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Journal of Environmental Quality 31:1868-1874 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Ground Water Quality

Leaching of Nitrogen and Phosphorus during Production of Forest Seedlings in Containers

Marja-Liisa Juntunen*,a, Taina Hammarb and Risto Rikalaa

a The Finnish Forest Research Institute, Suonenjoki Research Station, FIN-77600 Suonenjoki, Finland
b North Savo Regional Environment Centre, P.O. Box 1049, FIN-70101 Kuopio, Finland

* Corresponding author (marja-liisa.juntunen{at}metla.fi)

Received for publication July 31, 2001.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Little information is available concerning the contamination risk caused by forest seedling nurseries to local surface and ground waters compared with agricultural and horticultural production. Leaching of nitrogen (N) and phosphorus (P) through peat growing medium in containers and nutrient uptake of seedlings were monitored in production of silver birch (Betula pendula Roth), Norway spruce [Picea abies (L.) Karst], and Scots pine (Pinus sylvestris L.) seedlings. About half of the applied nutrients (total amount applied = 149 to 260 kg N ha-1 and 60 to 108 kg P ha-1) was premixed into the peat medium, as is usual in Finnish nursery practice, and the other half was applied to seedlings in liquid form with mobile booms. Depending on tree species, 11 to 19% of the applied N was recovered in leachates and 15 to 63% in seedlings. The undiscovered proportion varied from 19 to 71%. The amounts of leached N were 19 to 41 kg ha-1. Only 5 to 31% of the applied P was recovered in seedlings; 16 to 64% (11 to 56 kg ha-1) was found in leachates. Total N and P load to the environment may increase substantially if nutrients applied in liquid fertilization outside container trays are included. Consequently, it is important to determine the sources of nutrient load in container seedling production to mitigate the risk of environment contamination.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
GROUND WATER pollution and eutrophication of surface waters due to agricultural practices have been reported worldwide. The risk posed by production of forest tree seedlings, however, is not well known (Landis et al., 1991). Although the total use of fertilizers in forest nursery production is small compared with that in agriculture and horticulture, there can be at least local risks because some nurseries are situated on areas where ground water reservoirs form and/or near lakes and rivers. Seedling production is a part of the forest industry, and therefore knowledge of the environmental impacts of nurseries is also needed, for example, in life cycle analyses of wood products (Aldentun, 2002).

Mälkki et al. (1988) reported that production of bareroot seedlings may contaminate ground water with N compounds from fertilizers. Container seedling production has, however, largely replaced bareroot production in Norway, Sweden, Finland, and Canada. Since the early 1980s, the proportion of container production has increased from 30% to more than 90% in Finland. Concurrently, annual seedling production has decreased from 250 to 150 million seedlings (Rikala, 2000). These changes have decreased the total amount of fertilizers used in Finnish forest nurseries; in 1976, nurseries used about 650 Mg of fertilizer (Rikala and Westman, 1979) and in 1996 about 200 Mg (Juntunen and Rikala, 2001). On the other hand, the amounts of nutrients applied per unit area in production of container seedlings were higher than in production of bareroot seedlings.

Container seedling production is more intensive than bareroot production due to higher seedling densities and shorter growing times. In 1996, more than 90% of all container-grown pine and birch and 43% of spruce seedlings were delivered for planting as one-year-old seedlings, and the rest mostly as two-year-old seedlings in Finland (Juntunen and Rikala, 2001). Growing of container seedlings starts at the end of March in heated greenhouses and later in so-called season greenhouses. The first crops are moved outdoors in early June when the night frosts are over, but many nurseries keep spruce seedling lots in greenhouses until autumn (Juntunen and Rikala, 2001). Over winter most seedlings are stored outdoors under snow cover, which comes between mid-October and mid-December.

During production of container seedlings, a potential risk for nutrient leaching exists, as large volumes of irrigation water are used, and in many countries nearly all fertilizers are applied in liquid form through irrigation systems (fertigated) (Landis et al., 1989; Dumroese et al., 1995). Among other things, the influence of irrigation volume, application method, and fertilizer type on leaching of water and nutrients has been studied with different horticultural crops (Hershey and Paul, 1982; Rathier and Frink, 1989; McAvoy et al., 1992; Fare et al., 1994; Broschat, 1995; Andersen and Hansen, 2000). However, most of the crops studied have been grown in large, individual pots, one to five liters in volume, while forest seedlings are grown in small pots, 40 to 300 mL in volume. The individual pots, in forest nursery terminology often referred to as containers, are usually produced in aggregates called trays (Landis et al., 1990).

