Long-term effects of nitrogen fertilization on soilchemistry in three Scots pine stands in Sweden

Eva Ring, Staffan Jacobson, and Lars Hogbom

Abstract: Adding nitrogen to coniferous forests on mineral soils will increase stem-wood growth in most boreal forests.The addition of nitrogen affects soils and waters as well. This investigation was conducted to evaluate the long-term ef-fects of nitrogen fertilization at different intensities on soil chemistry in nitrogen-limited ecosystems. The study was per-formed at three experimental sites that were originally established around 1980 in Scots pine (Pinus sylvestris L.) stands.Fertilization regimes with applications ranging from conceivable commercial rates to very intensive rates (3� 150 kgN�ha–1 up to 12� 150 kg N�ha–1) had been applied. Samples were collected from the FH horizon at all sites and 0–20 cmin the mineral soil at two sites and analyzed for pH and major nutrients. The carbon to nitrogen ratio in the FH horizondecreased with increasing total nitrogen application, while the concentrations and contents of nitrogen and exchangeablemagnesium and phosphorus increased. The concentration and contents of exchangeable potassium decreased in both theFH horizon and the mineral soil. In general, larger effects on soil chemistry were observed with increasing fertilization in-tensity.

Resume : L’ajout d’azote dans les forets de coniferes etablies sur des sols mineraux augmentera la croissance du troncdes arbres dans la plupart des peuplements boreaux. L’ajout d’azote affecte aussi le sol et l’eau. Cette etude visait a eva-luer les effets a long terme de differentes intensites de fertilisation azotee sur les caracteristiques chimiques du sol dansles ecosystemes pauvres en azote. L’etude a ete effectuee dans trois stations experimentales ou les traitements initiaux ontete appliques vers 1980 dans des peuplements de pin sylvestre (Pinus sylvestris L.). Des regimes de fertilisation avec destaux d’application variant d’un taux correspondant a une application commerciale a un taux tres eleve (de 3 a 12� 150 kgN�ha–1) avaient ete utilises. Des echantillons ont ete preleves dans l’horizon FH de toutes les stations et a une profondeurde 0 a 20 cm dans le sol mineral de deux stations. Ces echantillons ont ensuite ete analyses pour determiner le pH et laconcentration des principaux nutriments. Le rapport entre le carbone et l’azote de l’horizon FH diminuait avec la quantitetotale d’azote qui avait ete appliquee alors que la concentration et le contenu en azote, en magnesium echangeable et enphosphore echangeable augmentaient. La concentration et le contenu en potassium echangeable avaient diminue dans l’ho-rizon FH et dans le sol mineral. D’une facon generale, l’ampleur des effets sur les caracteristiques chimiques du sol aug-mentait avec l’intensite de la fertilisation.

[Traduit par la Redaction]

Introduction

Nitrogen is the primary limiting nutrient for stem-woodgrowth on mineral soils in most boreal forests (cf. Tamm1991; Vitousek et al. 1997). Simultaneous supply of othernutrients (P, K, Ca, and Mg) generally does not increase thegrowth response on mineral soils (Pettersson 1994a; Binkleyand Hogberg 1997; Jacobson 2001). Furthermore, it hasbeen shown that N fertilization, with the addition of B, gen-erally does not cause any serious long-term (20–30 years)nutrient deficiencies in trees, even after substantial N addi-tions (Jacobson 2001).

Besides increasing stem-wood growth, N addition to bor-eal forests affects soils and waters. The environmental ef-fects of forest fertilization have been investigatedthoroughly (inter alia by Binkley et al. 1999; Nohrstedt2001; Saarsalmi and Malkonen 2001). Its effects on soil and

water depend on many factors, including the applicationrate, type of fertilizer, application method, application time,weather, and soil type. Moreover, there may be both short-and long-term effects. Shortly after the application ofNH4NO3, the concentrations of NO3

– and NH4+ increase in

the soil solution and in draining streams (Hogbom et al.2001; Nohrstedt 2001; Saarsalmi and Malkonen 2001; Ringet al. 2006).

The majority of the N applied at typical fertilization ratesis incorporated into the soil of boreal ecosystems and thesoil N pool is increased (Melin and Nommik 1988; Malko-nen 1990; Nohrstedt 1990). The C pool and the amount oforganic matter in the mor layer generally also increase (Mal-konen 1990; Nohrstedt 1990; Makipaa 1995). However, theC/N ratio in the mor layer is often reduced by fertilization(Nohrstedt 1990; Sjoberg et al. 2003; Prietzel et al. 2004;Hyvonen et al. 2008). This reduction seems to persist for

Received 5 May 2010. Accepted 22 October 2010. Published on the NRC Research Press Web site at cjfr.nrc.ca on 18 January 2011.

