The Potential of the Soil for Stabilisation of Organic Carbon in Soil Aggregates

Agriculture (Poľnohospodárstvo), 61, 2015 (2): 50−60 DOI: 10.1515/agri-2015-0010

THE POTENTIAL OF THE SOIL FOR STABILISATION OF ORGANIC CARBON IN SOIL AGGREGATES

ERIKA TOBIAŠOVÁ

Slovak University of Agriculture in Nitra

TOBIÁŠOVÁ, E.: The potential of the soil for stabilisation of organic carbon in soil aggregates. Agriculture (Poľnohos- podárstvo), vol. 61, 2015, no. 2, pp. 50–60.

Carbon stabilisation in soil is the result of interaction of carbon in the soil macro-aggregates. Ps was in an ex- between the chemical and physical mechanisms of pro- ponential dependence (r = 0.942; P < 0.01) with produc- tection and the dominance of the mechanism depends not tion potential of the soil, and the fractions of dry-sieved only on the long-term constant characteristics of soil but aggregates larger than 3 mm play an important role in also on the properties, which can be partly influenced by the first stages of the carbon stabilisation. The suitable human activities. In this study, the potential of the soil parameter, which reflects the changes in carbon stability for stabilisation of carbon (Ps) in different soil types de- in the soil is the ratio of the labile carbon and non-labile pending on soil properties was compared. Experiment in- carbon in the soil macro-aggregates (L/NL). Lower values cluded six soils (Eutric Fluvisol, Mollic Fluvisol, Haplic of L/NL that indicate a higher stability of carbon were Chernozem, Haplic Luvisol, Eutric Cambisol, and Rend- determined at a higher pH, at the higher content of car- zic Leptosol) of different land uses (forest, meadow, ur- bonates and exchangeable basic cations, and at a higher ban, and agro-ecosystem) in Slovakia. Ps was determined portion of humic acids free and bound with mobile se- with dependence on the ratio of labile and stable fractions squioxides R2O3.

Key words: carbon stabilisation, humus substances, labile carbon, non-labile carbon, soil aggregates, soil type

The potential of each soil for the physical sta- only in the case of the macro-aggregates, but also in bilisation of soil organic matter depends directly the case of the carbon in them (Oades 1984). This on its properties. Most of them are given by the soil stabilisation is the result of interaction between the genesis itself, but many of them vary depending on chemical and physical mechanisms of organic mat- land use and soil management (Debska et al. 2012; ter protection (Jastrow & Six 2006). The dominance Tobiašová et al. 2014). The stability of the soil of mechanism depends not only on a long-term con- structure is determined primarily by water-resistant stant properties such as the particle size distribution aggregates (Tisdall 1996; Tobiašová 2014), howev- (Caravaca et al. 1999; Eustehues et al. 2003; Moni er, in relation to the stabilisation of organic matter, et al. 2010), on a primary content of carbonates or especially in its early stages, are important main- sesquioxides (von Lützow et al. 2006), the over- ly dry-sieved aggregates, which play a much more all production potential of soil (Džatko & Ilavská important role in the early stages when deciding on 2005), but also on the properties that can be par- the follow up mechanisms for its next stabilisation tially influenced, such as the soil pH (Regelink et and are its primary repository. The organic matter al. 2015), content of exchangeable basis cations or is gradually decomposing; the products of metabo- nature of organic inputs through the land use (Ce- lism are releasing and the stability is increasing not sare Barbosa et al. 2015; Chrenková et al. 2014).

doc. Ing. Erika Tobiašová, PhD., Department of Soil Science, Slovak University of Agriculture in Nitra, Tr. A. Hlinku 2, 94976 Nitra, Slovak Republic, E-mail: [email protected]