Little information is available concerning the total annual amounts of nutrients leached in forest nurseries that use containers. To our knowledge, only Dumroese et al. (1991)(1995) have published results on the discharge of water and N during production of container conifer seedlings in a nursery. The aim of this case study was to determine the leaching of N and P through seedling container trays into the ground in nursery production.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Study Material and Growing Practices
Leaching of N and P was monitored in container production of silver birch, Scots pine, and Norway spruce seedlings at Suonenjoki Nursery in Finland (62°39' N, 27°03' E) in 1995.

Birch seedlings were grown in Plantek hard plastic container trays (Lännen Plant Systems, Säkylä, Finland), and the spruce and pine seedlings in Ecopot plastic-laminated paper container trays (Lännen Plant Systems) (Table 1). Containers were filled with medium-grade Sphagnum peat (Finnpeat M6; Kekkilä Corp., Tuusula, Finland) premixed with 0.8 kg base fertilizer (16–8–16 N–P–K) to one cubic meter peat (65 kg dry peat). Of the premixed N, 44, 38, 16, and 3% was in the form of ammonium, ureaformaldehyde, nitrate, and urea, respectively. The amounts of N and P applied per unit area or per seedling are given in Tables 2 and 3. The area-based amounts of N and P were calculated by weighing the amounts of peat in the container trays after filling. The dry mass of peat was determined by drying peat samples at 100°C for 24 h.


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  		Table 1. Dimensions of the container types.

 

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  		Table 2. Number of replicate trays for leachate analyses of nitrogen compounds (N) and phosphorus (P) and number of sample seedlings per tray for height monitoring and morphological measurements.

 

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  		Table 3. Amounts of N applied and recovered in leachate and in seedlings by tree species in 1995 and in 1996. The values are given in both per unit area and per seedling.

 
Growing was started in greenhouses (Fig. 1A) . Birch seeds were first sown on peat-filled flats on 2 May and the germinants, or "small seedlings," were transplanted into containers beginning on 25 May. About one month later the birch container trays were transported outdoors and placed 20 cm apart. Conifer seeds, on the other hand, were sown directly into the containers.



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  		Fig. 1. (A) Scheduling of culturing practices for birch, pine, first- and second-season spruce, and (B) height development of seedlings during the 1995 growing period. {blacksquare}, sowing; {diamondsuit}, transplanting; {square}, separation of trays. Symbols for tree species in (B) describe the dates of liquid fertilization in (A).

 
Seedlings were irrigated and fertigated with manually controlled mobile-boom sprayers. Both birch and conifer seedlings were irrigated about 20 times a month. At each irrigation session, the birch seedlings were given, on average, 8 mm of water (50 mL per seedling) and conifer seedlings 5 mm of water (about 10 mL per seedling). Due to rain, the second-season spruce seedlings, which were grown outdoors during the whole season, were irrigated only about six times a month. At each irrigation, 8 mm of water (about 20 mL per seedling), on average, was applied. The water content of the growing medium was monitored weekly by weighing container trays; the aim was to maintain the water content of the peat medium at an optimum level (40–55% v/v; Puustjärvi, 1977).

The timing of fertigations was determined by measuring the electrical conductivity of the press water from peat medium weekly as in the normal nursery routine (Juntunen and Rikala, 2001). The seedlings were fertigated three to eight times (Fig. 1A) with Superex fertilizers (Kekkilä Corp.), in which about 50% of the N was applied as urea, 37% as nitrate, and more than 10% as ammonium.

In 1996, the leaching studies were replicated in the production of one-year-old pine seedlings grown in Ecopot and Plantek container trays (Table 1). The seedlings were grown in the same manner as pine seedlings in 1995 (Fig. 1A), except that the plastic cover was removed one week earlier. Due to the greater filling density of peat in Plantek than in Ecopot container trays, the amounts of premixed N and P per seedling were the same even though the volume of Plantek containers was smaller (Table 1). Obviously, the tapering form of the Plantek containers was the reason for the greater filling density. Both container types were fertigated three times with the same amount of Superex fertilizer per unit area (Tables 3 and 4).


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  		Table 4. Amounts of P applied and recovered in leachate and in seedlings by tree species in 1995 and in 1996. The values are given in both per unit area and per seedling.

