Litter decomposition and nitrogen mineralization in newly formed gaps

in a Danish beech (Fagus sylvatica) forest

Eva Ritter*

Danish Centre for Forest, Landscape and Planning, KVL, Hørsholm Kongevej 11, 2970 Hørsholm, Denmark

Received 19 April 2004; received in revised form 18 November 2004; accepted 24 November 2004

Abstract

Gap formation is suggested as an alternative forest management approach to avoid extreme changes in the N cycle of forest ecosystems

caused by traditional management practises. The present study aimed to investigate the effect of gap formation on N availability in beech

litter and mineral soil on sites, which experienced only little soil disturbance during tree harvest. N pools, litter decomposition, and N

mineralization rates in mineral soil were studied in two gaps (17 and 30 m in diameter) in a 75-year-old managed European beech (Fagus

sylvatica L.) forest in Denmark and related to soil temperature (5 cm depth) and soil moisture (15 cm depth). Investigations were carried out

during the first 2 years after gap formation in measurement plots located along the north–south transect running through the centre of each

gap and into the surrounding forest.

An effect of gap size was found only for soil temperatures and litter mass loss: soil temperatures were significantly increased in the

northern part of the large gap during the first year after gap formation, and litter mass loss was significantly higher in the smaller gap. All

other parameters investigated revealed no effect of gap size. Nitrification, net mineralization, and soil N concentrations tended to be increased

in the gaps. Cumulative rates of net mineralization were two fold higher in the gaps during the growing season (June–October), but a

statistically significant increase was found only for soil NH4–N concentrations during this period. Forest floor parameters (C:N ratios, mass

loss, N release) were not significantly modified during the first year after gap formation, neither were the total C content nor the C:N ratio in

mineral soil at 0–10 cm depth.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: Fagus sylvatica L.; Gap; Litter decomposition; Mineralization; Nitrogen release; Soil core method; Soil moisture; Soil temperature

1. Introduction

Research in soil nitrogen (N) availability has important

relevance for the understanding of processes in forest

ecosystems (Gessel et al., 1973; Attiwill and Leeper, 1987;

Tamm, 1991; Pettersson and Hogbom, 2004). Nitrogen

availability depends highly on the decomposition of litter

material and subsequent release of N in plant-available

forms by mineralization, converting organic N into

inorganic N compounds (Davidson et al., 1992; Myrold,

1999). The major factors controlling decomposition

in forest ecosystems are soil moisture, temperature,

and substrate quality (Lousier and Parkinson, 1975; Gilliam

0038-0717/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.soilbio.2004.11.020

* Present address: The Agricultural University of Iceland, Hvanneyri, 311

Borgarnes, Iceland. Tel.: C354 433 5019; fax: C354 433 5001.

E-mail address: eva@hvanneyri.is.

et al., 2001; Trofymow et al., 2002). Thus, N availability

and the N cycling in forests can be affected by forest

management, as it may alter these factors (Vitousek and

Melillo, 1979; Binkley, 1984; Likens and Bormann, 1995).

Forest management which removes the majority of

mature trees in a large area may change the forest

microclimate drastically (Childs and Flint, 1987), often

resulting in increased N mineralization rates (Matson and

Vitousk, 1981; Frazer et al., 1990; Smethurst and Nambiar,

1990). With decreased N uptake by roots, this can cause

increased losses of nitrate (NO3K) by leaching or denitrifica-

tion (Keenan and Kimmins, 1993; Nohrstedt et al., 1994;

Piirainen et al., 2002). To avoid these and other undesirable

effects, new approaches in forest management are gaining

increasing interest (Attiwill, 1994; Fuhrer, 2000). One

possible approach is to harvest trees group-wise, thereby

creating small gaps. Microclimatic changes in such small

Soil Biology & Biochemistry 37 (2005) 1237–1247

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E. Ritter / Soil Biology & Biochemistry 37 (2005) 1237–12471238

canopy gaps are less pronounced than in large areas of open

space (Mosandl, 1988). Gap formation used as an alternative

approach in forest management may therefore have a less

severe impact on forest ecosystems than traditional

management practices. If applied on forest stands with a

homogeneous forest structure, gap formation may addition-

ally help to develop a more heterogeneous forest structure

(Lorimer, 1989). Especially for temperate forest ecosystems,

this is considered as positive in forestry circles (Kilian and

Fanta, 1998; Hockenjos, 1999). However, studies on this

topic are still scarce. Most investigations of gap processes

focus on tree regeneration, species diversity or microclimatic

gradients (e.g. Bray, 1956; Runkle, 1989; McClure and Lee,

1993; Gray et al., 2002). Many gap studies are restricted to

only few measurement plots, e.g. gap centre, edge, and under

closed canopy (Bauhus, 1996; Bartsch, 2000). Including a

larger area in and around a canopy gap may help to

understand the effects of gap formation better.

