Organic matter composition and dynamics in a northern hardwood forest ecosystem 15 years after...

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Forest Ecology and Management 240 (2007) 131–142

Organic matter composition and dynamics in a northern hardwood forest

ecosystem 15 years after clear-cutting

David A.N. Ussiri *, Chris E. Johnson

Department of Civil and Environmental Engineering, Syracuse University, 151 Link Hall, Syracuse, NY 13244, USA

Received 8 August 2006; received in revised form 15 December 2006; accepted 15 December 2006

Abstract

Soil organic matter (SOM) plays an important role in governing soil properties and nutrient cycling in forest ecosystems. Clear-cutting alters the

SOM cycle by changing decomposition rates and organic matter (OM) inputs to the forest ecosystem. We studied the 15-year clear-cutting response

on the properties and composition of SOM at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire. Solid-state 13C nuclear

magnetic resonance (NMR) spectroscopy was used to study the structural chemistry of SOM in whole soils and extracted humic substances.

Overall, alkyl C and O-alkyl C were the dominant C fractions in soils and humic substances. Alkyl C accounted for 38–49% of the total NMR signal

intensity in soils and 33–56% in humic substances. O-alkyl C accounted for 32–45% of the signal intensity in soils and 20–31% in humic

substances. Following clear-cutting, the contribution of O-alkyl C increased in whole soils and humic acids of the Oa horizon, while alkyl C

decreased in whole soils and humic acids of Oa and Bh horizons. Thus, the ratio of alkyl C to O-alkyl C, an index of the degree of decomposition of

SOM, decreased in whole soils and humic acids after clear-cutting, indicating that the SOM in post-harvest soils is less decomposed relative to pre-

harvest soils. On average, humic substances accounted for 47% of SOM. The concentration of humic acid decreased by up to 25% in Oa, E and Bh

horizons after clear-cutting, while the concentration of fulvic acid decreased by more than 40% in the Oa and E horizons. Together, these results

indicate that clear-cutting resulted in the loss of humic substances from the forest floor and upper mineral horizons, which was replaced by less

decomposed OM in the post-clear-cut soils under the regrowing forest.

# 2007 Elsevier B.V. All rights reserved.

Keywords: 13C CPMAS NMR; Forest soils; Spodosol; Humic substances; Forest clear-cutting; Hubbard Brook Experimental Forest

1. Introduction

Soil organic matter (SOM) is a heterogeneous mixture of

organic compounds of plant, animal and microbial origin in

various stages of decomposition (Rice and MacCarthy, 1991;

Stevenson, 1994). Soil organic matter forms a large and

dynamic reservoir of C, which is a major part of the global C

cycle. Global estimates of the C content of SOM ranges from

1100 to 3000 Pg (Schlesinger, 1997). As the major pool of

terrestrial C resides in soil (Schlesinger, 1997; Jobbagy and

Jackson, 2000), understanding the mechanisms and factors that

govern SOM release and retention is central to terrestrial

ecosystem research.

* Corresponding author. Current address: School of Environment and Natural

Resources, The Ohio State University, 2021 Coffey Road, Columbus, OH

43210, USA. Fax: +1 614 292 7432.

E-mail address: [email protected] (D.A.N. Ussiri).

0378-1127/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.foreco.2006.12.017

Clear-cutting alters C cycling through removal of above-

ground tree biomass for wood products. This induces changes

in the forest ecosystem that extend to a wide range of

biogeochemica1 processes (Likens et al., 1970; Boyle et al.,

1973; Bormann et al., 1974; Mann et al., 1988). For example,

the input of litter to the forest floor decreases for several years

following clear-cutting (Hughes and Fahey, 1994). The removal

of the forest canopy also causes decreased interception of

atmospheric moisture, reduced transpiration, and increased soil

temperatures due to increased solar radiation input to the soil

surface. These conditions will likely stimulate microbial

activity and therefore increase the rate of OM decomposition

(Bormann et al., 1974; Clayton and Kennedy, 1985; Hen-

dricksen et al., 1989; Mattson and Swank, 1989; Moore, 1989;

Dahlgren and Driscoll, 1994; Johnson et al., 1995). Changes in

decomposition rates and patterns after clear-cutting are likely to

influence the amount, structure, and chemistry of OM in forest

ecosystems.

Experimental Watershed 5 (W5) at the Hubbard Brook

Experimental Forest (HBEF) in New Hampshire was clear-cut

mailto:[email protected]

http://dx.doi.org/10.1016/j.foreco.2006.12.017

D.A.N. Ussiri, C.E. Johnson / Forest Ecology and Management 240 (2007) 131–142132

harvested in the winter of 1983–1984 for a long-term

investigation aimed at the determination of the patterns of

nutrient reorganization after clear-cutting. Previous research

showed that changes in forest floor and mineral soil C were not

statistically detectable for the first 3 years after clear-cutting

(Huntington and Ryan, 1990). After 8 years, pools of C in the

forest floor had declined by 27% ( p = 0.05), however, there were

no detectable changes in pools of C in the mineral soil (Johnson

et al., 1995). Increased OM concentrations were observed in E,

Bh, and Bs1 horizons between post-harvest years 3 and 8, but

remained unchanged in Bs2 horizon which comprises the bulk of

the solum (Johnson et al., 1995, 1997). In the mineral soils C:N

and C:OM ratios declined after clear-cutting, suggesting that

SOM was enriched in N relative to C, and C was preferentially

lost from mineral soils in the first 8 years after clear cutting

(Johnson, 1995). Dissolved organic carbon (DOC) concentra-

tions in zero-tension lysimeter solutions from Oa, Bh, and Bs

horizons and stream water increased in the first 9 years after

clear-cutting relative to uncut watershed 6 nearby (Johnson et al.,

1995; Dahlgren and Driscoll, 1994; Dai et al., 2001). Overall,

these findings suggested that changes in the chemistry of SOM

and dissolved organic matter in soil solution and stream water

following clear-cutting are the result of changes in both the

quality and quantity of SOM.