 
Collection and Analyses of Leachate Samples
For collecting leachate waters, sloped polystyrene plates (size of container trays, either 40 x 40 cm for Plantek or 60 x 40 cm for Ecopot) equipped with a hole and a sampling vessel were placed under the container trays (Table 2). The plates were exactly the size of the container trays and therefore only the leachate was collected; neither irrigation water nor rainfall could hit the plates. Collection of leachates began from the time the conifers were sowed and the birch germinants transplanted and continued until the peat growing medium froze in late October.

The volume, electrical conductivity (CDM80; Radiometer, Brønshøj, Denmark), and pH (3020; Jenway, Dunmow, England) of leachates were measured daily (excluding Saturday and Sunday) during 1995 and weekly in 1996. Samples were stored frozen for approximately 3 to 4 mo before nutrient analyses. When amounts of leachate were small and electrical conductivity values were at the same level, samples from two to four successive sampling times were pooled before nutrient analyses.

Total N in the leachate samples was measured according to the standard SFS-EN ISO 11905-1 (Finnish Standards Association, 1998) and PO4–P according to SFS 3025 (Finnish Standards Association, 1986). The sum of NO3–N and NO2–N was determined by the FIA (flow injection analysis) method (Lachat [Milwaukee, WI] Quick Chem 8000) according to the standard SFS-EN ISO 13395 (Finnish Standards Association, 1997). The NH4–N was analyzed spectrophotometrically (SFS 3032 [Finnish Standards Association, 1976]) and the fraction of organic N was determined by subtracting inorganic N from total N.

Seedling Measurements
During the growing season, the height growth of seedlings was measured weekly (Fig. 1B, Table 2). When leachate collection was stopped, seedlings were harvested (Table 2), and leaves, stems with branches (later referred to as stems), and roots were separated and dried (roots after washing) for 48 h at 60°C before they were weighed. For nutrient analysis, the leaves, stems, and roots were pooled separately by trays. Nitrogen concentration was determined with a LECO (St. Joseph, MI) CHN-600 analyzer, and P concentration was determined from dry-digested (2 M HCl) samples (Halonen et al., 1983) using plasma emission spectrophotometric analysis (ICP, ARL 3580; Applied Research Laboratories S.A., Ecublens, Switzerland).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Monitoring
1995
On average (standard deviation in parentheses), 25 (3), 122 (23), 141 (10), and 175 (30) mm of water leached through first-season spruce, second-season spruce, pine, and birch container trays, respectively (Fig. 2A–D) . Depending on tree species, 11 to 31% of the applied water (irrigation + precipitation) leached through the trays. During the greenhouse period, leaching was less than 10%. In the autumn, 50 to 70% of rainwater leached through the trays, because during rainy periods with decreased evapotranspiration the water content of peat medium exceeded the container capacity (data not shown). From all tree species, more than 50% of the total leachate water was collected during July and August after the container trays were moved outdoors.



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  		Fig. 2. Monthly amounts of precipitation, irrigation and leachate (mm) for (A) first-season spruce, (B) second-season spruce, (C) pine, and (D) birch in 1995, and for pine in (E) Ecopot and (F) Plantek containers in 1996.

 
The amounts of leached N per unit area varied from 23 to 41 kg ha-1, depending on tree species (Table 3). The same proportion (13 to 19%) of applied N was recovered in all leachates regardless of species. In birch seedling production, almost 50% of the total N recovered in the leachates occurred during May and in early June. About 50 to 90% of the N leachate from conifer trays was measured during July and August (the fertigation period) (Fig. 3) . After the beginning of September less than 15% of the total N leached through trays of any tree species.



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  		Fig. 3. Leachate volume (mm, bars), N concentration (mg L-1, dots), and amount (kg ha-1, bars) of N leached from birch and pine containers from May to October in 1995.

 
The highest concentrations of N, approximately 300 mg L-1, were measured in the leachates from birch trays just after the germinants were transplanted but then decreased to less than 10 mg L-1 at the end of August (Fig. 3). In leachates from conifer trays, N concentrations were usually 20 to 50 mg L-1, while some occasional peaks of 100 to 200 mg L-1 were measured (Fig. 3). In September and October the N concentrations were less than 10 mg L-1.

Depending on tree species, 24 to 54% of the N leached as NO3–N, 7 to 46% as NH4–N, and 24 to 59% as organic N (Fig. 4) . The largest amounts of NO3–N (16 kg ha-1) were found in the leachates of second-season spruce. From birch and first-season spruce containers N leached mainly as ammonium. Small amounts of organic N were measured in leachates throughout the growing season; but after the end of August, in all water samples almost all the leached N was in the organic form.