The objectives of this study were to examine changes in N

availability in litter and mineral soil in and around newly

formed gaps in a managed European beech (Fagus sylvatica

L.) forest. The study was conducted in a traditionally

managed, homogeneous, and even-aged forest, as this may

represent the kind of forest stand on which gap formation

typically will be applied. Soil disturbance in the gap areas

was kept to a minimum. It was hypothesised that decompo-

sition and N release would be stimulated in the gaps due to

microclimatic changes, i.e. increased temperature and

moisture, caused by the canopy opening. The study included

investigations of soil temperature, soil moisture, litter

(mass loss, C:N ratio, and N release), and mineral soil

(N concentrations, nitrification, and net N mineralization) in

the first 1–2 years after gap formation. Ground vegetation

cover and regeneration height were recorded once during the

study period.

2. Methods and materials

2.1. Site description

The study was carried out in two gaps in the managed

forest Ravnsholte (ca. 200 ha) which is part of

Table 1

Selected properties of mineral soil (0–10 cm depth)

n pH Claya (%) Si

Large gap site

Gap 3 3.3 (0.04) 7.6 (1.6) 12

Forest 4 3.3 (0.05) 9.1 (0.1) 13

Small gap site

Gap 3 3.3 (0.02) 8.5 (1.1) 13

Forest 4 3.5 (0.03) 9.6 (0.5) 14

Values are means (Gstandard error) of three plots located in the gap and four pla Clay !2 mm, siltZ2–20 mm, sandZ20–2000 mm.

the Skjoldenæsholm forest area situated in central

Zealand in the eastern part of Denmark (55831 0N,

11854 0E). The area is gently rolling (ca. 85 m a.s.l.). The

climate is cool-temperate with annual mean temperature of

8 8C and annual mean precipitation of about 600 mm. An

even-aged 75-year-old European beech stand (3.7 ha) with

advanced, but scattered regeneration of sycamore maple

(Acer pseudoplatanus L.) was chosen for the study. The

average canopy height of the stand was 27 m. Stem density

was 184 trees haK1, and basal area was 20.1 m2 haK1.

Ground vegetation was primarily Rubus idaeus, Stellaria

nemorum, Galium odoratum, and Milium effusum. The litter

layer was about 2 cm thick. The nutrient rich soil

developed on loamy glacial till is classified as Ultic

Hapludalf (Soil Survey Staff, 1998). Selected soil properties

are given in Table 1. Input of inorganic N with precipitation

or throughfall to the forest floor was about 15 kg haK1 yK1

at the study site (Ritter et al., in press-a).

2.2. Experimental design

In January 2001 two almost circular gaps, a large

(ca. 30 m in diameter) and a small (ca. 17 m in diameter)

one, were established in the stand by felling 12 and 4 trees,

respectively, which crowns made up part of the canopy

layer. No other trees remained in the gaps. All unnecessary

soil disturbances in the gap areas were avoided. The trees

were felled by hand and removed from the gap areas by

pulling them with a winch from outside the gaps. Stems

were removed from the study site, but tree crowns of the

felled trees were left at the site close to the gap edges in

the understorey of the surrounding stand. In February 2001

the study site was fenced to avoid browsing of deer. The

larger gap was located north–east of the small gap in a

distance of 64 m between the midpoints of the two gaps. It is

acknowledged that the experimental design does not allow a

generalisation for conditions in Danish beech forests of this

region, given the closeness of the two gap sites.

Subplots for measuring soil moisture, soil temperature,

and litter decay (litter plots) as well as subplots for mineral

soil sampling (soil plots) were located close to each other

in measurements plots placed along a transect running

north–south through the approximate centre of each gap.

lt (%) Sand (%) Bulk density

(g cmK3)

Total C (mg gK1)

.4 (1.5) 80.0 (3.1) 0.96 (0.1) 48.9 (5.4)

.9 (0.4) 77.1 (0.4) 0.94 (0.1) 48.4 (5.0)

.0 (1.4) 78.5 (2.3) 0.98 (0.04) 43.3 (1.6)

.9 (0.6) 75.6 (1.0) 1.00 (0.04) 45.5 (4.6)

ots located under closed canopy at each site.

