Plant and Soil 128: 83-89, 1990. © 1990 Kluwer Academic Publishers. Printed in the Netherlands.

PLSO FE09

Remarks on the effects of nitrogen deposition to forest ecosystems HEINZ W. ZOTTL

Institut fiir Bodenkunde und Walderniihrungslehre, Albert-Ludwigs-Universitiit, Bertoldstra]3e 17, D-7800 Freiburg i.Br., FRG Key words:

ammonium toxicity, forest fertilization, mycorrhiza, nitrogen saturation

Abstract

The effects of increased deposition of nitrogen compounds on forest sites are discussed based on literature data and own results from both earlier fertilization experiments and the ARINUS study area in the Black Forest. The influence on mycorrhiza is stated suppressive as well as stimulating so that no general conclusion can be drawn. The nitrogen nutrition status of coniferous forests is still sub-optimal over wide areas with a yearly deposition of 10-20 kg N ha- 1. Under considerable higher input rates the insufficient supply of other nutrients and imbalances in the nutritional status of trees are possible. When discussing 'nitrogen saturation' of ecosystems, the nitrogen storage capacity of soils has to be considered as a decisive factor which varies from site to site. Any actual input/output balance is strongly influenced by the internal turnover processes resulting from former land use.

Introduction

The deposition of air-borne nitrogen compounds (NH3, N O x or NH4, NO3, respectively) has certainly risen within the last decades and is a new ecological factor in many forest ecosystems e.g. in Central Europe and Southern Scandinavia. But the measured yearly deposition rates show pronounced regional differences. They vary from 0.8 k g N h a -1 in Northern Finland to 35 k g N h a ~ in some spruce stands in Southern Germany (Kreutzer, 1989; Malkonen, 1989). An extremely high input (40-80 kg N hayr 1) occurs in small forests in Northwest Germany and the Netherlands due to high NH 3emissions from bio-industries (intensive stock breeding) and heavy application of organic manure on arable land. However, the mean deposition over large forest areas in Central Europe is 10-20 kg/ha (Evers, 1985; Hueser and Rehfuess, 1988; Mies, 1987). Nevertheless this moderate input level clearly exceeds the yearly net incorporation in the grow-

ing biomass of a forest stand which can be roughly calculated to 5 - 1 0 k g h a -~ (Glatzel, 1989; Raisch, 1983). From simple comparisons of these parameters some authors, e.g. Schulze et al. (1987), concluded that there was an oversupply of nitrogen in the forests concerned. The hypothesis was suggested that trees have become over-saturated with nitrogen, and this was the key factor that triggers forest dieback (Nihlgard, 1985). Another explanation that has been offered is based on the high percentage of ammonia in the nitrogen deposition. Reviewing selected literature, Mohr (1986) concluded that toxic concentrations of ammonium in soil solution damage the mycorrhiza of the trees, leading to more dieback of rootlets with the result of reduced uptake of water and nutrients and finally dieback of the trees. All this sounds rather logical, but it is not a valid explanation for the actual situation in our forests as will be shown below. Three topics will be discussed: influence of higher nitrogen input on mycorrhiza; effect of

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long-term nitrogen deposition on tree nutrition; criteria for nitrogen saturation in forest ecosystem.

Nitrogen input and mycorrhiza The mycorrhizal infection of forest trees occurs under quite different soil conditions and levels of fertility. Bjrrkmann (1942) found high levels of mycorrhizal infection in the best growing stands on fertile soils as well as in stands of low increment on poor sites. Naturally, the mycorrhiza species vary with site and stand conditions. It is evident that mycorrhiza can develop on quite different levels of nitrogen supply. On the other hand, it is also known that trees may grow very well without mycorrhizal infections. Recent studies investigated the influence of varying mineral nitrogen supply on the formation of mycorrhiza and the growth of conifer seedlings under greenhouse conditions. It was found that higher concentrations of ammonium reduced the growth of pine seedlings (Mehrer et al., 1987) and the rate of mycorrhizal infection in spruce (Haug et al., 1988; Meyer, 1985). The negative effects occurred at ammonia concentrations of 15 mmol (Mehrer et al., 1987) or at 37.7 mmol (Haug et al., 1988). The experimental data given by Meyer (1985) indicate effects at concentrations up to 100mmol. Such concentrations are commonly used in laboratory studies or horticulture with highly nitrogen demanding species but they are 10-100 fold higher than the values measured in the soil solution of forests. There we normally find much less than 1 mmol. Soil solution and surface water of Black Forest sites are lower than 0.1 mmol (Brahmer and Feger, 1987). Under extremely high atmospheric input rates of ammonium (40-60 kg N h a -1 yr -1) van Breemen et al. (1987) reported NH4-concentrations of 0.53 mmol in throughfall of oak-birch stands on acid sandy soils and 0.07-0.09 in soil solution. Thus, the results of high treatment doses in greenhouse experiments cannot be transferred to forest conditions. Under field conditions, the effects of mineral nitrogen fertilizers have been studied intensively focusing on growth responses. However, there are many data and many observations available on the reaction of mycorrhiza forming fungi.

