There was a sharp increase in glutamine content when tomato plants were fumigated in daylight and a sharp increase in ammonia content when fumi- gated in the dark. Thus a high degree of injury coincides with a low content of carbohydrates, a high ammonia content and no change in glutamine con- tent. In other experiments there was only a small effect of fumigation on the starch and protein contents and an effect on asparagine similar to that on glutamine content but not as sharp. The same sort of biochemical ef- fects occurred in other plant species.

Concentrations for no adverse effect

Fig. 2 and Table VI summarize the results of some fumigation and exposure tests with several plant species. Data from the literature are ment ioned in ad- dition to m y own results. In Fig. 2 the position of every circle indicates the concentrat ion and exposure period of a test. A black circle means a signifi- cant adverse effect and an open circle no adverse effect. In this figure only visible tissue injury is mentioned.

Towards the right or to the top in Fig. 2 the exposure level increases and with it the risk of adverse effects. The curve separates the exposures which were not toxic (left of the curve) from those that were potentially toxic (right of the curve). Exposures at the right side, bu t very near to the curve caused injury only to sensitive plants under 'sensitive' conditions. With higher concentrat ions or longer exposure times the risk of adverse effects in- creases from almost 0% (on the curve) to 100% dependent on plant species and environmental circumstances. Consequently the line indicates the limit- ing concentrat ion for no adverse effect with different exposure times. Be- tween 5 h and 100 days the curve can be described rather well by the formula (1 + log T) (--1 + log C) = 4.0089 where T is in hours, and C in ug/m 3.

The curve is obtained for long-term exposures by the sensitivity of coni- fers and for short-term exposures by the sensitivity of crops such as cauli- flower, tomato and sunflower.

Ammonia and emissions from intensively managed livestock

In various experiments lasting 16--100 days, conifers, cabbage species and grass were fumigated with NH3 at concentrations of 600--1000 ~g/m 3. We found no differences in tissue injury, growth reduction and change in roo t / shoo t ratio between fumigation with NH3 and with 'chicken air' con- taining the same concentrat ion of NH3. Conifers exposed near poul t ry and pig farms showed the same type of tissue injury as conifers fumigated with NH3. In the introduct ion it was ment ioned that a mixture of amines, or- ganic acids and NH3 in a ratio as indicated in Table I had the same toxici ty as NH3 by itself.

In summary, NH3 is the main toxic component of livestock air and its toxici ty can be simulated with NH3 fumigation.

230

EXPOSURE TIME ( h )

' ' 0,025 0,050 0,10 1,0 10,0 100

CONCENTRATION OF N H 3 1 ' r n g / m 3 )

Fig. 2. Effect o f a m m o n i a w i t h exposure t ime and mass concentrat ion on a log scale. To the right o f the l ine, adverse ef fects are l ikely, o = no adverse effects; • = adverse effects . See Table VI for explanat ion o f the codes at the data points .

TABLE VI

231

Effects of ammonia on different species in the literature and in local experiments (codes refer to the data points in Fig. 2)

Code Plant species Remarks Source

a Sunflower Fumigated, nitrogen deficient Failer (1972) plants

b Various deciduous Near a poultry farm Garber and Schi~r- trees mann (1971)

c Sunflower, cauli- Fumigated; sunflower and cauliflower Lamberts (1977) flower, lettuce injured but lettuce not peru. commun.

