Plant and Soil 249: 319–330, 2003.© 2003 Kluwer Academic Publishers. Printed in the Netherlands.

319

Changes in soil properties after afforestation of former intensivelymanaged soils with oak and Norway spruce

Eva Ritter1, Lars Vesterdal & Per GundersenDepartment of Forest Ecology, Danish Forest and Landscape Research Institute Hørsholm Kongevej 11, DK-2970Hørsholm, Denmark. 1Corresponding author∗

Received 1 May 2002. Accepted in revised form 17 September 2002

Key words: afforestation, C/N ratio, nitrogen, Norway spruce, oak, pH

Abstract

In many European countries, surplus agricultural production and ecological problems due to intensive soil cultiva-tion have increased the interest in afforestation of arable soils. Many environmental consequences which might risefrom this alternative land-use are only known from forest establishment on less intensively managed or marginalsoils. The present study deals with changes in soil properties following afforestation of nutrient-rich arable soils.A chronosequence study was carried out comprising seven Norway spruce (Picea abies (Karst.) L.) and sevenoak (Quercus robur L.) stands established from 1969 to 1997 on former horticultural and agricultural soils in thevicinity of Copenhagen, Denmark. For comparison, a permanent pasture and a ca. 200-year-old mixed deciduousforest were included. This paper reports on changes in pH values, base saturation (BSeff), exchangeable calcium,soil N pools (Nmin contents), and C/N ratios in the Ap-horizon (0–25 cm) and the accumulated forest floor. Theresults suggest that afforestation slowly modifies soil properties of former arable soils. Land-use history seemsto influence soil properties more than the selected tree species. An effect of tree species was only found in theforest floor parameters. Soil acidification was the most apparent change along the chronosequence in terms of apH decrease from 6 to 4 in the upper 5 cm soil. Forest floor pH varied only slightly around 5. Nitrogen storagein the Ap-horizon remained almost constant at 5.5 Mg N ha−1. This was less than in the mineral soil of the ca.200-year-old forest. In the permanent pasture, N storage was somewhat higher in 0–15 cm depth than in afforestedstands of comparable age. Nitrogen storage in the forest floor of the 0–30-year-old stands increased in connectionwith the build-up of forest floor mass. The increase was approximately five times greater under spruce than oak.Mineral soil C/N ratios ranged from 10 to 15 in all stands and tended to increase in older stands only in 0–5 cmdepth. Forest floor C/N ratios were higher in spruce stands (26.4) as compared to oak stands (22.7). All standsexcept the youngest within a single tree species had comparable C/N ratios.

Introduction

During the last half century, agricultural managementhas been intensified in many European countries. Thishas resulted in increased production, but has alsoled to negative environmental effects such as freshand ground water pollution, erosion, and loss of bio-diversity. Surplus production in the late 1980s gener-ated interest in planting forest, especially on marginal

∗ FAX No: +45-45-763233.E-mail: evr@fsl.dk

arable land. Later, calls for reduced environmental im-pacts from agriculture put focus on afforestation as ameans to reduce environmental impacts in the agri-cultural landscape like nitrate leaching to ground andsurface water (e.g., Kubiniok and Müller, 1993). Bythe 1990s, afforestation has become an actual policywithin many European countries (Helles and Linddal,1996).

It is conceivable that ceasing of the intense cul-tivation (high fertilizer input, annual tillage, regularliming, weed control, etc.) of modern arable land

320

will better water quality and lead to accumulation ofsoil organic matter (Jug et al., 1999; Post and Kwon,2000). Nevertheless, rates of changes in soil propertiesand water quality are not well documented. Especially,the long-term patterns are unknown.

Studies of arable soils afforested 50–100 years agohave shown that nitrogen (N) cycling characteristics(mineralization, nitrification rates) and organic mat-ter C/N ratios differ significantly from those of oldgrowth forest soils even up to 100 years after affor-estation (Compton et al., 1998; Jussy, 1998; Koerneret al., 1999). Arable soil that is afforested currentlyand in the future is different from soil afforested 50–100 years ago with respect to N status, but also withrespect to other nutrient contents, tillage intensity anddepth, lime content, etc. Thus, the knowledge fromformer studies on afforestation may not be applicableto present-day conditions.

A well-known effect of afforestation is enhancedacidification of soil and soil solution (Alriksson andOlsson, 1995; Heinsdorf, 1994; Jussy, 1998; Kubin-iok and Müller, 1993). Even ceased cultivation andhomogenization may alone lead to a pH decrease inthe top mineral soil (Dick, 1983). Jug et al. (1999) re-ported a pH decrease in the top mineral soil (0–30 cm)within 7–10 years after afforestation of arable soil. Al-though changes may be fast in the initial phase afterafforestation, Kubiniok and Müller (1993) suggest atleast 80–100 years stand development to obtain pHvalues and nutrient and humus contents as found inold growth forest soils.

As a first step towards prediction of the effectof afforestation on ground and surface water quality,changes in the physical and chemical soil propertiesin a formerly intensively managed plow layer after af-forestation were examined in the present study. It washypothesized that (i) afforestation will slowly reducesoil pH, faster in conifers than in broadleaved spe-cies, (ii) N will accumulate in the soil organic matterafter an initial phase of losses, and (iii) soil C/N ratiowill increase due to the accumulation of new organicmatter. A chronosequence approach (time for spacesubstitution) with Norway spruce and oak stands wasused to test these hypotheses. This paper focuses onchanges in soil pH, effective base saturation (BSeff),exchangeable calcium (Ca2+), soil N pools, and C/Nratios with time after afforestation. Changes in carbon(C) storage have been reported in a previous paper(Vesterdal et al., 2002).