Fig. 1. Location of soil temperature sensors and measurement plots for soil

moisture, litterbags (litter plots) and sampling of mineral soil (soil plots) in

the large gap (left) and the small gap (right). Note that the figure does not

illustrate the location of the two gaps relative to each other.

E. Ritter / Soil Biology & Biochemistry 37 (2005) 1237–1247 1239

Soil moisture measurement, mineral soil sampling, and

investigations of litter decay were carried out in a total of

seven measurement plots at each gap site (Fig. 1). Three of

the seven measurement plots were placed in the gap (under

the open sky), two under the closed canopy north of the gap,

and two under the closed canopy south of the gap. Soil

temperature was measured with nine and eight temperature

sensors at the large and small gap site, respectively (Fig. 1).

The measurement plots were located up to 38 m from the

gap centres. A forest road about 25 m south of the small gap

site made it impossible to extend the transect in the small

gap further south than 19 m from the gap centre.

2.3. Sampling and measurements

2.3.1. Soil temperature

Soil temperature sensors P100 (Delta-T Devices Ltd,

Cambridge, UK) were placed at 5 cm depth from the top of

in the mineral soil. The soil temperature was measured

every 10 min, and 2-h averages were recorded in a data

logger (DL2e data logger system, Delta-T Devices Ltd,

Cambridge, UK). These temperature records are in the

following simply referred to as maximum, minimum, and

mean soil temperature, even though they represent 2-h

averages. Statistical analysis was based on the daily

maximum, minimum, and mean temperature from 31

August 2001 to 26 November 2002 (only of completely

(24 h) reported days).

2.3.2. Soil moisture

Instantaneous measurements of volumetric soil moisture

content of the upper 15 cm soil (including the litter layer)

was measured approximately weekly from June 2001 to

December 2001 using time-domain reflectometry (TDR)

(Topp et al., 1980; Ledieu et al., 1986). Two parallel steel

rods (diameter 6 mm, distance 5 cm) were embedded

vertically in the soil from the undisturbed soil surface

close to each of the 14 soil plots. A portable TDR cable

tester (Tektronix 1502C/1502B, Tektronix, Inc., Berkshire,

UK) and a hand-held computer (Husky FS/2, Coventry, UK)

were used for data collection. Measurement values were

converted to volumetric water content (%) according to the

calibration of Topp et al. (1980) with the software

AUTOTDR (Thomsen, 1994). Data analysis was carried

out on soil water content converted to depths of soil

water (mm).

2.3.3. Vegetation cover

Ground vegetation cover and the height of regenerating

trees at the approximately 5 m2 area around the different

measurement plots along the transects at the two gap sites

were recorded in the second year after gap formation, on

25 June 2002. Percent vegetation cover (herbs and bushes)

was classified in four groups: (i) 0, (ii) 1–25, (iii) 26–50,

(iv) 51–100%, of which the average was used for illustration

in Fig. 3. Height of regenerating trees was recorded as

average height.

2.3.4. Litter material

Mass loss and N release from beech foliar litter were

measured by the litterbag technique (Wieder and Lang,

1982). In November 2001 freshly fallen beech foliar litter

was collected from the study site and dried at 60 8C.

Litterbags made of polyester tissue with a mesh size of

2 mm were filled with 2 g dry litter material. Three litter

bags were placed within an area of ca. 1 m2 in each of the 14

litter plots and fixed by small stainless steel pins. In

November 2002, after 1 year of decomposition, the

litterbags were collected again, the litter was weighed, and

a subsample was dried at 60 8C to determine dry weight and

litter mass loss of each litter bag. Prior to analysis of total C

and N content, the three subsamples from each litter plot

were mixed thoroughly, as the aim was not to determine the

variation within each plot, but between plots in the gaps and

under closed canopy.

