Soil Use and Management (2004) 20, 65±73 DOI: 10.1079/SUM2004224

Methane consumption in a frequentlynitrogen-fertilized and limed spruce forest soil

after clear-cutting

A. Saari1,*, A. Smolander2 & P.J. Martikainen1

Abstract. The effects of especially frequent nitrogen (N) additions (from 1959 to 1986, totalling860 kg N ha±1) and liming (in 1958 and 1980, totalling 6000 kg CaCO3 ha±1) on CH4 uptake by a boreal forestsoil were studied in a stand of Norway spruce. Except for a forested reference plot, the stand was clear-cutin January 1993 and the following year one-half of each clear-cut plot was prepared by mounding. Fluxes ofCH4 were measured with static chambers in the autumn before clear-cutting and during the following foursummers. The average CH4 uptake during 1993±96 in the forested reference plot was 82 mg CH4 m±2 h±1

(ranging from 10 to 147 units). In the ®rst summer after clear-cutting, the cleared plot showed 42% lowerCH4 uptake rate than the forested reference plot, but thereafter the difference became less pronounced. Theshort-term decrease in CH4 consumption after clear-cutting was associated with increases in soil NH4

+ andNO3

- concentrations. Mounding tended at ®rst to stimulate CH4 uptake but later to inhibit it. Neither lim-ing nor N-fertilization had signi®cant effects on CH4 consumption. Our results suggest that over the longterm, in N-limited upland boreal forest soils, N addition does not decrease CH4 uptake by the soil.

Keywords: Boreal, clear-cutting, forest soil, liming, mounding, methane uptake, nitrogen fertilization

I N T R OD U C T I O N

Upland forest soils contain microbes capable of oxidizingatmospheric CH4, which is the second most important

greenhouse gas after CO2. The global soil sink of about30 Tg CH4 yr±1 is close to the annual increase of CH4 (about22 Tg CH4 yr±1) in the atmosphere (IPCC 2001). Thus, adecrease in the soil CH4 sink as a result of land managementor environmental changes would further accelerate globalwarming.

Clear-cutting is a common silvicultural practice inFinland, accounting for about 30% of the total felled areaduring the 1990s (Finnish Forest Research Institute 2000),and has been shown to affect both carbon (C) and nitrogen(N) cycles. Clear-cutting can initiate nitri®cation as a resultof increased availability of NH4

+ following reduced plantnutrient uptake and increased net mineralization of N andsoil pH, which enhances N losses (Smolander et al. 1998).The greater supply of mineral N, especially NH4

+, in soilafter felling can lead to decrease in CH4 uptake (Bradfordet al. 2000; Castro et al. 2000; KaÈhkoÈnen et al. 2002;

Huttunen et al. 2003), but the long-term effects on CH4

uptake remain unclear.After felling, site preparation such as `mounding' is often

used in reforestation to promote establishment of seedlings.Mounding usually involves digging a ditch and lifting thesoil to form mounds. As a result, the logging residue as wellas organic horizons of the original and inverted soil pro®lesare buried beneath the mineral soil of the mounds.Mounding can alter the physical (e.g. temperature andmoisture) and chemical (e.g. larger amount of organicsubstances) conditions in the mineral soil beneath themound. However, the effects of site preparation on soilconsumption of CH4 are poorly understood.

In boreal coniferous forests nitrogen cycling is slow.Nitrogen is bound primarily in organic matter, and onlyabout 1% of the total N in forest soils is in inorganic form.Thus, in such forests with low N deposition, N becomes themain factor limiting tree growth. In the 1970s N-fertilizationwas a rather common forest practice in Finland used topromote tree growth, but since then the area N-fertilizedannually has decreased signi®cantly, in the 1990s being onlyabout 10% (<22 000 ha) of that in the 1970s (Finnish ForestResearch Institute 2000).

The N-fertilization experiments established in the 1950soffer an excellent opportunity to evaluate the long-termeffects of elevated nitrogen inputs on forest soils, which arean important net biospheric sink for atmospheric CH4. The

1Department of Environmental Sciences, University of Kuopio, PO Box1627, FIN±70211 Kuopio, Finland. 2Finnish Forest Research Institute,Vantaa Research Centre, PO Box 18, FIN±01301 Vantaa, Finland.*Corresponding author. Tel: +358 17 163588. Fax: +358 17 163750.E-mail: Anne.Saari@uku.®

A. Saari et al. 65

large N additions in these experiments have been criticizedfor not simulating the deposition of air-borne N compounds(NOx and NHx), which are deposited in signi®cantly smallerquantities and much more frequently than fertilizer inputs(Bradford et al. 2001). On the other hand, long-term N-fertilizer experiments have the advantage that the effects ofmany chemical components other than N, which are alsopresent in the deposition, can be excluded, thereby allowingconclusions about the effects of N alone to be made moreeasily.