d Fruit trees, wheat Literature survey Garber (1935) e Rose Literature survey Garber (1935) f Cauliflower, lettuce, Fumigated; cauliflower injured, Wielard (1979)

radish lettuce and radish not peru. commun. g Deciduous trees Fumigated Ewert (1979) h Various crops Literature survey EPA (1978) i Various crops incl. Fumigated Thomas and

tomato, buckwheat, Hendricks (1956) sunflower

j Mustard, sunflower Fumigated Benedict and Breen (1955)

kl, 2 Conifers Near a pig farm Own results (kl median, k2 98 percentile)

k3, 4 Conifers Near a poultry farm Own results (k3 median, k4 98 percentile)

k5 Conifers Fumigated at 8--12°C Own results k6, 7 Conifers Fumigated at 8--12 ~ C with NH3 Own results

in chicken air (k6) and clean air (k7) k8, 9 Conifers Fumigated at 8--12°C with NH3 in Own results

chicken air (k8) and clean air (k9) kl0 Conifers Fumigated in open-top chambers Own results

in mid-winter, assessed after 1, 3, 6 and 12 weeks Fumigated at 20°C and 70% R.H. kll Cauliflower, tomato,

lettuce k12 Conifers

k13 Fruit trees

k14 Tomato

l Lettuce, tomato ml, 2 Conifers

m3 Conifers

m4 Beans (Phaseolus vulgaris )

n Italian rye grass and annual blue grass

p Cauliflower q Chinese cabbage

Fumigated in open-top chambers in mid-winter Fumigated in open-top chambers in mid-summer Fumigated at 20°C daylight (no in- jury) and in the dark (injury) Fumigated at 20°C and 50% R.H. Exposure near a pig farm (ml median, m2 peak conc.) Fumigated in open-top chambers in spring Fumigated at 20°C and 60% R.H.

Fumigated at 8--12°C and 70% R.H. (growth increment) Fumigated at 20°C and 50% R.H. Fumigated at 20°C and 50% R.H.

Own results

Own results

Own results

Own results

Van Raay (1974) Geuskens and Knol peru. commun. Geuskens and Knol (1979) peru. commun. Geuskens and Knol (1979) peru. commun. Own results

Own results Own results

232

DISCUSSION AND CONCLUSIONS

The data in the literature on uptake of atmospheric NH 3 suggest that up- take is determined mainly by atmospheric concentration, diffusion resistance of the boundary layer of the leaf and opening of the stomata. Whether in practice atmospheric NH3 can contribute significantly to the total nitrogen supply is still an open question.

The NH3 (+NH4*) taken up through leaves is potentially toxic for the fol- lowing reasons:

(1) NH3 is a well known inhibitor of photosynthetic phosphorylation and therefore decreases carbohydrate production and so can reduce growth. Losada and Arnon (1963) used ammonia in solution at 1 mmol/1 to inhibit photosynthet ic phosphorylation, which is about comparable with a content in dry plant material of 150/~g/g. The injured tomato plants (Fig. 1) contained 200/~g/g and those that were uninjured 40/~g/g. It therefore appears that photosynthetic phosphorylation was inhibited in that experiment.

(2) Probably NH4* can saturate the lipids in the cell membranes, so increas- ing permeability (plasmolysis and necrosis) and decreasing flexibility. One result of this can be increased frost sensitivity. However, NH4* can also be converted into amino acids and amides (mainly glutamine and asparagine) and so detoxified; inhibition of photosynthetic phosphorylation and other adverse effects are removed. Conditions for detoxification are that the metab- olic activity of the plant is high enough and that enough carbohydrates are available for this reaction. Metabolic activity and carbohydrate availability

UPTAKE

I l I

~TABOLISM [

I I I

EFFECTS [

I I

N~3

I - boundary layer resistanc~ sto~ta, ectodesmata

,oo little carbohydrates sufficient carbohydrates available available

NH3,~4+ left amino acids, ~ides (detoxificat ion)

~ncoupling of saturation liplds photophosphorylatlon in me~ranes

~ contribution to N I pply less carbohydrates

increased decreased permeability flexibility

growth reduction necrosis frost injury growth stimulation

Fig. 3. Metabolism and effects of NH3.

233

are low at low temperatures. This mechanism could explain the high sensitiv- ity of conifers in winter (Table III). The carbohydrate content is low at night, and so sensitivity is high in the dark (Fig. 1). All this is summarized in Fig. 3.