Recent advances in solid-state 13C nuclear magnetic

resonance (NMR) spectroscopy have made possible its routine

application to studies of soil organic matter (Baldock et al., 1992;

Baldock and Preston, 1995; Preston, 1996, 2001; Kogel-

Knabner, 1992, 1997, 2000). This technique affords the

opportunity to examine the distribution of soil C among classes

of structures, such as alkyl C, O-alkyl C, aromatic C, and

carbonyl C. Furthermore, solid-state 13C NMR spectroscopy

enables direct study of SOM in whole soil samples without

extraction. We used 13C NMR spectroscopy and chemical

methods to examine the chemistry and structure of SOM from

watershed 5 at the Hubbard Brook Experimental Forest. The

objective of this study was to assess the effects of clear-cutting on

the chemistry of SOM. To do this, we examined both soils and

humic substances of pre-harvest (1983) and post-harvest (1986,

1991, and 1998) watershed 5 at HBEF. The results from 1983

(pre-clear-cutting) have been published previously in greater

detail (Ussiri and Johnson, 2003), but they are included in this

paper as reference point since this manuscript builds on the

earlier work due to the time series nature of the study. In this

paper we examine the following research questions: (i) What are

the effects of clear-cutting on the chemical structure and

composition of SOM? (ii) Did clear-cutting change the

concentration and/or composition of various SOM fractions?

(iii) Did clear-cutting result in preferential loss of any SOM

fractions?

2. Materials and methods

2.1. Site description and soil sampling

The HBEF lies in the southern White Mountains of New

Hampshire (458560N and 718450W). Detailed description of

climate, geology and hydrology at the HBEF may be found

elsewhere (Federer et al., 1990; Likens and Bormann, 1995).

Vegetation in the study area is mostly northern hardwood,

dominated by American beech (Fagus grandifolia), sugar

maple (Acer saccharum), and yellow birch (Betula allegha-

niensis). Pockets of the spruce-fir forest type, consisting of red

spruce (Picea rubens), balsam fir (Abies balsamea) and white

birch (Betula papyrifera) are found at the highest elevations,

and on exposed slopes (Whittaker et al., 1974).

Soils at the HBEF are predominantly well-drained, coarse-

loamy, mixed-frigid Typic Haplorthods with a 3–15 cm organic

layer at the surface (Likens and Bormann, 1995). These soils

are acidic (pH < 4.5) with low base saturation and cation

exchange capacity (Johnson et al., 1991b). Average soil depth

to C horizon (or bedrock) is about 60 cm (Johnson et al.,

1991a). This experiment was conducted in W5, a 23-ha

watershed, which was clear cut in a whole-tree harvest in the

winter of 1983–1984. All trees with diameter at breast height

�5 cm were cut with chain saws and feller-bunchers, except in

a buffer strip along the watershed boundary. Whole trees were

removed from most of W5 in the spring and summer of 1984

with rubber tired skidders. More details of the clear-cutting

operation may be found in Ryan et al. (1992).

A stratified random sampling strategy was used in the study

to ensure coverage of the watershed. Sixty sampling locations

were chosen, 10 locations in each of 6 elevation bands of W5.

At each location, four sampling sites were chosen. Soils were

sampled from W5 prior to clear-cutting (1983) and in post-

harvest years 3, 8 and 15 (1986, 1991 and 1998). Sixty pits were

excavated in each sampling year at one site on each location.

Soil samples were collected by depth and by genetic horizons.

A more detailed description of the sampling plan and

methodology can be found elsewhere (Huntington et al.,

1988; Johnson et al., 1991a,b, 1997). Samples were air-dried to

a constant weight, sieved through a 5 mm (O horizons) or 2 mm

(mineral horizons) screen, and archived in glass jars. A subset

of the genetic horizon samples was used in this study. We used

samples collected from six pits in each sampling year, selected

to represent the range of elevation found in W5. Each of the

selected pits exhibited the full suite of horizons typically found

in HBEF Spodosols (i.e., Oi + Oe, Oa, E, Bh, Bs1, and Bs2).

Selected properties of the soil samples used in this study are

presented in Table 1.