The high deposition rates of acidic compounds induceacidi®cation of forest soils, resulting in losses of soilnutrients and the release of Al3+ ions that are potentiallytoxic to roots and microorganisms. Liming can be used toalleviate acidi®cation caused by N-deposition. However,large doses of lime may disturb nutrient cycling causingaccumulation of organic matter in acid coniferous forest soils(Derome 1990). The reported effects of liming on CH4

consumption by soil are contradictory (e.g. Kasimir-Klemedtsson & Klemedtsson 1997; Butterbach-Bahl &Papen 2002).

We studied the long-term effects of especially large N-application and liming, and subsequent clear-cutting andmounding, on CH4 uptake in a Norway spruce stand ineastern Finland. We expected to ®nd an interaction betweenthe N status of the soil and clear-cutting, which would thenaffect soil uptake of CH4.

M E T H O DS

Study site and treatmentsThis investigation was carried out between 1992 and 1996 inKerimaÈki, southeastern Finland (61°51¢N, 29°22¢E, 85 ma.s.l.), in a Norway spruce (Picea abies (L.) Karst.) standgrowing on ®ne sand till on an Oxalis±Myrtillus site, type(OMT) (Cajander 1949). The soil type was podzol and thehumus type was mor. The thickness of the organic horizonvaried from 2 to 5 cm. The study area has a nitrogendeposition rate of 4 kg N ha±1 yr±1. Continuous data on airtemperature and rainfall data were recorded at a weatherstation 7 km distant from the area.

The study site has been the subject of many micro-biological studies in which the effects of N-fertilization andliming and subsequent clear-cutting and mounding on Nand C transformations have been demonstrated (e.g.Smolander et al. 1994; Smolander et al. 1998; Smolanderet al. 2000). The stand had experimental plots of 30 3 30 m(after mounding, 15 3 30 m) with liming and N-fertilizer

treatments. The experimental design and treatment key areshown in Table 1 and the further soil characteristics aregiven in Table 2.

A total of 6000 kg ha±1 of ®nely ground limestone hadbeen spread over the site in 1958 and again in 1980. The N-plot had received N on 7 occasions during the years 1958±86giving a total of 860 kg N ha±1, added ®rst as (NH4)2SO4,then as urea, and later as NH4NO3 with dolomite. The N-fertilization history of the site, referred to as Exp. 31, hasbeen described in more detail by Smolander et al. (1994).Throughout the study there was a forested untreatedreference plot (0-plot) with a natural stand.

The ®rst ¯ux measurements were made in October 1992.The 60-year stand, except for the forested reference plot, wasclear-cut in January±February 1993. The logging residueconsisting of dead and living branches and the tops of stemswas evenly distributed on the surface of each plot. In October1993, half (15 3 30 m) of each clear-cut plot was mounded.The mounds were made with an excavator and formed byinverting a scoop of soil on top of the adjacent soil and loggingresidue. The mounds were about 70 cm in diameter and about15 cm high. The tops of the mounds were formed from the Bhorizon soil and underlain by the upper mineral soil layers (A,E) and the organic horizon (O). The inverted organic horizonlay on top of the logging residue and organic horizon of theundisturbed soil, forming a double organic horizon. In May1994, 1-year-old silver birch (Betula pendula L.) seedlingswere planted on top of the mounds except where ¯uxmeasurements were made.

Soil sampling and physical, chemical and microbiologicalanalysesDuring most of the ¯ux measurements (11 times out of 14),soil samples were also taken from the organic horizon. Thethickness of the organic horizon was 2 cm in the clear-cut(CC) plot and also in the clear-cut N-fertilized (CC-N) plot,and 5 cm in the clear-cut limed (CC-L) plot and in theforested reference plot (0). In addition, soil samples weretaken once in the summers of 1993, 1995 and 1996 from themineral soil layers: in the unmounded plots from theuppermost 5 cm of mineral soil, and in the mounded plotsfrom the top of the mounds and from mineral soil beneaththe mounds. The 5±10 cm mineral soil layer was alsosampled in the 0-plot. Soil sampling and analytical methodshave been published, and the following parameters havebeen used as background information for the gas ¯uxes:gravimetric water content, organic matter content, pH, netammoni®cation and nitri®cation, CO2-C production and

Table 1. Experimental design in the Patasalo forest stand.

