Introduction

Dead trees are associated with many key functions in forest ecosystems (Harmon and Hua 1991). Because water soluble and nutrient-rich organic compounds are abundant in coarse woody debris (CWD), interstitial water and leachate (Yavitt and Fahey 1985), it has ben suggested that CWD decomposition and leaching may affect soil organic matter (e.g., Hafner et al. 2005). However, Laiho and Prescott (2004) found CWD to be of minor importance in the nutrient cycles of northern coniferous forests. More recently, CWD effect on organic matter composition in the mineral soil was shown to diminish rapidly with depth (Krzyszowska-Waitkus et al. 2006). Also, thus far, a significant CWD-produced soil organic carbon (SOC) increase has not been reported. The absence of SOC imprint may relate to limited availability of adsorption sites, as soil particle surfaces eventually become saturated (Mayer 1994). Therefore, our search focused on andic soils, whose adsorption properties in relation to various compounds have been intensively investigated (e.g., Tanikawa and Takenaka 1999). With regard to SOC, andic soils contain allophanes and amorphous Fe-compounds as important SOC-binding agents (Shoji et al. 1993), which contribute to the formation of highly stable metal–humus–clay complexes. In addition, additional andic soils properties make them suitable candidates for possible detection of a quantitative, CWD-produced SOC increase: Andosols are characterized by uniform solute transport (Magesan et al. 2003) and physical protection of SOC inside soil aggregates (Chevallier et al. 2010). In addition, the abundance of polygalacturonase-producing fungi causing wood pectin degradation (Sato 1976; Green et al. 1996) could lead to a faster than usual decomposition of the downed logs of beech. It has, therefore, been our hypothesis that these characteristics would facilitate the formation of an at least temporary CWD-produced organic carbon increase in soil with beech CWD.

Materials and methods

Study area

Our investigation was conducted in Vtáčnik, an important and strict beech (Fagus sylvatica L.) primeval forest preserve located in the Tertiary Volcanic Range of the West Carpathian Arc, Slovakia (48º37′32″N, 18º38′49″E, 1,100 m a. s. l.). The area falls into a cool mountainous and humid region with mean annual temperature and precipitation, averaging 3.0 °C and 950 mm (1961–1990), respectively (Faško and Štastný 2002; Šťastný et al. 2002). Local soil developed on andesite slope deposits. The soil unit according to FAO (2006) is Dystric Andosol with a clayey loam texture. The clayey fraction represents ca. 20–29 % of the fine earth mass. The amorphous/crystalline Fe2O3 ratio is high (0.75) within the top 50 cm and the percentage of allophane in the clayey fraction increases with depth from ca 0.30 to 0.34 (Šály and Mihálik 1977). The surface humus form is moder with a litter horizon (OL), a fragmented horizon (OF), and a thin humus horizon (OH). Three downed logs of beech were selected within the forest segment the age of which varies from 180 to 210 years. All selected logs were at the stage of transiting from decay class (DC) 4 to DC 5 sensu Maser et al. (1979) and Sollins (1982). This means that the logs were resting on the ground, devoid of bark and twigs, and having a round to oval shape. Their texture contained some blocky pieces and their wood was soft, but without powdery structure. The log fragments were 6–12 m long and 50–60 cm in diameter.

Soil sampling and analyses

Three soil profiles were opened beneath the downed logs in the plane perpendicular to their axial orientation. Each soil profile with CWD was 0.6 m deep and paired with another soil profile without CWD, ca. 3–5 m apart. Stoniness within the concerned soil depth (0–60 cm) was determined as the sum of: (1) the relative volume of stones (20–200 mm) and boulders (>200 mm), which is proportional to their relative area on the profile wall in soil pits (Folk 1951; Alexander 1982), and (2) the volume of gravel (2–20 mm). The latter volume was measured in undisturbed soil samples (200 cm3), also collected for the determination of fine earth bulk density from 5 depths (0–10, …, 40–50 cm).

Soil samples for organic carbon content determination, weighing approx. 100 g were collected from 0.1 m soil layers (0.0–0.1 m, …, 0.4–0.5 m) along three vertical lines, 0.2 m apart. The central sampling line intersected the respective longitudinal axis of each downed log. A total of 90 samples were collected. Soil samples were air-dried, ground, and passed through a 2-mm mesh sieve. Visible plant residues were removed manually with a pair of tweezers and by electrostatically charged stick. The C content in the fine earth (<2 mm) was determined by Vario MACRO Elemental Analyzer (CNS Version; Elementar, Hanau, Germany), which employs the dry combustion method. Because we knew the bulk density and volume of the soil (i.e. fine earth plus soil pores, without coarse fragments with diameter >2 mm), SOC stock was computed by summing the C content in all five 0.1-m layers, separately for the topsoil (0.0–0.3 m) and subsoil (0.3–0.5 m) at each sampling point according to

$$ {\text{SOCS}} = \sum\limits_{i = 1}^{j} {{\text{BD}}_{i} } \times {\text{SOCC}}_{i} \times d_{i} \times (1 - {\text{cf}}_{i} ) $$
(1)

where SOCS, i, j, BD i , SOCC i , d i , and cf i are, respectively, SOC stock (kg m−2), respective soil layer index, number of soil layers within topsoil and subsoil (j = 3 or j = 2, respectively), soil bulk density (kg m−3), SOC concentration (kg kg−1), i-th soil layer thickness (d i  = 0.1 m for all i), and volumetric fraction of gravel, stones, and boulders (m3 m−3).

