Production of dissolved organic carbon in forest soils along the north–south European transect

Applied Geochemistry 24 (2009) 1686–1701

Contents lists available at ScienceDirect

Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem

Production of dissolved organic carbon in forest soils along the north–south European transect F. Buzek *, T. Paces, I. Jackova Czech Geological Survey, Geologicka 6, 152 00 Prague 5, Czech Republic

a r t i c l e

i n f o

Article history: Received 12 September 2008 Accepted 28 April 2009 Available online 9 May 2009 Editorial handling by S. Bottrell

a b s t r a c t The aim of this study is to estimate the C loss from forest soils due to the production of dissolved organic C (DOC) along a north–south European transect. Dissolved organic matter (DOM) was extracted from the forest soils incubated at a controlled temperature and water content. Soils were sampled from forest plots from Sweden to Italy. The plots represent monocultures of spruce, pine and beech and three selected chronosequences of spruce and beech spanning a range of mean annual temperature from 2 to 14 °C. The DOM was characterized by its DOC/DON ratio and the C isotope composition d13C. The DOC/DON ratio of DOM varied from 25 to 15 after 16 days of incubation and it decreased to between 16 and 10 after 126 days. At the beginning of incubation the d13C values of DOC were 1‰ or 2‰ less negative than incubated soils. At the end of the experiment d13C of DOC were the same as soil values. In addition to DOC production heterotrophic respiration and N mineralization were measured on the incubated soils. The DON production rates decreased from 30 to 5 lgN gC1 d1 after 16 days of incubation to constant values from 5 to 2 lgN gC1 d1 after 126 days at the end of experiment. The DIN production rates were nearly constant during the experiments with values ranging from 20 to 4 lgN gC1 d1. DOC production followed first-order reaction kinetics and heterotrophic respiration followed zero-order reaction kinetics. Kinetic analysis of the experimental data yielded mean annual DOC and respiration productions with respect to sites. Mean annual estimates of DOC flux varied from 3 to 29 g of C m2 (1–19 mg C g1 of available C), corresponding to mean DOC concentrations from 2 to 85 mg C L1. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Dissolved organic C (DOC) is an important fraction of the C pool in soils. As was summarized by Michalzik et al. (2001), as much as 10–40 g of DOC m2 a1, is translocated from the organic surface layer to deeper mineral horizons in forest soils. Dissolved organic matter (DOM) is the most bioavailable fraction of soil organic matter, since all microbes utilize aqueous uptake mechanisms (Marschner and Kalbitz, 2003). DOC plays a central role during podzolisation (Lundstrom et al., 1995), and the adsorption of DOC on mineral surfaces is critical for the stabilisation and degradation of soil organic compounds (Guggenberger and Kaiser, 2003). Aerobic heterotrophic respiration is the major mechanism of C loss from the soils and DOM mineralization (Raich and Schlesinger, 1992). DOM fluxes leaving the litter layer consist mostly of easily degradable simple carbohydrates and amino acids, while fluxes through the mineral horizons often contain slowly degradable complex lignocellulose-associated biopolymers (Neff and Asner, 2001). The most stable forms of DOM are further stabilized by sorption on mineral soil. For example, the mean residence time * Corresponding author. Fax: +420 251 818 748. E-mail address: [email protected] (F. Buzek). 0883-2927/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2009.04.036

of stable DOM increased from 28 a in solution to 91 a after sorption as was measured by Kalbitz et al. (2005). Stability of DOM is critical for evaluation of its degradation. Generally, labile forms of DOM are biodegraded in days, refractory forms on timescales of one hundred days to years (McDowell et al., 2006). Variation in d13C values among biochemical fractions within a source plant is relatively large and can be used for differentiation of DOM fractions. Difference between the d13C value of a fraction and the plant reflects the lability of the fraction to decomposition. Difference is positive for labile, metabolic fractions such as pectins, amino acids and sugars, close to zero for moderately labile structural fractions such as cellulose and hemicellulose and negative for recalcitrant materials such as lignin and cutin (Table 1). The main objective of this study was to quantify net DOC production in incubated forest soils of different origin, climate and litter quality. DOC production and DOC loss by heterotrophic respiration were simultaneously evaluated to generate more reliable estimates of DOC fluxes than simple measurement of DOC concentration or CO2 production. The DOC and CO2 production were evaluated from the kinetic analysis of closed experimental mesocosms. The DOC was characterized by the (C/N) ratio and C isotope composition d13C. The estimated DOC fluxes were tested for the effects of a C pool size, C quality, C mean residence time and climatic conditions.

F. Buzek et al. / Applied Geochemistry 24 (2009) 1686–1701 Table 1 The d13C values of the plant biochemical fractions calculated as the differences of d13C of the whole plant from which they were extracted – data from Dienes (1980). Fraction

d13C fraction–d13C plant (‰)

Pectin Hemicellulose Amino acids Sugars Cellulose Lignin Lipids

from from from from from from from

2.5 to 4.5 0.5 to 3.5 0.5 to 4 0.5 to 3 2 to 0.2 4.5 to 0.5 7.5 to 2.5

2. Study sites and soil sampling and processing The sampled sites were, from north to south: Flakaliden (northern Sweden), Soroe (Denmark), Loobos (The Netherlands), Hainich (Germany), Hesse (France), Rocca and Collelongo (Italy) (Fig. 1). Ten sites include five monocultures of beech, three monocultures of spruce and single pine and oak monocultures. Three chronosequences (sites with the same climate and soil type) were included to follow a possible tree age effect on DOC production. The chronosequences were located in Germany. They were: Tharandt (5, 24, 42 and 97 a old spruce), Leinefelde (30, 62, 111 and 153 a old beech) and Wetzstein (10, 60 and 120 a old spruce). The sampled sites are characterized in Table 2. Sites correspond to a north– south climatic transect, with all chronosequence sites in a temperate climate. All sites belong to the FORCAST project as a part of the CARBOEUROPE project. Sites were sampled during the project campaign in years 2000–2001. Nine sampling plots were randomly selected within each site and age-group stand. The same plots were used by all FORCAST participants for all methods (Schulze, 2003). Nine samples from

1687

each plot were pooled to form three groups of samples for each site and age-group stand. The litter (L + F layer) was sampled from a 400-cm2 area. Humus (H) was collected from well-developed profiles only. Soil was sampled with a 14-cm2 corer. Four cores in the area of litter sampling were made at each sampled plot and combined to a single sample for depth ranges 0–5, 5–10, 10–20 and 20–30 cm, independent of generic soil horizons. Litter, 0–5 and 5–10 cm layers were used for incubation experiments. The samples were transported to a laboratory within 1–3 days, where all samples were dried at 50 °C to a constant weight. Dried samples were sieved through a 2-mm sieve to remove roots, stones and other large particles. Dried and sieved samples were used for the incubation experiments. A part of the sieved samples was homogenized to a fraction below 0.063 mm for elemental and isotopic soil measurements. Carbon-14 dating and soil mineralization experiments of sampling plots were performed by A.P. Rowland, NERC, UK and T. Persson, University of Uppsala, Sweden (Schulze, 2003). 3. Methods 3.1. Incubation experiment Net DOC production as a function of time was measured simultaneously with loss of CO2. To specify possible effects of C quality on DOC production the sample set was treated in subsets according to tree type and tree age. The tree type sets included four coniferous and six deciduous sites with different climatic conditions. Samples of 100 g of air dried, sieved (<2 mm), pooled soils were moistened to a water content of 60% of soil water saturation (Persson et al., 2000; Persson, 2003) and kept in the dark and at a constant temperature of 15 °C. Microlysimeter experimental units were used according to Hagedorn et al. (2004). They consisted of

Fig. 1. Map of the sampled sites.