Forestedreference

Clear-cut Clear-cut+ limed

Clear-cut+ N-fertilized

Clear-cut + limed+ N-fertilized

Plot / treatment keyWithout mounding 0 CC CC-L CC-N CC-L-NWith mounding CCm CC-Lm CC-Nm CC-L-Nm

Amendments (total amount)Liming (kg ha±1) 6000 6000N-fertilization (kg ha±1) 860 860

Methane consumption in a clear-cut forest soil after N-fertilization and liming66

microbial biomass N and C (Smolander et al. 1994;Smolander et al. 2000). Fresh bulk density (g cm±3) of thesoil was determined from sieved samples in the laboratory.On gas sampling days, temperatures were recorded insidethe chambers, of ambient air, at the soil surface and atdepths of 3, 6 and 12 cm in the soil pro®le.

Flux measurementsMethane ¯uxes were measured with opaque closed chambers(diameter 32 cm, headspace 20±23 dm3) made of steel. Fluxmeasurements were made from 3 replicate chambers in eachplot once before clear-cutting (in October 1992) and 14

occasions after clear-cutting (4 times in 1993, 1994 and 1995and twice in 1996), all during the growing season(May±October).

On mounded areas, chamber measurements were madefrom the top of the mound (no vegetation) and onunmounded areas distant from the seedlings in order toavoid their effects on gas ¯uxes. To ensure gas-tight sealing,the open ends of the chambers were pushed into the mineralsoil to a depth of about 4 cm prior to measuring. Gassamples from the headspace of the chambers were with-drawn with 60 mL plastic syringes 3 or 4 times during the30 min incubation.

Table 2. Characteristics of the mineral soils of the study plots.

Characteristics and sampling dates Study plotsa

0 CC CC-L CC-N CC-L-N

Water content (%)12 Oct 1992b 18.1 16.4 20.9 18.2 19.47 Oct 1993c 22.3 20.9 26.6 22.2 27.87 Aug 1996c 24.4 17.5 33.6 27.1 28.9Mounds:above the mound 7 Aug 1996 15.1 17.8 17.5 19.2below the mound 7 Aug 1996 19.2 21.9 20.1 26.3

pH (H2O)12 Oct 1992b 3.9 4.2 4.3 3.8 4.27 Oct 1993c 4.3 4.8 4.8 4.8 5.07 Aug 1996c 4.0 4.6 4.6 4.4 5.0Mounds:above the mound 7 Aug 1996 5.0 5.1 5.0 5.2below the mound 7 Aug 1996 4.5 4.8 4.4 4.7

Total C (%)12 Oct 1992 2.8 2.5 3.0 3.3 3.1

Total N (%)12 Oct 1992 0.18 0.16 0.20 0.19 0.18

NH4+-N (mg cm±3)

12 Oct 1992b 2.3 2.0 6.7 2.7 4.07 Oct 1993c 2.2 14.8 10.9 14.1 12.67 Aug 1996c 1.3 1.95 5.1 2.9 4.1Mounds:above the mound 7 Aug 1996 0.85 1.2 3.6 0.96below the mound 7 Aug 1996 2.1 2.3 3.8 3.0

(NO3±+ NO2

±)-N (mg cm±3)12 Oct 1992b 0 0.02 0.11 0.01 0.057 Oct 1993c 0 0.66 0.27 1.1 0.527 Aug 1996c 0 0.55 4.4 0.88 3.11Mounds:above the mound 7 Aug 1996 0 0 0 0below the mound 7 Aug 1996 0.44 0.52 0.37 0.75

Basal respiration (mg CO2-C cm±3 h±1)12 Oct 1992b 0.25 0.25 0.24 0.22 0.227 Oct 1993c 0.52 0.71 0.59 0.78 0.557 Aug 1996c 0.39 0.31 0.41 0.32 0.54Mounds:above the mound 7 Aug 1996 0.19 0.24 0.33 0.34below the mound 7 Aug 1996 0.20 0.23 0.27 0.36

Net nitri®cation (mg [NO2±+ NO3

±]-N cm±3)12 Oct 1992b 0.08 0.10 0.60 0 0.207 Oct 1993c 0 0.30 1.07 9.9 20.47 Aug 1996c 0 6.1 24.2 6.1 32.6Mounds:above the mound 7 Aug 1996 1.7 4.3 6.4 8.5below the mound 7 Aug 1996 2.3 1.8 3.4 2.1

aFor key to treatment codes, see Table 1.bThe determinations in the year 1992 were done before clear-cutting using the uppermost 10 cm of mineral soil.cIn 1993 and 1996 the soil was sampled from the uppermost 5 cm of mineral soil.