These findings commonly refer to the number of species and to the frequency of the fruiting bodies. A rough correlation between the numbers of epigeous basidiocarps and the numbers of mycorrhizal root tips can be reasonably assumed. Nevertheless, the sporosphore production by mycorrhizal fungi does not necessarily reflect the frequency of these species as symbionts in mycorrhiza (Menge and Grand, 1978). Keeping these reservations in mind a review of reported data from coniferous forests leads to the following evaluation. Heavy mineral nitrogen fertilizations (urea, ammonium sulphate, ammonium nitrate) as single or repeated applications of 100-300 kg N ha -1 may reduce mycorrhizal infection frequency but increase the number of mycorrhizal types (Alexander and Fairley, 1983; Menge and Grand, 1978; M/iller and Oberwinkler, 1988). Specific successions in fungi population occur but no general negative effects of nitrogen inputs have been observed. In nutrient poor forest types, in particular, the yield of edible basidiomycetes increased after nitrogen fertilization. Ohenoja (1978) reported increased fruit body production of Lactarius rufus and Paxillus involutus in coniferous forests in Finland after nitrogen fertilization. Earlier observations in fertilization experiments in Scots pine stands on very acid sandstone podzols have shown a striking increase in PaxiUus involutus and Boletus badius (Z6ttl and Kennel, 1962). Summarizing the impact of nitrogen on mycorrhiza it can be stated that suppressive as well as stimulating effects are observed. Different site conditions considerably influence the picture. A general conclusion that increased nitrogen (ammonium) supply in European forests reduces the formation of mycorrhiza cannot be drawn.

Nitrogen status of trees With an intention to answer the question 'how do trees react to higher nitrogen supply' the knowledge derived from fertilization experiments in Central Europe and Scandinavia should be applied. Following on from Wittich's (1951) finding that the formerly wide-spread practice of removing litter had resulted in greatly reduced nitrogen reserves of the forests, a wave of nitro-

Effects of nitrogen deposition to forest ecosystems gen fertilization started in the 1950's. Application rates of 50-200 kg N ha -1 improved the nitrogen level in the foliage and considerably raised the rate of wood increment (Kenk and Fischer, 1988; ZSttl, 1964). Repeated applications of low doses were less effective than a higher single dose (Z6ttl and Kennel, 1963). Long-term fertilization experiments in Sweden with yearly application rates of 30 or 6 0 k g N h a -1 influenced tree growth positively (Tamm, 1985). Based on nutrition experiments with seedlings (Ingestad, 1962) the optimum nitrogen levels under field conditions were determined for Norway spruce (Tamm, 1968) and Scots pine (Kennel and Wehrmann, 1967). A rough estimate of the utilization percentage of the applied fertilizer nitrogen suggested rather low values, generally in the order of 20% (Z6ttl, 1959). Evidently, the applied nitrogen is to a large extent taken up and accumulated by microorganisms, fungi, soil fauna, and ground vegetation before tree roots can take it up. Nitrate losses e.g. leaching from the rooted soil were not studied in these older field experiments. With regard to all these nitrogen fertilization results the actual nitrogen deposition of 102 0 k g N h a i appears more beneficial than harmful- at least so far as the nutrition level and growth of forest trees is concerned. Furthermore, only a small fraction of the nitrogen which reaches the soil by throughfall can be expected to contribute to tree nutrition, slightly improving the wide-spread sub-optimal or even insufficient quantities of available nitrogen in these forests. This situation is shown by comparing foliar analysis data from different periods and areas with the threshold values for optimum nutrition. For SW-Germany, no significant trend in the level of nitrogen nutrition in Norway spruce and silver fir was seen (HiJttl, 1985; Z6ttl and Hfittl, 1985). Young spruce in a network of nearly 150 plots comprising the 'diagnostic fertilization project' contained between 12 and 1 6 m g N g d . m . -1 in current needles (Liu, 1988). The addition of nitrate fertilizer to a series of plots generally resulted in higher needle nitrogen contents and better height growth (Liu, 1988). These interactions of nutrient and growth show that under the actual conditions optimum nitrogen levels are seldom reached and supra-optimum nitrogen supply certainly is not found in SW-Germany.