1688

F. Buzek et al. / Applied Geochemistry 24 (2009) 1686–1701

a StericupÒ infiltration system 250 or 500 mL (Millipore filter 0.45 lm) enclosed in a doubled 2 L Zip-LockÒ polyethylene bag sealed by CellotapeÒ. Bags were doubled to prevent possible loss of the gas phase by permeation. Blind tests with base solution inside single sealed bags confirmed no CO2 diffusion from the outside atmosphere. A sealed bag with base solution inside has no overpressure of respired CO2 and can leak only by microdifussion through bag foil (a pressure test of the sealed bag for leaks is recommended). The StericupÒ units enabled precise and easy weigh-

ing of added and evacuated water. Enclosing of the infiltration units in plastic bags enabled trapping of respired carbon CO2 into a base solution (beaker with Ba(OH)2 inside the bag) and control analysis of the gas phase above the incubated soil. Samples were incubated for 4 months and sampled four times after a period of 16, 38, 76 and 126 days. The 60% of soil water saturation corresponded to approximately 1-month average precipitation reduced by 55% evaporation. The 100% soil water saturation was defined as the water content of saturated soil allowed to drain up to the sur-

Table 2 Sample site descriptions. Country

Site name

Code

Elevation a.s.l./m

Dominant species

Climate type

Annual precipitation (mm)

Annual mean air temperature (°C)

Soil type (FAO)

Stand age in 2000/years

Sweden

Flakaliden

Fl

310–320

Picea abies

Boreal/oceanic

587

1.9

38

Denmark

Soroe

So

40

Temperate/oceanic

510

8.1

Netherlands

Loobos

Lo

52

Temperate/oceanic

786

9.8

Sandy dunes

80

Germany

Leinefelde

Li

440

7

Cambisol to Luvisol

Tharandt

Th

380

Temperate/continental (subatlantic/submontane) Temperate/continental

700–750

Germany

820

7.5

Brown earth (rhyolith)

Germany

Hainich

Ha

445

750

7

Cambisol

Chronosequence site 30–153 Chronosequence site 5–97 131

Germany

Wetzstein

We

785

France

Hesse

He

300

Italy

Rocca

Ro

120–190

Italy

Collelongo

Co

1550

Fagus sylvatica Pinus sylvestris Fagus sylvatica Picea abies Fagus sylvatica Picea abies Fagus sylvatica Quercus cerris Fagus sylvatica

Spodosol on sandy glacial till of gneissic origin Mollisol

59.7

temperate/continental (subatlantic/submontane) Temperate/continental (subatlantic/submontane) temperate/suboceanic

780

7

Dystric Cambisol

885

9.2

Gleyic luvisol

Chronosequence site 6–160 34

Mediterranean

936

14.4

Cambisol

0–16 coppicea

Mediterranean/montane

1180

6.3

Humic alisol (brown earth)

110

a Coppicing is a traditional method of woodland management in which young tree stems are cut down to near ground level. In subsequent growth years, many new shoots will emerge and after a number of years the coppiced tree is ready to be harvested again.

Table 3 Soil data – sites deciduous forested. Site

C poola (kgC m2)

N poola (kgN m2)

C content (%)

N content (%)

C/N

d13C soil (‰)

14

Li/30/L + F Li/30/0–5 cm Li/30/5–10 cm Li/62/L + F Li/62//0–5 cm Li/62/5–10 cm Li/111/L + F Li/111/0–5 cm Li/111/5–10 cm Li/153/L + F Li/153/0–5 cm Li/153/5–10 cm Ha/L + F Ha/0–5 cm Ha/5–10 cm He/L + F He/0–5 cm He/5–10 cm So/L + F So/0–5 cm So/5–10 cm Ro/L + F Ro/0–5 cm Ro/5–10 cm Co/L + F Co/H Co/0–5 cm Co/5–10 cm

0.379 1.404 1.125 0.359 1.349 1.031 0.359 1.207 0.974 0.368 1.431 1.399 0.289 2.622 1.752 0.450 1.336 0.857 1.320 3.319 3.179 0.780 2.285 1.124 0.855 1.221 2.605 2.149

0.013 0.107 0.089 0.013 0.100 0.089 0.013 0.082 0.075 0.012 0.096 0.100 0.009 0.189 0.152 0.015 0.086 0.062 0.049 0.215 0.206 0.026 0.184 0.098 0.028 0.066 0.171 0.157

40.7 4.5 2.3 34.9 4.8 2.6 41.8 4.3 2.1 43.6 6.6 4.0 43.5 7.2 4.1 34.0 2.9 2.3 39.3 12.5 7.8 42.7 4.6 2.0 39.7 21.6 13.4 10.8

1.40 0.35 0.18 1.25 0.35 0.22 1.47 0.29 0.16 1.39 0.45 0.29 1.41 0.52 0.35 1.10 0.19 0.17 1.45 0.81 0.50 1.45 0.37 0.17 1.32 1.17 0.88 0.79

29.1 13.1 12.7 27.9 13.5 11.5 28.4 14.7 13.0 31.3 14.9 14.0 30.9 13.9 11.5 30.9 15.5 13.9 27.1 15.4 15.4 29.4 12.4 11.5 30.1 18.5 15.2 13.7

27.9 26.8 26.3 28.0 26.3 26.1 28.1 26.8 25.9 27.6 26.6 26.3 27.2 26.4 26.2 28.6 27.2 26.1 28.3 27.3 27.1 27.7 26.8 26.2 26.7 26.1 25.7 25.1

9 60 140 6 100 220 3 90 220 5 90 140 5 75 165 5 55 65 9 80 275

a b

Data from Persson in Schulze (2003). Data from Rowland in Schulze (2003).

C MRTb (years)

1689

F. Buzek et al. / Applied Geochemistry 24 (2009) 1686–1701

face of the soil in the microlysimeter unit without any bubbles. Afterwards, 40% of the water was evacuated and the unit was closed for incubation (Persson et al., 2000). During sampling, 110 mL of acidified water (pH = 4.8) was added to the microlysimeter. After reaching equilibrium (usually 1 h), the added volume of solution was evacuated to keep the remaining water content of the soil in the microlysimeter close to the starting value of 60% saturation. A part of the extracted volume (20 mL) was separated for  þ NO 3 and NH4 analyses. The NO3 content was analysed by ion chromatography (Shimadzu LC-6A, detection limit 0.3 mg 1 NO 3 L ). Ammonium was analysed spectrophotometrically with 1 sum of the a detection limit of 0.02 mg NHþ 4  L . The þ andNH contents was considered to be the dissolved inorNO 4 3 ganic N (DIN). To measure C and N contents of DOM, samples were acidified to remove inorganic C, filtered (0.45 lm) and the filtrate evaporated to produce a dry sample for analysis using a Fisons 1108 elemental analyzer. The error in C content was 5%, in N content 8% of measured values. Calculated DOC concentration was randomly controlled (in 5% of samples) using a DOC analyzer Shimadzu TOC 5000A with a detection limit of 0.1 mg L1. Calculated and directly measured DOC concentrations agreed within a 10% range. The same extracts were used to measure d13C with a standard error ±0.15‰. The C isotopic composition of extracts were measured by sample combustion in a Fisons 1108 elemental analyzer coupled to a Mat 251 mass spectrometer. The dissolved organic N (DON) concentration was calculated by subtraction of the þ NO 3 and NH4 contents from the total N content of DOM. The BaCO3 precipitate was filtered under a N2 atmosphere, washed with distilled water and dried at 105 °C. The resulting weights were used for the calculation of CO2 production. An aliquot of BaCO3 was decomposed by concentrated orthophosphoric acid in vacuum to measure the d13C of CO2 (with ±0.1‰ error). The advantage of the alkali trapping method is in zero partial pressure