2.2. Extraction, fractionation and purification of humic

substances

Humic substances were extracted from soil samples of the

Oa, E, Bh, Bs1 and Bs2 horizons. The procedures used for

extraction, fractionation, and purification of humic substances

in this study are summarized in Ussiri and Johnson (2003),

which includes a flow-chart. Briefly, humic substances were

extracted with 0.5 M NaOH at soil mass (g) to solution volume

(ml) ratios of 1:10 and 1:20 for mineral horizon soils and

organic horizons, respectively, under an N2 atmosphere

(Stevenson, 1994). The extracted humic substances were

fractionated into humic acid and fulvic acid by acidifying the

Table 1

Selected chemica1 properties of the soils used in this study

Horizon Year Soil propertiesa

pHw pHs OM (g kg�l) Tota1 C (g kg�l) Tota1 N (g kg�l) CEC (cmolc kg�l) Exch. acidity (cmolc kg�l)

Oa 1983 3.82 3.16 513.4 309 a 14.2 a 15.6 ab 7.1

1986 3.78 3.22 463.6 194 b 8.3 b 13.6 b 8.5

1991 4.04 3.55 542.3 283 b 14.7 a 19.1 a 9.4

1998 3.78 2.88 552.0 331 a 14.2 a 16.7 a 7.8

E 1983 4.34 3.43 35.1 22 1.0 a 2.5 2.0

1986 3.91 3.27 25.5 15 0.5 b 1.8 1.5

1991 4.12 3.53 41.6 20 1.1 a 2.1 1.6

1998 3.76 2.97 27.7 19 1.0 a 2.3 2.0

Bh 1983 4.18 3.37 153.2 65 b 3.4 b 6.5b 5.7 b

1986 3.79 3.36 123.4 74 b 3.5 b 11.0 a 10.0 a

1991 4.08 3.68 129.8 90 ab 5.0 a 13.6 a 12.2 a

1998 3.80 3.05 148.5 125 a 5.9 a 12.6 a 11.0 a

Bs1 1983 4.10 3.58 127.7 62 ab 2.8 6.7 b 6.2 b

1986 4.14 3.74 110.0 50 b 2.3 9.0 ab 8.4 ab

1991 4.22 3.76 173.0 72 a 3.6 12.8 a 11.7 a

1998 4.02 3.27 142.6 77 a 3.7 11.7 a 10.5 a

Bs2 1983 4.52 4.10 77.5 32 b 1.3 3.4 3.2

1986 4.51 4.16 75.7 33 b 1.3 4.5 4.3

1991 4.70 4.25 101.0 44 a 2.0 5.9 5.5

1998 4.50 3.96 88.5 48 a 1.9 5.3 4.8

Values are means of six samples. Watershed 5 was clear-cut in the winter of 1983–1984. pHw: pH measured in deionized water; pHs: pH measured in 0.01 M CaCl2solution, Exch. acidity = exchangeable acidity.

a For each set of four values in a column, means followed by the same letter (or none) are not significantly different ( p = 0.05) by Duncan’s Multiple Range Test

(DMRT).

D.A.N. Ussiri, C.E. Johnson / Forest Ecology and Management 240 (2007) 131–142 133

extract to pH 2 using 6 M HCl. The acidified extract was

allowed to settle overnight, centrifuged at 3000 � g to isolate

the humic acid, which was purified by the Stevenson (1994)

method. Polysaccharides were isolated from the fulvic acid

supernatant using a modification of the method described by

Cheshire et al. (1992). Purified humic acid, fulvic acid, and

polysaccharide samples were freeze-dried. The residual soils

(‘‘humin’’) were rinsed with DI water until the suspension pH

was neutral, then freeze-dried.

2.3. Elemental analysis

The elemental composition of the isolated and freeze-dried

humic substances was determined by dry combustion, followed

by gas chromatography, using an elemental C, H and N

analyzer (Carlo Erba model EAl108) and adjusted for ash

content. Ash content was determined by combustion overnight

in a muffle furnace at 500 8C.

2.4. Solid-state 13C CPMAS NMR analysis

The structural chemistry of whole soils, residual soils (i.e.

humin), and freeze-dried humic and fulvic acids was analyzed

using solid-state 13C NMR, with cross polarization and magic-

angle spinning (CPMAS). Four replicates each of freeze-dried

humic and fulvic acid from Oa, Bh and Bs1 horizons were

analyzed for each sampling year. In addition, four replicates of

whole soils and three replicates of humin from Oa, Bh and Bs1

horizons from 1983 (pre-harvest) and 1998 (15 years after

harvest) were analyzed by solid-state 13C CPMAS NMR. Prior to

analysis, whole-soil samples from Bh and Bs1 horizons were

treated with 2% HF to remove paramagnetic materials (Dai and

Johnson, 1999). The treated samples were freeze-dried and

ground to pass a 400-mm screen. Oa horizon and humin samples

were ground to pass a 400 mm screen without HF treatment.

Solid-state 13C CPMAS NMR analyses were conducted on a

Bruker AMX 300 spectrophotometer operating at 75.47 MHz.

Samples were spun at 5 kHz in 7 mm zirconia rotors with Kel-F

caps. The number of transients required for an acceptable signal

to noise ratio ranged from 3918 to 99,224. The contact time for

each spectrum was 1 ms, and recycle time was 1 s. The

acquisition time was 61 ms, number of decay curves 4096, and

spectral width 33,112 Hz. No attempt was made to remove or

correct for spinning side bands. Chemical shift assignments

were externally referenced to the glycine resonance at 176 ppm.