A. Saari et al. 67

Soil incubation experiments to measure CH4 oxidationpotentialThe capacity of the soil samples, taken in August 1996, tooxidize CH4 was determined in triplicate 600 mL ¯asks with30 mL soil and 10 ppmv CH4 in the headspace. The ¯askswere sealed with rubber septum and incubated at 20 °C. Theincubation period varied from 24 to 115 h, depending on theCH4 oxidation rate of the sample. At the beginning of theexperiment each ¯ask had an air overpressure of 15 kPa. Gassamples (30 mL) for CH4 analyses were taken through theseptum with a gas-tight syringe equipped with a three-waystopcock and a needle (26G). Because CH4 consumption didnot follow ®rst-order reaction kinetics in all soil samples, therates for all mineral soil samples were calculated fromthe linear decrease in CH4 concentration in the headspace ofthe ¯asks during the ®rst 24 h and for organic soil samplesduring the ®rst 42 h.

Methane analysesCH4 concentration in gas samples was analysed with a gaschromatograph (HP 5890 Series 2) equipped with a ¯ameionization (FI) detector (Saari et al. 1997). Gas samples wereanalysed within 24 h after sampling.

Expression of the results and statistical methodsBecause soil samples were taken from mineral soil andorganic horizons with different bulk densities, the resultswere calculated on a soil volume basis. To avoid the problemof possible diurnal variation in ¯ux rates, ¯uxes in situ wereexpressed as mg m±2 h±1, not as daily averages. Becausesampling intervals differed between years, we also calculatedtime-weighted means for the time period from 15 May to15 October.

Field data were generally analysed separately for 0-plot,clear-cut unmounded plots and mounded plots. Tests ofsigni®cance of differences in the CH4 ¯uxes between thedifferent treatments (clear-cutting, mounding, liming andN-fertilization) as well as comparisons between different

study years were performed using one-way analysis ofvariance if the data were normally distributed according tothe Kolmogorov±Smirnov test. For non-parametric ¯uxdata, temperature and moisture data, and for data fromincubation experiments, the Mann and Whitney U-test andthe Bonferroni correction were used. Relations between CH4

consumption rates and environmental variables wereanalysed by Spearman's rank correlation analysis. All testswere done with SPSS Windows 9.0.

RE SUL TS

Weather conditionsThe average air temperature at the weather station,measured from June to September at 9 a.m. to 3 p.m.,was equal to (in 1993) or higher than (in 1994±96) thelong-term (1971±2000) average of 15.1 °C (FinnishMeteorological Institute 2002). The average air tempera-tures in 1994, 1995 and 1996 exceeded the long-term averageby 5.6 °C, 3.5 °C or 8.4 °C, respectively.

The 30-year average (1971±2000) rainfall of the study areafrom June to September was 256 mm (FinnishMeteorological Institute 2002). The summers of 1993(+17%) and 1994 (+14%) were wetter and the summersof 1995 (±15%) and 1996 (±23%) were drier than the long-term average. In 1994 shortly after mounding, the organichorizons in the mounded plots were drier than insubsequent years.

Effect of clear-cutting and mounding on soil temperature andmoistureAt 6 cm, that is, in the soil layer showing the maximum CH4

oxidation, mean soil temperature in the mounded plots, intheir unmounded counterparts and in the 0-plot were18.2°C, 13.0°C and 11.0°C, respectively; the differencebetween the mounded plots and the 0-plot was statisticallysigni®cant (P <0.05).

Figure 1. Mean uptake of CH4 in unmounded (a) and mounded (b) plots in the summers of 1992±96. The mounded plots lack data for 1992 and1993, because the mounds were made in October 1993. SE of the means, shown by bars, is for sampling times (replicate chambers have ®rst been aver-aged).

Methane consumption in a clear-cut forest soil after N-fertilization and liming68

The organic horizons of clear-cut plots were slightly(P >0.05) drier than that of the 0-plot (results not shown).Furthermore, the organic horizons (data not shown) andmineral soils (Table 2) in the mounded plots were drier thanthose of the unmounded plots, because mounding causedmixing with mineral soil.

CH4 ¯uxesThe soils were generally net sinks for CH4. During 1993±96,the average uptake rates ranged from 10 to147 mg CH4 m±2 h±1 for the 0-plot (mean 82 units), from 3to 177 mg CH4 m±2 h±1 for clear-cut unmounded plots (mean65 units) and from 13 to 172 mg CH4 m±2 h±1 for moundedplots (mean 67 units during 1994±96). CH4 emission up to93 mg CH4 m±2 h±1 was observed in 4 out of the 287 chambersmeasured. Non-weighted summer means of CH4 ¯uxes withstandard errors are shown in Figure 1. The standard error

of time-weighted means cannot be calculated. The time-weighted summer means of CH4 ¯uxes differed from thenon-weighted summer means by ±9% to +35%.