of CO2 over the incubation bath, i.e. very low dissolution of CO2 in solution. Random samples were controlled for the gas phase composition in the bags. Gas samples were measured for absence of CO2 and traces of CH4 by gas chromatography (Chrom 5 GC with Porapak Q column and TCD and FID detectors). Controlled gas samples were combusted over a Pd catalyst at 900 °C and the resultant CO2 was measured for the CH4 d13C value. 3.2. Soil solution Soil water samples were collected from zero tension lysimeters at Flakaliden and from suction lysimeters at the Collelongo site. The depth of the lyzimeters at the Flakaliden site was 15 cm, and from 5 to 90 cm at the Collelongo site. Samples for DOC determination were collected in June and October at Flakaliden, and in May at Collelongo. The water samples were stored overnight in a refrigerator at 4 °C and transported for analysis within 2 days. Samples from tension lysimeters were filtered (0.45 lm) prior to analysis, samples from suction lysimeters were used directly. The analysis was performed using a Shimadzu TOC 5000A analyser. The d13C was measured in evaporated (at 50 °C), acidified samples on a Finnigan Mat 251 spectrometer coupled with a Fisons 1108 elemental analyser. The d18O of water in soil solutions was measured by the standard equilibration method (Buzek, 1984). The d18O value of the soil water was used to specify time delays between precipitation events and infiltration to the lysimeters. 3.3. Elemental and isotope measurements in soils Three pooled samples from individual depth horizons were used for the elemental and d13C measurements at each site. Soils were tested for carbonate content by addition of dilute HCl (10%). The pH values were measured only on the samples evolving

Table 4 Soil data – sites coniferous forested. Site

C poola (kg C m2)

N poola (kg N m2)

C content (%)

N content (%)

C/N

d13C soil (‰)

14

Th/5/L + F Th/5/0–5 cm Th/5/5–10 cm Th/24/L + F Th/24/0–5 cm Th/24/5–10 cm Th/42/L + F Th/42/0–5 cm Th/42/5–10 cm Th/97/L + F Th/97/0–5 cm Th/97/5–10 cm Fl/L + F Fl/0–5 cm Fl/5–10 cm Lo/L + F Lo/H Lo/0–5 cm Lo/5–10 cm We/10/L + F We/10/H We/10/0–5 cm We/10/5–10 cm We/60/L + F We/60/H We/60/0–5 cm We/60/5–10 cm We/120/L + F We/120/H We/120/0–5 cm We/120/5–10 cm

1.700 2.075 1.695 0.973 1.976 1.045 1.124 1.881 1.715 1.037 2.249 1.561 0.808 1.800 1.700 2.601 2.231 1.734 0.354 1.200 1.719 2.782 1.403 2.499 2.710 2.263 1.471 1.600 2.456 3.092 2.418

0.066 0.102 0.077 0.040 0.086 0.044 0.046 0.065 0.049 0.043 0.082 0.059 0.020 0.064 0.055 0.107 0.073 0.064 0.015 0.062 0.084 0.122 0.056 0.100 0.105 0.091 0.061 0.079 0.113 0.137 0.090

46.7 5.0 1.9 45.0 9.4 4.3 44.5 7.9 4.0 47.9 6.0 3.5 37.5 2.5 1.2 49.5 31.6 3.9 0.5 45.2 38.1 24.4 8.1 49.4 42.9 8.4 5.3 43.1 33.0 19.6 10.8

1.82 0.24 0.09 1.86 0.41 0.18 1.81 0.27 0.12 1.98 0.22 0.13 0.92 0.09 0.04 2.04 1.04 0.14 0.02 2.35 1.87 1.07 0.32 1.97 1.66 0.34 0.22 2.14 1.52 0.87 0.40

25.7 20.3 22.1 24.2 23.1 23.6 24.5 28.9 35.0 24.2 27.5 26.5 40.5 28.0 31.0 24.3 30.4 27.1 23.3 19.2 20.4 22.8 25.0 25.0 25.8 24.9 23.9 20.1 21.7 22.6 26.9

27.0 26.7 26.4 28.2 26.6 26.2 26.7 26.2 26.1 26.7 26.2 26.3 28.3 26.7 26.6 28.5 27.9 26.8 26.7 27.9 26.8 27.0 26.8 27.5 26.6 26.6 27.0 27.8 27.6 27.5 27.4

2 276 323 2 280 250 20 65 365 15 80 330 7 135 160 20 20 125 165

a b

Data from Persson in Schulze (2003). Data from Rowland in Schulze (2003).

C MRTb (years)

1690

F. Buzek et al. / Applied Geochemistry 24 (2009) 1686–1701

gas. A 0.01 M CaCl2 solution with a soil/solution ratio of 1:2.5 (in weight) was used. Samples with a pH higher than six were acidified with 10% HCl, washed with distilled water and dried at 50 °C. The acid treatment did not change the isotopic composition

of the studied soils as was shown by Midwood and Boutton (1998). The d13C measurements were performed by flash combustion in a Fisons 1108 elemental analyzer coupled with a Mat 251 isotope ratio mass spectrometer in a continuous flow regime. Sample size

Fig. 2. Plots of DOC (A) and CO2 (B) production rates, C/N ratio of DOM (C) and d13C of DOC relative to the d13C values of soil layers (D) vs time of incubation. Data sets were chronosequences, coniferous and decidous soils. Points were calculated as average values, error bars represent ±SE of replicates (n = 3).

1691

F. Buzek et al. / Applied Geochemistry 24 (2009) 1686–1701

was adjusted to contain a sufficient amount of C to obtain external reproducibility of 0.15‰ for all types of samples. The soil data are summarized in Table 3 for deciduous sites and in Table 4 for coniferous sites. 3.4. Data quality Pooled samples from three sample sites were used with two replicate samples in each layer (L + F layer from Soroe, 0–5 cm from Hesse and 5–10 cm from Hainich site). The effect of pooling on sample heterogenity was evaluated by comparing the elemental and isotopic composition of pooled and non pooled samples. For non pooled samples the standard deviation of C and N content determination was from 9% to 10.5%, and 9.8–11% of measured value (three replicates), respectively. Repeated measurement of pooled samples had standard deviations from 4.5% to 7% for C, and from 5.6% to 8% for N content determination. This means that the pooling of samples decreased inhomogeneity inside sampled sites significantly. The d13C measurements had similar standard deviations for both the single and pooled samples from 5% to 8.5% of the measured value. The replicate samples had the highest standard deviation for the first DOC and CO2 estimates (after 16 days of incubation) about 10% below the replicates average. At the end of the experiment (after 126 days of incubation) the differences between replicates were about 11% of DOC and 13% of the measured values of CO2. The estimate of confidence interval for normal distribution and three replicates was calculated from Eq. (1):

pffiffiffi X ¼ Xav g  T a s= 3

3.5. Kinetic analysis The CO2 production decreased linearly with time for the majority of soils. It suggests that the reaction proceeds according to zeroorder kinetics. The DOC concentration decreased exponentially with time of incubation, which suggests first-order kinetics. The reaction is zero-order if the concentration data are plotted versus time and the result is a straight line with the slope of rate constant k0 (Eq. (2)):

½At ¼ k0 t þ ½A0

ð2Þ

where [A]0 represents the initial concentration of the reactant and [A]t concentration at a particular time. Eq. (3) gives the half-life of the zero-order reaction:

T 1=2 ¼ ½A0 =2k0

ð3Þ

Note, that half-life of a zero-order reaction depends on the initial concentration of reactant. Reaction is first-order if a plot of ln[A] vs. time t gives a straight line with a slope equal to the reaction rate constant k1 (Eq. (4)):

ln½At ¼ k1 t þ ln½A0

ð4Þ

where [A]0 represents the initial concentration of the reactant and [A]t concentration at a particular time. The half-life of a first-order reaction is independent of the starting concentration and is given by the Eq. (5):

T 1=2 ¼ lnð2=k1 Þ:

ð5Þ

ð1Þ

where Xavg is the mean value of measurement, s is the standard deviation and Ta equals 4.30 for 95% confidence interval and normal distribution, i.e. standard deviation should be multiplied by 2.5 (Meloun and Milicky, 2004).