Materials and methods

Study site

The study was conducted at the afforestation areaVestskoven, 15 km west of Copenhagen (55◦70′ N,12◦35′E) in Denmark. Preceding afforestation, thearea had been intensively used as arable fields or fornurseries for several 100 years. The soil nutrient statusof the whole area is high, thus there are no system-atic differences between soil nutrient contents of theearliest and the latest afforested stands. Afforestationwas started in this area in the late 1960s and is stillongoing. Today, Vestskoven covers a total area of 1340ha with forest stands of about 1–10 ha in size and largeopenings of permanent pastures. Stands are mainlymonocultures of oak (Quercus robur L.), beech (FagusSylvatica L.), pine (Pinus sylvestris L.), and Norwayspruce (Picea abies (L.) Karst).

The climate is temperate with an average annualtemperature of 7.7 ◦C and a mean annual rainfall ofabout 600 mm. The elevation of the gently rolling areais 20–28 m. The nutrient-rich soils have developedfrom calcareous weichselian till material and appearto be relative homogeneous over the area. The textureis sandy loam, and soils are classified as Alfisols orInceptisols with an Ap-horizon overlying a B or Bt-horizon. In some stands, slight pseudogley propertiescould be recognized below 45 cm. Particle size dis-tribution of the area is given in Table 1. In additionto the nutrient supply derived from preceding agricul-ture, current N input from atmospheric deposition inthe region is approximately 14 kg N ha−1 year−1 inmature oak stands and 18 kg N ha−1 year−1 in maturespruce stands (Andersen et al., 2002).

For the study, seven oak and seven spruce standswere selected to form a chronosequence ranging from1 to 30 years since afforestation (Table 2). Trees wereplanted at an age of 2 years (oak) and 3–4 years(spruce) in the original plow layer of 25–30 cm depthwithout any soil preparation. Coherent forest floorshad accumulated after ca. 8 years, and canopy closureis reached in stands after approximately 10 years.

A field from the study area, which was conver-ted to permanent pasture in 1977, was included asan example of pasture as an alternative to afforest-ation. Similar studies in a ca. 200-year-old mixeddeciduous forest located about 1 km west of Vest-skoven should provide information on soil propertiesof old forest areas. This 5-ha large forest, LedøjePlantage, is dominated by oak and sycamore maple

321

Table 1. Soil properties of the youngest and oldest Norway spruce stand at Vestskoven and the 200-year-old mixeddeciduous forest Ledøje Plantage, determined on samples from soil pits

Horizon Depth Claya Silt Sand pHCaCl2 C N Pb Cac Mg K

(cm) (%) (%) (%) (%) (%) (mg kg−1) cmol+ kg−1)

Vestskoven, Norway spruce (1 year)

Ap 0–22 15.1 15.2 69.7 5.4 1.79 0.18 191 10.3 0.44 0.25

Btg 22–65 23.9 15.2 60.9 6.1 0.19 0.03 75 14.4 0.88 0.26

BC 65–85 8.3 7.1 84.6 7.4 0.04 0.01 235 7.1 0.29 0.12

Ckgd 85–100 16.7 12.3 71.0 7.7 – 0.01 241 – 0.42 0.22

IICkgd 100–120 20.5 15.5 64.0 7.7 – 0.01 204 – 0.62 0.19

Vestskoven, Norway spruce (29 years)

O −3–0

Ap 0–20 14.9 16.9 68.2 3.7 2.54 0.19 223 3.9 0.53 0.22

Bt 20–32 27.9 16.4 55.7 5.3 0.41 0.05 42 10.8 1.12 0.27

Bt(g) 32–48 27.0 15.3 57.7 6.0 0.32 0.04 58 11.3 1.15 0.30

Btg 48–85 22.9 16.2 60.9 6.6 0.18 0.03 173 9.9 0.85 0.23

Ckge 85–95 21.8 17.0 61.2 7.5 – 0.02 298 – 0.68 0.25

IICk(g)e 95–120 19.7 16.3 64.0 7.6 – 0.01 246 – 0.15 0.04

Ledøje Plantage, mixed deciduous (∼ 200 years)

A 0–45 20.5 17.5 62.0 4.0 1.63 0.15 84 0.45 0.75 0.14

Bwg 45–73 22.1 15.9 62.0 5.9 0.32 0.03 72 0.12 1.21 0.23

Ckgf 73–120 20.2 17.2 62.6 7.5 0.27 0.02 231 – 0.68 0.29

aClay = <2 µm, silt = 2–20 µm, sand = 20–2000 µm.bP was extracted in 0.1 M H2SO4.cCa2+, Mg2+ and K+ were exchanged in 1 M NH4NO3.dThe contents of CaCO3 were 7.0% in Ckg and 19.5% in IICkg.eThe contents of CaCO3 were 9.3% in Ckg and 9.1% in IICk(g).fThe content of CaCO3 was 3.5% in Ckg.

(Acer pseudoplatanus L.), but also a few ash (Fraxinusexcelsior L.) and beech trees (Fagus sylvatica L.) maybe found. Soil data are provided in Table 1. This forestis not considered as a part of the chronosequence sincesoil management preceding afforestation is assumed todiffer widely from that in the younger forest stands.Furthermore, it must be taken into consideration thatspace is substituted for time in a chronosequencestudy. Inevitable differences in soil conditions amongthe investigated stands may have affected results tosome extent.

Sampling

In each forest stand and on the permanent pasture,three circular plots (10 m radius, 0.031 ha) were es-tablished, providing three replicates per stand or field.Samples were taken at the four cardinal points of eachcircular plot and later pooled to one sample per plotand horizon.

Forest floor was sampled in September 1998 in or-der to represent the annual minimum of oak standsprior to leaf litter fall. Samples were taken on areabasis using a 25 × 25-cm frame. Roots were removedand samples were divided into a non-foliar fraction(branches – and cones in the case of spruce) anda foliar fraction (foliar litter including Oi and Oe-horizons). The mixed samples of each plot were driedat 60 ◦C to determine dry weight (± 10 mg). Forfurther chemical analysis, they were ground to passthrough a mesh screen of 0.75 mm.