The quantitative reliability of solid-state 13C CPMAS NMR

spectroscopy of SOM has been a matter of concern (Wilson,

1987; Frund and Ludemann, 1989; Kinchesh et al., 1995a,

1995b; Preston, 1996; Mathers et al., 2000). There are several

factors that affect the signal intensity in CPMAS experiments,

including: (1) varying efficiency of cross-polarization for

different C structures, (2) lack of detection of C close to

paramagnetic centers, and (3) spinning sidebands (Kinchesh

et al., 1995a). Several methods for improving the quantitation

of 13C NMR have been proposed by researchers (Kinchesh

et al., 1995b; Conte et al., 1997; Smernik and Oades, 2000; Mao


et al., 2000; Smernik, 2005). However, these methods require

extended time of analysis and/or additional analyses, therefore

limiting the number of samples that can be analyzed. Studies of

soils and humic substances have indicated that the fraction of C

in the aromatic and carbonyl regions of 13C CPMAS NMR

spectra may be underestimated, causing overestimation of alkyl

and O-alkyl C fractions (Kinchesh et al., 1995a; Mao et al.,

2000). Consequently, the integrated 13C CPMAS NMR spectra

may not reflect the true distribution of C in an individual

sample. Kinchesh et al. (1995a), however, suggested that the

results of 13C CPMAS NMR could be used for comparison of

samples of similar origin. Our goal was to analyze as many

samples as possible to detect changes in the composition of

specific horizons and SOM fractions with time; therefore we

limited our analyses to 13C CPMAS NMR.

3. Statistical methods

The Student’s two-sample t-test was used to test the

significant differences between pre- and post-harvest samples.

After testing for the equality of the variances, the appropriate

formulation of the t-test was applied. Mean comparison was

conducted using Duncan’s Multiple Range Test (DMRT) when

means were different by t-test ( p � 0.05).

4. Results

4.1. Soils

Chemical properties of the soils used in this study are given in

Table 1. Since we used only a subset of the samples collected, the

values in Table 1 may be somewhat different than the overall

Fig. 1. Solid-state 13C CPMAS NMR spectra acquired from whole soils of various ho

15 years (1998) after whole-tree clear-cutting.

averages published elsewhere (Johnson et al., 1991a, 1991b,

1997; Johnson, 1995). The 1998 data have not been published

before. Carbon and N concentrations of the Oa horizon decreased

after clear-cutting ( p < 0.05). Nitrogen concentrations returned

to pre-harvest levels in the first 8 years, while C concentrations

returned to pre-harvest levels after 15 years (Table 1). In the Bh,

Bs1 and Bs2 horizons, C and N concentrations increased after

clear-cutting (Table 1). In addition, the cation exchange capacity

(CEC) and exchange acidity of Bh and Bs1 horizons increased

after clear-cutting (Table 1).

Examples of solid-state 13C CPMAS NMR spectra of whole

soils from the Oa, Bh and Bs1 soil horizons are presented in

Fig. 1. The spectra of pre- and post-harvest Oa horizons

exhibited a large peak at 33 ppm, corresponding to alkyl C in

long-chain polymethylene structures (e.g. fatty acids, waxes,

and resins) (Kogel et al., 1988; Preston et al., 1994). In the Bh

and Bs1 horizons, this peak shifted to 30 ppm in both pre- and

post-harvest soils (Fig. 1), suggesting shorter chains than in the

Oa horizons (Baldock and Preston, 1995).

In the O-alkyl C range, (50–110 ppm), the major peak at

73 ppm corresponds to C2, C3, and C5 carbons in carbohydrates.

A side peak at 57 ppm indicates methoxyl C, and the resonance

at 105 ppm is due to C1 carbons in carbohydrates. In the Oa

horizons the peaks in the O-alkyl C region were large and

intense, while in Bh and Bs1 soil, the peaks were smaller. Both

the methoxyl C (57 ppm) and anomeric C (105 ppm) peaks

were larger in the post-harvest soils, while the carbohydrate C

peak at 73 ppm was slightly smaller in the post-harvest soils.

In the aromatic C region, peaks between 110 and 145 ppm

are attributed to H-substituted C, while peaks between 145 and

160 ppm arise from O-substituted aromatic C (Baldock and

Preston, 1995). The aromatic C peaks were more intense in the

rizons of watershed 5 at Hubbard Brook Experimental Forest before (1983) and

Fig. 2. Carbon distribution in whole soils of watershed 5 at Hubbard Brook

Experimental Forest based on 13C CPMAS NMR analysis before (1983) and 15

years (1998) after whole-tree clear-cutting. Error bars represent standard errors

of the means.


post-harvest Oa horizon soils (Fig. 1). In the carbonyl C region

(160–220 ppm), there was only one peak at 173 ppm. This peak

originates primarily from carboxyl C, but amides and esters can

also contribute (Baldock and Preston, 1995). The carboxylic C

peaks were larger and more intense in the post-harvest Oa

horizons (Fig. 1).

Integration of the major regions of 13C resonance indicated

that the major C components in whole-SOM were alkyl C (38–

49%) and O-alkyl C (32–45% of total C). Together, alkyl C and

O-alkyl C typically accounted for about 80% of C in Hubbard

Brook SOM (Fig. 2). The contributions of aromatic and

carbonyl C were much smaller (10–15% and 5–10% of total C,

respectively, Fig. 2). These ranges are consistent with ranges for

forest soils reported by Mahieu et al. (1999). The proportion of

alkyl C was greater in mineral soils than in Oa horizons,

increasing from 39% in the Oa horizon to 45% in the Bs1

horizon of the pre-harvest profile, and from 38% to 44% in the

post-harvest profile. The aromatic C fraction also increased

with increasing soil depth from 11% to 15% in the pre-harvest

profile, and from 10% to 14% in the post-harvest profile. In

contrast, the O-alkyl C fraction decreased from 45% of total C

in the Oa horizon to 32% of total C in the pre-harvest Bs1

horizon, and from 41 to 31% in the post-harvest soils. The

proportion of carbonyl C did not show any significant changes

with increased soil depth (Fig. 2).