3.6. Statistics The TableCurveÒ (SSI) program pack was used for correlation and regression calculation. Correlation coefficient R2 was calcu-

Table 5 Kinetic parameters of incubation experiments for sites forested by deciduous trees: half-lives of reactions and initial concentrations evaluated with regard to C quality (lgC gC1) and C pool (mgC m2). d13C of CO2 was measured as BaCO3 after 38 days of incubation. Site

d13C BaCO3 38 days (‰)

[DOC]0 (lgC gC1)

[CO2]0 (lgC gC1)

[DOC + CO2]0 (lgC gC1)

T1/2 (DOC) (days)

T1/2 (CO2) (days)

T1/2 (DOC + CO2) (days)

[DOC]0 (mgC m2)

[CO2]0 (mgC m2)

[DOC + CO2]0 (mgC m2)

Li/30/L + F Li/30/0–5 cm Li/30/5–10 cm Li/62/L + F Li/62//0–5 cm Li/62/5–10 cm Li/111/L + F Li/111/0–5 cm Li/111/5–10 cm Li/153/L + F Li/153/0–5 cm Li/153/5–10 cm Ha/L + F Ha/0–5 cm Ha/5–10 cm He/L + F He/0–5 cm He/5–10 cm So/L + F So/0–5 cm So/5–10 cm Ro/L + F Ro/0–5 cm Ro/5–10 cm Co/L + F Co/H Co/0–5 cm Co/5–10 cm

42.0 41.4 28.5 31.0 38.4 40.1 42.9 30.3 38.7 30.0 37.0 30.9 25.8 36.7 30.4 34.6 35.7 29.0 36.1 33.3 30.7 28.5 39.6 31.6 34.2 31.2 36.6 35.6

139 341 544 458 389 881 568 653 370 180 141 195 27 117 182 226 205 126 421 134 166 101 247 50 194 49 140 76

1439 423 290 1031 527 256 1454 515 277 1689 426 172 1251 232 243 1569 572 570 1068 53 52 1789 301 182 1154 301 218 123

1466 717 752 1405 921 747 1926 1046 725 1952 541 325 1309 357 340 1738 797 823 1449 183 211 1879 479 182 1322 388 334 194

83 21 21 28 24 16 22 16 23 55 28 23 55 28 23 21 23 23 35 53 45 25 18 41 38 99 19 21

243 113 129 363 111 196 580 102 107 229 115 126 177 130 82 171 97 94 177 180 210 182 95 68 317 95 103 124

341 65 53 169 62 82 173 51 40 103 92 134 184 77 61 156 65 53 124 87 74 189 57 89 253 79 64 65

53 478 612 165 525 908 204 788 361 66 202 273 8 306 318 102 274 108 555 446 526 79 564 56 166 60 364 163

546 595 326 370 710 264 522 622 270 621 609 241 360 608 426 707 764 489 1410 176 165 1396 688 205 987 367 568 265

556 1007 846 504 1242 770 691 1263 706 718 774 454 377 936 595 783 1064 705 1913 606 671 1465 1095 205 1130 474 871 418

1692

F. Buzek et al. / Applied Geochemistry 24 (2009) 1686–1701

Table 6 Kinetic parameters of incubation experiments for sites forested by coniferous trees: half-lives of reactions and initial concentrations evaluated with regard to C quality (lgC gC1) and C pool (mgC m2). d13C of CO2 was measured as BaCO3 after 38 days of incubation. Site

d13C BaCO3 38 days (‰)

[DOC]0 (lgC gC1)

[CO2]0 (lgC gC1)

[DOC + CO2]0 (lgC gC1)

T1/2 (DOC) (days)

T1/2 (CO2) (days)

T1/2 (DOC + CO2) (days)

[DOC]0 (mgC m2)

[CO2]0 (mgC m2)

[DOC + CO2]0 (mgC m2)

Th/5/L + F Th/5/0–5 cm Th/5/5–10 cm Th/24/L + F Th/24/0–5 cm Th/24/5–10 cm Th/42/L + F Th/42/0–5 cm Th/42/5–10 cm Th/97/L + F Th/97/0–5 cm Th/97/5–10 cm Fl/L + F Fl/0–5 cm Fl/5–10 cm Lo/L + F Lo/H Lo/0–5 cm Lo/5–10 cm We/10/L + F We/10/H We/10/0–5 cm We/10/5–10 cm We/60/L + F We/60/H We/60/0–5 cm We/60/5–10 cm We/120/L + F We/120/H We/120/0–5 cm We/120/5–10 cm

34.9 39.0 30.6 28.9 38.4 29.4 40.2 34.0 37.7 31.6 34.8 30.6 34.0 30.4 28.6 28.1 37.6 37.0 37.4 33.1 29.1 40.4 32.4 33.8 40.7 31.8 27.7 33.8 36.8 42.5 39.8

262 191 241 419 21 128 342 200 194 796 64 107 81 124 452 45 157 102 569 278 94 57 70 97 171 22 95 266 43 65 73

1551 106 86 1235 115 150 1250 91 145 1583 114 100 1569 487 309 1290 166 249 341 1085 285 116 55 850 162 81 45 1231 225 95 73

1883 319 235 1858 154 346 1279 337 387 2345 218 259 1689 682 600 1470 326 350 990 1436 359 180 119 981 313 104 154 1521 292 162 135

234 47 20 23 40 25 25 38 38 40 26 36 275 62 28 86 33 40 34 47 25 36 39 37 33 97 35 39 43 49 37

220 81 157 200 91 111 320 85 71 179 84 88 171 139 149 160 187 192 120 479 126 130 266 218 163 146 230 191 115 109 84

231 51 58 102 64 48 359 47 43 114 45 43 187 110 66 215 81 124 52 175 99 80 107 180 81 197 63 147 87 77 53

445 397 409 408 41 134 384 377 333 826 144 166 66 224 768 116 351 177 201 334 161 159 99 243 464 50 140 425 106 201 178

2637 221 146 1202 228 157 1405 172 249 1642 256 157 1266 877 526 3355 370 432 121 1302 490 323 77 2125 439 174 84 1969 553 295 176

3201 661 398 1808 305 362 1438 633 664 2432 491 404 1363 1228 1019 3828 727 606 350 1722 616 500 166 2453 848 234 226 2434 716 502 327

Table 7 Mean annual estimates of DOC, CO2, DON and DIN production for sites forested by deciduous trees. Applied correction coefficients and infiltration amounts used for the calculation of mean DOC concentration are presented. Site Li/30/L + F Li/30/0–5 cm Li/30/5–10 cm Li/62/L + F Li/62//0–5 cm Li/62/5–10 cm Li/111/L + F Li/111/0–5 cm Li/111/5–10 cm Li/153/L + F Li/153/0–5 cm Li/153/5–10 cm Ha/L + F Ha/0–5 cm Ha/5–10 cm He/L + F He/0–5 cm He/5–10 cm So/L + F So/0–5 cm So/5–10 cm Ro/L + F Ro/0–5 cm Ro/5–10 cm Co/L + F Co/H Co/0–5 cm Co/5–10 cm a b c

MAPa (mm)

ACFb

MAIc (mm)

[DOC]365 (gC m2)

[CO2]365 (gC m2)

[DOC]365 (mgC gC1)

[CO2]365 (mgC gC1)

[DON]365 (mgN gC1)

[DIN]365 (mgN gC1)

[DOC]365 (mgC L1)

750

0.30

338

750

0.30

338

885

0.30

398

510

0.37

230

936

0.39

421

1180

0.29

531

3 5 6 2 6 5 2 7 4 2 4 3 0 5 4 1 3 1 15 19 19 1 6 3 4 4 3 2

49 39 24 46 48 18 40 40 16 42 41 19 30 46 20 58 46 30 151 22 22 150 53 11 91 22 35 16

7 3 6 7 5 5 6 6 4 6 2 2 1 2 2 2 2 1 11 6 6 2 2 2 4 3 1 1

128 28 21 129 36 18 113 33 16 115 29 13 104 18 11 128 34 35 114 7 7 193 23 10 106 18 13 8

284 241 401 352 342 376 310 541 352 287 184 163 57 126 159 108 185 119 883 451 451 92 156 196 282 213 102 61

948 1712 2117 1281 1743 1542 2524 1729 1614 1172 2203 1832 1255 1460 1948 1394 2829 1518 2330 569 373 1022 2022 2115 530 116 1825 1164

8 14 19 7 19 16 7 21 13 7 11 9 1 14 12 2 8 3 65 85 83 3 14 6 7 7 6 3

MAP = mean annual precipitation. ACF = annual correction for the field. MAI = mean annual infiltration.