Methane uptake correlated positively with air tempera-ture (P = 0.0001, r = 0.36) and negatively with the NH4

+

concentration in the organic horizon (P = 0.048, r = ±0.21)and mineral soil (P = 0.016, r = ±0.63). Methane uptake alsocorrelated negatively with nitri®cation potential (P = 0.020,r = ±0.25) and respiration (P = 0.023, r = ±0.24) in theorganic horizon but did not correlate with soil moisture,pH (in H2O), NO3

± concentration or organic matter content.

Effect of clear-cutting on CH4 uptakeDuring the ®rst study summer (1993) after clear-cutting, allcleared plots showed lower rates of CH4 uptake than the 0-plot (Figure 1). In 1993 the mean CH4 uptake rate in theclear-cut plot (CC) was 42% lower than that of the 0-plot

Figure 3. Methane uptake and concentration of NH4+ in the organic horizons of the forested reference plot (0) and the clear-cut (CC) plot.

Figure 2. Effect of clear-cutting and mounding on the mean uptake rates of CH4 in the forest soils. SE of the means, shown by bars, is for study plots(a) or for sampling times (b). The unmounded plots (n = 4) were sampled 14 times (in 1993±96) and the mounded plots (n = 4) 10 times (in 1994±96).

A. Saari et al. 69

(P <0.05). During the next summer (1994) the mean CH4

uptake of the CC-plot was similar to that of the 0-plot.The mean rates of CH4 uptake in unmounded plots

showed an increasing trend from 1993 to 1996 (Figure 2),being signi®cantly lower in 1993 and 1994 than in 1995(P <0.01) or 1996 (P <0.05). In contrast, there were nosigni®cant differences between years in the 0-plot or in themounded plots.

The short-term decrease in CH4 consumption after clear-cutting was linked to the increase in soil NH4

+ concentra-tion (Table 2; Figure 3). In 1993, that is, the ®rst summerafter clear-cutting, the NH4

+ concentration in the organichorizon of the CC-plot was four times and that in themineral soil seven times the levels before clear-cutting(results not shown). In 1995, that is, 2.5 years after clear-cutting, NH4

+ concentration in the organic horizon of theCC-plot was two times that before clear-cutting. In 1996,the NH4

+ concentration in the mineral soil was the same asbefore clear-cutting (not analysed in 1994 and 1995).

Effect of mounding on CH4 uptakeImmediately after mounding, in 1994, the mounded plotsconsumed, on average, 33% more CH4 than theirunmounded counterparts (Figure 2a). Later, in 1995 and1996, the reverse occurred; the mounded plots consumed22% less CH4 than their unmounded counterparts. Thedifference between the mounded and unmounded plots wasstatistically signi®cant (P <0.05) in 1995, but not in 1994 or1996.

Effect of N-fertilization on CH4 uptakeN-fertilization had no signi®cant (P >0.05) long-term effectson rate of CH4 uptake either before clear-cutting orafterwards when the N-fertilized plot was compared to itsrespective control plot or when all N-fertilized plots,

including the plot that was both limed and N-fertilized,were compared to all non-fertilized plots in the whole dataset or grouped by mounding. The possible short-termeffects of N-fertilization on CH4 uptake were not studied.

Effect of liming on CH4 uptakeWhen all limed plots, including the plot that had been bothlimed and N-fertilized, were compared to all unlimed plotsbefore clear-cutting, liming had no signi®cant effect on CH4

uptake. After clear-cutting, however, liming signi®cantly(P <0.05) decreased CH4 uptake in the clear-cut,unmounded plots, but signi®cantly (P<0.05) increaseduptake in the mounded plots (Figures 1 and 2).

The unmounded plot that had received both lime and N(CC-L-N) took up 12±34% less (P >0.05) CH4 than theCC-L treatment, except in 1993 (56% increase, P >0.05). Inthe mounded plots there were no clear differences in CH4

uptake between the CC-L-Nm and CC-Lm treatments.

CH4 oxidation potential in laboratory incubationsIn the laboratory incubations, organic horizons generally hadlower CH4 oxidation potentials (from 0.4 to 3.1 ng CH4 cm±3

soil h±1) than did mineral soils (from 1.2 to 6.8 units)(Figure 4). The average CH4 oxidation potential in theorganic horizons of the mounded plots (2.2 ngCH4 cm±3 soil h±1) was greater (P <0.001) than that of theunmounded plots (0.96 units).