50 Agriculture (Poľnohospodárstvo), 61, 2015 (2): 50−60

After changes in the land use, there occurs not only ern border of the Danube Basin. Geological struc- the reduction of carbon content, but also a change ture is characterised with neogene strata, which con- in the ratio of stabile and labile forms of soil or- sist mainly of claystones, sandstones and andesites, ganic matter in the ecosystem, and after a period of which are covered with younger quaternary rocks time, a new balance is created (van Noordwijk et al. that are represented by different fluvial and aeolian 1997). The formation of organo-mineral complexes sediments (Pristaš et al. 2000). The natural vege- is a key factor in the stabilisation and sequestration tation consists mostly of ash-oak-elm-alder forests, of organic carbon (Baldock & Skjemstad 2000) and and along the river, there are willow-poplar forests its incorporation into the soil aggregates seems to be and floodplain forests. In the elevated areas and a key factor that controls the stock and dynamics of dunes, xerophilic communities of oak-elm forests soil organic matter (Oades 1988). are dominant (Korec et al. 1997). Eutric Fluvisol The objectives of this study were as follows: (i) and Haplic Luvisol come from the locality Vráble to find out the relationship between the soil type, (48°24’N, 18°31’E), which is situated in the Dan- respectively, production ability and potential of soil ube lowland, in the unit of the Danube plain, con- for carbon stabilisation, (ii) to determine the pa- cretely in the Nitra upland. Geological structure is rameter, which more characterizes the stability of characterised with neogene sediments – loess and carbon and reflects changes in more soil properties, loess loamy. Neogene bedrock is formed by the lake and (iii) to specify the soil factors that are in direct and brackish sediments (clays, gravels, and sands) relation to this parameter. (Hók et al. 2001). In the lower parts, oak forests are dominant and in the higher parts, there are beech forests. Rendzic Leptosol comes from the locality MATERIAL AND METHODS Pružina (49°02’N, 18°47’E), which is situated at the northeastern foot of the hill Strážov, in the valley of Characteristics of the territory the river Pružina. Geological structure is charac- The studied areas are located in different parts terised with the core mountains of the outer arc of of Slovakia (Figure 1). Haplic Chernozem and Mol- the Central Western Carpathians. A substantial part lic Fluvisol come from the locality Horná Kráľová of the mountain is composed of nappes with a high- (48°24’N, 17°92’E), which are situated on the north- ly variable resistance of rocks. The core is formed

1,2 3,4 Bratislava

Figure 1. Localities of the soils: 1 – Haplic Chernozem, 2 – Mollic Fluvisol, 3 – Eutric Fluvisol, 4 – Haplic Luvisol, 5 – Rendzic Leptosol, and 6 – Eutric Cambisol

51 Agriculture (Poľnohospodárstvo), 61, 2015 (2): 50−60 of the crystalline slates, granites, amphibolites and lomites in the east (Pristaš et al. 2000). In the for- in the south and southeast, there are mesozoic do- ests, spruce dominates, with the addition in some lomites, limestones and slates, which are folded parts of these forests of deciduous trees (Korec et and stored in the form of nappe debris (Pristaš et al. 1997). al. 2000). In the forests, beech and oak dominate, Experimental details with the addition in the higher parts of these for- The experiment included four types of eco- ests of fir and higher number of conifers (Korec et systems, which present different land use (forest, al. 1997). Eutric Cambisol comes from the locality meadow, urban, and agro-ecosystems) of six soil Selce (48°76’N, 19°20’E), which is situated at the types (Eutric Fluvisol, Mollic Fluvisol, Haplic slopes of mountain Starohorské vrchy. Geological Chernozem, Haplic Luvisol, Eutric Cambisol, and structure is characterised with the core mountains Rendzic Leptosol). These are the soils (Table 1) of of the Central Western Carpathians. This part of the lowlands and uplands, which have the largest pro- mountain is composed of palaeozoic conglomerates, portion in Slovakia and are intensively agricultural- breccias, shales, and sandstones in the south and do- ly used. The forest ecosystems were natural forests