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Optimum nitrogen supply combined with excellent growth of Norway spruce is reported under input conditions of 35-40 kg N ha 1 in the HSglwald (SW-Bavaria) by Kreutzer (pers. comm.). There is an actual wood increment of 24m 3 ha --~. This productivity will probably be maintained as long as the supply of other nutrients remains sufficient. Under higher nitrogen input conditions nutrient imbalances (P, Cu) appear. For example, in Scots pine or Douglas fir stands on nutrient poor sandy soils, the foliar nitrogen content may reach 27 mg/g d.m. due to the impact of high ammonia emissions from bio-industries (van den Burg, 1988; Kaupenjohann, 1988). Under specific conditions of an overall good supply of nutrients with the exception of one element, e.g. magnesium, a strong imbalance was induced after application of a high dose of ammonium sulphate. This experimental application raised the needle nitrogen content (first whorl) from 17 to 2 1 m g g d . m . -1 and reduced the magnesium level to 0.3 mg g d.m. -1 with consequent appearance of pronounced deficiency symptoms (H/ittl, 1988). Additional examples are given by H/ittl (1989). Another type of induced deficiency is that of boron which has been observed in Finnish and Scandinavian fertilizer trials where biomass increment was considerably increased by application of N(PK) on poor sandy soils and peatland (M611er, 1983). In summary, it can be stated that under the present condition of a deposition of 1020 kg N ha ~ over wide areas coniferous forests still show a sub-optimal nitrogen nutrition status. However, growth is often better than in former times (Kenk and Fischer, 1988). Severe nutrient imbalances occur only when input rates are considerably higher and when the supply of other nutrients is insufficient. Valuable knowledge can be drawn from nitrogen fertilization experiments. However, there is an important proviso in that 'atmospheric fertilization' first affects the canopy where a certain portion of the nitrogen compounds may be absorbed by the foliage (Grennfelt and Hultberg, 1986). With fertilization, the mineral nitrogen has always been applied directly to the soil. Other important aspects, such as the long-term effect of nitrogen deposition on soil acidification

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or on the species composition of plant communities cannot be discussed here.

Nitrogen saturation of forest ecosystems As mentioned above, only a rather small part of the atmogenic nitrogen input is directly taken up by the trees and incorporated into the biomass. Much is stored in the soil organic matter. These nitrogen reserves vary considerably from site to site. In Bavarian forest soils (1 m depth) Emberger (1965) found 2000-16000 kg N ha -I. Emberger could show that the clay mineral content of the soil and secondly the phosphorus content are the dominant factors determining the level of soil nitrogen reserves. A key process is the formation of clay-humus-complexes by biological activity, producing compounds with a high stability against further microbial mineralization. The nitrogen pool in the organic floor is much less stable due to rather rapid changes of the mineralization rate. The varying set of site factors and soil properties has led to different amounts of nitrogen reserves. This means that the nitrogen storage capacity of soils (ecosystems) varies widely, also in relation to atmogenic depositions. Clay-rich soils of a high fertility level can accumulate high amounts of nitrogen in stable form. Sandy soils, mostly of a low base saturation, can only store certain amounts of nitrogen in the organic floor as long as decomposition is low. From this it must be concluded that under a given input nitrogen saturation of ecosystems will be reached at very different levels of nitrogen reserves and these levels are subject to changes within the lifetime of a forest stand. Therefore, the absolute amount of nitrogen stored in soils (ecosystems) cannot serve as a parameter to define the saturation level. A better insight is provided when considering the input/output-relation of an ecosystem, including the reaction of the vegetation compartment. Comparisons of the input/output data of ecosystems or catchments give some hints about the actual nitrogen balance, but do not necessarily answer the central question of the role of increased deposition in the nitrogen cycle. This is because the mineralization rate of organic nitrogen can change considerably during the lifetime