Mineral soil was collected down to the basis of theplow layer by using a soil auger with 5 cm in diameter(Westman, 1995). In the laboratory, soil cores weredivided into subsamples of three depth strata: 0–5, 5–15 cm, and 15 cm to maximum depth. Calculationof nutrient storage in the deepest layer of the min-eral soil was made for an average depth of 25 cm.Samples were air-dried, ground, and passed througha 2-mm sieve to remove gravel. The < 2-mm fraction

322

Table 2. Data from the stands in the chronosequence study at Vestskoven. Stand data estimatesare based on yield data. (Data from the National Forest and Nature Agency). (n.a.= no dataavailable; DBH = diameter at breast height)

Tree species Planting Age at sampling Area Height DBH Volume

year (years) (ha) (m) (cm) (m3 ha−1)

Mixed deciduous ? ∼200 5.0 20.2 27.7 370

Oak 1970 28 7.2 12.4 15.1 99

1976 22 4.3 7.1 7.6 50

1977 21 2.9 8.4 8.5 53

1979 19 5.0 5.3 5.8 23

1988 10 1.5 2.8 1.9 11

1993 5 1.2 1.7 1.6 8

Norway spruce 1969 29 7.2 16.0 21.1 206

1970 28 1.0 15.0 17.1 242

1975 23 5.0 7.2 8.6 86

1976 22 0.7 9.8 11.0 161

1988 10 0.7 2.6 2.0 43

1990 8 2.3 2.1 2.0 35

1997 1 2.6 n.a. n.a. n.a.

was dried at 60 ◦C and weighed. For further chemicalanalyses, the four samples per plot were pooled byhorizon.

Bulk density of mineral soil <2 mm of each layerwas determined on one sample per plot by correct-ing bulk density of the fresh soil (mass of soil corevolume in the augar) for water content. For determin-ation of water content, a subsample of 10 g of eachlayer was dried at 105 ◦C. Volume of roots and stones>2 mm was taking into account, which was <4% inall samples. No greater stones were found within thedepth of the Ap-horizon.

Chemical analysis

The pH of foliar forest floor material and the mineralsoil was determined in a 0.01 M CaCl2 solution, using1 g of the forest floor (ratio 1:10), and 12 g of the min-eral soil sample (ratio 1:2.5) of each depth strata. Thesuspension was shaken for at least 2 h before meas-uring the pH value with a glass electrode. Organicsamples were analyzed manually, while an automatedRadiometer set-up consisting of a SAC90 SampleChanger, a VIT90 Video Titrator and a GK2401 com-bination pH electrode was used for the mineral soilsamples.

For determination of BSeff, 3 g mineral soil wasextracted with 30 mL 1 M NH4NO3 by shaking thesuspension for 20 h. After centrifugation, the soilwas extracted two additional times with 10 ml 1 MNH4NO3 for 10 min. The total extract was adjustedto 50 mL by 1 M NH4NO3. Base cations (Na+, K+,Ca+, Mg+, and Mn+) in the extract were measured byplasma-atom emission spectrometry (ICP- AES) usinga Perkin-Elmer Optima 3000 instrument. Exchange-able acidity of the extract was analyzed by end-pointtitration with 0.01 M NaOH until pH 6.5, i.e., basesaturation at soil pH (BSeff). BSeff was then calculatedby

BSeff(%) = ( base cations / ( base

cations + exchangeable acidity )) × 100

Total N concentrations were analyzed on 0.2 g ofthe organic samples and 0.5 g of the mineral soilsamples by dry combustion (Dumas method) using aLeco-CSN-2000 Analyzer (Matejovic, 1993). Prior toanalysis, the mineral fraction was ground further topass through a mesh screen of 0.49 mm. The stonecontent was taken into consideration when calculatingN contents on base of the measured concentrations.

323

Statistics

All statistical analyses were conducted using SAS stat-istical software (SAS Institute, 1990). Effects of treespecies and age on the examined soil parameters, andinteraction between tree species and stand age weretested by analysis of covariance (ANCOVA). If no sig-nificant effect of tree species was detected, the data forboth tree species were related to stand age by usingsimple linear regression. The accepted level of signi-ficance was P < 0.05. Data transformation was notnecessary since variances were homogenous and re-siduals were normally distributed. The standard errorof the mean (SEM) for each stand was based on resultsfrom the three circular plots.

Results

Mineral soil bulk density

Bulk density was lowest in the top 5 cm of the mineralsoil, increasing slightly downwards the profile from1.3 to 1.6 g cm−3. Bulk density was not affected bytree species, and a correlation with increasing standage was only found in 0–5 cm depth in terms of asignificant decrease (P = 0.01, r2 = 0.41). In contrast,mineral soil of both the permanent pasture and the oldmixed forest Ledøje Plantage had a lower bulk densitythan the afforested stands, ranging from only 1.0 to 1.5g cm−3.

Bulk density was lower and N concentrations werehigher in the 23-year-old spruce stand than in soils ofthe other stands. That part of Vestskoven was less well-drained before cultivation, and probably, the soil in-herited organic material from the more wet conditionsalready at the time of afforestation. Therefore, thisstand is considered to be an outlier in the following.This was also supported by statistical tests.

pH

In the foliar fraction of the forest floor, pH valuesranged from 4.7 to 5.6 (Table 3). There was no signi-ficant effect of tree species and no tendency of changesin pH along the chronosequence. Thus, the pH of thefoliar fraction remained fairly constant during the first30 years following afforestation.