T a b l e 1

Basic chemical and physical properties of the soils in different ecosystems

TOC CEC Clay Silt Sand Soil type Ecosystem pH [g/kg] [mmol/kg] [%] FE 6.56 21.65 406.00 14.61 32.34 53.05 Haplic ME 7.18 20.13 499.88 15.50 39.01 45.49 Chernozem UE 7.27 18.22 500.21 12.22 49.75 38.03 AE 7.10 17.62 412.75 17.44 50.13 32.43 FE 6.14 20.95 377.29 54.43 35.13 10.44 Mollic ME 7.39 14.96 500.21 54.64 31.20 14.16 Fluvisol UE 7.50 17.66 501.54 47.11 32.33 20.56 AE 6.77 11.44 347.93 46.68 36.78 16.54 FE 6.33 19.85 393.20 11.17 64.55 24.28 Eutric ME 6.75 17.11 371.49 11.49 72.46 16.05 Fluvisol UE 6.78 15.11 322.78 10.74 73.65 15.61 AE 5.78 11.29 265.91 12.52 75.43 12.05 FE 5.11 11.07 386.38 16.68 73.87 9.45 Haplic ME 5.87 8.63 311.44 17.47 73.68 8.85 Luvisol UE 7.17 9.23 453.32 11.74 78.93 9.33 AE 6.06 10.09 320.12 14.66 69.45 15.89 FE 7.25 31.68 504.46 15.31 41.11 43.58 Rendzic ME 7.36 17.65 504.04 19.41 37.18 43.41 Leptosol UE 7.26 11.58 503.21 18.50 40.08 41.42 AE 7.04 13.12 442.65 15.53 50.48 33.99 FE 5.63 22.30 150.96 8.50 49.49 42.01 Eutric ME 6.87 18.43 193.83 5.07 35.64 59.29 Cambisol UE 6.45 19.49 277.19 6.51 45.65 47.84 AE 6.21 18.48 205.99 6.13 41.93 51.94 FE – forest ecosystem; ME – meadow ecosystem; UE – urban ecosystem; AE – agro-ecosystem; TOC – total organic carbon; CEC – cation exchangeable capacity

52 Agriculture (Poľnohospodárstvo), 61, 2015 (2): 50−60

with human control; the meadow ecosystems were with 30% H2O2. After repeated washing, samples created by man 30 years ago; and the urban ecosys- were dispersed using Na(PO3)6. Silt, sand, and clay tems presented soils of urban landscape (grasses in fractions were determined according to the pipette a town influenced by human activities). The fields method (van Reeuwijk 2002). In the soil and soil in agro-ecosystems were located in different farms macro-aggregates (dry and wet sieve), the total or- under real production conditions. The vegetation in ganic carbon (TOC) by wet combustion (Orlov & the agro-ecosystem is given by the crop rotations. Grišina 1981); the labile carbon (CL) by KMnO4 ox- Dominated crops on the fields were as follows: win- idation (Loginov et al. 1987), were determined and ter wheat, spring barley, sugar beet, sunflower, corn, non-labile carbon (CNL) and the lability of carbon soybean (Eutric Fluvisol, Haplic Luvisol); winter (LC) were calculated. In the soil, the next parameters wheat, oil rape, alfalfa, sugar beet, sunflower (Mol- were determined: the fractional composition of hu- lic Fluvisol, Haplic Chernozem), winter wheat, triti- mus substances (Ponomarevova & Plotnikova 1975); cale, corn, alfalfa (Eutric Cambisol), and winter rye, the pH of the soil was potentiometrically measured winter wheat triticale, corn, mix of clover and grass in a supernatant suspension of a 1:2.5 soil:liq- (Rendzic Leptosol). uid mixture. The liquid is either 1 mol/dm3 KCl (pH ) (van Reeuwijk 2002); carbonates were de- Soil samples and analytical methods used KCl termined by volumetric methods (using a simple cal- The soil samples used to determine the chemical cimeter), based on the CO evolving after reactions and physical properties were collected in three rep- 2 with HCl (diluted with water in a 1:3 ratio) (Allison licates to depth of 0.30 m, and dried in a constant & Moodie 1965). The cation exchangeable capaci- temperature room of 25±2°C. The soil samples for ty (CEC) was determined according to the Pfeffer determination of chemical properties were ground. method (Jackson 2005), and the sum of exchange- To determine the fractions of soil aggregates, the able alkaline cations (S) was calculated from the soil samples were divided by the sieve (dry and CEC and hydrolytic acidity (van Reeuwijk 2002). wet sieve) to size fractions of the net aggregates The obtained data were analysed using Statgraph- (Sarkar & Haldar 2005). The particle size distribu- ic Plus statistical software. A multifactor ANOVA tion was determined after dissolution of CaCO with 3 model was used for individual treatment compari- 2 mol/dm3 HCl and oxidation of the organic matter sons at P < 0.05, with separation of the means by