2.3.5. Mineral soil

Three months before gap formation, three mineral soil

samples (0–10 cm depth) were collected in four randomly

distributed places at each gap site to get an estimate of the N

status of the study site, using plastic tubes (length 20 cm,

internal diameter 6.9 cm). After gap formation soil samples

were collected in the 14 soil plots by the sequential soil core

method (Adams et al., 1989; De Boer et al., 1993) for

determination of net nitrification and net mineralization

rates. Sampling was carried out every 4–5 weeks from 20

April 2001 to 7 May 2002. The December incubation was

prolonged to the end of January 2002, because soil at

E. Ritter / Soil Biology & Biochemistry 37 (2005) 1237–12471240

sampling depth was frozen in the weeks around New Year

2002. After removal of the litter layer, three plastic tubes

(as described above) were inserted into the A-horizon to a

depth of 10 cm in each soil plot (ca. 2 m2). Tubes were

covered with a plastic lid on the top to avoid that

precipitation would enhance leaching and perforated in

the upper part to enable gas exchange. With the onset of

each incubation period, a set of three control samples was

taken in each soil plot to determine the initial ammonium–

nitrogen (NH4–N) and nitrate–nitrogen (NO3–N) status. The

three subsamples of the control samples and the incubated

soil of each soil plot, respectively, were mixed thoroughly

before chemical analysis.

Nitrification rates for each incubation period were

calculated by subtracting soil NO3–N concentrations of the

initial soil samples from the final, incubated samples, and net

mineralization rates by subtracting the initial total inorganic

N concentrations (Nmin) (sum of NO3–N and NH4–N) from

the final, incubated Nmin concentrations. It should be noted

that this method does neither measure microbial immobil-

ization of NO3–N and NH4–N nor denitrification during

incubation periods. Furthermore, the exclusion of roots from

the soil inside the tubes or roots being cut off when inserting

the tubes into the soil reduces the competition between roots

and micro-organisms and might enhance nitrification

(Vitousek et al., 1982). Thus, the method determines only

net rates, not gross rates, of nitrification and mineralization.

For simplicity, the terms mineralization and nitrification are

used in the following instead of net mineralization and net

nitrification, respectively. The period 11 July to 13 August

2001 was excluded from data analysis of nitrification and

mineralization rates because of missing data of incubated soil

samples at the small gap site.

2.3.6. Chemical analyses

Litter material and mineral soil samples used for

determination of total C and N contents were ground finely

to pass through a mesh screen of 0.49 mm prior to analysis

by dry combustion (DUMAS method, Matejovic, 1993)

with a Leco-CSN-2000 Analyzer. Mineral soil samples

were frozen (K18 8C) until chemical analysis to avoid

further mineralization due to disturbance and increased

temperatures. Gravimetric water content was determined by

drying 10 g mineral soil sample of each soil plot at 60 8C.

Another 10 g mineral soil of each soil plot were extracted in

20 ml 1 M KCl for 1 h, centrifuged, and filtered (0.45 mm).

Determination of NH4–N and NO3–N in the extract was

carried out with a Flow Injection Analyzer (FIA) (Perkin

Elmer FIAS 300). The intensity in colour change was

measured spectrophotometrically at 590 nm (Perkin Elmer

UV/VIS spectrometer Lamda 2).

2.4. Statistics

Plot effects on daily maximum and mean soil

temperatures, respectively, were tested for every month by

one-way ANOVA with repeated measures followed by

Tukey pairwise multiple comparisons (SigmaStat, Vers.

2.03, SPSSw, Chicago, IL). The factor of the ANOVA was

measurement plot, and Julian day was the repeated measure.

When equal variances or normal distribution could not be

achieved by transformation, the Friedman Repeated

Measures ANOVA on ranked data was conducted.

To detect a possible influence of gap size in the growing

season and the dormant season, respectively, data on soil

moisture, litter (mass loss, C:N ratio, N release), soil N

concentrations, mineralization, and nitrification of each

season were tested by the Student’s t-test between the two

gaps (SAS statistical software, Vers. 8.02 SASw Institute,

Inc., Cary, NC, USA). The seasons were defined by the

developmental status of the tree leaves, with growing season

as the period when trees were in full leaf (June–October),

and dormant season as the period when trees were without

or with not yet fully developed leaves (November–May).

This assignment was chosen due to the possible influence of

roots on soil N and soil moisture content after leaf flush

(root uptake). The Student’s t-test was applied on the

seasonal averages of soil moisture and soil N concentrations

of the different gap plots, and on plot-wise cumulative rates

of nitrification and mineralization of each season. If no

effect of gap size was confirmed, the two gaps and their

surrounding forest were considered as replicates, and a one-

way ANOVA using the GLM procedure in SAS statistical

software with gap site as a block factor was applied. The

independent factor at two levels was: (i) plots located in

the gaps, and (ii) plots located under the closed canopy. The

analysis was carried out on the averages of the plots in each

gap and forest compartment to avoid pseudo-replicates.