In contrast to organic horizons, the average CH4 oxidationpotential (5.4 ng CH4 cm±3 soil h±1) of the mineral soils in theunmounded plots was higher than that of the mineral soilsbeneath the mound in the mounded plots (4.7 units;P >0.05) or in the 0-plot (3.4 units; P <0.01). In themounded plots, mineral soils on top of the moundsconsumed less CH4 (2.4 ng CH4 cm±3 soil h±1) (P<0.01)than mineral soils beneath the mounds (4.7 units). The

Figure 4. CH4 oxidation in samples from organic horizon (a) and mineral soil layer (b). Means are for triplicate samples. SE of the means is shown bybars for mineral soil (b). Organic matter content (%) in soil samples from organic horizons is shown above the respective columns (a).

Methane consumption in a clear-cut forest soil after N-fertilization and liming70

highest potential (6.8 ng CH4 cm±3 soil h±1) was found in themineral soil of the CC-plot (Figure 4), while the mineralsoils of the CC-L-Nm-plot had the smallest potentials: 2.3units beneath the mound and 1.2 units on top of the mound.

CH4 oxidation potentials in the incubated soil samples didnot correlate with the CH4 uptake in situ measured on the dayof sampling. In the mineral soils, CH4 oxidation potential didnot correlate with soil NH4

+ or NO3± concentrations,

moisture, organic matter content, production of CO2,nitri®cation or ammoni®cation rates but was negativelycorrelated with soil bulk density (P = 0.022, r = ±0.63). Inthe organic horizons, CH4 oxidation potential was negativelycorrelated with organic matter content (P = 0.005, r = ±0.83),production of CO2 (P = 0.020, r = ±0.75), microbial biomassN (P = 0.016, r = ±0.78) and was correlated positively withsoil bulk density (P = 0.004, r = 0.85).

D I S C U S S I O N

CH4 oxidation potential in laboratory incubationsIn general, CH4 oxidation activity in soil samples from theorganic horizon was small, and the maximum CH4 oxidationoccurred in the uppermost mineral soil layers, as has beenfound previously (Adamsen & King 1993; Saari et al. 1998).Several factors have been suggested to explain the low levelsCH4 oxidation in organic horizons, such as presence ofmonoterpenes (Amaral et al. 1998), large ¯uctuation inmoisture conditions, competition for nutrients, low pH andhigh levels of NH4

+ and NO3± as a result of atmospheric

deposition (Adamsen & King 1993) and absence of clay(Bender & Conrad 1994). In our studies some organichorizons showed relatively high rates of CH4 oxidation. Thelow content of organic matter and high bulk density in thesesoils suggest that mixing of mineral soil into the organichorizon may explain the CH4 oxidation rates obseved. Inmineral soil samples, those taken at the top of the moundshad the lowest rates of CH4 oxidation and the highest bulkdensities. CH4 oxidation had no direct correlation with soilNH4

+ concentration but in the organic horizons it correlatednegatively with respiration, which was strongly associatedwith the NH4

+ concentration (P = 0.002, r = 0.87).

CH4 ¯uxesThe mean rate of CH4 uptake (82 mg m±2 h±1) in the 0-plot ofthis study is close to that in a Finnish pine forest whenmeasured with the same chamber method (Saari et al. 1998).The CH4 emissions may result from temporary anaerobicconditions in the soil, for example, after heavy rainfall.However, only a few of the CH4 emissions occurred afterrainfall. Production of CH4 has also been measured in someother aerobic forest soils, especially in the organic horizonswhere there may be anoxic microsites that supportmethanogenesis (Yavitt et al. 1990).

Effects of soil temperature and moisture on CH4 consumptionTemperature regulated CH4 uptake in the clear-cut plots,but not in the 0-plot. Air and soil temperatures and theirvariation were lowest in the 0-plot as a result of shading bytrees and were highest in the mounded plots, where the CH4

uptake was also the most temperature-dependent. Diffusion

of CH4 into the soil is generally considered to be a key factoraffecting CH4 uptake in forest soils. Temperature controlsCO2 ¯uxes more than it affects CH4 ¯uxes (Gulledge &Schimel 2000). However, positive correlations between CH4

uptake and temperature have also been found in previousstudies conducted in temperate (Steinkamp et al. 2001) andin boreal (Saari et al. 1998) forest soils. The effect oftemperature has been considered to be important, especiallyduring spring and autumn when air temperatures are <10 °C(Steinkamp et al. 2001).

Clear-cutting only slightly reduced the water content inthe organic horizons. An increase in soil moisture mighthave been expected when the transpiration of trees wasremoved (Bradford et al. 2000). In slash pine plantations inFlorida, the soil moisture increased after clear-cutting, thuscreating anaerobiosis that induced CH4 production (Castroet al. 2000). In the present study, the higher temperature ofthe clear-cut plots may have caused a higher rate ofevaporation than in the 0-plot. Alternatively, the smallerorganic matter content resulting from clear-cutting andmounding may explain the lower water content in theorganic horizons of the clear-cut plots.