T a b l e 2

Correlations between lability of carbon (LC) in soil aggregates and selected soil properties

2- Macro-aggregates H S pH/KCl CO3 Sand Silt ++ +++ +++ +++ +++ +++

>7 0.46 −0.50 −0.57 −0.65 −0.50 0.52 5–7 0.44++ −0.55++ −0.52+++ −0.72+++ −0.57+++ 0.61+++ 3–5 0.38+ −0.49++ −0.51+++ −0.68+++ −0.58+++ 0.62+++ +++ +++ +++ +++ ++ +++ [mm] 1–3 0.50 −0.54 −0.57 −0.71 −0.49 0.54

in dry sieved ++ +++ +++ +++ + ++ C 0.5–1 0.41 −0.52 −0.53 −0.72 −0.39 0.41 L 0.25–0.5 0.34+ −0.37+ −0.43++ −0.69+++ −0.43++ 0.46++ >5 0.45++ −0.56+++ −0.55+++ −0.55+++ −0.47++ 0.51+++ 3–5 0.37+ −0.49++ −0.48++ −0.62+++ −0.35+ 0.40++ 2–3 0.37+ −0.43++ 0.45++ 0.54+++ −0.47++ 0.52+++

in wet in wet ++ +++ +++ +++ ++ +++

C 1–2 0.49 −0.56 −0.52 −0.56 −0.48 0.57 L +++ +++ +++ +++ ++ +++ sieved [mm] 0.5–1 0.53 −0.65 −0.60 −0.62 −0.47 0.56 0.25–0.5 0.33+ −0.45++ −0.43++ −0.62+++ −0.33+ 0.40++ H – hydrolytic acidity; S – content of alkaline exchangeable cations; +++P < 0.001; ++P < 0.01; +P < 0.05

53 Agriculture (Poľnohospodárstvo), 61, 2015 (2): 50−60

Tukey multiple-range test. Correlation analysis was of organic matter in the soil, had a different influ- used to determine the relationships between the ence on the LC values (Table 3). Lower values of chemical properties and parameters of soil structure LC were determined at a higher portion of humic stability. Significant correlation coefficients were acids, in particular the fraction of humic acids free tested at P < 0.05, P < 0.01, and P < 0.001. and bound with mobile sesquioxides R2O3 (HA 1) and vice versa, in the case of fulvic acids free and

bound with mobile sesquioxides R2O3 (FA 1), the

RESULTS AND DISCUSSION values of LC were higher. The fraction of aggressive

fulvic acids was in a negative correlation with LC. The obtained results show on a higher impor- HS are considered an important element in the for- tance of the ratio of labile and stabile components mation of stabile soil aggregates (Tisdall & Oades rather than their actual contents. The ratio of labile 1982), but their individual fractions participate in and non-labile carbons, called by Blair (1995) as the the different degree of the stabilisation. According lability of carbon (LC), was as in the case of the frac- to Tobiašová et al. (2013), the fraction of HA 1 was tions of dry-sieved macro-aggregates (DSA), even in a positive correlation with the hydrolytic acidity in the fractions of wet-sieved macro-aggregates (H), therefore, higher H was associated with higher