1693

F. Buzek et al. / Applied Geochemistry 24 (2009) 1686–1701

lated supposing normal distribution of data. Significance level P was calculated for P > t, or P > F where t and F are values of Student and Fischer tests. R2 and P values were shown with every calculation. Regression lines were plotted with a 95% significance level interval to illustrate the relationship of samples and the applied regression equation. 4. Results 4.1. DOC and CO2 production The DOC and CO2 production rates during the incubation experiment were plotted together with changes in elemental and isotopic composition of DOM (Fig. 2). The DOC production rates varied from 100 to 600 lg C g C1 d1 after 16 days of incubation to about 60 lg C g C1 d1 after 126 days. The CO2 production rate varied from 1100 lg C g C1 d1 (forest floor values) to 100 lg C g C1 d1 (10 cm depth horizon values) after 16 days of incubation. The range of CO2 production rate decreased to 900 and 60 lg C g C1 d1 after 126 days of incubation for the forest floor and mineral soil samples, respectively. The C/N ratios in DOM were similar to the values of the source soils at the beginning of incubation. With increasing incubation time the C/N ratio decreased to nearly constant values from 15 to 10 at the end of the experiment. The difference between the d13C value of DOC and d13C of soil in the same layer was positive (from 1‰ to 2‰) after 16 days and it was close to zero (from 0.5‰ to 1‰) after 126 days. It means that d13C values of DOC were closer to source organic d13C at the end of incubation.

The microlysimeter unit is a well-defined closed reaction system allowing kinetic analysis of the proceeding reactions and their parameters. Using Eqs. (2)–(5) initial concentrations of DOC and CO2 [A0] and the rate constants k0 and k1 and half-life of the reactions were evaluated. For the majority of the experiment the correlation coefficient R2 was 0.94 or better. The sum of initial values [DOC]0 and [CO2]0 proved to be the most sensitive variable to DOC production. The sum of DOC and CO2 production rates decreased with time exponentially suggesting first-order kinetics. Estimates of initial value [DOC + CO2]0 (calculated from the sum of DOC and CO2) differed from the sum of initial values [DOC]0 and [CO2]0 (calculated from DOC and CO2 separately) within 7% of value or less (Tables 5 and 6). Heterotrophic respiration was the primary but not the only process of CO2 production. The second process was oxidation of microbial CH4. All d13C of BaCO3 were measured after 38 and 76 days of soil incubation (38 day data are in Tables 5 and 6). The d13C values of random samples after 16 and 126 days were similar to the 38 and 76 days data. The BaCO3 includes all CO2 produced during incubation. The d13C values of BaCO3 were generally more negative than would be predicted for simple decomposition of organic matter. Instead of expected values around 26‰ or 28‰, d13C values as negative as 43‰ were measured. Such values resulted from the contribution of CO2 from oxidation of microbial CH4. It means that part of the organic C was reduced first and subsequently oxidized. The gas phase inside the incubation units contained no CO and only tens of ppm of methane and its d13C values were within 30‰ to 22‰. Thus there must have been extensive CH4 reoxidation.

Table 8 Mean annual estimates of DOC, CO2, DON and DIN production for sites forested by coniferous trees. Applied correction coefficients and infiltration amounts used for the calculation of mean DOC concentration are presented. Site

MAPa (mm)

ACFb

MAIc (mm)

[DOC]365 (gC m2)

[CO2]365 (gC m2)

[DOC]365 (mgC gC1)

[CO2]365 (mgC gC1)

[DON]365 (mgN gC1)

[DIN]365 (mgN gC1)

[DOC]365 (mgC L1)

Th/5/L + F Th/5/0–5 cm Th/5/5–10 cm Th/24/L + F Th/24/0–5 cm Th/24/5–10 cm Th/42/L + F Th/42/0–5 cm Th/42/5–10 cm Th/97/L + F Th/97/0–5 cm Th/97/5–10 cm Fl/L + F Fl/0–5 cm Fl/5–10 cm Lo/L + F Lo/H Lo/0–5 cm Lo/5–10 cm We/10/L+F We/10/H We/10/0–5 cm We/10/5–10 cm We/60/L + F We/60/H We/60/0–5 cm We/60/5–10 cm We/120/L + F We/120/H We/120/0–5 cm We/120/5–10 cm

820

0.30

369

587

0.20

264

786

0.30

354

780

0.30

351

29 12 4 6 1 2 8 11 8 20 2 4 4 9 8 6 7 4 4 10 2 3 2 6 8 4 3 10 3 6 4

232 11 12 103 13 11 133 9 14 137 13 9 69 43 27 277 32 37 9 123 35 24 7 187 35 15 6 168 38 19 9

17 6 2 7 1 2 7 6 5 19 1 2 5 5 5 2 3 2 12 9 1 1 2 2 3 2 2 7 1 2 2

136 5 7 106 7 10 118 5 8 132 6 6 85 24 16 106 14 22 24 103 20 9 5 75 13 7 4 105 15 6 4

1,829 378 106 309 38 102 485 351 311 1,357 62 171 248 223 298 180 170 189 902 407 88 84 119 189 233 110 140 444 76 134 112

2,249 222 161 1,073 274 169 2,384 276 268 1,826 172 156 143 40 141 538 322 1,313 818 591 476 500 218 1,236 203 175 147 1,295 220 171 83

77 32 10 17 3 5 21 30 21 55 6 10 15 33 30 17 19 12 12 29 6 9 7 17 24 10 9 30 8 18 11

a b c

MAP = mean annual precipitation. ACF = annual correction for the field. MAI = mean annual infiltration.

1694

F. Buzek et al. / Applied Geochemistry 24 (2009) 1686–1701

Annual average production rates of DOC and CO2 were estimated from the cumulative flux data. They are linear in time – the CO2 flux with a correlation coefficient R2 0.97 or better and (CO2 + DOC) flux with a correlation coefficient R2 0.95 and better. Linear correlations of CO2 and (DOC + CO2) fluxes with time were extrapolated in time to 1-year value and average CO2 and DOC production rates were calculated. Averages were calculated with respect to C quality i.e. in mg of C/g of soil C, and with respect to the soil C pool, i.e. in g of C/unit area. Extrapolation of the experimental data to the field is difficult due to dissimilarity of sites and their climatic conditions. The method of Persson et al. (2000) was adopted because some of their measured sites were evaluated in this study. Based on additional measurements of temperature and soil water content at the site a correction factor for the site was calculated. Experimental production rates were corrected by factor multiplication. The value of the correction factor varied from 0.2 (Flakaliden site) to 0.39 (Rocca site) (Tables 7 and 8). The annual average DOC and CO2 losses from measured sites are summarized in Table 6 for deciduous sites and in Table 7 for coniferous sites. The annual losses of DOC and CO2 varied widely. The maximal DOC loss was 29.2 gC m2 in (Th5 L + F) of the Tharandt site and 19.4 mgC gC1 in (Th95 L + F), the minimal DOC loss was 0.3 gC m2 in the Hainich site (L + F) and 0.8 mgC gC1 in the Collelongo plot (5–10 cm layer). The variation in the CO2 losses was even wider. The maximum losses were 276.7 gC m2 in (L + F) of the Loobos site and 192 mgC gC1 in (L + F) of the Rocca site. The minimum losses were 6.4 gC m2 in the 5–10 cm layer of the