However, it must be considered that the two gaps and their

surrounding forest are no true replicates, because of their

location next to each other. The conclusions of the study

must be tempered according to this. If an effect of gap size

was found, the Student’s t-test was applied separately on the

two gap sites and their respective surrounding forests as

described before. Also differences between average

monthly soil moisture content in plots in the gaps and

under closed canopy were tested by the Student’s t-test.

A correlation between mean soil temperature or soil

moisture contents at the beginning of an incubation period

and monthly nitrification and mineralization, respectively,

was tested by the Pearson correlation statistics (CORR

procedure, SAS statistical software, Vers. 8.02, SASw

Institute, Inc., Cary, NC, USA). The level of significance for

all analyses was P!0.05.

3. Results

3.1. Soil temperatures

Daily maximum and mean soil temperatures in the

northern part of the large gap and up to 4 m into the adjacent

Fig. 2. Soil moisture content of mineral soil at 0–15 cm depth at: (a) the

large, and (b) the small gap site. White circles represent the monthly

average of the three measurement plots located in the gap and black

circles of the four measurement plots located under closed canopy.

Black bars indicate the period with trees in full leaf. Error bars are 1

standard error.

E. Ritter / Soil Biology & Biochemistry 37 (2005) 1237–1247 1241

forest were significantly higher than in all other plots from

September 2001 to January 2002 (Tukey: P!0.05). This

area was also the most sun exposed part of the gap site

(between 6 and 18 m north of the gap centre) where direct

irradiation reached the forest floor at noon on a day in mid-

June. The difference between the monthly average soil

temperature in the warmest plot as compared to plots with

significantly lower soil temperatures was up to 2 8C. After

January 2002 this clear gap effect on soil temperatures had

disappeared. At the small gap site, direct irradiation

reached only into the forest understorey north of the gap

(about 10–14 m from the gap centre) at noon on a day in

mid-June. Soil temperatures did not appear to be affected by

gap formation to the same extent as at the large gap site, as

similar high maximum and mean soil temperatures were

measured both in the gap and under closed canopy.

At both sites absolute lowest mean soil temperatures

(monthly averages) were measured in plots under the closed

canopy or in the southern part of the gap. Differences

between daily maximum and minimum soil temperatures

ranged from 0.2 to 3.9 8C (monthly averages) in all

measurement plots. Daily differences were greatest in

spring from March to May (ANOVA: P!0.0001).

3.2. Soil moisture

Averages soil moisture content was 60 and 50 mm in the

growing season and 64 and 60 mm in the dormant season in

the gaps and under closed canopy, respectively. Thus,

seasonal averages were not significantly different between

the two gap sizes in either season (t-test: PR0.3), and there

was no gap effect in either season (ANOVA: PR0.23).

However, monthly averages revealed that soil moisture

content was consistently high in the gaps while it decreased

under closed canopy with the beginning of the growing

season (Fig. 2). Soil moisture content in the forest increased

again during September. Greatest differences between soil

moisture in gaps and forest were found in August 2001 and

smallest in December 2001. Analysed separately by site, soil

moisture content in the large gap differed by up to 30% from

the surrounding forest in all months of the measurement

period (t-test: P%0.01). At the small gap site, soil moisture

content was significantly higher in the gap area only in July

and August 2001 and no more than 19% (t-test: P%0.02).

3.3. Vegetation cover

In the second summer after gap formation (25 June

2002), regenerating sycamore maple trees had reached an

average height of at least 80 cm in the two gaps. Height

distribution was quite regular at the large gap site, but

tended to be taller in the gap than under the closed canopy

(Fig. 3a). At the small gap site, regeneration was more

scattered and its height more variable (Fig. 3b). Average

cover of ground vegetation was at least 50% at both

gap sites.

3.4. Litter

Mass loss of beech foliar litter after 1 year of

decomposition was significantly higher in the small gap

than in the large gap (t-test: PZ0.02). At the small gap site,

average losses were 39% in the gap and 33% under closed

canopy (ANOVA: PZ0.057, Fig. 4). At the large gap site,

litter mass decreased in average by 35% of initial start

weight in the gap and by 31% under closed canopy and

revealed no gap effect (ANOVA: PZ0.14).