of a stand and therefore periods of higher nitrate leaching produce a negative balance. The nutritional status of the vegetation compartment in relation to growth parameters must be considered in this context. Nitrogen saturation occurs when primary production is not further raised by an increased nitrogen supply and the system loses considerable amounts of nitrogen (Nilsson, 1986). Here it should also be remembered that an insufficient supply of some other nutrient element could be limiting growth, a situation which can change within a short period of time. Certainly, foliage analysis to determine the nutritional status in relation to established optimum threshold values is very helpful. These few remarks may underline the complexity of the nitrogen saturation concept. In the following section these ideas will be used to discuss the nitrogen balance data from experimental areas of the ARINUS project (Z6ttl et al., 1987). These results also illustrate the actual situation of representative forest sites in SW-Germany. The site at 'Schluchsee' (upper part of Fig. 1 and left part of Table 1) is typical for the submontane zone of the southern Black Forest. A large earthworm (Lumbricus badensis) ensures a high biological turnover in the coursely weathered granite podzol in spite of the strong tendency for raw humus formation (Lamparski, 1985). The managed spruce stand has a rather good nitrogen nutritional status. This is in contrast to the low current increment (see Table 1). Such a situation has been described as characteristic of high elevation forests where the nitrogen supply by microbial mineralization is not fully used for biomass production due to a temperature limited short growing season (Erhardt, 1961; Ferraz, 1985). The site at 'Villingen' (lower part of Fig. 1 and right part of Table 1) is characterized by loamy acid brown earth derived from a quartz-rich and base-poor sandstone which forms the middle and northern part of the Black Forest. The relation between the present nitrogen nutritional status and the growth of the managed spruce stand still fits well into the frame found in Bavarian spruce forests 30 years ago by Strebel (1960). The rather poor nitrogen status of the trees, as revealed by needle analysis, is mainly a result of

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Table 1. ARINUS catchment balance June 1987-May 1988 (from Brahmer and Feger, 1989) Site details

Catchment Schluchsee

Villingen

Parent rock Soil type 0~ soil horizon pH 0~ soil horizon C:N Altitude Mean ann. temp Precipitation Tree crop Site index

Granite Podzol 2.9 29.5 1150-1250 m a.s.l. 5°C 2301 mm Norway spruce 20-40 yrs 6-7*

quartz-rich sandstone acid brown earth 2.8 36.5 800-960 m a.s.1. 6°C 1638 mm Norway spruce 80-100 yrs 8-9*

NH4_N kg ha i NO3-N kg ha- ~ SO4-N kgha -1

Open land 7.2 4.9 8.9

Throughfall 4.6 5.1 11.1

Streamwater 0.1 10.7 26.1

Open land 6.0 4.4 7.6

* Mean annual increment in m 3 related to total production in 100 years.

Throughfall 2.6 3.2 12.4

Stream and Groundwater 0.3 0.5 16.2

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former anthropogenic degradation (cattle grazing, litter removal) of this forest (Rothfuss, 1988). In both areas the open land bulk deposition of ammonium, nitrate and sulphate is low and very similar. But the output rates are very different. In 'Schluchsee' the N (and S) output exceeds considerably the atmospheric input whereas in 'Villingen' almost no nitrogen losses are found and the S output is lower than at 'Schluchsee'. Thus, there are totally different nitrogen balances in spite of the fact that the low input rates are comparable and the soil nitrogen reserves do not differ greatly between the two sites. It seems evident that a very active soil fauna and intensive microbial turnover processes play the key role at 'Schluchsee'. This has been illustrated in more detail by field and laboratory incubation tests (Z6ttl et al., 1989). At least at the present time, the 'Schluchsee' catchment appears to be nitrogen saturated, whereas the catchment 'Villingen' is still accumulating nitrogen. At the latter site the spruce stands are low in foliage nitrogen level and the C:N-ratio of the organic floor is high (see Table 1). The trees appear to be taking up nitrogen by foliar absorption as is suggested by the difference between the nitrogen open land deposition and throughfall. 'Villingen' is certainly far from being nitrogen saturated.