Wetzstein 60 plot and 3.9 mgC gC1 in the 5–10 cm layer of the Wetzstein 120 plot. 4.2. DON and DIN production The DON production rates varied from 30 to 5 lgN gC1 d1 after 16 days of incubation to constant values from 5 to 2 lgN gC1 d1 after 126 days at the end of experiment. The DIN production rates were nearly constant during the experiments with values ranging from 20 to 4 lgN gC1 d1 (Fig. 3). Cumulative DON and DIN production gives an estimate of annual average production. Annual cumulative DIN flux was linear in time (R2 0.96 and better) as well as (DIN + DON) flux (R2 0.93 and better). The cumulative DON flux itself does not correlate well with time because of a significant decrease during first 2 months of incubation. Linear correlations of DIN and (DIN + DON) fluxes with time were extrapolated in time to a 1 year value to obtain mean annual DIN and DON productions. Mean values are given in lg of N/g soil C, (reflecting C quality effect – Tables 7 and 8). The experimental data on DIN and DON were extrapolated to the field conditions using the same correction factors as for C mineralisation (Persson et al., 2000). Correction factors convert mineralisation rates obtained in the laboratory at 15 °C and 50–60% of water saturation content to mineralisation rates in the field. The average DON loss per year varied from 1358 lgN gC1 in (L + F) of the Tharandt 95 site to 57 lgN gC1 in (L + F) of the Hainich site. The DIN losses were in the range from 2830 lgN gC1 in the 0–5 cm layer of the Flakaliden site to 40 lgN gC1 in the 0–5 cm layer of the Hesse site.

Fig. 3. Time plot of DON (A) and DIN (B) production rates. Data sets were chronosequences, coniferous and decidous soils. Points were calculated as average values, error bars represent ±SE of replicates (n = 3).

F. Buzek et al. / Applied Geochemistry 24 (2009) 1686–1701

4.3. Lysimeter DOC DOC concentration, d13C of DOC and d18O of water were measured in soil water collected in lysimeters of Flakaliden and Collelongo. The DOC concentrations in tension lysimeters in Flakaliden varied from 25 mg L1 in June to 30 mg l1 in October, with d13C value between 28‰ and 29‰. The values of d13C were quite close to the 28‰ of the litter values (Fig. 4). The d18O in lysimeter water was 13.3‰ in June and it changed to a more positive value

1695

of 10.8‰ in October. The d18O value 13.3‰ corresponds to melted snow and 10.8‰ corresponds to early summer precipitation in the area. It means that the time delay between precipitation and infiltration to lysimeters was about 2 months. The DOC in the soil water at the Collelongo site was sampled in early May. Water with short transit time in the soil profile was collected in zero tension lysimeters from 5, 15 and 25 cm depth. This shallow water contained DOC from the litter of the forest floor. The deeper soil water from 55 to 90 cm suction lysimeters with

Fig. 4. Concentration (A) and d13C (B) of DOC measured in the field and estimated from incubation experiment. Error bars represent ±SE of replicates of field samples (n = 2). Arrows indicate range of DOC concentration estimates. Variation of the d18O values in lysimeters is presented (C).

1696

F. Buzek et al. / Applied Geochemistry 24 (2009) 1686–1701

longer transit time contained DOC from both the organic and mineral horizons. The difference in the transit time was reflected by

the d18O of water. The d18O was 10‰ at 5 cm and 7.5‰ at 90 cm. The less negative d18O of water corresponds to late sum-

Fig. 5. Comparison of half-lives of CO2 and (DOC + CO2) production from incubated L + F layers with half-lives of litter decay of deciduous measured on sites. Data on litter decay are from Cotrufo (2003). Regression equations (bold dashed lines) are presented with correlation coefficient and significant probability level P values. Arrows indicate range of 95% significance level of regression (fine dashed lines).

Fig. 6. C pool specific regression for initial [CO2]0 (A) and [DOC + CO2]0 (B). Chronosequence sites were used as single points with average values. Data on C pool are from Persson (2003). Regression equations (bold dashed and dotted lines) are presented with correlation coefficient and significant probability level P values. Arrows indicate range of 95% significance level of regression (fine dashed lines).

F. Buzek et al. / Applied Geochemistry 24 (2009) 1686–1701

mer precipitation. The more negative d18O value corresponds to early spring precipitation. Soil water contained DOC concentrations from 2 to 6 mg L1. The d13C value of DOC in soil water varied from 23‰ to 27.5‰. The d13C of DOC in shallow lysimeters was close to the d13C of litter, i.e. more negative. The d13C of DOC at 55 cm depth was less negative than in shallow lysimeters. The DOC from the deepest lysimeter at 90 cm had d13C values similar to shallow lysimeters. This DOC was probably leached from the litter layer by infiltrated precipitation during the previous summer. Arrows on Fig. 4 indicate a range of DOC estimates extrapolated from the incubation data. In the case of the Collelongo and Flakaliden sites the same estimates of d13C were obtained from incubation experiments as measured in lysimeter water.

5. Discussion Results of the kinetic analysis of incubation experiments can be compared with results of kinetic analysis of litter decay measured directly on deciduous sites by Cotrufo (Schulze, 2003) – see Fig. 5. Half-lives of reaction from incubation experiments were compared with half-lives of litter decay on the site. The best correlations between litter decay and DOC and CO2 production were obtained for half-lives estimated from [CO2] and [DOC + CO2] data (R2 equal 0.98 and 0.89). Half-lifes estimated from [DOC] only did not correlate with litter decay. This means that for deciduous sites the rates of DOM degradation during incubation are highly proportional to the actual rates of litter decay. At 15 °C and experimental conditions, the rate of DOM decomposition is from 5 to 7 times higher than annual litter decomposition rate on the site.

1697

Biodegradation of DOM encompasses two alternative or sequential processes: (i) the breakdown and transformation of DOM to products that can be used as precursors for the biosynthesis and (ii) a complete mineralization to obtain energy and inorganic nutrients (Marschner and Kalbitz, 2003). The processes are manifested by a decline in DOC and CO2 production. Using the data an effect of microbiota on the residual DOC can be traced. Kinetic analysis of the DOC consumption and CO2 production give estimates of the concentration of DOC and CO2 production at the beginning of the incubation experiment, when the degradation of DOM started. Such kinetic analysis of DOC degradation by measuring of CO2 production was used for example by Kalbitz et al. (2003) to differentiate labile and stable DOC pools. As the half-life of the degradation of the DOC pool in the present experiments was evaluated in tens of day only, it is the labile pool degradation that is included in the estimate of the initial CO2 production. The initial concentrations [DOC]0, [CO2]0, [DOC + CO2]0 and the extrapolated values after 1 year of degradation [DOC]365, [CO2]365, [DON]365 and [DIN]365 were compared with soil and site-specific properties of individual sampled sites. The soil-specific properties were C pool size and C mean residence time (MRT). The site-specific properties were mean annual temperature of the site (MAT), tree species and age of trees. 5.1. Effect of the carbon pool size The initial CO2 and DOC evaluated as [CO2]0 and [DOC + CO2]0 correlate well with the litter C pool in all sites (Fig. 6). Plots are separated into deciduous trees and conifers. The larger the C pool size is, the more litter and the labile organic matter is available

Fig. 7. C pool specific regression for annual production estimates of [DOC]365 (A) and [CO2]365 (B). Chronosequence sites were used as single points with average values. Data on C pool are from Persson (2003). Regression equations (bold dashed lines) are presented with correlation coefficient and significant probability level P values. Arrows indicate range of 95% significance level of regression (fine dashed lines).