Oa horizons did not show detectable changes in alkyl C and

aromatic C concentration 15 years after clear-cutting, but mean

O-alkyl C decreased from an average of 45% of total C in 1983

to 40% of total C in 1998 ( p = 0.1). Carbonyl C increased by

nearly two-fold from 5% of total C in 1983 to 9% of total C 15

years later ( p = 0.01, Fig. 2). In the Bh horizon, the alkyl C

decreased significantly ( p = 0.05) from 49% of total C in 1983

to 43% of total C in 1998. In addition, carbonyl C nearly

doubled in the Bh horizon 15 years after-clear-cutting (Fig. 2).

In the Bs1 horizon, changes in the alkyl C, O-alkyl C, aromatic

C and carbonyl C fractions were not statistically significant.

4.2. Isolated humic substances

4.2.1. Concentration of humic substances extracted

Extractable humic substances ranged from 307 to 624 g kg�1

of SOM in HBEF soils (Table 2). Both humic and fulvic acid

concentrations decreased significantly after clear-cutting in the

Oa horizon ( p < 0.05, Table 2). Humic acid decreased by nearly

24% in the first 3 years and did not return to the pre-harvest

concentrations by year 15. Fulvic acid decreased by nearly 47%

and did not return to pre-harvest concentrations during first 15

years (Table 2). In the E horizon, the fulvic acid concentration

decreased by 51% in the first 8 years after harvest ( p < 0.05).

Similar decreases in humic acid were also observed. In the Bh

horizon, humic acid concentrations decreased significantly by

about 25% in the first 3 years and remained lower than pre-

harvest concentrations after 15 years.

4.2.2. NMR analysis of humic and fulvic acid

Examples of solid-state 13C CPMAS NMR spectra of

isolated humic and fulvic acid fractions are presented in Figs. 3

and 4, respectively. Spectra of humic and fulvic acids from Oa,

Bh, and Bs1 horizons exhibited distinct peaks in the alkyl C (0–

50 ppm), O-alkyl C (50–110 ppm), aromatic C (110–160 ppm),

carboxyl C (160–185 ppm) and ketone (185–220 ppm) regions.

In the alkyl C region humic acid extracted from the Oa horizon

had a broad alkyl C peak centered around 33 ppm (Fig. 3) while

in the fulvic acid this peak centered around 29 ppm (Fig. 4). In

Bh and Bs1 horizons, the alkyl peak in humic acid shifted

downward to around 29 ppm (Fig. 3), while in fulvic acid this

peak shifted upwards to 38 ppm (Fig. 4). The decrease in

chemical shift of the alkyl C peak towards 25 ppm in humic

acid indicates a reduction in long-chain polymethylene

structures and/or an increase in short-chain polymers (Baldock

and Preston, 1995). This pattern indicates that the alkyl C of

humic acids extracted from mineral soils had shorter chains

than those from the Oa horizon, possibly due to the breakdown

of long-chain structures over time in the mineral horizons.

Methoxyl C and carbohydrate C peaks at 57 and 73 ppm,

respectively, dominated the O-alkyl C region of humic and

fulvic acid spectra. These peaks were large and intense in

humic acid extracted from the Oa horizons, and decreased in

intensity in the mineral horizons (Bh and Bs1). In fulvic acid,

however, the methoxyl peak at 57 ppm was much less

prominent than in corresponding humic acid samples, and

only minor variations were observed with soil depth.

Table 2

Humic substance concentrations in soils of watershed 5 at the Hubbard Brook Experimental Forest, NH

Horizon Year Humic substances concentrationa

Humic acid (g kg�1 of OM) Fulvic acid (g kg�1 of OM) Polysaccharides (g kg�1 of OM) Total extractable (g kg�1 of OM)

Oa 1983 371 a 87 a 44 502

1986 281 b 48 b 42 371

1991 336 b 66 b 38 440

1998 339 b 60 b 40 439

E 1983 315 a 139 a 66 520

1986 227 b 82 b 54 363

1991 161 b 67 b 79 307

1998 216 b 77 b 81 374

Bh 1983 334 a 254 36 624

1986 250 b 227 25 502

1991 255 b 295 27 577

1998 262 b 296 50 608

Bs1 1983 141 ab 312 21 474

1986 112 b 337 25 474

1991 122 b 296 20 438

1998 161 a 312 32 505

Bs2 1983 91 302 25 b 418

1986 86 268 48 b 402

1991 80 297 57 b 434

1998 107 296 93 a 496

Watershed 5 was clear-cut in the winter of 1983–1984.a For each set of four values in a column, means followed by the same letter (or none) are not significantly different ( p = 0.05) by DMRT.


Signals in the aromatic region tended to be broader and less

intense than in other spectral regions. Fulvic acid spectra

exhibited prominent H-substituted aromatic peaks (110–

145 ppm), whereas many of the humic acid samples showed

Fig. 3. Solid-state 13C CPMAS NMR spectra acquired from humic acid isolates of va

(1983) and 3, 8, and 15 years after (1986, 1991 and 1998) whole-tree clear-cuttin

only minor signals in this region. The carbonyl C peak at

173 ppm was generally larger in fulvic acid than in humic acid

in both pre- and post-harvest samples. In addition, samples

from the mineral soils had larger carboxyl peaks than those

rious soil horizons of watershed 5 at Hubbard Brook Experimental Forest before

g.