Data evaluation

Exponential function y = a e −kx, where a, k represent parameters affecting location and concaveness, respectively, was fitted on the SOC concentration (y) versus soil depth (x) relationship. The Shapiro–Wilck test (Shapiro and Wilk 1965) and Anderson–Darling test (Snedecor and Cochran 1991) were used to assess whether sampled SOC concentration data followed normal or log-normal distribution. If the sample distribution significantly deviated from normal towards log-normal distribution, log-normal transformation was applied. As a result, SOC concentration and SOC stock differences between soils with and without CWD were tested by Student’s t test (Sokal and Rohlf 1995) on log-transformed or original data, respectively.

Results

Average soil bulk density and stoniness, as important variables that affect SOC stock, are given in Table 1. With regard to these properties, no significant differences between soils underneath and next to downed logs were detected. Besides showing the complete SOC concentration dataset, Fig. 1 also displays SOC concentration (y) versus soil depth (x) relationship, fitted by exponential equations

Table 1 Student’s t test of differences in bulk density and stoniness between bare soils and soils underneath coarse woody debris (CWD)
Full size table
Fig. 1
figure 1

Soil organic carbon concentration decrease with depth in the Vtáčnik Andosol with and without coarse woody debris (CWD), fitted by exponential function

Full size image
$$ y = 0.24 {\text{e}}^{ - 1.97x} $$
(2)

(Eq. 2: soil with CWD)

and

$$ y = 0. 20 {\text{e}}^{ - 2. 10x} $$
(3)

(Eq. 3: soil without CWD)

Mean squared error (MSE) for Eqs. 2 and 3 was 4.4 × 10−3 and 3.7 × 10−3, respectively.

A good match between organic carbon concentration data from soil with CWD and the log-normal distribution (P = 0.51), but a disagreement with the normal distribution (P = 0.05), was suggested by the Anderson–Darling test and Shapiro–Wilck test (P = 0.01). Our assessment of the differences between organic carbon concentrations in soils with and without CWD by the t test, applied to log-transformed data (Table 2), indicated that CWD effect on SOC concentration persisted within the top 0.5 m layer. After inserting SOC concentration data, soil bulk density, and stoniness into Eq. (1), we obtained SOC stock values (also Table 2). The corresponding organic carbon content differences between soils with CWD and without CWD were statistically significant for both topsoil and subsoil.

Table 2 Student’s t test of differences in soil organic carbon concentration and stock between bare soils and soils underneath coarse woody debris (CWD)
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Discussion

In terms of SOC concentration and its change with depth, our data are very similar to the results reported for Andosols in other parts of the world, e.g., in Costa Rica (Aran et al. 2001) and Ecuador (Tonneijck and Jongmans 2008). Small absolute values of the k (exponential) coefficients in both Eqs. 2 and 3 and an almost linear appearance of the relationship is in contrast with the typically exponential decrease of SOC concentration with soil depth (e.g., Bernoux et al. 1998). One reason for such a moderate non-linearity of the SOC decrease probably consists in the rising concentration of allophane and Fe-compounds with soil depth. At the same time, MSE is bigger for Eq. 2, apparently owing to pronounced log-normality of SOC distribution in the topsoil (0.0–0.3 m), with several values approaching 0.30 kg kg−1.

Remarkably, the differences between organic carbon concentrations in soils with and without CWD at Vtáčnik are in sharp contrast to results obtained during similar investigations in non-andic soils. For instance, Busse (1994) and Spears et al. (2003) detected only topsoil-limited (e.g., within 0.00–0.05 m), small or no organic carbon increase in soil with CWD in temperate forests. In fact, the respective differences at Vtáčnik are several times larger than the long-term land use history effects on forest soils (e.g., Stevens and van Wesemael 2008). Data on SOC contents in soils underneath downed logs are extremely rare, but Kahl (2011) found no difference in organic carbon pools in soil with CWD and control (0.0–0.2 m). However, his isotopic study based on δ13C showed that the dead wood decay led to the formation of a clear isotopic footprint in the soil with CWD, in that a portion of the SOC under dead wood had been exchanged. Our observations indicate that such qualitative imprint may materialize into a quantitative imprint in the presence of mineral surfaces, protecting SOC from intense respiration, such as in andic soils. When considered in SOC stock terms, fluctuating stoniness and soil bulk density values must be considered. Otherwise, intrinsic connections between SOC concentration and stoniness (Schaetzl 1991; Tate et al. 2005; Ahmed et al. 2012) can obscure the true CWD effect. In our case, the calculated SOC stock difference between soil with CWD and without CWD primarily resulted from SOC concentration, since the stoniness in both plots was practically identical.

Factors mainly responsible for affecting the CWD-produced SOC increase into subsoil probably include leaching and vertical mixing by soil organisms. Harmon and Sexton (1995) showed that CWD may release considerable amounts of leachate, especially under humid conditions. We speculate that organic matter was immobilized and stabilized during its transport in the form of metal–humus–clay complexes, preferably inside soil aggregates—a feature typical of Andosols (Chevallier et al. 2010). It is therefore very likely that quantitative SOC imprint is substrate-specific, determined by the presence of free adsorption sites provided by allophanes and amorphous Fe-compounds.

Conclusions

A significant organic carbon increase in soil with CWD, especially in the subsoil, has not been detected before. Therefore, our investigation aimed at the possible CWD imprint in an andic soil featuring noticeable amounts of allophane and amorphous Fe-compounds. Soil organic carbon analysis showed that concentration differences in soils with and without CWD reached 0.04–0.07 kg kg−1 within the top 0.5 m soil layer, corresponding to 5.51–15.84 kg m−2 in terms of SOC stock. Their significance was supported by the t test. Our results underscore the status of Andosols as valuable natural history archives, enabling the study of even subtle soil-forming processes. Appropriate measures should be taken to protect and conserve these soils as proposed by Kleber and Jahn (2007), not least because of their role as an active carbon sink.