1698

F. Buzek et al. / Applied Geochemistry 24 (2009) 1686–1701

for dissolution and mineralization. The [DOC]0 values themselves did not correlate with C pool size. In spite of low drying temperature, the [DOC]0 estimates could be affected by the results of the first extraction. During soil sample drying and re-moistening, part of the lysed bacteria could be washed out into DOM thus increasing the [DOC] value. Consequently, the loss of bacteria might suppress microbial activity (Zsolnay et al., 1999). However, recently Akagi et al. (2007) found that pre-incubation of dried soils preceding the sampling of DOM can turn microbial populations back to their standardized state. Correlation of the [CO2]0 and [DOC + CO2]0 values of the mineral layers 0–5 cm (Fig. 6) as well as 5–10 cm (not plotted) with the C pool is negative. In the case of mineral layers the higher C pool does not mean more easy degradable C for dissolution and mineralization. The annual mineralization loss [CO2]365 estimates correlated positively with litter pool (Fig. 7). The [DOC]365 estimates correlated positively with the litter pool of the deciduous sites only. Mineral pool 0–5 cm and 5–10 cm did not correlated with C pool size. 5.2. Effect of the carbon residence time Mean residence time (MRT) of the soil C estimated from 14C content is an important characteristic of the C pool encompassing time processes of the C accumulation and transformation. The data on MRT are taken from Rowland (2003). To eliminate possible ef-

fects of the soil type on MRT chronosequence sites were evaluated first. The variations in the initial [DOC] and [CO2] with the MRT in soils under coniferous (Tharandt site) and decidous trees (Leinefelde site) of different ages are plotted in Fig. 8. The diagrams show that the age of trees does not influence the values of the initial DOC and CO2 significantly. The [DOC]0 in soils with very low MRT vary over a wide range from 100 to 600 lg gC1 and [CO2]0 varies from 1000 to 1800 lg gC1. With increasing MRT the variability in the concentrations is reduced. Both the concentrations are between 100 to 700 lg gC1 at the Leinefelde site and between 0 and 250 lg gC1 at the Tharandt site. There is no obvious trend in the initial concentration of DOC with increasing MRT at both sites. The initial concentration of CO2, on the other hand drops sharply with increasing MRT in all plots. The best correlation between [CO2]0 and C MRT was obtained for the exponential regression (R2 = 0.86, P 0.02 for Leinefelde, R2 = 0.89, P 0.05 for Tharandt site). Carbon MRT and annual C mineralization [CO2]365 of chronosequences are plotted in Fig. 9. Whereas annual loss of DOC decreases irregularly with MRT the annual mineralization loss can be fitted with an exponential decrease with MRT. For all sites both the initial and annual C mineralization loss decreased with C MRT (Fig. 10). There is a principal difference between the trends in DOC and C mineralization with ‘‘age” of C. The initial and annual mineralization decreased with the MRT of C. It is due to a decline in available labile organic matter within the soil profiles. The residual concen-

Fig. 8. Plot of initial [DOC]0 and [CO2]0 production on C mean residence time for Leinefelde (A) and Tharandt (B) chronosequence sites. Examples of [CO2]0 exponential regression are presented. Data on C mean residence time are from Rowland (2003). Regression was calculated from all stands on the site. Regression equations (bold dashed lines) are presented with correlation coefficient and significant probability level P values. Arrows indicate range of 95% significance level of regression (fine dashed lines).

F. Buzek et al. / Applied Geochemistry 24 (2009) 1686–1701

tration of DOC does not change with the MRT of C in spite of the increase in MRT, which includes a complex transformation of litter

1699

to more complex organic compounds. It infers that the measured DOC in the incubation experiments is not primary derived from

Fig. 9. Plot of annual [CO2]365 production estimates on C mean residence time for Leinefelde and Tharandt chronosequence sites. Data on C mean residence time are from Rowland (2003). Regressions were calculated from all stands on the site. Regression equations (bold dashed lines) are presented with correlation coefficient and significant probability level P values. Arrows indicate range of 95% significance level of regression (fine dashed lines).

Fig. 10. Plots of initial [CO2]0 (A) and annual [CO2]365 (B) production on C mean residence time for all sites. Data on C mean residence time are from Rowland (2003). Regression equations (bold dashed and dotted lines) are presented with correlation coefficient and significant probability level P values. Arrows indicate range of 95% significance level of regression (fine dashed lines).

1700

F. Buzek et al. / Applied Geochemistry 24 (2009) 1686–1701

the labile C pool which is preferentially consumed by mineralization. The remaining DOC originates from the decomposition of complex organics present in all tested soil layers. This result agrees with recent conclusions by Hagedorn et al. (2004), that DOC is produced during an incomplete decomposition of recalcitrant native C in the soils, whereas microbes rapidly consume easily degradable organics. In this way the DOC production is related to overall degradation or stabilization of organic matter in the soil. The older soil produced less DOC per available organic matter. This is supported by the DOC/DON ratio and d13C values of DOC during the incubation experiment. At the end of the incubation experiment the DOC/DON ratio was constant and d13C values were close to or more negative than source soil corresponding to controlled decomposition of lignin-derived compounds (Fig. 2). 5.3. Effect of annual temperature The mean annual temperature (MAT) and duration of growing season have a major influence on mineralization rates. Temperature and water content in the soil control accumulation and decomposition of the soil organic matter. The relationship between initial [DOC + CO2]0 in the litter layer and MAT of the site is shown in Fig. 11a. The initial DOC and the mineralization of labile organic C increased with increasing MAT up to about 10 °C. They stay constant above 10 °C up to 14 °C at the Rocca site. The coniferous Flakaliden and Loobos sites differ significantly from that behaviour. The very high [DOC + CO2]0 in the Flakaliden site with the lowest MAT 1.9 °C resulted from a low temperature effect. Lower temper-

ature favours photosynthesis rather than decomposition (Trumbore, 1993). It results in a higher accumulation of humus in colder climates and a higher C/N ratio (see Table 4). This is because of less complete decomposition of plant material. A short growing period and low soil temperature retarded the decomposition and transformation of soil organic matter there. Higher temperature of incubation speeds up the decomposition and releases high amounts of DOC and CO2. The decomposition in the Loobos site is affected by the soil properties of sand dunes. A fast infiltration of precipitation promotes transport of DOM from the forest floor to mineral soil. It results in exceptionally low forest floor initial concentration of DOC and CO2. The mean annual temperature affects the size and quality of the litter pool of C (Trumbore, 1993). There is a positive correlation between C pool size and the MAT (Fig. 11b). The labile pool mineralization indicated by [CO2]0 that formed the significant part of [DOC + CO2]0 increases with MAT. 5.4. Effect of tree species The soils under coniferous stands produced more [DOC]0 than soils under decidous stands. The mineralization rate is comparable for both types of litter, however the degradation of [DOC]0 in litter from deciduous trees is faster than in the litter from coniferous trees (Tables 5 and 6). Chronosequence stands with the same type of soils were considered only to evaluate possible tree age effect on DOC production. In the Leinefelde chronosequence the higher [DOC]0

Fig. 11. Effect of mean annual temperature on initial [DOC + CO2]0 production in litter layer with respect to C quality (A) and C pool size (B). Data on MAT are from Schultze (2003). Regression equation (bold dashed line) is presented with correlation coefficient and significant probability level P values. Fine dashed lines define 95% significance level of regression.