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  		Fig. 4. Leached amounts of NO3–N, NH4–N, and organic N by (A) tree species in 1995 and (B) container types in 1996.

 
The amount of leached P and its proportion of the applied P differed greatly among tree species (Table 4). In leachates of birch and pine more than 50% of the applied P was recovered, but in leachates of spruces only 25% or less. About 80% of the P premixed into peat leached through birch containers in May and in early June.

The PO4–P concentrations in the leachates from birch containers followed the pattern for N concentrations; they were highest, 200 to 300 mg L-1, just after the germinants were transplanted and decreased gradually during the summer (not shown). The concentrations of PO4–P in the leachates from conifer trays varied greatly, from 5 to 100 mg L-1, and there was no clear trend over time. After August, however, the concentrations were always less than 10 mg L-1.

About the same proportions, 52 to 63% of the applied N and 25 to 31% of applied P, were recovered in seedlings (Tables 3 and 4). The first-season spruce seedlings were an exception; they took up only 15% of the applied N and 5% of the applied P.

1996
On average (standard deviation in parentheses), 53 (9) and 47 (14) mm of water leached through Ecopot and Plantek pine trays (Fig. 2E,F). More than half of the leachate was collected in July and August after container trays have been moved outdoors. Due to a dry autumn, the amounts of water leached in September were very small. The amounts of N leached through the two container types were the same (19 kg ha-1) (Fig. 4B). Nearly all the N leached in late July and August. A greater amount of P leached through the Plantek than Ecopot trays (Table 4). Seedlings grown in Ecopot container trays took up more N and P than those grown in Plantek containers (Tables 3 and 4).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The annual amounts of leached N and P, 19 to 41 and 11 to 56 kg ha-1, respectively, were small compared with those measured in studies of horticultural crops (van der Boon and Niers, 1983; Broschat, 1995). Comparison with horticultural studies was difficult, however, because the amounts of nutrients leached have often been given per pot instead of per unit area, and the fertilizer system and methods used were different from those used in forest nurseries. Based on the results by Dumroese et al. (1995), we calculated that in production of ponderosa pine (Pinus ponderosa P. Lawson & C. Lawson) the total N discharge was about 130 N kg ha-1.

Obviously, the irrigation method used in this nursery study explains the small amounts of leachate and partly explains the small amounts of N and P leached. An attempt was to made to keep the water content of the peat medium at an optimum level, 40 to 55% v/v (Puustjärvi, 1977), by monitoring container weight weekly, as is the practice in most Finnish nurseries (Juntunen and Rikala, 2001). Therefore, the amounts of leachate (25–175 mm) were small, especially during the greenhouse period. In the study of Dumroese et al. (1995), the amounts of discharged water were as large as 450 to 800 mm. Finnish forest nurseries do not irrigate and fertigate in excess of container capacity as do some forest and ornamental nurseries (McAvoy et al., 1992; Yelanich and Biernbaum, 1993; Dumroese et al., 1995) to reduce the potential for accumulation of soluble salts and nutrient imbalance (Landis et al., 1989; Biernbaum, 1992). The greater the leachate fraction, the greater is the risk of nutrient contamination of the environment (McAvoy et al., 1992).

The amounts of nutrients applied by Suonenjoki nursery were virtually the same as the mean total amounts of nutrients applied by Finnish nurseries in 1996 (Juntunen and Rikala, 2001). The frequency of fertigations was, however, lower than the average (once a week) for Finnish nurseries. The few fertigations must have caused high peaks in the nutrient content of the peat medium. When the nutrient content of peat is large, irrigation water and rain could leach nutrients. In 1995, for example, a heavy rainfall (10 mm) that occurred soon after one fertigation leached about one-third of the total N leached from the pine and second-season spruce container trays during the whole collection period. The experiences of van der Boon and Niers (1983) in an ornamental nursery were similar. In growing of first-season spruce, almost all leaching of N and P occurred in connection with fertigations. Due to the few fertigation sessions, the fertilizer doses and the amounts of water applied were large, which increased leaching.

The other situation, where the nutrient content of peat was large, was at the beginning of seedling growth because of the fertilizer premixed into peat medium. In production of birch seedlings, large amounts of N and P leached with small amounts of water in May and in early June. The large volume of the birch containers might have increased the leaching of nutrients, because during the first growing month birch and pine did not differ either in fertilization or in amounts of leachate water.