The sensitivity of the plant thus depends strongly on several environmental condit ions as well as on its genetic properties. Tables IV and V can be used as a guide for planting at various distances from ammonia sources. In general, conifers are sensitive and most other horticultural crops are moderately sensi- tive (most arable crops seem to be insensitive). As far as there is an overlap, Tables IV and V fit quite well with data in the literature (EPA, 1978; Tesche and Schmidtchen, 1978; Ewert, 1979). There seems to be some disagreement with sensitivity ment ioned by Temple et al. (1979), but they only studied effects of very high concentrat ions of NHs for very short periods.

Criteria of air quality can be based on sensitive species growing under con- ditions in which the plant is sensitive (the curve in Fig. 2). An estimate of the limiting concentrat ion for no adverse effect as a function of exposure t ime (for sensitive species under 'sensitive' conditions) is 75 pg/m 3 for a year, 600 ~g/m s for 24 h and 10 000 #g/m s for 1 h. An estimate that entails more risk of adverse effects (in other words that does not protect all plants under all circumstances) is higher. For example, a curve parallel to the curve in Fig. 2 which has not 100% but only 90% of the tests with adverse effects on the right of it (and so separates the tests in which adverse effects were caused by a rare coincidence of unfavourable conditions) indicates higher limiting concentrations: 100 ~g/m 3 for a year, 850 ~g/m s for 24 h and 20 000 #g/m 3 for 1 h.

There are a few standards for maximum advisable average concentrations of NH3: in Czechoslovakia 100 ~g/m s for 24 h; in Canada, 3500 pg/m 3 for 30 rain; and in the Soviet Union 200 pg/m s (for conifers 100 ~g/m 3) for 24 h. These standards are quite safe for vegetation, because they are well to the left of the curve in Fig. 2.

In another paper, the limiting concentrations ment ioned in this paper will be combined with emission data and a dispersion model. In this way, one can evaluate potential effects of an NH3 source as a function of emission and distance between source and vegetation (Harssema et al., in preparation).

ACKNOWLEDGEMENTS

Thanks are due to A. van Raay who initiated the NH3 research in our department , to J. Mooi for his help in the fumigations, to M. Gremmen and J. van Achterberg for their assistance in the experiments, and to the students E.T. Lamberts, M.J. Wielard, R. Geuskens and M. Knol.

REFERENCES

Benedict, H.M. and Breen, W.H., 1955. The use of weeds as a means of evaluating vegeta- tion damage caused by air pollution. In: Proc. 3rd Nat. Air Pollut. Symp., Los Angeles, pp. 177--190.

234

Boehringer (Editor), 1980. Methods of Enzymatic Food Analysis. Mannheim, Fed. Rep. Germany, 48 pp.

Buck, M. and Stratmann, H., 1965. Die Anwendung des Impinger-Princips bei der Bestim- mung von Ammoniak und Ammoniumionen in der Atmosph~ire. Anal. Chem., 4: 241--250.

Denmead, O.T., Hulsen, R. and Thurtell, G.W., 1979. Ammonia exchange over a corn crop. Soil Sci. Soc. Am. Proc., 5: 840--842.

De Temmerman, L., 1980. Acute en subacute plantenschade als gevolg van een accidentele ammoniak-lozing. Landbouwtijdschrif t Belgie, 33 :753- -766 (with English summary).

EPA, 1978. Diagnosing vegetation injury caused by air pollution. USA-EPA 450/3-78- 005, Environmental Protect ion Agency, pp. 6015--17.

Ewert, E., 1978. Vegetationssch~iden in der Umgebung landwirtsehaftlicher Tierproduk- tionsanlagen. Luft K~iltetech., 6: 218--220.

Ewert, E., 1979. Phytotoxicit~it yon Ammoniak. Hercynia, 16: 75--80. Failer, M., 1972. Schwefeldioxid, Schefelwasserstoff, nitrose Gase und Ammoniak als

aussehliessliche S- bzw N-Quellen der hSheren Pflanze. Z. Pflanzenern~ihr. Bodenkd., 131: 120--130.