Mineral soil pH was also not affected by treespecies. A strong negative correlation of pH with in-creasing stand age (P < 0.001) was found in the twoupper layers (0–5 and 5–15 cm) (Figure 1). pH values

Table 3. pH in the foliar fraction and nitrogen concentrations inthe foliar and non-foliar fraction of the organic layer in affores-ted stands where forest floors had developed. SEM is given inparentheses. n = 3

Age at sampling Foliar pH Foliar N Non-foliar N

(years) (%) (%)

Mixed deciduous

∼200 – 1.56 (0.03) 1.01 (0.05)

Oak

28 5.24 (0.10) 1.29 (0.08) 1.08 (0.11)

28 5.32 (0.09) 1.31 (0.14) 0.97 (0.05)

22 5.05 (0.08) 1.60 (0.07) 0.90 (0.06)

21 4.88 (0.05) 1.75 (0.13) 1.02 (0.07)

19 4.90 (0.05) 1.86 (0.04) 1.08 (0.09)

10 4.93 (0.12) 1.71 (0.02) 1.02 (0.08)

Norway spruce

29 4.71 (0.10) 1.46 (0.08) 0.69 (0.03)

28 5.29 (0.11) 1.36 (0.04) 0.76 (0.10)

23 5.62 (0.13) 1.35 (0.03) 0.98 (0.01)

22 5.56 (0.05) 1.44 (0.06) 0.71 (0.08)

10 5.18 (0.12) 0.93 (0.05) 1.02(−)

decreased by almost two units from 6 to 4 in the 0–5 cm soil layer, and slightly less in 5–15 cm depth. Inthese two upper layers, pH values were lower than inthe organic layer accumulated on top of the afforestedmineral soil, except for the 10-year-old oak stand. pHincreased with depth at all sites except for the per-manent pasture and for stands aged younger than ca. 5years. The pH of the permanent pasture was generallyabout one unit higher than in the same depth of foreststands of comparable age. At Ledøje Plantage, pH wasabout 4–5 and did not differ much between the threedepth strata.

BSeff and exchangeable cations

Effective base saturation in mineral soil of the affores-ted stands revealed no differences between the two treespecies (Figure 2). In the first 10 years after afforest-ation, BSeff remained 100 % in all three layers of theAp-horizon. Later on, BSeff tended to decrease in theupper two soil layers, and most clearly in the top 5 cm.However, the decrease was not significant. In contrastto the >20-year-old forest stands, the permanent pas-ture still had 100 % BSeff in all three depths 21 yearsafter conversion.

324

Figure 1. pH in three different layers of the Ap-horizon in affores-ted oak and Norway spruce stands of different ages, a permanentpasture, and the ca. 200-year-old mixed deciduous forest. (a)0–5 cm, (b) 5–15 cm, (c) 15–25 cm depth. One Norway sprucestand was excluded from regression. Error bars are one SEM.

Of all determined base cations, only exchangeableCa2+ showed a significant change with time, at leastin the upper two soil layers (Figure 3). ExchangeableCa2+ was reduced to an average of 7 cmol+ kg−1

along the chronosequence, both in 0–5 and 5–15 cmdepth. However, there was no effect of tree species.The permanent pasture had a higher Ca2+ concentra-tion than all forest stands, except for the 5-year-oldoak stand in the upper 15 cm.

Figure 2. Effective base saturation (Bseff) (%) of the Ap-horizon ofan afforestation chronosequence of oak and Norway spruce at 0–5,5–15 and 15–25 cm depth, and of a permanent pasture.

Figure 3. Exchangeable calcium (cmol+ kg−1) in (a) 0–5 cm depth,(b) 5–15 cm depth, and (c) 15–25 cm depth of the Ap-horizon inafforested oak and Norway spruce stands, and a permanent pasture.

325

Figure 4. C/N ratio of the foliar forest floor material of oak and Nor-way spruce stands at Vestskoven, and of a ca. 200-year-old mixeddeciduous forest. Error bars are one SEM.

Nitrogen and C/N ratios

At a stand age of 30 years, forest floor mass underspruce (23 Mg ha−1) exceeded that of oak (5 Mg ha−1)by almost five times. In the spruce stands and the 10-year-old oak stand, the foliar fraction made up morethan 80 % (dry weight) of the forest floor material. Inthe other oak stands, the mass of foliar material wasequal to non-foliar material. Nitrogen concentrationsof the total forest floor material did not change withincreasing stand age. Forest floor material of oak re-vealed slightly higher N concentrations than that ofspruce. Both tree species had higher N concentra-tions in the foliar fraction (1–2 %) as compared to thenon-foliar fraction (≤1 %) (Table 3).

C/N ratios of the foliage fraction did not cor-relate with time after afforestation, but values wereclearly different for oak and spruce (P = 0.006). Themean C/N ratio of the foliar fraction of oak was 22.7,whereas it was 26.4 in spruce stands (Figure 4). The10-year-old spruce stand was left out, because of itsthin forest floor.

Increase in N storage in the foliar fraction due toaccumulation of forest floor was higher under sprucethan oak with a significant interaction (P < 0.01)between tree species and stand age (Figure 5). Thisinteraction effect was somewhat weaker when the non-foliar fraction was included. The total accumulation ofN in the forest floor (both fractions) was highly signi-ficant in the spruce stands (P = 0.002, r2 = 0.88) andgreater as compared to oak stands. In the oak stands,accumulation observed along the chronosequence wasonly weakly significant (P = 0.088, r2 = 0.47). Thirty

Figure 5. Nitrogen storage of the forest floor (foliar and non-foliarfraction) (kg ha−1) of afforested oak and Norway spruce stands ofdifferent ages, and of a ca. 200-year-old mixed deciduous forest.Error bars are one SEM.

years after afforestation, almost 300 kg N ha−1 werestored in the organic layer in the spruce stands, whichis more than five times more than in the oak stands.Additionally, the amount of N stored in the forest floorof oak appears to level off around 30 years after affor-estation, while it could still be increasing in the sprucestands.