In many studies only the leaching of NO3–N has been measured (Rathier and Frink, 1989; Fare et al., 1994; Broschat, 1995; Andersen and Hansen, 2000), obviously because the NO3–N levels of 10 mg L-1 (USEPA, 2001) or NO3 levels of 50 mg L-1 (NO3–N 11.3 mg L-1) (European Community, 1998) in drinking water are considered unsafe for humans. In our study, however, the amounts of NO3–N in leachates covered only 25 to 54% of the total N leached. Ammonium and organic N compounds can also increase the risk of ground water contamination, since they can later be transformed to nitrate in the soil (Addiscott et al., 1991; Colangelo and Brand, 2001). Therefore, apparently NO3–N analyses are not enough; when the risk of contamination is evaluated, at least total N analyses are also needed.

The proportion of N that leached through the peat medium was fairly constant at 11 to 19% of applied N, while the proportions of P (16 to 64%) recovered in leachates varied greatly. The large amounts of P in leachates do, however, indicate that P leaches easily from peat medium. Based on measurements of P adsorption isotherms, Marconi and Nelson (1984) came to the same conclusion.

The proportion of N and P not recovered in leachates and seedlings was generally high, greater than 50%, which means that nutrient losses into the nursery environment due to leaching cannot be monitored with precision using only the nutrient content of seedlings. With the nutrient content of the crop, only the magnitude of the risk of leaching can be estimated. Because the growing medium was not analyzed, it is not possible to know whether the nutrients remained in the peat medium in late October or whether N had volatilized into the atmosphere during the summer and autumn. However, we speculate that very little soluble P and inorganic N were in the peat medium after the end of August because the amounts of these elements in the leachates were small.

In the production of container seedlings, the total nutrient load consists of the amounts of nutrients leached from containers into the ground and fertigated directly into the ground outside the seedling trays. Based on the dimensions of the greenhouses and the coverage of seedling trays, we estimated that about 10 to 20% of the water volume was irrigated alongside the conifer containers. It can be concluded that in pine production fertigation outside seedlings, 17 kg N ha-1 (20% x 87 kg N ha-1), and leaching through seedling trays, 18 kg N ha-1 (80% x 23 kg N ha-1), caused an equal N load. In birch production about half of the fertigation water fell outside container trays because of the separation of birch trays. In this situation the load caused by fertigations directly on the ground was much greater, 88 kg N ha-1 (50% x 175 kg N ha-1), than the load of N leached, 20 kg N ha-1 (50% x 41 kg N ha-1).

The annual leaching of N through container medium into the ground was the same order of magnitude as the mean losses of N, about 18 to 20 kg ha-1, from Finnish agricultural fields (Rekolainen et al., 1993). The P leached in substantially greater amounts than what had been measured from agricultural fields, 0.95 to 1.7 kg ha-1 (Rekolainen et al., 1997). The amounts of N and P fertigated outside the container trays, however, increased the total load of N and P per unit area. In addition, the same type of production continuing at the same places for years and most of annual nutrient leaching occurring during one or two months could increase the risk of contamination. It is difficult to say when the amounts of nutrients discharged are so great that there could be a risk of harmful environment contamination. Many factors related to a nursery, such as depth of the water table, characteristics of soil layers between the ground and ground water reservoirs, and nearness of rivers and lakes, influence the risk of contamination.

Changes in fertilization methods, for example, increasing the frequency of applications and/or applying the nutrients in exponentially increasing additions instead of at constant rate of addition (Timmer, 1997), could increase the nutrient-use efficiency of seedlings and decrease the amount of nutrients applied and discharged. By premixing slow-release fertilizers, also known as controlled-release fertilizers (Landis et al., 1989), into growing medium it may be possible to avoid fertigations and decrease the amounts of nutrients applied directly on the ground. The elimination of rain by covering outdoor areas with movable roofs could also be worth studying. Making changes in nursery practices to decrease the environmental impacts of forest nurseries may increase production costs. At the same time, however, it can be assumed that many of these measures will improve seedling quality.


   ACKNOWLEDGMENTS
 
We thank the staff of Suonenjoki Nursery and Research Station for good cooperation during the experiment and the laboratory personal of North Savo Regional Environment Centre for making the chemical analyses. Special thanks to Sirpa Metsärinne and Mikko Maukonen for technical assistance. We also thank Dr. H. Heinonen-Tanski, Dr. H. Smolander, R. Richardson, and anonymous referees for valuable comments on the manuscript, and Dr. J. von Weissenberg for revising the English language. We gratefully acknowledge financial support from Pohjois-Savon Liitto and Metsäosaamiskeskus-hanke.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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