The C:N ratio of 58 for fresh beech foliar litter material

decreased equally in both gaps (t-test: PZ0.4). After 1 year

of decomposition, the average C:N ratio of the litter samples

was 28 (standard error 1) both in the gaps and under the

closed canopy (ANOVA: PZ0.6). There was no net release

of N from litter after 1 year in either of the two gap sites,

instead N was immobilized (Fig. 4). Net N immobilization

was similar in the two gaps (t-test: PZ0.6). The average

amount of N immobilised in litter was 2.4 (standard error

0.2) and 3.0 mg N gK1 litter (standard error 0.3) in the gaps

and under closed canopy, respectively. This difference was

not significant (ANOVA: PZ0.3).

Fig. 4. Mass loss of litter material (%) and immobilization of nitrogen in

litter (mg N gK1 litter) after 1 year of decomposition at: (a) the large, and

(b) the small gap site. Error bars for mass loss are 1 standard error.

Fig. 3. Average plant cover of ground vegetation (herbs and bushes) (%)

and average height of regenerating trees (cm) at: (a) the large, and (b) the

small gap site in the second summer after gap formation (25 June 2002).

E. Ritter / Soil Biology & Biochemistry 37 (2005) 1237–12471242

3.5. Mineral soil

C:N ratios of the soil samples were similar in the two

gaps (t-test: PZ0.09) (range 18–20) and did not reveal any

gap effect either (ANOVA: PZ0.9). The average C:N ratio

in all plots in the gaps and under closed canopy was 19.

Total C content in gap plots was not affected by gap size

(t-test: PZ0.4). Total C content ranged from 42 to

60 mg C gK1 soil in the gaps and from 35 to 58 mg C gK1

soil under closed canopy (Table 1).

Monthly nitrification and mineralization rates varied

much among soil plots. In several incubation periods both

net release and immobilisation were measured in different

soil plots. However, for seasonal cumulative rates,

variations were less distinct (Figs. 5 and 6). There was no

significant effect of gap size on cumulative rates of both

nitrification and mineralization in neither growing nor

dormant season (t-test: PR0.09). The incubation period

July–August 2001 was excluded from the cumulative rates

of the growing season 2001 due to too many missing values

at the small gap site. Cumulative rates for this season are

therefore probably underestimated, but allowed a quantitat-

ive comparison between gaps and closed forest. During the

growing season nitrified N was on average 28 kg NO3–N

haK1 in the gaps and 18 NO3–N kg haK1 under

closed canopy (Fig. 7a). Mineralization resulted in 40

Nmin kg haK1 and 20 kg Nmin haK1 in gaps and under

the canopy, respectively, in the same period (Fig. 7b).

During the dormant season 14 and 15 kg NO3–N haK1 was

nitrified in the gaps and under closed canopy, respectively,

while mineralized N was on average 22 kg Nmin haK1 in the

gaps and 24 Nmin kg haK1 under closed canopy (Fig. 7).

Thus, despite the indication of increased nitrification and

mineralization in the gaps during the growing season,

differences between gaps and closed canopy were not

significant in either season for cumulative rates of both

nitrification (ANOVA: PR0.5) and mineralization

(ANOVA: PR0.14).

At the large gap site, mean soil temperatures were

positively correlated to nitrification rates in August 2001

(RZ0.76) and September 2001 (RZ0.88), and to mineral-

ization rates in September 2001 (RZ0.86). At the small gap

site, mean soil temperatures were positively correlated to

nitrification in March 2002 (RZ0.75) and to mineralization

in August 2001 (RZ0.90). A strong positive correlation of

the initial soil moisture content of the incubated soil samples

to nitrification rates was found at the large gap site in August

2001 (RZ0.82) and a negatively correlation in April 2002

(RZK0.77). In all other months the Pearson correlation

statistics were below 0.7 at either gap site.

Average soil N concentrations before gap formation

(November 2000) were 0.1 mg NO3–N kg soil and 8.9 mg

NH4–N kg soil. After gap formation average soil N

concentrations were not different between the two gaps in

Fig. 5. Amount of nitrified N (kg NO3–N haK1) and seasonal averages of maximum soil temperatures in soil plots along the transect at the large and the small

gap site in the growing season (left) and the dormant season (right), respectively. Error bars are 1 standard error.

Fig. 6. Amount of mineralized N (kg Nmin haK1) and seasonal averages of maximum soil temperatures in soil plots along the transect at the large and the small

gap site in the growing season (left) and the dormant season (right), respectively. Error bars are 1 standard error.

E. Ritter / Soil Biology & Biochemistry 37 (2005) 1237–1247 1243

Fig. 7. Comparison of the amount of: (a) nitrified N, and (b) mineralized N

under closed canopy (nZ8) and in the gap (nZ6) of the growing season

and the dormant season, respectively. Error bars are 1 standard error.