Tree species affected neither concentration norstorage of N in the mineral soil. Therefore, mineralsoil N data of both tree species were pooled to onedata set. Nitrogen concentrations varied from about0.1 to 0.3 % in all three depth strata. Concentrationswere highest in the upper 5 cm of the mineral soil anddecreased slightly downwards in the soil profile. Therewas no significant change in N concentrations in 0–15 cm depth with increasing stand age (P > 0.1),whereas N concentrations tended to decrease in soildepths below 15 cm (P = 0.067, r2 = 0.27).

Mineral soil C/N ratios ranged from 9.5 to 18.8 inthe different soil layers of all investigated plots. AtVestskoven, the highest C/N ratios were observed inthe top 5 cm soil (Figure 6). A weak effect of treespecies could be found (P = 0.07), where the averageC/N ratios were lower for oak (12) than for spruce(14). Pooling the results of both tree species, a slightincrease in C/N ratio with stand age was observed inthe upper 5 cm mineral soil (P = 0.029). Neverthe-less, when considering the total Ap-horizon down to25 cm, the C/N ratio was about 11, and no effect oftree species or time was apparent.

C/N ratios at Ledøje Plantage did not differ fromthose in the young afforestation stands. The C/N ra-

326

Figure 6. Mineral soil C/N ratios in three different layers of theAp-horizon in afforested oak and Norway spruce stands as well asin a permanent pasture. (a) 0–5 cm, (b) 5–15 cm, (c) 15–25 cm. Onespruce stand was excluded from regression.

tios of the permanent pasture were closer to those inthe same soil depths of oak stands than to those ofspruce stands of comparable age. Compared to foreststands, the permanent pasture had lower C/N ratios inthe upper 0–5 cm soil, with an average of 11.

Storage of N remained almost constant over thechronosequence in 0–5 and 5–15 cm depth of the Ap-

Figure 7. Nitrogen content (Mg ha−1) in three different layers ofthe Ap-horizon in afforested oak and Norway spruce stands, a per-manent pasture, and a ca. 200-year-old mixed deciduous forest. (a)0–5 cm, (b) 5–15 cm, (c) 15–25 cm. One spruce stand was excludedfrom the dataset used for regression analysis. Error bars are oneSEM.

horizon (Figure 7). The 15–25 cm layer revealed aslight decrease in N content with stand age (P < 0.05,r2 = 0.34) when the 23-year-old spruce stand was ex-cluded as an outlier based on the mentioned reasons.The average N storage in the total former plow layer(0–25 cm) of the forest stands was about 5.5 Mg ha−1.This value did not change much when including theamount of N stored in the forest floor. The 0–25 cmsoil layer of the permanent pasture contained almost2 Mg ha−1 more N than afforested soil of the sameage (7.1 Mg N ha−1). Nitrogen storage in the old forestsoil (0–25 cm) in Ledøje Plantage was 6.6 Mg N ha−1.

327

Discussion

pH and base cations

Tree species or time after afforestation did not affectthe pH of the foliar fraction of the forest floor ma-terial. This is in contrast to studies reporting a moreacid humus layer under coniferous than under decidu-ous trees (e.g., Vesterdal and Raulund-Rasmussen,1998). Differences in pH are generally ascribed toinitial differences in leaf litter quality and thus vari-ations in the rate and processes of decomposition ofthe surface litter (Ovington, 1953). Decompositionand nutrient dynamics may after some time reduce theinitial effect of the nutrient status of litter on forestfloor pH (Ovington, 1953; Rustad, 1994). Alrikssonand Eriksson (1998) studied pH values in the com-bined forest floor and organic layer of 27-year-oldstands of coniferous and deciduous tree species, andfound no significant difference between pH values un-der Norway spruce and birch (Betula pendula Roth).The observed pH values were similar to the results inthe present study, and also at Vestskoven, the smallrange in pH values of the forest floors indicates anabsence of species effect at this nutrient rich site.

The permanent pasture had higher pH values thanforest stands of comparable ages. In the afforestedstands, mineral soil pH in the 0–5 cm layer decreasedfrom pH 6 by almost two units within 30 years afterafforestation. The decrease in pH was weaker in thedeeper soil layers. These results support the well-known pattern that afforestation in general enhancesacidification of mineral soil. The greatest increase inacidity in the upper 0–15 cm of the profile is consistentwith results from other studies (e.g., Hallbäcken andTamm, 1986; Ovington, 1953). It may be attributed tothe greater influence of vegetation on the soil layersnear the surface.

In young stands, high pH values are found due toformer liming, but after about a decade, pH valuesdecrease because of acidifying processes in the upperlayers. Dick (1983) observed a pH decrease directlyafter ceased soil cultivation. An age dependence ofsoil pH has also been reported in several other stud-ies. Ovington and Madgewick (1957) observed thegreatest change in soil pH within the first 25 years afterforest establishment, but they found no evidence thatthe change would continue or be intensified through-out the life of a plantation stand. It may be assumedthat the observed decrease in pH in the present studywill level off after some more decades. pH values

observed in the ca. 200-year-old forest may supportthis assumption, since pH is not much lower than inthe 30-year-old afforested stands, at least in 0–5 and5–15 cm depth (Figure 1). Furthermore, changes insoil pH were not significantly affected by the differ-ent tree species. This is in accordance with a studyon afforestation in England and Wales, which showedlittle influence of tree species on soil pH (Ovingtonand Madgewick, 1957). Time after afforestation maytherefore be more important for short-term changes insoil pH than the selection of tree species.

An important factor regulating mineral soil pH isthe capacity of the soil to buffer protons producedby plant metabolism, biological transformations inthe soil, or imported to the area with deposition. Nochanges in pH will occur despite a high proton inputas long as the buffer capacity of the soil is suffi-cient. An empirical model of Binkley et al. (1989)indicated that a decline in pH results largely from thereduction in BS of the exchange complex with time.In the present study, the reduction in BSeff was notsignificant but still most apparent in 0–5 cm depth(Figure 2), which was the soil layer with the greatestpH decrease (Figure 1a). Since high proton input en-hances the reduction in BSeff, proton production mustbe comparable for the studied oak and spruce stands.This may point to a common proton source under thetwo tree species. Nitrification and subsequent leachingof nitrate from the top soil may be the proton pro-ducing process common to both tree species on suchan old agricultural soil with no restriction in soil Navailability. Nitrification may be enhanced even sev-eral decades after afforestation, which was reported byJussy (1998).