Farquhar, G.D., Wetselaar, R. and Firth, M., 1979. Ammonia volatil ization from seneseing leaves of maize. Science, 4386: 1257--1258.

Garber, K., 1935. lJber die Physiologie der Einwirkung yon Ammoniakgase auf die Pflanze. Landwirtsch. Vers. Wes., 123: 277--343.

Garber, K. and Schtirmann, B., 1971. Wirkung und Nachweiss yon Ammoniak-Immissionen in der N~ihe yon Gross-Stallungen. Landwirtsch. Forsch., 26(1), Sonderheft 20: 36-- 40.

Hunger, W., 1978. Uber Absterbeerscheinungen an ~iltern Fichtenhest~nden in der N~ihe einer Schweinemastanlage. Beitr. Forstwirtsch., 4: 188--189.

Hutchinson, G.L., 1974. Foliar absorption of atmospheric ammonia. Diss. Abstr. Int. B, 11 : 5289--5290.

Hutchinson, G.L., Millington, R.J. and Peters, O.B., 1972. Atmospheric ammonia: ab- sorpt ion by plant leaves. Science, 4023: 771--772.

Krause, G., 1979. Relative Phytotoxicit~it von Schwefelwasserstof. Staub Reinhalt. Luft, 39: 165--167.

Kiihne, H., 1966. Absterbeerscheinungen an Koniferen in der N~he yon Htihnerstallen mit Entluftung durch Ventilatoren. Nachrichtenbl. Dtseh. PflanzensehutzdJenstes (Braun- sehweig), 18(8): 121--123.

Losada, M. and Arnon, D.J., 1963. Selective inhibitors. In: R.M. Hochster and J.H. Quastel (Editors), Metabolic Inhibitors. Vol. 2, New York, NY, pp. 503--611.

Meyer, M.W., 1973. Absorpt ion and release of ammonia from and to the atmosphere by plants. Diss. Abstr. Int. B, 6: 2404.

Porter, L.K., Viets, F.G. and Hutchinson, G.L., 1972. Air containing N-15 ammonia: foliar absorption by corn seedlings. Science, 175: 759--761.

Temple, P.J., Harper, D.S., Pearson, R.G. and Linzon, S.N., 1979. Toxic effects of am- monia on vegetation in Ontario. Environ. Pollut., 4: 297--301.

Tesche, M. and Schmidtchen, A., 1978. Sch~idigungen an Koniferen in der Umgebung yon Anlagen der Industriem~issigen Hiihnerhaltung. Arch. Phytopathol . Pflanzenschutz, 5: 327--332.

Thomas, M.D. and Hendricks, R.H., 1956. Effect of air pol lut ion on plants. In: P.L. Magill, F.R. Holden and C. Ackley (Editors), Air Pollution Handbook. McGraw-Hill, New York, NY, pp. 901--944.

Thompson, R.D. and Kats, G., 1978. Effects of continuous H2S-fumigation on crop and forest plants. Environ. Sci. Technol., 12: 550--553.

Van der Eerden, L.J., 1978. Invloed van stallucht afkomstig van intensieve veehouderij- bedrijven en van ammoniak op planten. IPO-Report R 203, 50 pp.

235

Van Haut, H. and Prinz, B., 1979. Beurteilung der relativen Pflanzensch~idlichkeit organischer Luftverunreinigungen im LIS-Kurzzeittest. Staub Reinhalt. Luft, 39: 408--413.

Van Raay, A., 1974. Enkele gegevens m.b.t, kunstmatige begassingsproeven met NH 3 en/ of amines in kasjes op het IPO te Wageningen. IPO-Report R 139, 11 pp.

Vetter, H., and Kowalewsky, H.H., 1979. H2S-Emissionen aus Tierhaltungsbetrieben. Staub Reinhalt. Luft, 39: 181--182.