Fig. 5. Carbon distribution in humic acid isolates of various soil horizons of

watershed 5 at Hubbard Brook Experimental Forest based on 13C CPMAS NMR

analysis before (1983) and 3, 8, and 15 years after (1986, 1991 and 1998) whole-

tree clear-cutting. Error bars represent standard errors of the means.

Fig. 4. Solid-state 13C CPMAS NMR spectra acquired from fulvic acid isolates of various soil horizons of watershed 5 at Hubbard Brook Experimental Forest before

(1983) and 3, 8, and 15 years after (1986, 1991 and 1998) whole-tree clear-cutting.


from the Oa horizon, especially in the fulvic acid samples

(Fig. 4).

Results of the integration of 13C spectra of humic acid and

fulvic acid are presented in Figs. 5 and 6. In the humic acid, the

proportions of alkyl C, O-alkyl C, aromatic C, carboxyl C and

ketone C were 40–56%, 20–31%, 12–17%, and 7–10% and 2–

6% of total C, respectively (Fig. 5), while in the fulvic acid

alkyl C, O-a1kyl C, aromatic C, carboxyl C and ketone C

accounted for 33–38%, 24–31%, 13–17%, 14–19% and 4–9%

of total C, respectively (Fig. 6). These ranges are consistent

with ranges for forest-soil humic and fulvic acids compiled by

Mahieu et al. (1999).

In humic acid, alkyl C decreased, while O-alkyl C increased,

after clear-cutting in the Oa horizon (Fig. 5). The response was

somewhat delayed. In the first 3 years after clear-cutting, the

alkyl C fraction in Oa horizon humic acid remained nearly

unchanged. A decreasing trend was observed after 8 years, and

after 15 years alkyl C was about 17% lower than before

harvesting (Fig. 5). In contrast, the O-alkyl C fraction increased

steadily after clear-cutting; from 25% of total signal intensity in

1983 to 31% in 1998, about a 24% increase. There were no

major changes in humic acid of other horizons after clear-

cutting. Clear-cutting did not produce any consistent or

significant changes in the distribution of C among spectral

regions in fulvic acid (Fig. 6).

4.2.3. NMR analysis of humin

Examples of solid-state 13C CPMAS NMR spectra of the

humin of Oa, Bh and Bs1 horizons are presented in Fig. 7. The

spectral quality of the humin isolated from mineral soils was

somewhat poorer that those from Oa horizon, and the peaks

Fig. 6. Carbon distribution in fulvic acid isolates of various soil horizons of

watershed 5 at Hubbard Brook Experimental Forest based on 13C CPMAS NMR

analysis before (1983) and 3, 8, and 15 years after (1986, 1991 and 1998) whole-

tree clear-cutting. Error bars represent standard errors of the means.

Fig. 7. Solid-state 13C CPMAS NMR spectra acquired from humin isolates of vario

(1983) and 15 years (1998) after whole-tree clear-cutting.


were not well resolved, due to lower C concentrations and/or

interferences from paramagnetic materials, particularly Fe.

Low signal-to-noise ratios in mineral horizon spectra obscured

all peaks except the alkyl C peak and a small O-alkyl C peak at

73 ppm. Humin from Oa horizons has more prominent peaks in

the O-alkyl C region, compared to the alkyl C region, than

whole soils (Fig. 1). In all horizons, the carboxyl peak

(173 ppm) in humin is much less pronounced than in the whole

soils.

4.2.4. Elemental composition of humic and fulvic acid

fractions

The elemental concentrations in pre- and post-harvest humic

and fulvic acids are presented in Table 3. The C concentration

of humic acid ranged from 488 to 618 g kg�1, with a mean of

568 g kg�1 and standard error of 61.6 g kg�1. Carbon contents

of fulvic acids ranged from 477 to 543 g kg�1 with a mean of

500 g kg�1 and standard error of 27.2 g kg�1 (Table 3). These

ranges are consistent with values for soil humic and fulvic acids

compiled by Rice and MacCarthy (1991). Humic acids had

higher C concentrations than fulvic acids (Table 3). Carbon

concentrations in humic and fulvic acids varied little among

horizons or sampling years. None of the between-year or

between-horizon differences were greater than 5% (Table 3).

Nitrogen concentrations in humic acid ranged from 22.6 to

43.3 g kg�1 with a mean of 33.5 g kg�1. The C:N ratio ranged

from 17.2 to 23.1 (average = 20.2; Table 3). The N concentra-

tion and the C:N ratio of fulvic acid ranged from 9.5 to 21.9 and

31.4 to 49.4 g kg�1, respectively (Table 3). These ranges are

within the published values of humic and fulvic acids from soils

(Steelink, 1985; Rice and MacCarthy, 1991). The concentration

us soil horizons of watershed 5 at Hubbard Brook Experimental Forest before

Table 3

Elemental content and atomic ratios of purified, ash-free humic and fulvic acids isolated from soils of watershed 5 at the Hubbard Brook Experimental Forest, NH