E. Ring,1 S. Jacobson, and L. Hogbom. Skogforsk, The Forestry Research Institute of Sweden, Uppsala Science Park, SE-751 83Uppsala, Sweden.

1Corresponding author (e-mail: eva.ring@skogforsk.se).

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Can. J. For. Res. 41: 279–288 (2011) doi:10.1139/X10-208 Published by NRC Research Press

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many years and to depend on the total amount of N applied(Nohrstedt 1990; Sjoberg et al. 2003; Prietzel et al. 2004). Areduction in the C/N ratio increases the probability of NO3

–

leaching on N-poor sites (Nohrstedt 1990; Ring 1996).When summarizing data from a large number of Europeanforest sites, Gundersen et al. (2006) reported a thresholdC/N ratio of 25, below which NO3

– leaching generally iselevated in coniferous forests. By using nonparametricclassification and regression trees, Rothwell et al. (2008)identified six key environmental parameters that influencethe leaching of dissolved inorganic N from European for-ests. However, these parameters did not include the C/Nratio in the organic horizon. Rothwell et al. (2008) con-cluded that the C/N ratio in the organic horizon is impor-tant under some circumstances but is not a ubiquitouspredictor of inorganic N leaching.

Fertilization may give rise to long-term effects in forestecosystems, for example by increasing the leaching of Nafter final felling. This issue was identified by Tamm et al.(1974) and the elevation of NO3

– leaching after the felling offorest stands at two low-productivity sites treated withNH4NO3 fertilizer was subsequently reported (Ring 1996;Berden et al. 1997). In contrast, NO3

– leaching after finalfelling has been found to be lower from urea-fertilized plotsthan from unfertilized plots at a fertile forest site in southernSweden (Ring et al. 2003). This was probably partly due toan increase in field layer vegetation biomass (as a conse-quence of previous fertilization) and its storage of N.

The present study was conducted to investigate the long-term effects of N fertilization at different intensity in N-lim-ited ecosystems. The study was based on observations fromthree field experiments that commenced around 1980 inScots pine (Pinus sylvestris L.) stands. The investigationswere initiated in mature forest stands, of which two weresubsequently harvested. The monitoring continues followingfinal felling at the harvested sites. In the present paper, theeffects observed near the end of the rotation period follow-ing total application levels between 450 and 1800 kg N�ha–1

on soil chemistry are reported. Our main hypothesis was thatthe C/N ratio in the organic layer would decrease with in-creasing fertilization intensity due to a buildup of the or-ganic layer N pool.

Materials and methods

Soil samples were collected from three field experimentalsites located along a south to north gradient across Sweden(Table 1). The sites are designated 165 Hagfors, 132 Nissa-fors, and 171 Asele, hereafter referred to by location alone,and they represent different climate, N deposition, and sitequality classes. The sites were originally established to studythe effects of N fertilization on stem-wood growth. The ob-served increases in stem-wood increment agreed with ex-pected values (Pettersson 1994b; Jacobson and Pettersson2010). The addition of 150 kg N ha–1 (hereafter referred toas 150N), applied at different intensities in the present study,normally results in an average relative increase in stem-wood growth of 30%–55%, corresponding to 12–20 m3�ha–1, over a period of 7–11 years (Pettersson 1994b).Results from earlier investigations at the study sites have

been reported by Nohrstedt (1988b, 1989, 1998), Hogbomand Nohrstedt (2000), and Jacobson and Pettersson (2001).

Experimental designA randomized block design had been applied in all three

experiments, each with three replicates. Study plots were30 m � 30 m, except for block 1 at Nissafors, where theplots were 25 m � 25 m. The original purpose of the Hag-fors and Asele experiments was to determine changes instem-wood growth at different N-fertilization intervals, withand without the addition of other nutrients. The control (0N)and three out of six regimes were included in the presentstudy in which 450, 900, and 1800 kg N�ha–1 (1650 kgN�ha–1 at Asele) had been applied (these fertilization re-gimes are subsequently referred to as 450N, 900N, 1800N,and 1650N, respectively). Beginning in the early 1980s,150N was applied as NH4NO3 at 8-, 4-, and 2-year intervalsto achieve total application levels of 450N, 900N, and1800N, respectively, at Hagfors and 450N, 900N, and1650N at Asele (Table 1). At each N application, 1 kgB�ha–1 was also applied. From 1991 onwards, dolomite wasadded to the NH4NO3 to change the N, Ca, Mg, and B con-centrations of the fertilizer to 27%, 5%, 2%, and 0.2%, re-spectively, reflecting a change in the chemical compositionof fertilizer used in operational forestry. At the same time,the application scheme for Mg and micronutrients waschanged, affecting only the 1800N and 1650N regimes inthe present study. A new scheme was systematically appliedto plots that had received varying loads of Mg and micronu-trients to even out potential effects of previous applications.Thus, the applied loads of Mg and micronutrients differamong the plots representing the 1800N (or 1650N at Asele)regimes (Table 1). The measured increases in stem volumegrowth over a 20-year period were significant for all appli-cation regimes at both Hagfors and Asele (Jacobson and Pet-tersson 2010), and the highest application regimes resultedin increases in the average stem volume growth of morethan 50% and 100%, corresponding to 60 and 95 m3�ha–1, atHagfors and Asele, respectively.