(WSA), influenced by soil type and was in a direct content of HA 1 and a higher value of LC. Overall, correlation with more soil properties (Tables 2 and at lower pH, there is more carbon in a labile form 3). In the case of all macro-aggregates fractions, (Tobiašová 2010). In the case of FA 1, this corre- lower values of LC were at lower soil acidity, at a lation was a positive. The reason can be a differ- higher content of exchangeable alkaline cations and ent solubility of HS. Fulvic acids are soluble in the carbonates (Table 2). These soil properties can be whole range of pH, but the solubility of humic acids to some extent controlled, which increases the po- decreases with decreasing of pH values. Decreasing tential of the soil for carbon stabilisation in the soil. of pH values results in the blockade of exchange- Humus substances (HS), such as a stabile fraction able positions by cations of Al3+ and Fe3+ or their

T a b l e 3

Correlations between lability of carbon (LC) in soil aggregates and fractions of humus substances

Macro-aggregates HA 1 HA 2 HA 3 ΣHA FA 1a FA 1 FA 2 FA 3 ΣFA >7 −0.33+ n.s. n.s. −0.41++ −0.40++ 0.35+ n.s. n.s. n.s. 5–7 −0.41++ −0.36+ n.s. −0.49++ −0.43++ 0.47++ n.s. n.s. n.s. 3–5 −0.43++ n.s. n.s. −0.43++ −0.45++ 0.49++ n.s. n.s. n.s. ++ + ++ ++ +++ [mm] 1–3 −0.41 −0.32 n.s. −0.49 −0.45 0.51 n.s. n.s. n.s.

in dry sieved + + + + C 0.5–1 −0.38 n.s. n.s. −0.36 −0.38 0.35 n.s. n.s. n.s. L 0.25–0.5 −0.45++ n.s. n.s. −0.40++ −0.43++ 0.38+ n.s. n.s. n.s. >5 −0.43++ −0.32+ n.s. −0.49++ −0.34+ 0.49+++ n.s. n.s. n.s. 3–5 −0.48++ n.s. n.s. −0.51+++ −0.49++ 0.44++ −0.31+ n.s. n.s. 2–3 −0.44++ −0.31+ n.s. −0.50+++ −0.48++ 0.47++ n.s. n.s. n.s. ++ +++ +++ ++ ++ [mm] 1–2 −0.42 −0.51 n.s. −0.62 −0.48 0.43 n.s. n.s. n.s.

in wet sieved + +++ ++ +++ + +++ C 0.5–1 −0.31 −0.55 −0.41 −0.63 −0.34 0.50 n.s. n.s. n.s. L 0.25–0.5 −0.50+++ −0.41++ n.s. −0.60+++ −0.42++ 0.42++ n.s. n.s. n.s. 2+ HA 1 – humic acids free and bound with mobile R2O3; HA 2 – humic acids bound with Ca ; HA 3 – humic acids bound with mineral component of soil and stabile R2O3; ΣHA – sum of humic acids; FA 1a – free aggressive 2+ fulvic acids; FA 1 – fulvic acids free and bound with mobile R2O3; FA 2 – fulvic acids bound with Ca ; FA 3 – fulvic acids bound with mineral component of soil and stabile R2O3; ΣFA – sum of fulvic acids; +++P < 0.001;++P < 0.01;+P < 0.05

54 Agriculture (Poľnohospodárstvo), 61, 2015 (2): 50−60 hydroxides (Brady 1990), especially in the case of therefore, even after their incorporation into aggre- free unprotected humic acids and fulvic acids. HA 1 gates, the lability of carbon does not change and the become more stabile by binding to the sesquioxides correlation between LC in aggregates and HA 1 is and by the loss of solubility (Boudot et al. 1989), also a positive. Conversely, the fraction of FA 1 is