The decrease in Ca2+ concentration with time afterafforestation starting from a rather high level can beattributed to the ceased agricultural management. Juget al. (1999) found a similar decrease in Ca2+ sat-uration in short-rotation plantations on former arablesoils. During agricultural land-use, liming activitiescause a high to neutral mineral soil pH. This is mostpronounced in the plow layer. After afforestation, theinitiated acidification starts from the top soil, which iswhy the Ca2+ concentration decrease most rapidly inthe upper soil layers. However, Alriksson and Olsson(1995) observed higher Ca2+ concentrations in the top10 cm of a 55-year-old stand planted on former farmland. They suggested that a transfer of cations fromdeeper soil layers to the top soil had occurred.

328

Nitrogen and C/N ratio

After three decades, five times more N is stored inthe forest floor of spruce stands as compared to oak,despite similar N concentrations. This can probablybe attributed to differences in the amount and com-position of litter fall. The effect of plant litter qualityon decomposition rates of organic material has beendescribed by several authors (e.g., Aber et al., 1990;Anderson, 1991; Palm and Sanchez, 1991). Deciduouslitter may be decomposed already within one vegeta-tion period depending on site conditions and species.In contrast, the slowly decomposing litter of conifersremains on the soil surface for several years. There-fore, forest floors in spruce stands may to some extentbe considered a sink for N. However, compared to theamount of soil N stored in 0–25 cm depth in the 30-year-old spruce stand, the 300 kg N ha−1 found in theforest floor is still less than 7 % of the total N storageper hectare of the upper soil layer of spruce stands.

Forest floor C/N ratios differed between oak andspruce, but there was no apparent change with timeafter afforestation (Figure 4). C/N ratios were gener-ally lower (22.7) in oak stands compared to spruce(26.4). Vesterdal and Raulund-Rasmussen (1998) re-ported C/N ratios > 28 for forest floors of oak andNorway spruce on nutrient rich soil of similar soil tex-ture in Denmark. Some of the sites had been forestsfor centuries. This suggests that forest floor C/N ratiosmay be affected by land-use history, since litter qualityis found to be correlated with the nutrient status ofmineral soil (Chappell et al., 1999). The lower C/N ra-tios of the forest floor at Vestskoven may be attributedto the high soil N capital derived from fertilizationduring agricultural use. Prescott et al. (2000) reportssimilar observations.

Forest floor C/N ratio is suggested as an indicatorfor the risk of nitrogen leaching with seepage water(Gundersen et al., 1998a). Therefore, the C/N ratioof the forest floor material in the afforested standsis of special interest when judging a possible effectof afforestation on soil water quality. A low forestfloor C/N ratio enhances nitrification and mineraliza-tion rates of the organic matter, resulting in the releaseof plant available N (Gundersen et al. 1998b; John-son, 1992). Nitrification was found to increase withdecreasing C/N ratio in a nonlinear way, suggestingthat soils with forest floor C/N ratios > 25–30 do notnitrify (Kriebitzsch, 1978). Judged by this and thelow C/N ratio in mineral soil (10–15) in most of thestands, high rates of nitrification may be assumed at

Vestskoven. When nitrification is high and N supplyexceeds the demand of trees, there is a risk of NO3

−losses by leaching. In several studies it has been shownthat NO3

− output with seepage water under forest eco-systems occurs at sites with low forest floor C/N ratio(Ågren and Bosatta, 1988; Matzner and Grosholz,1997; Gundersen et al., 1998a). Thus, some NO3

−leaching may be expected at Vestskoven, especially inthe oak stands, which have the lower forest floor C/Nratios and a higher precipitation surplus and light andenergy input to the forest floor. Additionally, N storagein the accumulated organic layer under spruce mightprevent N leaching.

No significant age dependence of N concentrationsin the mineral soil was found within 30 years of affor-estation. Only in 15–25 cm depth of the Ap-horizon,N concentrations tended to decrease slightly with age.However, there was no significant redistribution of Nbetween layers with time as found for carbon (Vester-dal et al., 2002). The unchanged N concentrations areconsistent with results reported by Hamburg (1984).Nevertheless, he found a decrease in the amount oftotal N stored in the former plow layer of an old-fieldchronosequence with spruce and hardwood. This wasnot the case in the present study where no signific-ant change in N storage with time could be shown.Jug et al. (1999) also observed no significant changeor a small decrease in mineral soil N in 10-year-oldafforested stands in different parts of Germany. Adecrease in the N pool with time would have beenexpected, since there is no input of mineral N throughfertilizers, and N from the previous agricultural useis incorporated in the growing biomass. There is anestimated accumulation of N in the structural biomassof 0.18 Mg ha−1 for oak and 0.28 Mg ha−1 for spruce30 years after afforestation, calculated from standingvolume (Statens forstlige Forsøgsvæsen, 1979) and anassumed N concentration of 0.17 %. The cumulativeN input with deposition within this period would havebeen about 0.5 Mg N ha−1 (Andersen et al., 2002).Hence, the N demand of the plants could be satisfiedwith the N deposition without leading to a reduction inthe N pool of the mineral soil.