The purpose of the Nissafors experiment was to evaluatethe change in stem-wood growth following either a singleapplication of 150N (given as NH4NO3) or split applicationsof the same total amount. Of the five fertilization regimes atNissafors, one (450N) and the control (0N) were selected forthe present study. In 1977 and 1978, the plots in blocks 1and 2 were fertilized with a single application of 75 kgN�ha–1�year–1, while the plot in block 3 was fertilized withsix applications of 25 kg N�ha–1 in 1977. The applicationscheme subsequently changed so that in 1984 and 1989, asingle application of 150N was made to each plot, with theaddition of 20 kg P�ha–1 in 1989. The measured increases ingrowth after N addition were significant throughout thestudied periods, but the addition of P had no discernible ef-fect on tree growth (Jacobson and Pettersson 2001).

Soil chemistryTwenty subsamples of the FH horizon were taken from

each selected plot using a 50 mm diameter soil auger and40 subsamples of mineral soil (0–20 cm) were obtainedfrom each of the selected plots at Hagfors and Nissaforswith a 27 mm diameter soil auger (Table 1) but not from

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Table 1. Description of the study sites and sampling programs.

Nissafors Hagfors Asele

Site descriptionLocation 57824’N, 13837’E 60800’N, 13842’E 64807’N, 17833’EYear of establishment 1977 1981 1982Altitude (m above mean sea level) 170 190 330Soil Sandy sediment Sandy–silty till Sandy sedimentSoil type Podzol Podzol PodzolFH* (mm) 41–59 34–37 ~20E* (mm) 60–71 64–79 ~30Precipitation (mm�year–1){ 949 728 560Annual mean air temperature (8C){ 5.4 ~4. 0.5Open-field deposition of inorganic N

(kg N�ha–1�year–1){8.5 6.2 1.5

Dominant tree species Pinus sylvestris Pinus sylvestris Pinus sylvestrisTree age (years) 40 (block 1) 65 75

46 (blocks 2 and 3)Stem density (stems�ha–1) 1865 (block 1) 800 1000

876 (blocks 2 and 3)Standing stem volume (m3�ha–1) 105 (block 1) 180 105

75 (blocks 2 and 3)Site quality (m3�ha–1�year–1) 5.5 5.9 3.4N fertilization regimes (kg N�ha–1) in-

cluded in the present study (years ofapplication)§

0N 0N 0N3� 150N (1977–1978||, 1984, 1989) 3� 150N (1981, 1989, 1997) 3� 150N (1982, 1990, 1998)

6� 150N (1981, 1985, 1989, ..., 2001) 6� 150N (1982, 1986, 1990, ..., 2002)12� 150N} (1981, 1983, 1985, ..., 2003) 11� 150N} (1982, 1984, 1986, ..., 2002)

Soil samplingDate of soil sampling 8–9 June 2005 13–15 June 2005 16–18 September 2002Sampled treatments

FH horizon 0N, 450N 0N, 450N, 900N, 1800N 0N, 450N, 900N, 1650N0–20 cm in mineral soil 0N, 450N 0N, 450N, 900N na

Years elapsed since last fertilization450N 16 8 4900N na 4 0.221800N and 1650N na 2 0.22

Chemical analyses pH, total C, total N, exchangeable K, Mg,Ca, P, NH4

+-N, NO3–-N

pH, total C, total N, exchangeable K, Mg,Ca, P, NH4

+-N, NO3–-N

Total C, total N

Note: All data concerning the tree stands refer to conditions at the time when the experiments were originally established. na, not available.*Hagfors and Nissafors: data from three control plots in 2005; Asele: data from 1982.{Data from nearby meteorological stations (Alexandersson et al. 1991).{Data from nearby stations operated by the Swedish Environmental Research Institute (http://www.krondroppsnatet.ivl.se/). Average for 1996–1997 to 2004–2005 (Hagfors), 1996–1997 to 2004–2005

(Nissafors), and 1996–1997 to 2001–2002 (Asele). NO3–-N accounted for 51%–61%.

§At each N application, 1 kg B�ha–1 was added at Hagfors and Asele but not at Nissafors.||150N was applied as 2� 75N (blocks 1 and 2) or 6� 25N (block 3).}At the first five applications, one plot received 150N, one plot received 150N and 1 kg of B, and one plot received 150N, 10 kg of Mg, and micronutrients (per hectare: 3.4 kg of Mn, 1.7 kg of Cu, 1 kg of Zn,

1 kg of B, 0.06 kg of Co, and 0.06 kg of Mo).