1/3 1/3

0 0 >7mm >5mm >7mm >5mm 5-7mm 3-5mm 1-3mm 3-5mm 2-3mm 1-2mm 5-7mm 3-5mm 1-3mm 3-5mm 2-3mm 1-2mm 0.5-1mm 0.5-1mm 0.5-1mm 0.5-1mm 0.25-0.5mm 0.25-0.5mm 0.25-0.5mm 0.25-0.5mm A dry sieved wet sieved B dry sieved wet sieved

1/3 1/3

0 0 >7mm >7mm >5mm >5mm 5-7mm 3-5mm 1-3mm 5-7mm 3-5mm 1-3mm 3-5mm 2-3mm 1-2mm 3-5mm 2-3mm 1-2mm 0.5-1mm 0.5-1mm 0.5-1mm 0.5-1mm 0.25-0.5mm 0.25-0.5mm 0.25-0.5mm 0.25-0.5mm C dry sieved wet sieved D dry sieved wet sieved

1/3 1/3

0 0 >5mm >5mm >7mm >7mm 3-5mm 2-3mm 1-2mm 3-5mm 2-3mm 1-2mm 5-7mm 3-5mm 1-3mm 5-7mm 3-5mm 1-3mm 0.5-1mm 0.5-1mm 0.5-1mm 0.5-1mm 0.25-0.5mm 0.25-0.5mm 0.25-0.5mm 0.25-0.5mm E dry sieved wet sieved F dry sieved wet sieved

‒ minimum value of L/NL ‒ range of variability Figure 2. The ratio of labile carbon and non-labile carbon (L/NL) in the soil macro-aggregates in: A – Eutric Fluvisol, B – Mollic Fluvisol, C – Haplic Chernozem, D – Haplic Luvisol, E – Eutric Cambisol, F – Rendzic Leptosol

55 Agriculture (Poľnohospodárstvo), 61, 2015 (2): 50−60 still in the soluble state at pH decrease and they sub- Soil pH and composition of cations in the soil ject more easily to degradation, thereby, the portion can be partially influenced; the fractional compo- of labile carbon increases, so the correlation is pos- sition of humus substances can be in a short time itive. Dominant charge is negative, similarly as in less influenced, but about the influence of the parti- the case of the mineral colloids, therefore through cle size distribution we can consider in decades or their incorporation into soil aggregates contribute hundreds of years. Silt and clay are, however, one to increase of the labile component. A correlation of the basic stabilising agents of organic substances between the LC and the fraction of aggressive ful- in the soil and are with them in a positive corre- vic acids (FA 1a) is negative. The fraction of FA 1a lation (Bosatta & Ågren 1997; Tobiašová 2011). mobilises from the sorption complex alkaline cat- Jastrow (1996) observed that clay not only provides ions and dissolves carbonates that contribute to the protection of organic matter, but also supports the carbon stabilisation. The mechanism of stabilisation formation of very stabile macro-aggregates. A cor- through the cation bridges (von Lützow et al. 2006) relation LC with the fraction of clay, however, has is also often. This strong influence of the mentioned not been recorded. This indicates the fact, that not components on carbon stability is confirmed also by all carbon in the aggregates is stabilised by binding a negative correlation of LC with the content of ex- to this fraction, but the stabilisation mechanism can changeable alkaline cations (S) and carbonates. be different; or there is a carbon in the aggregates

T a b l e 4

Statistical evaluation of the lability of carbon (LC) in dry-sieved and wet-sieved macro-aggregates in different soil types