Mineral soil C/N ratios tended only to differslightly between tree species (P = 0.07). As in forestfloors, C/N ratios were somewhat greater under sprucethan under oak stands. Compton et al. (1998) reporteda significant effect of the present vegetation on mineralsoil C/N ratios in 0-15 cm depth, and coniferous spe-cies had higher C/N ratios than deciduous. In general,C/N ratios decrease with soil depth (Post and Mann,

329

1990), which was also the case in the present study. AtVestskoven, C/N ratios were apparently affected by in-herent soil conditions of the study site and influencedby the former agricultural treatment. C/N ratios werestill in the order of magnitude of cultivated soil, whichrange by average from 11 to 15. Compton et al. (1998)found a significant effect of land-use history on soilC/N ratio in 0–15 cm depth in 40-year-old mixed oakstands. Stands on formerly plowed soil had a lowerC/N ratio (25.6) than those on unplowed soil (29.6).However, the ratios were almost two times greater ascompared to the present study. This may probably be aconsequence of former less intensive agricultural man-agement practices, less N fertilization and a poorer soiltype. The influence of stand age on the mineral soilC/N ratio was significant in 0–5 cm depth (P = 0.029)(Figure 6). This may be explained by changes in bal-ances between litter input and decomposition ratesvarying with age and soil depth. Wells (1987) founda strong increase in C/N ratio in the top 8 cm soil 15years after afforestation of abandoned arable soil withloblolly pine. The trend was weaker in the 15–30 cmlayer, and no change was found below 30 cm.

In contrast to the forest floor, storage and concen-tration of N in the Ap-horizon was not affected by treespecies. This is in accordance with results of Comptonet al. (1998) who reported no influence of tree specieson soil N concentration after afforestation of arablesoils. This and our findings lead to the conclusion thatN storage in the mineral soil of afforestation standsmay probably not be influenced by the selection of treespecies, at least within a time scale of a few decades.Nitrogen storage in the 0–25 cm profile of the per-manent pasture was almost 2 Mg N ha−1 higher thanin forest stands of similar age classes. The increasedamount of N in the upper soil layers may be explainedby a high root litter production of the grass vegeta-tion on the field. In the afforested stands, the initiallymobilized N can be fixed in the biomass again, whichis an important sink for N in the very early phase ofafforestation (Heilman et al., 1995). Loss of N mightoccur when the mobilized mineral soil N exceeds theamount that can be stored in the biomass (Jug et al.,1999). This may particularly occur in the establish-ment period where trees are small and weed control isapplied.

In conclusion, afforestation modifies soil proper-ties derived from cultivation. Our results from a loamysoil indicate that site conditions inherited from parentmaterial and the former land-use determine the dir-ection and range of changes more than the choice of

tree species, at least during the first 30 years followingafforestation. Forest floor accumulation, C/N ratios inthe top 5 cm soil, and mineral soil pH changed signi-ficantly with time. Differences related to tree specieswere only found for the forest floor. The acidifyingaffect of afforestation on mineral soil was confirmedby a significant decrease in soil pH in the 0–5 cmlayer and a slightly weaker decrease in the 5–15 cmlayer. Spruce stands were characterized by a continu-ously increasing accumulation of forest floor material,whereas no further increase seemed to occur in oakstands after two decades. As a result, storage of forestfloor N was higher in the 30-year-old spruce stand,exceeding that in oak stands of the same age by fivetimes. Almost no changes in mineral soil N storagesuggest, that N supply from deposition was sufficientto satisfy the N demand of the trees without reducingthe N pool of the soil. Forest floor C/N ratios werelower in oak stands as compared to spruce stands. Incontrast, mineral soil C/N ratios were only weeklyaffected by tree species.

Three decades after afforestation, soil propertieswere still closer to those of arable soils than to thoseof old growth forests. This slow development indicatesthe necessity of long-term investigations in order tounderstand changes in soil properties and to predictfuture trends.

Acknowledgements

We thank the technicians Lena Byrgesen and MadsM. Krag for their skilful chemical analysis of nu-merous soil samples. Financial support came fromthe Nordic Forest Research Co-operation Committee(SNS), project SNS-67, the Danish Forest and NatureAgency and in a later stage the European Commissionin contract No: EKV1-CT1999-00020 (AFFOREST).

References

Aber J D, Melillo J M and McClaugherty C A 1990 Predicting long-term patterns of mass loss, nitrogen dynamics and soil organicmatter formation from initial fine litter chemistry in temperateforest ecosystems. Can. J. Bot. 68, 2201–2208.

Alriksson A and Eriksson H M 1998 Variations in mineral nutrientand distribution in the soil and vegetation compartments of fivetemperate tree species in NE Sweden. For. Ecol. Manage. 108,261–273.

Alriksson A and Olsson M T 1995 Soil changes in different ageclasses of Norway spruce (Picea abies (L.) Karst.) on afforestedfarmland. Plant Soil 168/169, 103–110.

330

Andersen H V, Hovmand M, Ro-Poulsen H, Asman W, Hansen K,Pilegård K, Mikkelsen T N, Hummelshøj P, Jensen N O and Sten-holt C 2002 Atmosfærisk deposition til skovene. In Stofkredsløbi skove – Ionbalancen. Ed. K Hansen. Forskningsserien. DanishForest and Landscape Research Institute, Denmark. (Submitted).

Anderson J M 1991 The effects of climate change on decomposi-tion processes in grassland and coniferous forests. Ecol. Appl. 1,326–347.

Binkley D, Valentine D, Well C and Valentine U 1989 An empiricalanalysis of the factors contributing to 20-year decrease in soilpH in an old-field plantation of loblolly pine. Biogeochemistry7, 139–154.

Chappell, H N, Prescott C E, and Vesterdal L 1999 Long-term ef-fects of nitrogen fertilization on nitrogen availability in coastalDouglas-fir forest floors. Soil Sci. Soca. Am. J. 63, 1448–1454.

Compton J E, Boone R D, Motzkin G and Foster D R 1998 Soilcarbon and nitrogen in a pine-oak sand plain in central Mas-sachusetts: Role of vegetation and land-use history. Oecologia116, 536–542.

Dick W A 1983 Organic carbon, nitrogen and phosphorus concen-trations and pH in soil profiles as affected by tillage intensity.Soil Sci. Soca. Am. J. 47, 102–107.