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any plots at Asele. The E and Bs horizons within each sub-sample were separated and combined, by horizon, to form acomposite sample of each horizon for each plot. The hori-zons in the soil profile occasionally showed signs of disturb-ance, e.g., the presence of two E horizons, or soil mixing.These locations were avoided as far as possible. At Hagfors,subsurface stones frequently prevented sampling of the low-est part of the mineral soil. On average, 81% (60%–91%) ofthe total soil column (40 samples � 20 cm per sample) wassampled per plot, and for comparisons when calculating theelemental contents in the mineral soil, the dry mass was nor-malized to the full sampling depth (i.e., 0–20 cm). However,at Nissafors, the mineral soil was sampled down to 20 cmfor all subsamples, so no correction was required. No cor-rection for stoniness was applied to the calculated elementalcontents in the mineral soil. Dry mass data for Asele werenot available.

Soil samples were sieved in the laboratory (mesh size5.6 mm for FH and 4.0 mm for mineral soil) and subsam-ples were withdrawn for chemical analyses. The pH was de-termined in aqueous extracts. Exchangeable K, Mg, Ca, andP were extracted with ammonium acetate (1 mol�L–1

CH3COONH4, pH = 7.0) and measured using an inductivelycoupled plasma – optical emission spectrometer. NH4

+ andNO3

– were extracted with KCl (2 mol�L–1) and quantified byflow injection analysis. The limits of detection correspondedto approximately 0.2 mg K�kg–1, 0.1 mg Mg�kg–1, 0.6 mgCa�kg–1, 0.3 mg P�kg–1, 0.3 mg NH4

+-N�kg–1, and 0.02–0.08 mg NO3

–-N�kg–1. Total C and N were measured in driedsamples (vacuum drying at 70 8C) after combustion.

StatisticsThe soil chemistry data were statistically analyzed using

SAS ver. 9.2 (http://support.sas.com). The data acquiredfrom each experimental site were separately statistically an-alyzed due to the different N fertilization regimes, samplingprograms, and time elapsed between sampling and last fertil-ization (Table 1). The data were analyzed by applying theMIXED procedure using the following mixed linear model(Model 1):

½1� yij ¼ mþ ai þ bj þ eij

where yij is the variable under consideration, m is the overallmean, ai is the fixed effect of N fertilization regime i, bj isthe random block effect for which j = 1, 2, or 3 (the replica-tion within N fertilization regime i), and eij is the residualterm. The denominator degrees of freedom for the tests offixed effects were estimated by a general Satterthwaite ap-proximation. The effects of N fertilization regime were com-pared for the Hagfors and Asele data by calculatingdifferences of least-square means and adjusting for multiplecomparisons according to Tukey–Kramer.

A linear relationship with total N application was appa-rent, so a single-factor linear model (Model 2) was sepa-rately applied to the Hagfors and Asele soil data as wellusing the GLM procedure:

½2� yij ¼ kxi þ mþ eij

where yij is the variable under consideration, k is the slope,xi is the total N dose for N fertilization regime i for which

j = 1, 2, or 3 (the replication within N fertilization regimei), m is the intercept, and eij is the residual term.

Concentrations below the limit of detection were treatedas equal to the limit of detection and statistical significancewas defined as p < 0.05. The concentrations of K and Mgwere all above the limit of detection, while single concentra-tions of NH4

+-N and Ca were below. The P concentrations inthe samples from the FH horizon were all above the limit ofdetection, while most of the P concentrations in the mineralsoil from Nissafors were below the limit of detection. Ef-fects on NO3

–-N and P in the mineral soil from Nissaforswere not statistically evaluated due to the large number ofconcentrations below the limit of detection.

ResultsFor the Hagfors site, an effect of N fertilization regime

(ai) was detected with Model 1 for the C/N ratio and theconcentration of NH4

+-N and exchangeable P, K, Mg, andCa (Table 2). Effects were mainly observed in the FH hori-zon (which was sampled in the plots subjected to the 0N to1800N regimes, while the mineral soil was sampled in plotsthat had been subjected to up to the 900N regimes). TheC/N ratio measured in the FH horizon was lower in the1800N regime than in the control at Hagfors and lower inthe FH horizon of the 900N and 1650N regimes than inthe control at Asele (Table 2). Accordingly, the 900N and1650N regimes increased (and the 450N regime tended toincrease) the N concentration in the FH horizon. At Hag-fors, fertilization resulted in a close to significant increasein the N concentration (p = 0.057), and according toModel 2, the N concentration in the FH horizon increasedwith increasing fertilizer dose at both of these sites(Table 3). At Nissafors, previous fertilizations with 450Nwere found to reduce the C/N ratio in the FH horizon andincrease the C content in 0–20 cm of the mineral soil (Ta-bles 2 and 4).