The organic horizons and mineral soils of the moundedplots were, in general, drier than those in the unmoundedplots or the forested reference plot. It is unlikely, however,that the CH4-oxidizing microbes in the mounded plotssuffered from water stress, because CH4 consumption inboreal soils is resistant to soil dryness; matric potentialsbelow ±1.0 MPa in mineral soils did not inhibit CH4 uptakein situ (Saari et al. 1998). At high soil water contents,diffusion of CH4 into the soil decreases, thus limiting CH4

oxidation (Adamsen & King 1993; Steinkamp et al. 2001). Inthe present study, there was no signi®cant correlationbetween CH4 uptake and soil moisture.

Effect of clear-cutting and mounding on CH4 consumptionIn the ®rst summer after clear-cutting, the CC-plot had alower CH4 uptake rate than the 0-plot; but during the nextsummer (15 to 19 months after clear-cutting), there was nodifference. Our results con®rm earlier ®ndings about theCH4 oxidation activity decrease after clear-cutting (Bradfordet al. 2000; Castro et al. 2000; KaÈhkoÈnen et al. 2002;Huttunen et al. 2003).

Most studies of the effects of clear-cutting on CH4 uptakehave not reported possible changes in soil NH4

+ or NO3±

concentrations. In forest soil in Florida, clear-cuttingincreased the NH4

+ concentrations in soil 10-fold but thishad no effect on CH4 ¯uxes (Castro et al. 2000). In the presentstudy, soil CH4 uptake and soil NH4

+ concentration werenegatively correlated. An earlier study at the site showed thatbefore clear-cutting net nitri®cation took place only in the soilof the L-N plot. Clear-cutting increased soil pH and initiatednet nitri®cation in all plots (Smolander et al. 1998). Theincrease in soil NH4

+ and NO3± concentrations after clear-

cutting were probably associated with the short-termdecrease in CH4 consumption, because both NH4

+

(Steudler et al. 1989; Adamsen & King 1993) and NO3±

(Dun®eld & Knowles 1995; KaÈhkoÈnen et al. 2002) are knownto inhibit CH4 oxidation. The inhibition of CH4 oxidationhad already ceased during the second summer after clear-

A. Saari et al. 71

cutting. However, the concentrations of NH4+ and NO3

±

were then still slightly higher than before clear-cutting.In addition to changes in inorganic N pools after clear-

cutting, an increase in bulk density through soil compactionduring cutting (forestry machines) may also contribute to thedecrease in CH4 uptake (Bradford et al. 2000). In the presentstudy, an increase in laboratory-determined bulk densitiesafter clear-cutting was also found in samples from the organichorizon and mineral soil (results not shown). If an increase inbulk density had been the main reason for the fall in CH4

uptake after clear-cutting, the inhibition caused by compac-tion would presumably have lasted longer than a year.

On some sites, preparation is necessary to assure survivaland growth of seedlings after clear-cutting. However, theeffects of different methods of site preparation on soil CH4

consumption are poorly understood. In the present study,mounding stimulated CH4 uptake during the ®rst summer;but thereafter mounded plots consumed less CH4 than theother plots. Surprisingly, in the incubation experiments, twoyears after mounding, the mineral soils on top of themounds, without any organic horizon as a diffusion barrierfor atmospheric CH4, oxidized less CH4 than did themineral soils below the mounds (i.e. one extra organichorizon and one extra mineral soil layer on the soil surface).Our results suggest that the initial increase in CH4 uptakeafter mounding results from a high capacity for CH4

oxidation in the mineral soil on the surface of moundwithout any organic horizon to act as a diffusion barrier.This corresponds to the positive effect of organic horizonremoval on CH4 uptake by soil (Saari et al. 1998). For somereason, however, in spite of the better supply of CH4 and O2

in the surface mineral soil on the mound, during thefollowing years the conditions for methanotrophic microbeswere less favourable there than in the uppermost layer ofmineral soil below the organic horizon. It is possible thatsome of the factors discussed above which limited CH4

oxidation in the organic horizons also reduced the CH4

oxidation activity in the above-mound mineral soil. Inaddition, the double organic horizons may have reduceddiffusion of atmospheric CH4 into the mineral soilbeneath the mound, thus inhibiting CH4 consumption inthat layer.

Effect of N-fertilization and liming on CH4 consumptionIn general, N-fertilization and atmospheric N depositionhave been found to reduce the activity of CH4-oxidizingbacteria (Saari et al. 1997; KaÈhkoÈnen et al. 2002) and CH4

uptake into the soil (Steudler et al. 1989; Kasimir-Klemedtsson & Klemedtsson 1997). The inhibition ofCH4 consumption after N addition can be short term(Steinkamp et al. 2001) or persist for years or even decades(Prieme et al. 1997).