F. Buzek et al. / Applied Geochemistry 24 (2009) 1686–1701

concentration in the forest floor (568 lgC gC1) is in the 111 a old stand. Stands in Leinefelde do not differ in the C pool size the for L + F layer, however the 111 a old stand had the lowest CO2 production rate. The Tharandt chronosequence having the higher [DOC]0 concentration is in the oldest (97 a) stand (796 lgC gC1). This stand produced the highest C pool. Tree age affects the C pool size rather than DOC production. A stand with young spruce, 5 a old (Th 5), produced rich under storey and a high labile pool in the forest floor. Previous clear-cutting stimulated N dissolution and mineralization (Table 8). 5.5. DOC and CO2 loss estimates The estimates of DOC and CO2 annual loss given should be considered as underestimates. This is because during the sieving of samples roots and litter particles above 2 mm were removed. In the case of heterotrophic respiration, such an underestimation could be about 20–30% of total CO2 production (Persson et al., 2000). The residual DOC is not much affected by the labile C pool size and underestimation would be significantly lower than for CO2 production. In contrast to the closed system conditions of the incubation experiments, the forest soil in situ is an open system with mass transport through its boundaries. Average values of DOC and CO2 losses evaluated at quasi-steady conditions can be affected by the rate of solution transport. The transit time of solute in the 5 cm soil layer is 3 weeks at the average infiltration rate or 3 months at low infiltration conditions. Resulting DOC degradation and CO2 production react to the differences in the infiltration rate differently. The fast labile pool mineralization is not significantly affected, while the DOC loss with the half-life of several tens or hundreds of days depends on the infiltration rate. Mean annual loss of DOC was evaluated as a mean amount transported out of the layer due to annual infiltration. Mean annual value is not affected by the immediate infiltration rate too much. In fact, the shorter residence time of DOC in the soil layer does not automatically mean a lower [DOC]. The [DOC] after a short incubation period is higher than after a longer incubation period. This results in higher [DOC] in the soil with a shorter residence time. 6. Conclusions DOC, d13C of DOC and mineralization data were determined in incubation experiments of soils from ten coniferous and deciduous sites along European north–south transect. Results of incubation experiments yield estimates of the concentration of DOC in soil solution and its loss by mineralization. Good concordance was obtained between experimental and field DOC concentrations and their d13C values in two sites. Carbon pool size and C quality affect the DOC and CO2 production. The DOC mineralization is derived from the availability of labile forms of DOC. Production of CO2 increases with the litter pool size and decreases with the size of the C pool in subjacent mineral layers. Residual DOC does not correlate with the size of the C pool. Only soils very rich in organics produced elevated DOC concentrations. The CO2 production is a function of C mean residence time; CO2 production decreased exponentially with C age. The residual DOC is not affected by the C age. This difference implies that DOC originates predominantly from the decomposition of complex structures, whereas CO2 is produced from easy degradable labile components. At low mean annual temperatures, increase in mean annual temperature increases (DOC + CO2) production from the litter,

1701

but above a mean annual temperature of 10 °C production does not increase. Soils with extreme accumulation or loss of C do not lie on the same trend. Tree age does not affect the (DOC + CO2) production. Carbon pool size had the dominant effect on the (DOC + CO2) production. Acknowledgements We thank Tryggve Persson, Francesca Cotrufo and Phil Rowland for sharing unpublished data from the FORCAST Report. Simon Bottrell as the Editor and two anonymous reviewers are gratefully acknowledged for their useful and constructive comments. The EC Contract EVK2-CT 1999-00035 and the Research Centrum Project No. 1M0554 supported this study. References Akagi, J., Zsolnay, A., Bastide, F., 2007. Quantity and spectroscopic properties of soil dissolved organic matter (DOM) as a function of soil sample treatments: airdrying and pre-incubation. Chemosphere 69, 1040–1046. Buzek, F., 1984. A rapid procedure for preparing oxygen-18 determination in water samples. Isotopenpraxis 19, 70–72. Cotrufo, F., 2003. C/N transfer from litter to SOM. In: Schulze, E.-D., (Ed.), FORCAST Final Report. Max-Planck Institute for Biogeochemistry, Jena. pp. 33–42. . Dienes, P., 1980. Reduced carbon. In: Fritz, P., Fontes, J.Ch. (Eds.), Handbook of Environmental Isotope Geochemistry. Vol. I, The Terrestrial Environment, A. Elsevier Scientific Publishing Company, New York, pp. 329–406. Guggenberger, G., Kaiser, K., 2003. Dissolved organic matter in soil: challenging the paradigm of sorptive preservation. Geoderma 113, 293–310. Hagedorn, F., Sauer, M., Blaser, P., 2004. A 13C tracer study to identify the origin of dissolved organic carbon in forested mineral soils. European Journal of Soil Science 55, 91–100. Kalbitz, K., Schmerwitz, J., Schwesig, D., Matzner, E., 2003. Biodegradation of soilderived dissolved organic matter as related to its properties. Geoderma 113, 273–291. Kalbitz, K., Schwesig, D., Rethemeyer, J., Matzner, E., 2005. Stabilization of dissolved organic matter by sorption to the mineral soil. Soil Biology & Biochemistry 37, 1319–1331. Lundstrom, U.S., van Bremen, N., Iongmas, A.G., 1995. Evidence for microbial decomposition of organic acids during podsolization. European Journal of Soil Science 46, 489–496. Marschner, B., Kalbitz, K., 2003. Controls of bioavailability and biodegradability of dissolved organic matter in soils. Geoderma 113, 211–235. McDowell, W.H., Zsolnay, A., Aitkenhead-Peterson, J.A., Gregorich, E.G., Jones, D.L., Jödemann, D., Kalbitz, K., Marschner, E., Schwesig, D., 2006. A comparison of methods to determine the biodegradable dissolved organic carbon from different terrestrial sources. Soil Biology & Biochemistry 38, 1933–1942. Meloun, M., Milicky, J., 2004. Statistical Analysis of Experimental Data. Academia, Prague (in Czech). Michalzik, B., Kalbitz, K., Park, J.-H., Solinger, S., Matzner, E., 2001. Fluxes and concentration of dissolved organic matter—a synthesis for temperate forests. Biogeochemistry 52, 173–205. Midwood, A.J., Boutton, T.W., 1998. Soil carbonate decomposition by acids has little effect on d13C of organic matter. Soil Biology & Biochemistry 30, 1301–1307. Neff, J.C., Asner, G.P., 2001. Dissolved organic carbon in terrestrial ecosystems: synthesis and a model. Ecosystems 4, 29–48. Persson, T., 2003. C/N pools, bulk density. In: Schulze, E.-D. (Ed.), FORCAST Final Report. Max-Planck Institute for Biogeochemistry, Jena. pp. 50–53. . Persson, T., Karlsson, P.S., Seyferth, U., Sjoberg, R.M., Rudebeck, A., 2000. Carbon mineralisation in European forest soils. In: Schulze, E.-D. (Ed.), Carbon and Nitrogen Cycling in European Forest Ecosystems. Ecol. Stud, vol. 142. SpringerVerlag, Berlin, Heidelberg, New York, pp. 257–275. Raich, J.W., Schlesinger, W.H., 1992. The global carbon dioxide flux in soil respiration and its relationship to climate. Tellus 44B, 81–99. Rowland, A.P. 2003. Mean residence time of SOM. In: Schulze, E.-D. (Ed.), FORCAST Final Report; Max-Planck Institute for Biogeochemistry, Jena. pp. 63–66. . Schulze, E.-D. (Ed.), 2003. FORCAST Final Report. Max-Planck Institute for Biogeochemistry, Jena. . Trumbore, S.E., 1993. Comparison of carbon dynamics in tropical and temperate soils using radiocarbon measurements. Global Biogeochemical Cycles 7, 275– 290. Zsolnay, A., Baigar, E., Jimenez, M., Steinweg, D., Saccomandi, F., 1999. Differentiating with fluorescence spectroscopy the sources of dissolved organic matter in soils subjected to drying. Chemosphere 38, 45–50.