The concentrations of NH4+-N and exchangeable Mg in the

FH horizon representing the 900N and 1800N regimes atHagfors were elevated compared with the control (Table 2).Their concentrations of exchangeable Mg were also greaterthan in the 450N regime. The concentration of exchangeableP in the FH horizon representing the 1800N regime wasgreater than in the 0N and 450N regimes, while the concen-tration of exchangeable K was lower. The concentrations ofexchangeable K were also lower in the E and Bs horizons(Table 2). The pH and the concentrations of N and C alsotended to be influenced by N fertilization regime at Hagfors(0.05 < p < 0.10).

The soil contents of N, NH4+-N, and exchangeable P, K,

Mg, and Ca at Hagfors were influenced by N fertilizationregime (Table 4) and reflected the effects on the concentra-tions of these elements. Analysis of the data using Model 1suggested that N fertilization regime had no significant in-fluence (p = 0.19) on the dry mass of the FH horizon, butanalysis using Model 2 indicated a positive correlation be-tween the fertilizer load and dry mass (Table 3). The totalamounts of exchangeable K and Ca were reduced by 34%and 26% in the mineral soil representing the 900N regimeand by 33% for Ca in the 450N regime (Table 4). The Ncontent of the 1800N FH horizon was 1.6-fold, the NH4

+-N

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Table 2. Concentrations of elements, C/N ratios, and pH in the FH, E, and Bs horizons at Hagfors, Nissafors, and Asele (means of three replicates).

HorizonHagforsp (ai) 0N 450N 900N 1800N

Nissaforsp (ai) 0N 450N

Aselep (ai) 0N 450N 900N 1650N

pH FH 0.083 4.1 4.1 4.1 4.4 0.46 4.3 4.2E 0.45 4.3 4.2 4.1 na 0.44 4.4 4.4Bs 0.063 4.4 4.2 4.2 na 0.60 4.5 4.5

N (%) FH 0.057 1.1 1.2 1.3 1.4 0.21 1.2 1.3 0.0047 0.97a 1.2ab{ 1.3b 1.4bE 0.14 0.041 0.039 0.049 na 0.74 0.059 0.061Bs 0.058 0.073 0.060 0.057 na 0.14 0.062 0.067

C (%) FH 0.90 44 44 43 46 0.32 38 41 0.11 42 48 48 46E 0.090 1.2 1.3 1.7 na 0.39 1.8 2.0Bs 0.052 2.2 1.8 1.7 na 0.054 1.6 1.8

C/N FH 0.027 39.a 37.ab 33.ab 32.b 0.048§ 32.a 31.b 0.0024 44.a 40.ab 37.bc 34.cE 0.23 28 34 35 na 0.37 30 33Bs 0.96 30 30 30 na 0.36 26 27

NO3– (mg N�kg–1) FH bd bd bd 0.84{ bd bd

E bd bd bd na bd bdBs bd bd bd na bd 0.057

NH4+ (mg N�kg–1) FH <0.0001 51.a 57.ab 0.10�103b 0.25�103c 0.30 60 76

E 0.19 2.4 1.1 2.4 na 0.80 1.3 1.4Bs 0.11 0.73 0.64 1.1 na 0.26 0.63 0.86

P (mg�kg–1) FH 0.0054 21.a 23.a 28.ab 34.b 0.67 14 13E 0.53 0.83 0.60 0.75 na bd bdBs 0.23 0.73 bd 0.47 na bd bd

K (mg�kg–1) FH 0.0002 0.48�103a 0.43�103a 0.38�103a* 0.23�103b 0.14 0.34�103 0.29�103

E 0.0057 24.a 19.b 18.b na 0.38 22 19Bs 0.033 19.a 15.ab 11.b na 0.14 11 10

Mg (mg�kg–1) FH 0.0029 0.20�103a 0.22�103a 0.34�103b 0.39�103b 0.21 0.24�103 0.18�103

E 0.065 5.0 4.3 5.8 na 0.18 5.4 4.4Bs 0.32 2.9 2.3 2.3 na 0.086 2.1 1.8

Ca (mg�kg–1) FH 0.21 1.7�103 1.5�103 1.7�103 1.9�103 0.88 0.76�103 0.75�103

E 0.061 29 21 25 na 0.35 13 14Bs 0.037 8.7a 4.6b 5.9ab na 0.71 2.3 2.5 .

Note: Significantly different values (p < 0.05) according to Model 1 are marked by different letters row-wise per site. p values <0.05 are highlighted in bold. na, not available; bd, most observations belowthe limit of detection.

*p = 0.059 in comparison with 0N.{p = 0.056 in comparison with 0N.{This corresponded to 0.04 kg NO3

–-N�ha–1.§The C/N ratios were 32.5 and 30.9 when using three significant figures.