Conclusions

The wide-spread nitrogen deposition rates of 10-20 kg ha -1 year -1 are without question an important ecological factor but they do not appear as the main cause of the 'new type forest damage'. In the light of the pronounced regional variations of the nitrogen input any generalization of results seems very problematic. In the long run the nitrogen storage capacity of soils has to be considered as a decisive factor, and this varies considerably from site to site. In order to estimate the degree of 'nitrogen saturation' of a forest stand (ecosystem, catchment) all available parameters have to be considered, inter alia nutritional status and growth of the trees, reserves stored in soil and biomass, input/output data, biological turnover processes, history of land use. Even if an agreement on a definition of 'nitrogen saturation' cannot be ex-

pected soon, further discussions are very useful to create a more detailed picture of the problem. Low to medium level nitrogen deposition can be seen as a challenge to forestry practice. It seems possible to make efficient use of the 'atmospheric fertilizer' by control of the nutritional status through foliar analysis, specific fertilization and favouring more nitrogen demanding species. However, certain low nitrogen demanding ecosystems can be maintained only under reduced input rates. Also, with a view to the long-term effects of higher nitrogen depositions on soil and ground water properties, all possible means should be applied to reduce relevant atmospheric emissions. In this respect ammonia emissions from 'bio-industries' and application of liquid organic manure deserve special attention.

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Nihlgard B 1985 The ammonium hypothesis-An additional explanation to the forest dieback in Europe. Ambio 14, 2-8. Nilsson J 1986 Critical loads for nitrogen and sulphur. Nordic Council of Ministers Report, Copenhagen. Ohenoja E 1978 Mushrooms and mushroom yields in fertilized forests. Ann. Bot. Fennici 15, 38-46. Raisch W 1983 Bioelementverteilung in Fichten6kosystemen der Barhalde (Sfidschwarzwald). Freib. Bodenkundl. Abh. 11, 1-238. Rothfuss I 1988 Menschlicher Einftuss auf den ViUinger Stadtwald: Geschichtliche Entwicklung und Auswirkungen auf den heutigen Bodenzustand. Dipl. Arbeit Forstwiss. Fak. Univ. Freiburg/Br. Schulze E D, Oren R and Zimmermann R 1987 Die Wirkung von Immissionen auf 30-j~ihrige Fichten in mittleren Hfhenlagen des Fichtelgebirges auf Phyllit. Allg. Forstz. 42, 725-730. Strebel O 1960 Mineralstoffern~ihrung und Wuchsleistung von Fichtenbest/inden (Picea abies) in Bayern. Forstwiss. Cbl. 79, 17-42. Tamm C O 1968 An attempt to assess the optimum nitrogen level in Norway spruce under field conditions. Studia For. Suec. 61, 1-67. Tamm C O 1985 The Swedish optimum nutrition experiments in forest stands - aims, methods, yield results. K. Slogs. -o. Lantbr. akad. tidskr. Suppl. 17, 9-29. Van Breemen N, Mulder J and Van Grimsven J J M 1987 Impacts of acid atmospheric deposition on woodland soils in the Netherlands. II. Nitrogen transformations. Soil Sci. Soc. Am. J. 51, 1634-1640. Van den Burg J 1988 N-Deposition, N~ihrstoffversorgung und Dfingungsversuche in den Niederlanden. Vortrag Tagung Sektion Waldern~ihrung, Wingst. Wittich W 1951 Der Einfluss der Streunutzung auf den Boden. Forstwiss. Cbl. 70, 65-92. Z6ttl H W 1959 Voraussetzungen fiir eine wirkungsvolle Verbesserung der Stickstoffversorgung von Nadelholzbest~inden. Z Pflanzenernaehr Dueng. Bodenkd. 84, 116122. Z6ttl H W 1964 Wirskamkeit der Forstdiingung in Siiddeutschland. 8. Int. Congr. Soil Sc. Bucharest V, 10091017. Z6ttl H W, Feger K H and Brahmer G 1987 Projekt ARINUS I. Zielsetzung und Ausgangslage. KfK-PEFBerichte 12, 269-281. Z6ttl H W, Feger K H and Simon B 1989 Untersuchung der Auswirkungen von Neutralsalzgaben im Projekt ARINUS. IMA-Querschnittseminar Bayreuth, KfK-PEF-Berichte, 55, 119-128. Z6ttl H W and Hiittl R F 1985 Schadsymptome und Ernfihrungszustand von Fichtenbestanden im sfidwestdeutschen Alpenvorland. Allg. Forstz. 40, 197-199. Z6ttl H W and Kennel R 1962 Die Wirkung von Ammoniakgas- und Stickstoffsalzdiingung in Kiefernbestfinden. Forstwiss. Cbl. 81, 65-91. Z6ttl H W and Kennel R 1963 Ernfihrungszustand und Wachstum von Fichten-Altbest~inden nach Ammoniakgasund Stickstoffsalzdiingung. Forstwiss. Cbl. 82, 76-100.