Differences were not significant.

Fig. 8. Seasonal averages of: (a) NO3–N concentrations, and (b) NH4–N

concentrations in mineral soil under closed canopy (nZ8) and in the gap

(nZ6). Error bars are 1 standard error. A significant difference between gap

and closed canopy was found for NH4–N concentrations during the growing

season.

E. Ritter / Soil Biology & Biochemistry 37 (2005) 1237–12471244

either season (t-test: PR0.4). Therefore, gaps were

considered as replicates. The statistical restrictions, which

inevitably arise due to the closeness of the two sites to each

other, have to be taken into account. Mean NO3–N

concentrations were about 2 mg NO3–N kgK1 under closed

canopy in both seasons. They tended to be lower than in the

gaps both in the growing season (ANOVA: PZ0.42) and in

the dormant season (ANOVA: PZ0.13). In the dormant

season the difference was almost two fold (Fig. 8a). Mean

NH4–N concentrations in the gaps were significantly higher

(6.4 mg NH4–N kgK1) than under closed canopy (4.3 mg

NH4–N kgK1) in the growing season (ANOVA: PZ0.04).

This difference was no longer significant during the dormant

season (ANOVA: PZ0.10) (Fig. 8b).

4. Discussion

4.1. Soil temperature

The stronger effect of the canopy opening on soil

temperatures found in the large gap than in the small gap

may be attributed to a higher input of radiant energy. The

close correlation between soil temperature and radiant

energy input at the soil surface was reported in a study by

Balisky and Burton (1993). An increase in radiant energy at

the forest floor as a result of increasing thinning of forest

stands was studied intensively by Mosandl (unpublished

manuscript). Hence, direct irradiation reaching the forest

floor in a gap and the adjacent forest can be expected to

affect the soil temperature in this area positively. In the

small gap sun light reached only into the forest understorey

north of the gap when the sun was at its highest. In the large

gap the area, which experienced direct irradiation at

the same time of the day was located in the northern part

of the large gap, and it was in this area that soil temperatures

were significantly increased from September 2001 to

January 2002. Similar observations were made in other

gap studies (Bauhus, 1996; Wright et al., 1998; Gray et al.,

2002). The low soil temperatures in plots in the southern

part of the gaps were probably both due to less incoming

direct irradiation during the day as compared to the northern

part of the gaps and at the same time higher nightly

radiation than in the stand, as also found by Bauhus and

Bartsch (1995).

In addition to light regimes, which are generally

characterised by a high degree of spatial and temporal

heterogeneity (Canham, 1988; Bazzaz and Wayne, 1993),

ground vegetation cover modifies the amount of radiant

energy reaching the soil (Balisky and Burton, 1993).

Vegetation prevents solar irradiation from reaching the

forest floor, and it insulates against energy loss by nightly

radiation. Thus, the gap effect found in the large gap might

have disappeared in the second year after gap formation due

to developing regeneration and ground vegetation in

the gap. However, in a gap study carried out in a very

E. Ritter / Soil Biology & Biochemistry 37 (2005) 1237–1247 1245

heterogeneous, semi-natural forest in Zealand, Denmark,

soil temperatures were highest in the eastern part of the gap

with least ground vegetation cover until 3 years after gap

formation (Ritter et al., in press-b). Presumably, soil

temperature is governed both by the location within gaps

and the ground vegetation cover. This should be considered

for the characterization of soil temperatures in gaps, as

temperature extremes may not always be gauged if only the

north–south transect of a gap or a few numbers of

measurement plots are included in gap studies.

4.2. Litter

Lower decomposition rates in the large gap compared to

the small gap may be attributed to the moisture content of

the litter layer. Moisture, temperature, and stage of decay

are key factors controlling litter decomposition rates

(Lousier and Parkinson, 1975; Moore, 1986). In cool-

temperate climates, as in this study, the litter layer may dry

out in warm periods of the summer. Hence, it cannot be

excluded that the litter layer in the sun exposed northern part

of the large gap dried out periodically and thereby delayed

the decomposition process, even though soil moisture levels

in the upper 15 cm of mineral soil were found to be

constantly high. This diverse effect of temperature on

decomposition rates—causing a decrease at the soil surface

due to drying and an increase in deeper soil layers—was

also emphasised by Yanai et al. (2003). Likewise, lower

litter decomposition rates were found in clear-cut areas

(1–97 ha) or in large gaps (O15 m diameter) than in small

gaps and or under closed canopy (Zhang and Liang, 1995;