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content sixfold, the contents of exchangeable P, Mg, and Catwo-, 2.5-, and 1.4-fold, respectively, and exchangeable Kcontent 0.6-fold the content in the control.

Data analysis using Model 2 suggested correlations be-tween several chemical parameters in the FH horizon andtotal N load, with coefficients of determination varyingfrom 0.35 to 0.91 (Table 3). The analysis of data relating tothe Hagfors E and Bs horizons (0N, 450N, and 900N re-gimes) indicated no correlation between total N load andpH, C/N ratio, or the concentrations of N, NH4

+-N, P, Mg,and Ca. A negative correlation was indicated for the concen-tration of N in the Bs horizon (p = 0.050, R2 = 0.44). Theconcentration of K decreased with increasing N load in theE and Bs horizons (p < 0.05, R2 = 0.59 and 0.69, respec-tively). The concentration of C in the E horizon was posi-tively correlated (p < 0.05) with N load but was negativelyrelated in the Bs horizon.

DiscussionSoil chemistry appeared to change progressively with in-

creasing total N load. However, the analysis did not conclu-sively demonstrate a direct relationship with the totalamount of N applied because the rate of application and thetime elapsed since the last application also varied. Stem-wood growth and leaching in coniferous forests reportedlydisplay distinct temporal patterns after the application of fer-tilizer; increases in stem-wood growth lasting 7–11 yearshave been observed (Pettersson 1994b), whereas the leach-ing of inorganic N is reportedly elevated for 1–2 years(Ring et al. 2006). To our knowledge, temporal changes in

soil chemistry have seldom been investigated, and manysoil studies provide data from one sampling occasion. How-ever, temporal changes probably differ between measuredvariables and soil depths. In the present study, it was notpossible to distinguish between the effects of the totalamount of N applied and the effects of application intervalor time elapsed since the last application. However, the totalfertilizer load probably influenced several variables (cf.Nohrstedt 1990).

Table 5 presents a simple N budget for the Hagfors exper-imental site. The total amounts of N in the trees were simu-lated using suitable biomass functions (Marklund 1988) andtypical N concentrations of aboveground tree components(stem wood, stem bark, living and dead branches, needles)for similar sites in Sweden. Assuming that the concentra-tions of N in different tree components were constant for allapplication regimes, the total amount of N in the above-ground part of the tree stand was approximately 200 kgN�ha–1 for the control and an additional 42–48 kg N�ha–1 forthe N fertilization regimes (Table 5). The additionalamounts corresponded to around 9%, 5%, and 3% of the to-tal N application for the 450N, 900N, and 1800N regimes,respectively. If it is assumed that the concentrations of N inbranches and needles increased by 10%, 15%, and 20% inthe 450N, 900N, and 1800N regimes, respectively, the addi-tional amounts of N in the stand would increase onlyslightly, from 42–48 to 55–72 kg N�ha–1. Thus, only a smallproportion of added N is likely to be found in the above-ground components of the tree stand, as previously reported(Melin and Nommik 1988; Johnson 1992; Nadelhoffer et al.1999). According to the calculated N budget, roughly 25%

Table 3. Linear relationships (Model 2) between soil parameters and total fertilizer appli-cation in the FH horizon at Hagfors and Asele (explicitly stated).

Modelsignificance, p Equation R2

pH and concentrationpH 0.043 pH = 0.0001818X + 4.036 0.35N 0.0040 N (%) = 0.0001719X + 1.127 0.58C 0.53 C (%) = 0.001124X + 43.38 0.041C/N 0.0036 C/N = –0.003883X + 38.40 0.59NH4

+-N <0.0001 NH4+-N (mg�kg–1) = 0.1159X + 23.41 0.88

P <0.0001 P (mg�kg–1) = 0.007570X + 20.18 0.81K <0.0001 K (mg�kg–1) = –0.1374X + 488.2 0.88Mg 0.0009 Mg (mg�kg–1) = 0.1111X + 199.0 0.68Ca 0.28 Ca (mg�kg–1) = 0.1487X + 1563 0.12N (Asele) 0.0012 N (%) = 0.0002315X + 1.040 0.66C (Asele) 0.34 C (%) = 0.001779X + 44.72 0.090C/N (Asele) 0.0001 C/N = –0.006050X + 43.02 0.79

Store in FHDry mass 0.044 Dry mass (kg�m–2) = 0.0005693X + 4.154 0.35N 0.0017 N (103 kg�ha–1) = 0.0001526X + 0.4653 0.64C 0.068 C (103 kg�ha–1) = 0.003207X + 17.92 0.30NH4

+-N <0.0001 NH4+-N (kg�ha–1) = 0.006333X + 0.6317 0.82

P <0.0001 P (kg�ha–1) = 0.0005111X + 0.8122 0.91K 0.0003 K (kg�ha–1) = –0.004274X + 20.40 0.74Mg <0.0001 Mg (kg�ha–1) = 0.007123X + 7.734 0.81Ca 0.0014 Ca (kg�ha–1) = 0.01730X + 63.02 0.66

Note: Data from 0N, 450N, 900N, and 1800N (1650N) regimes were included. X is the total Nload in kg N�ha–1. p values <0.05 are highlighted in bold.