Elements (g kg�1) Sampling year

1983 1998 1991 1986

Oa Bh Bs1 Oa Bh Bs1 O Bh Bs1 Oa Bh Bs1

Humic acid

C 579 577 574 567 571 565 567 562 564 560 563 565

N 29.9 36.9 35.8 34.0 36.9 35.9 33.6 38.3 38.2 32.9 35.0 35.2

H 47.0 51.5 49.2 53.8 52.5 50.0 50.2 55.8 56.0 51.6 53.1 52.4

H:C 0.97 1.06 1.02 1.13 1.10 1.06 1.05 1.08 1.18 1.10 1.12 1.10

C:N 23.0 18.9 19.0 19.6 18.1 18.6 19.8 17.2 17.5 20.0 19.0 18.7

Fulvic acid

C 503 512 513 481 491 492 487 491 489 479 493 489

N 16.1 14.4 12.6 16.0 18.4 12.4 14.9 18.5 13.8 15.7 14.9 11.6

H 44.0 35.7 34.6 39.1 37.3 32.9 42.4 35.6 34.7 41.1 36.7 32.1

H:C 1.03 0.83 0.80 0.97 0.91 0.79 1.04 0.86 0.84 1.02 0.90 0.78

C:N 38.0 41.9 48.3 35.2 31.6 48.3 38.2 31.0 43.0 35.7 38.0 49.4

Watershed 5 was clear-cut in the winter of 1983–1984.


of N increased ( p < 0.05) in Oa horizon humic acid after clear-

cutting, but there were no significant changes in Bh and Bs1

humic acids. The C:N ratio of humic acid decreased after clear-

cutting (Table 3). Humic acid contained more N and had lower

C:N ratio than fulvic acid. The N concentration of fulvic acid

increased in the Bh horizon after clear-cutting (Table 3). This

was accompanied by a decrease in C:N ratio, indicating N

enrichment in Bh horizon fulvic acids after-clear-cutting.

The H concentration ranged from 19 to 69 g kg�1 in humic

acid, and 29–49 g kg�1 in fulvic acid. The H:C atomic ratio –

an indicator of aliphaticity – ranged from 0.37 to 1.36 for humic

acid and 0.71–1.17 for fulvic acid (Table 3). Generally, humic

acid contained more H and had higher H:C ratios than fulvic

acid (Table 3). Fulvic acids extracted from Oa horizons had

greater H concentrations and higher H:C ratios than mineral

soil fulvic acids in all sampling years, suggesting that fulvic

acid of the Oa horizon is generally more aliphatic than in

mineral horizons. Clear-cutting did not significantly affect H

concentrations in either humic or fulvic acid.

5. Discussion

Covington (1981) hypothesized that decreased litter inputs

and accelerated decomposition following clear-cutting of

northern hardwoods would result in decreases of up to 50%

in forest floor SOM content in the first 15 years after harvest,

followed by a slow increase back to pre-cut levels. His

observations were based on samples from a chronosequence of

stands, ranging in age from 3 to 70 years, in the White

Mountains of New Hampshire. Concerns over the large

magnitude of the reported C loss from clear-cutting disturbance

and its implication regarding nutrient cycling in forest

motivated a large number of studies (Federer, 1984; Snyder

and Harter, 1985; Mattson and Swank, 1989; Brais et al., 1995;

Yanai et al., 2000). Many of these studies failed to support

Covington’s hypothesis. Thus, there are no consistent results of

the effects of harvesting on carbon dynamics and long-term site

productivity (Yanai et al., 2003). A meta-analysis of the effects

of forest management on soil C showed that forest harvesting

had little or no effects on soil C and N (Johnson and Curtis,

2001).

Research at the HBEF has indicated that changes in both the

quantity and quality of the OM occur after clear-cutting. For

example, direct measurements using soil pit sampling revealed

a 27% decline in forest floor C pool 8 years after clear-cutting,

while isolated forest floor blocks incubated in W5 lost 20–30%

of mass in the first 7 years after clear-cutting (Johnson et al.,

1995). Even though new litter inputs were excluded from the

forest floor blocks, their rate of mass loss was still slower than

that predicted by Covington (1981). These declines were the

result of decreases in both C concentration and the mass of

forest floor. The decline in C pools and concentrations were

attributed to increased decomposition and physical disturbance

during logging, which caused mixing of the upper mineral

horizons into the forest floor (Ryan et al., 1992). While

disturbance-related mixing undoubtedly played a role in the

redistribution of soil C after harvesting, our results indicate that

decomposition processes resulted in changes in OM quality.

Clear-cutting may alter both quantity and quality of OM by

reducing litter input and increasing decomposition rates. Clear-

cutting at the HBEF caused significant 3- and 8-year declines in

N and C concentrations in the Oa horizon, respectively (Table 1,

Johnson et al., 1995). In the Bh, Bs1 and Bs2 horizons,

however, C and N concentrations increased between years 8 and

15 (Table 1). In addition, the humic acid concentration

decreased by 24, 28 and 25% in the Oa, E, and Bh horizons,

respectively, in the first 3 years after clear-cutting, and

remained significantly lower than the pre-harvest concentra-

tions 15 years after clear-cutting (Table 2). Fulvic acid

concentrations decreased by 47 and 41% in the Oa and E

horizons, respectively, in the first 3 years and remained

significantly lower than pre-harvest concentrations 15 years

after clear-cutting (Table 2). This could be the result of the

export of soluble humic substances from the forest floor and the

Fig. 8. Indices of decomposition (R) in whole soil, humic acid and fulvic acid of

watershed 5 at Hubbard Brook Experimental Forest before (1983) and 3, 8, 15

years after (1996, 1991, and 1998) whole-tree clear-cutting. Error bars represent

standard error of the means.


upper mineral horizon to the lower mineral horizons and stream

waters. Consistent with this mechanism, the concentration of

dissolved organic carbon (DOC) in stream water from W5, and

soil solutions collected from Oa, Bh, and Bs horizons by

tension-free lysimeters, increased significantly relative to an

adjacent uncut watershed in the first 8 years after clear-cutting,

and remained elevated 15 years after clear-cutting (Johnson

et al., 1995; Dai et al., 2001).