Prescott et al., 2000). This was attributed to a higher

exposure of the forest floor in the open areas to incoming

radiation and wind. In contrast, Bauhus (1996) reported

similar forest floor mass and forest floor C and N

concentrations in gaps (30 m in diameter) and the forest

stand 21 months after gap formation in a European beech

forest. In a long-term gap study, no changes in litter

decomposition were found in gaps 8 years after gap

formation (Bauhus et al., 2004). This was explained by

too small soil temperature differences caused by the canopy

opening of 30 m diameter. Litter quality, which also affects

decomposition rates (Aber and Melillo, 1980; Vesterdal,

1999), did not differ among plots or gaps in the present study

(same fresh litter material used for all litter bags) and can

thus be excluded as a factor.

4.3. Mineral soil

Even though there was only a tendency of increased

nitrification and mineralization rates in the gaps as

compared to the closed forest during the growing season,

the up to two fold higher cumulative mineralization rates in

the gaps indicate that gap formation enhanced N turnover.

This may be attributed to favourable conditions for micro-

organisms in the gaps with more constant and higher soil

moisture levels during summer as compared to the stand, as

also found by Mladenoff (1987). The missing correlation

between mineralization rates and soil temperatures except

for a few months is in contrast to other studies, in which a

general influence of soil temperature regimes on N

mineralization is stressed (Cassman and Munns, 1980;

Ellert and Bettany, 1992). However, Bauhus and Barthel

(1995) reported in an in situ study on N mineralization rates

in gaps, measured in monthly intervals over a 1-year period

similar to the present study, that differences in mineraliz-

ation rates between gaps and forest could not be explained

by fluctuations in soil temperatures. Other studies found that

increased soil temperatures also had a stimulating effect on

microbial N immobilization, thus resulting in both positive

and negative mineralization rates (Schmidt et al., 1999,

2002) Furthermore, nitrification and N mineralization rates

are controlled by several other factors than soil moisture and

temperature, e.g. C availability, decomposer species, soil

acidity, soil texture, nutrient uptake by roots and return in

litter fall (Pastor et al., 1984; Persson et al., 2000). Since

many of these non-microclimatic factors were very

similar at the two gap sites, this may also explain why no

clear difference between the two gaps of different sizes

was found.

Increased substrate availability and favourable microcli-

matic conditions were shown to enhance mineralization and

increase soil N concentrations after tree felling and removal

of the canopy cover (e.g. Matson and Vitousek, 1981; Fisk

and Fahey, 1990). This, along with the reduced uptake by

tree roots, may have resulted in higher soil NH4–N

concentrations in the gaps than in the closed forest during

the growing season in the present study. This pattern

was not found for concentrations. However, leaching of

NO3K with soil solution was increased in the gaps (Ritter

et al., in press-a), and NO3K may furthermore be subject to

denitrification, keeping NO3–N concentrations in the gaps

on a lower level.

5. Conclusions

Within the range of gap diameters included in this study,

results have shown that gap size affected soil temperature,

being significantly increased only in the northern part of the

large gap. Despite this effect on microclimate, an impact of

gap size on processes of the N cycle was significant only for

litter decomposition in terms of lower decomposition rates

in the large than in the small gap. Overall, gap formation

resulted in two fold higher cumulative net N mineralization

rates during the growing season and a tendency of increased

cumulative nitrification rates in the gaps. Although not

statistically significant, this indicates a stimulation of N

mineralization processes in the gaps. A clearly significant

gap effect was found for NH4–N concentrations, which were

increased in the gaps during the growing season. No

alteration in forest floor parameters and soil C:N ratios was

E. Ritter / Soil Biology & Biochemistry 37 (2005) 1237–12471246

found 1 year after gap formation. Even though results need

to be tempered according to the restrictions of the

experimental design, this study has shown that N

availability in forest ecosystems was affected at gap sites

which experienced little soil disturbance during the

establishment of the gaps.

Acknowledgements

I acknowledge the technical assistance of L. Byrgesen, P.

Frederiksen, X. Haliti, A. Harder, M. M. Krag, S. Nasim,

and U. Vilhar during field work and in the laboratory. I also

thank I. Callesen for classification of the soil. For valuable

comments on the manuscript, I wish to thank M. Ingerslev,

I. K. Schmidt, and L. Vesterdal. The research was financed

by the Danish Centre for Forest, Landscape and Planning

and by the EU 5th Framework Program, project Nat-Man.

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