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Table 4. Dry mass, depth, and elemental content of the FH horizon and 0–20 cm of the mineral soil at Hagfors and Nissafors (means of three replicates).

Variable HorizonHagforsp (ai) 0N 450N 900N 1800N

Nissaforsp (ai) 0N 450N

Dry mass (kg�m–2) FH 0.19 4.2 4.4 4.6 5.2 0.32 6.8 7.30–20 cm 0.78 0.22�103 0.23�103 0.22�103 na 0.85 0.27�103 0.27�103

Depth (mm) FH 0.032 36.a 43.ab 43.b 43.ab 0.20 49 57N (103 kg�ha–1) FH 0.040 0.47a 0.52ab 0.60ab 0.74b 0.25 0.79 0.99

0–20 cm 0.53 1.2 1.1 1.2 na 0.33 1.7 1.8S 0.64 1.7 1.6 1.8 na 0.10 2.5 2.8

C (103 kg�ha–1) FH 0.33 18 20 20 24 0.30 26 300–20 cm 0.66 36 35 37 na 0.0051 45.a 51.bS 0.53 55 55 58 na 0.10 71 81

NH4+-N (kg�ha–1) FH 0.0003 2.2a 2.5a 4.6a 13.b 0.16 4.1 5.6

0–20 cm 0.17 3.3 1.9 3.4 na 0.42 2.3 2.9S 0.060 5.5 4.4 8.0 na 0.27 6.4 8.5

P (kg�ha –1) FH 0.0005 0.84a 1.0ab 1.3b 1.7c 0.97 0.97 0.960–20 cm 0.069 1.8 1.1 1.2 na bd bdS 0.26 2.6 2.1 2.5 na na na

K (kg�ha –1) FH 0.0010 19.a 19.a 18.a 12.b 0.46 23 210–20 cm 0.0083 46.a 38.ab 30.b na 0.42 39 36S 0.0026 65.a 57.b 48.c na 0.43 62 57

Mg (kg�ha –1) FH 0.0009 8.1a 9.4a 16.b 20.b 0.49 16 140–20 cm 0.21 8.3 7.1 8.0 na 0.098 8.4 7.5S 0.0003 16.a 17.a 24.b na 0.40 25 21

Ca (kg�ha –1) FH 0.014 68.a 63.a 80.ab 96.b 0.64 52 560–20 cm 0.0063 40.a 27.b 29.b na 0.36 15 17S 0.086 108 90 109 na 0.34 67 73

Note: S is the accumulated elemental content of the FH horizon and 0–20 cm of the mineral soil. Significantly different values (p < 0.05) according to Model 1 are marked by different letters row-wiseper site. p values <0.05 are highlighted in bold. na, not available; bd, most observations below the limit of detection.

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of the total added N can be accounted for. The remainingproportion may be found in the litter and bottom layer andthe root system and sequestered in the mineral soil; Melinand Nommik (1988) found that almost 50% of N added asNH4NO3 at 150 kg N�ha–1 was present as organic N in thelitter layer down to a depth of 85 cm in the mineral soil ayear after application.

In the present study, a progressive decrease in the C/N ra-tio with increasing total N load was observed in the FH ho-rizon. Similar effects have been observed in otherfertilization experiments located in northern Sweden (Nohr-stedt 1990, 1992; Hogberg et al. 2006). Notably, data fromone fertilization experiment indicate that NO3

– leaching canbe elevated despite high C/N ratios (38) when the site hasbeen subjected to intensive fertilization (Nohrstedt 1990).The data from Nissafors demonstrate that fertilization atconceivable commercial rates (3� 150N) can result in long-term changes in the C/N ratio of the FH horizon. Similarly,repeated application of urea to Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) stands in Washington, USA, ledto reductions in the C/N ratio of the soil organic layer thatwere detectable 8–15 years after the last application(Prietzel et al. 2004).

The concentration and total amount of exchangeable P inthe FH horizon at Hagfors increased with the level of fertil-izer application, even though P was not added with the fer-tilizer. A similar increase in the exchangeable P amount wasobserved at another fertilization experiment and was ex-plained by the accumulation of organic matter (Nohrstedt1990). The increase in the P content at Hagfors appeared tobe associated with increases in both the dry mass of the FHhorizon and its P concentration (Table 4). The P concentra-tion in current-year needles of year 2002 tended to be ele-vated in the studied fertilizer regimes (unpublished datafrom two blocks), suggesting that deposition of needle litterenriched in P may have contributed to the elevated level ofexchangeable P in the FH layer. There were indications of adecrease in the total available P in the top 20 cm of mineralsoil. The levels of exchangeable P decreased with increased

N application in the E horizon at two other fertilization ex-periments (Nohrstedt 1990, 1992).