Gundersen P, Callesen I and de Vries W 1998a Nitrate leaching inforest ecosystems is related to forest floor C/N ratios. Environ.Pollut. 102 (S1), 403–407.

Gundersen P, Emmett B A, Kjønaas O J, Koopmans C and TietemaA 1998b. Impact of nitrogen deposition on nitrogen cycling: asynthesis of NITREX-data. For. Ecol. Manage. 101, 37–55.

Hallbäcken L and Tamm C O 1986 Changes in soil acidity from1927 to 1982–1984 in a forest area of south-west Sweden. Scand.J. For. Res. 1, 219–232.

Hamburg S P 1984 Effects of forest growth on soil nitrogen andorganic matter pools following release from subsistence agri-culture. In Forest Soils and Treatment Impacts. Ed. E L Stone.pp.145–158. University of Tennessee, Knoxville, TN.

Helles F and Linddal M 1996 The Rothamstedt long-term experi-ments: are they still of use? Agron. J. 83, 2–10.

Heilmann B, Makeschin F and Rehfuess K E 1995 Vegetation-skundliche Untersuchungen auf einer Schnellwuchsplantage mitPappeln und Weiden nach Ackernutzung. Forstw. Cbl. 114,16–29.

Heinsdorf D 1994 Bemerkungen zur Aufforstung stillgelegter Ack-erflächen im nordostdeutschen Tiefland. Beitr. Forstwirtsch.Landsch. ökol. 28 (1), 29–34.

Johnson D W 1992 Effects of forest management on soil carbonstorage. Water Air Soil Pollut. 64, 83–120.

Jug A, Makeschin F, Rehfuess K E and Hofmann-Schielle C 1999Short-rotation plantations of balsam poplars, aspen and willowson former arable land in the Federal Republic of Germany. III.Soil ecological effects. For. Ecol. Manage. 121, 85–99.

Jussy J-H 1998 Minéralisation de l’azote, nitrification et prélève-ment radiculaire dans différent écosystèmes forestiers sur solacide. Effets de l’essence, du stade de développement du peuple-ment et de l’usage ancien des sols. Ph.D. thesis INRA-Nancy,France, 171 pp.

Koerner W, Dambrine E, Dupouey J L, and Benoît M 1999. δ15Nof forest soil and understorey vegetation reflect the former

agricultural land use. Oecologia 121, 421–425.Kriebitzsch W U 1978 Stickstoffnachlieferung in sauren Waldböden

Nordwestdeutschlands. In Nitrification in Forest Soils: effectsfrom nitrogen deposition on soil acidification and aluminum re-lease. Reviews of Environmental Contamination and Toxicology,Vol. 13. Ed. P Gundersen and L Rasmussen 1990. pp. 1–16.Scripta Geobotanica, Goltze, Göttingen, Germany.

Kubiniok J and Müller V 1993 Auf Böden des Rotliegenden imnordöstlichen Saarland: Bodenentwicklung und Nährstoffhaush-alt unterschiedlich alter Ackeraufforstungen. AFZ 5, 236–238.

Matejovic I 1993 Determination of carbon, hydrogen, and nitro-gen in soils by automated elemental analysis (dry combustionmehtod). Communi. Soil Sci. Plant Anal. 24, 2213–2222.

Matzner E and Grosholz C 1997 Beziehungen zwischen NO3−-

Austrägen, C/N-Verhältnissen der Auflage und N-Einträgen inFichtenwald (Picea abies Karst.)-Ökosystemen MitteleuropasForstw. Cbl. 116, 39–44.

Ovington J D 1953 Studies of the development of woodland condi-tions under different trees. I. Soils pH. J. Ecol. 4, 13–34.

Ovington J D and Madgewick H A I 1957 Afforestation and soilreaction. J. Soil Sci. 8 (1), 141–149.

Palm C A and Sanchez P A 1991 Nitrogen release from theleaves of some tropical legumes as affected by their lignin andpolyphenolic contents. Soil Biol. Biochem. 23, 83–88.

Post W M and Kwon C 2000 Soil carbon sequestration and land-usechange: processes and potential. Glob. Chan. Biol. 6, 317–327.

Post W M and Mann L K 1990 Changes in soil organic carbonand nitrogen as a result of cultivation. In Soils and GreenhouseEffect. Ed. A F Bouwman. pp. 401–416. John Wiley & Sons Ltd1990.

Prescott C E, Chappell H N, and Vesterdal L 2000 Nitrogen turnoverin forest floors of coastal Douglas-fir at sites differing in soilnitrogen capital. Ecology 81 (7), 1878–1886.

Rustad L E 1994 Element dynamics along a decay continuum in ared spruce ecosystem in Maine, USA. Ecology 75, 867–879.

SAS Institute 1990 The GLM procedure. In SAS User’s Guide:Statistics Version 6. pp. 891–996. SAS Institute Inca., Cary, NC,USA.

Statens forstlige Forsøgsvæsen 1979 Skovbrugstabeller. JJ tryk-teknik a/s København. 270 pp.

Vesterdal L and Raulund-Rasmussen K 1998 Forest floor chemistryunder seven tree species along a soil fertility gradient. Can. J.For. Res. 28 (11), 1636–1647.

Vesterdal L, Ritter E and Gundersen P 2002 Changes in soil organiccarbon following afforestation of former arable land. For. Ecol.Manage. (In review)

Wells C G 1987 In Properties and Management of Forest Soils. Ed.W L Pritchett and RF Fisher. p. 106. John Wiley & Sons, NewYork.

Westman C J 1995 A simple device for sampling of volumetricforest soil. Cores Silva Fennica 29, 247–251.

Ågren G and Bosatta E 1988 Nitrogen saturation of terrestrialecosystems. Environ. Pollut. 54, 185–197.

Section editor: R.F. Hüttl