The ammonium ion can inhibit CH4 oxidation by acting asa competitive inhibitor of methane monooxygenase (MMO),by stimulating production of hydroxylamine and NO2

- or bya salt (osmotic) effect, and sometimes several of thesemechanisms are involved in the inhibition (reviewed byGulledge & Schimel 1998). CH4 oxidation is not alwaysinhibited by NO3

±, but NO3± may inhibit CH4 oxidation by

having a common salt effect (Dun®eld & Knowles, 1995) or

by some other non-competitive mechanism (Wang & Ineson2003).

In the present study, N-fertilization had been extremelyhigh. Nitrogen was added every 3±6 years at rates of82±180 kg N ha±1. Thus, in the year of fertilization, the N-plot received a single dose of N that was 20±45 times theannual N deposition rate in the region (4 kg N ha±1 yr±1).Over the whole period, the average annual rate of N additionwas 31 kg N ha±1yr±1. However, the N-fertilization hadcaused no long-term (measured 6±10 years after fertiliza-tion) inhibition in CH4 uptake, either before or after clear-cutting. On the other hand, short-term inhibition of CH4

oxidation after N-fertilization may have occurred, eventhough no long-term effects were found.

There are some other forest soils where high natural Nconcentration (Maljanen et al. 2003), N deposition (Saariet al. 1997; Bradford et al. 2001a) or fertilization (Gulledgeet al. 1997; Gulledge & Schimel 2000) has had no signi®canteffects on CH4 consumption by the soil. Usually, theenvironments where N addition does not inhibit CH4

oxidation are N limited. In those environments, recoveryfrom the inhibition in CH4 oxidation after N-fertilization israpid (Steinkamp et al. 2001) and, in the long term, N-fertilization can even stimulate CH4 oxidation (BoÈrjesson &Nohrstedt 1998).

One reason for different responses of soil CH4 consump-tion to mineral N in soil may be the distribution ofphysiologically diverse CH4-oxidizing populations acrossvarious sites (Gulledge et al. 1997). Ammonium-insensitivemethanotrophs may occur in certain soils, especially thosesubjected to repeated ammonium inputs, and account forreports of negligible inhibition due to added NH4

+ inagricultural soils (e.g. HuÈtsch et al. 1993). Shifts in therelative importance of methanotrophs in CH4 oxidation,compared to that of ammonia-oxidizing bacteria, have beensuggested to occur in response to NH4

+ additions (Castroet al. 1994). However, recent results show that NH4

+

oxidizers do not contribute signi®cantly to CH4 consump-tion in soils (Jiang & Bakken 1999; Klemedtsson et al. 1999).

Long-term ®eld experiments in Finland showed thatliming (2 t ha±1) increases the pH in the organic horizon andto some extent in the uppermost mineral soil, and alsocauses accumulation of organic matter in the organic horizon(Derome 1990). In the present study, however, liming hadonly a minor effect on CH4 consumption after clear-cutting.The uppermost mineral soil layers of the plots (CC, 0, CC-Land CC-N) are known to have a broad pH optimum range (4to 6.5) for CH4 oxidation (A. Saari et al., unpublishedresults), and in all of the plots studied the natural pH in themineral soils was in the optimum range. The insensitivity ofsoil CH4 consumption to changes in soil pH has also beenobserved by others (e.g. Benstead & King 2001), and thereported effects of liming on CH4 consumption are contra-dictory. In some forest soils liming has increased CH4

consumption (Kasimir-Klemedtsson & Klemedtsson 1997;Saari et al. 1997), while in other soils liming has had noeffect (Yavitt et al. 1993) or a negative effect on soil CH4

consumption (Butterbach-Bahl & Papen 2002).In conclusion, clear-cutting led to a short-term reduction

in CH4 uptake, which was linked to increased concentration

Methane consumption in a clear-cut forest soil after N-fertilization and liming72

of NH4+ in the soil. However, in this N-limited boreal forest

soil even a heavy N load had no long-term inhibitory effect onCH4 uptake. Liming had only a minor effect on CH4 uptakeboth before and after clear-cutting. The combination ofliming and N-fertilization had no signi®cant effects on CH4

uptake either before or after clear-cutting. Mounding at ®rststimulated but later decreased CH4 uptake by the soil.

A C K N O W L E D G E M E N T S

This work was supported by the Academy of Finland, theMaj and Tor Nessling Foundation and the KemiraFoundation. We thank Hannu NykaÈnen, Dr Laura HoÈijerand the personnel of Finnish Forest Research Institute forassistance in the ®eld and laboratory and Joann vonWeissenberg for revising the language.

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Received July 2003, accepted after revision November 2003.

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