The total amounts of exchangeable Mg and Ca werefound to have increased in the FH horizon representing the900N (Mg only) and 1800N regimes; the addition of dolo-mite after 1991 could have contributed. About 80 kg Ca�ha–1

and 40 kg Mg�ha–1 were added in the 900N regime andaround 180 kg Ca�ha–1 and 90 kg Mg�ha–1 in the 1800N re-gime. An increase in exchangeable Mg was also found inthe FH horizon after intensive fertilization, possibly due toincreased uptake and translocation by the trees (Hogberg etal. 2006). The decreased contents of exchangeable K and Cain the mineral soil at Hagfors corresponded to observationsreported by Hogberg et al. (2006) but conflicted with find-ings at another fertilization experiment where no change inthe contents of exchangeable base cations was detected inthe mor layer to a depth of 10 cm in the mineral soil (Nohr-stedt 1990).

In comparison with the control, the fertilization treatmentsreduced both concentrations and total amounts of exchange-able K in the FH horizon and the mineral soil at Hagfors.Similar results have been reported by Nohrstedt (1992). Thedecrease in exchangeable K could be attributable to the in-creased leaching of K during the first year after fertilizer ad-dition (Ring et al. 2006). Neither a previous study atHagfors (in 1996–1997) (Nohrstedt 1998) nor the presentstudy detected fertilization-related changes in K concentra-tion in the deep soil solution (data not shown). However, inboth cases, the time since the last application was 4 years ormore and the increased leaching of K had probably ceasedby then.

The pH of the Bs horizon at Hagfors tended to decreasedue to fertilization, as previously reported by Nohrstedt(1990). The reduction in pH may have been partially causedby decreases in the mineral soil content of exchangeable Kand Ca. The pH of the FH horizon at Hagfors tended to in-crease after the 1800N treatment, but no correspondingchange was reported by Nohrstedt (1990). In another Swed-ish study, the pH of the mor layer was found to be elevated

Table 5. Estimated N budget for the Hagfors experimental site.

N fertilization regime

Change in N 450N 900N 1800NDLoss by leaching* +22 +45 +90DLoss by denitrification{ 0 0 0DN fixation{ 0 0 0DN content in FH horizon +50 +130 +270DN content in 0–20 cm in mineral soil§ 0 0 naDN content in tree stand +42 +46 +48DN content in field layer vegetation|| 0 +9 +15Accumulated change 114 230 423Accumulated change of added N (%) 25 26 24

Note: All values (D and accumulated change) are expressed as kg N�ha–1; D refersto a change resulting from fertilization. na, not available.

*A 5% loss at every application was assumed (cf. Nohrstedt 2001).{No effect of fertilization was assumed (cf. Nohrstedt 1988a).{No effect of fertilization was assumed, although a reduction can occur (Nohrstedt

1988a).§No effect was detected (Table 3).||Similar effects to those reported by Nohrstedt (1990) were assumed.

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1 year after the last of five applications of 162 kg N�ha–1

(Nohrstedt 1992). Annual additions of NH4NO3 at an experi-ment in northern Sweden, to a total level exceeding 2000 kgN�ha–1, also affected the pH of both the mor layer and themineral soil; further, a positive correlation was observed be-tween the pH of the mor layer and extractable NH4

+ (Hog-berg et al. 2006). The NH4

+ concentration and pH of the FHhorizon representing the 1800N regime at Hagfors were ofthe same order of magnitude as reported by Hogberg et al.(2006).

Significant effects of N application were detected fornearly all of the measured variables in the present study.However, the environmental consequences of the reportedeffects are unclear. Ongoing measurements of soil solutionchemistry, seedling growth, and field layer vegetation devel-opment following final felling will help to elucidate thelong-term effects of the reported changes.

AcknowledgementsThe authors wish to thank the companies Bergvik and

Stora Enso (Hagfors), Sveaskog (Nissafors), and SCA(Asele) for hosting the experiments and for adapting theirprocedures to accommodate the experimental design. Thefield work was excellently conducted by Sten Nordlund,Lars-Ake Dahl, Thomas Hjerpe, and Hagos Lundstrom. Thestudy was financed by the Swedish Research Council forEnvironment, Agricultural Sciences and Spatial Planning,the Skogforsk framework program, and Future Forests, amultidisciplinary research programme supported by theFoundation for Strategic Environmental Research, the Swed-ish Forest Industry, the Swedish University of AgriculturalSciences, Umea University, and Skogforsk.

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