The changes in the elemental composition of humic and

fulvic acid fractions observed in this study (Table 3) suggest

that C was preferentially lost from humic and fulvic acid while

the N concentration increased or did not change in post-clear-

cut soils. These changes were consistent with the decrease in

total soil C and the increase in total N concentration observed in

the first 8 years after clear-cutting (Johnson, 1995; Johnson

et al., 1995). Johnson (1995) attributed the increase in the N

concentration of OM in the forest floor to the predominance of

pin cherry (Prunus pennsylvanica L.f.) and raspberry (Rubus

spp.) in the early years after clear-cutting. These species have

high tissue N concentration (Marks, 1974; Mou et al., 1993),

and decomposition of their litter likely results in higher N

concentration and lower C:N ratios in the forest floor. Data from

this study indicate that the changes observed in total soil C and

N were reflected in humic substances, even relatively stable

fractions such as humic acid, and have persisted through the

first 15 years after clear-cutting.

Studies of decomposition in forest soils have indicated that

in general, the relative amount of alkyl C increases during

biodegradation while the amount of O-alkyl C decreases. This

has been indicated in studies of forest-soil profiles, peat, and

litterbag experiments (Wilson et al., 1983; Kogel-Knabner,

1993; Baldock and Preston, 1995; Gressel et al., 1996; Dai

et al., 2001; Ussiri and Johnson, 2003). In this study, O-alkyl C

decreased in whole soils of Oa horizons (Fig. 2) after clear-

cutting, suggesting accelerated decomposition of carbohydrates

in the forest floor.

In contrast, the O-alkyl C content of Oa-horizon humic acid

increased after clear-cutting (Fig. 5). This suggests that the

humic acid pool acted to stabilize carbohydrates and other O-

alkyl compounds after clear-cutting, protecting them from

chemical and microbial attack. Baldock and Preston (1995) and

Baldock et al. (1997) suggested the use of the ratio of alkyl C to

O-alkyl C (R) as an index of the degree of decomposition of

SOM. However, they also pointed out that this should be done

only within a soil profile or set of organic materials originating

from the same source materials due to the varying nature of the

alkyl to O-alkyl ratio in different tissues. In whole soils, R

decreased significantly in Bh and Bs1 horizons after clear-

cutting, but changes in the Oa horizon were not significant. In

humic acid, R decreased significantly in Oa Bh and Bs1

horizons after clear-cutting (Fig. 8). There were no significant

variations of R in fulvic acid. Overall, these responses can be

attributed to the contribution of decomposing roots and logging

residues, as well as the change in species composition in the

young stand.

Hughes and Fahey (1994) reported that in the first 4 years after

clear-cutting of W5, pin cherry leaves accounted for nearly half

of the mass of leaf litterfall, and remained the single most

dominant species, contributing more than 40% of leaf mass

litterfall in a 15–17-year-old stand at the HBEF (W4, strip-cut).

They also reported high N and non-structural carbohydrates, and

lower lignin concentrations, in litterfall of young stands of 3 and

15 years at HBEF, which could affect the chemistry of OM in

post-clear-cut soils. Consistent with this, we observed lower C

concentrations and C:N ratios, accompanied by increasing N

concentrations, after clear-cutting in humic and fulvic acid in Oa,

Bh, and Bs1 horizons (Table 3).

The increases in the carbonyl C fraction of whole soils in Oa

and Bh horizons are consistent with increase in CEC observed

in post-clearcut soils (Table 1, Johnson et al., 1997). At

Hubbard Brook and other recently glaciated soils in the

northeast USA, soil clay content is relatively low (generally

<5% at HBEF); and most of the CEC of the soil is provided by

deprotonated acidic functional groups associated with SOM

(Federer and Hornbeck, 1985; Ross et al., 1991; Ross and

Bartlett, 1995; Johnson et al., 1997; Johnson, 2002).

6. Conclusions

Forest harvesting in northern hardwood ecosystem induces

complex ecological, biogeochemical, and hydrological

changes. Results from this study suggest that following

clear-cutting there is a dynamic redistribution and restructuring


of SOM pools and fractions in these soils. In addition,

accelerated decomposition in the forest floor resulted in the

preferential decomposition of O-alkyl C compounds and the

loss of humic substances from the soil, probably through

increased leaching as DOC and increased respiration losses.

Changes in the composition of the humic acid fraction after

harvesting indicate that humic acid plays an important role in

the response of soils to clear-cutting disturbance. The

magnitude of changes in the structure and composition of

SOM after clear-cutting were relatively small. The absence of

large changes may be attributed to large C pool sizes and longer

turnover times of SOM. More labile fractions such as humic

and fulvic acids, and more dynamic pools of OM such as soil

solution and stream water DOC may be more sensitive

indicators of disturbance effects in the short and medium terms.

Acknowledgements

This work was supported by grants from USDA-National

Research Initiative Competitive Grants Program and National

Science Foundation-Long Term Ecological Research (NSF-

LTER) Program. We thank David Kiemle of the State

University of New York, College of Environmental Science

and Forestry for help with NMR acquisitions. This is a

contribution to the Hubbard Brook Ecosystem Study. Hubbard

Brook Experimental Forest is operated and maintained by

USDA Forest Service, Northeastern Forest Experiment Station,

US Department of Agriculture, Radnor, Pennsylvania, USA.

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