L in dry sieved [mm] L in wet sieved [mm] Soils C C >7 5–7 3–5 >5 3–5 2–3 Eutric Fluvisol 0.20b 0.19b 0.20b 0.22b 0.20b 0.21b Mollic Fluvisol 0.12ab 0.09a 0.10ab 0.12a 0.12ab 0.11ab Haplic Chernozem 0.12ab 0.12ab 0.13ab 0.17ab 0.17ab 0.17ab Haplic Luvisol 0.19ab 0.19b 0.19ab 0.21b 0.19ab 0.18ab Eutric Cambisol 0.13ab 0.15ab 0.14ab 0.15a 0.17ab 0.15ab Rendzic Leptosol 0.09a 0.10a 0.09a 0.11a 0.10a 0.10a Different letters (a, b) between the factors show statistically significant differences (P < 0.05) – Tukey test

T a b l e 5

Statistical evaluation of the fractions of dry-sieved and wet-sieved macro-aggregates in different soil types

Dry sieved [mm] Wet sieved [mm] Soils >7 5–7 3–5 >5 3–5 2–3 Eutric Fluvisol 27.57ab 13.10a 17.92ab 17.87a 13.13a 16.45ab Mollic Fluvisol 17.27ab 23.34b 21.22bc 5.83a 10.60a 13.04a Haplic Chernozem 17.35ab 24.05b 24.24c 5.75a 13.36a 22.26b Haplic Luvisol 30.77b 14.50a 17.95ab 19.85a 13.54a 13.16a Eutric Cambisol 21.42ab 10.51a 13.49a 11.74a 11.64a 18.56ab Rendzic Leptosol 16.35a 24.25b 23.96c 12.45a 18.90a 22.82b Different letters (a, b, and c) between the factors show statistically significant differences (P < 0.05) – Tukey test

56 Agriculture (Poľnohospodárstvo), 61, 2015 (2): 50−60

[%] (L ) were in the soils with overall a higher quality 25 C a ab b ab of the organic matter (Mollic Fluvisol, Haplic Cher- nozem, and Rendzic Leptosol) (Figure 2) that means 20 in the soils in which the stability of the organic sub- 15 stances is the highest. This potential reflects higher inputs of labile sources in the connection with a 10 higher biodiversity that results in a higher amount of the microbial biomass or plant biomass with richer 5 root exudates, which can be stabilised in the soil aggregates. Moreover, in the case of these soils, the pH 0 plays an important role. AE FE ME UE The most significant differences were recorded in Figure 3. Statistical evaluation of proportion of dry the largest fractions of the macro-aggregates (>3 mm) sieved macro-aggregates (> 3 mm) (Table 4), and right in the portions of these frac- AE – agro-ecosystem, FE – forest ecosys- tions, the statistically significant differences - be tem, ME – meadow ecosystem, UE – urban ecosystem. Different letters (a, b) between tween the soil types were recorded (Table 5). These the factors show statistically significant dif- are the first potential sink of carbon entering into the ferences (P < 0.05) – Tukey test soil and already in this stage, it is deciding about his next fate. Higher contents of carbon are in the larger fractions of the macro-aggregates, but this carbon is that has not been meanwhile stabilised. In relation also subjecting to larger changes, particularly min- to the parameter of LC, the contents of sand and silt eralisation (Jastrow 1996; Sohi et al. 2001). More- seem to be more important rather than more often over, the portion of DSA of the 3–5 mm size fraction mentioned a fraction of clay (Table 2). LC takes into is strongly influenced by land use (Figure 3), which account the existence of other stabilising mecha- is the next potential for an increasing of carbon sta- nisms that are a part of the relationship between the bilisation in the soil. carbon and its stability in the soil aggregates. LC was With respect to the land use, the composition in a negative correlation with the fraction of sand. of vegetation has primary influence, which is con- More sand means a higher intensity of the oxidation, firmed by the results of more authors (Barreto et al. which results in the loss of carbon from the soil, but 2009; Cantón et al. 2009; Tobiašová 2011). The vice versa, the stability of the organic substances increases. Higher stabilisation of humic acids through their increased aromatisation in stronger oxidative conditions is also described by Madari et al. (1998). Ps A correlation of L with the fraction of silt was a C 2 positive. Christensen and Sörensen (1985) show that huge amount of the organic matter is bound right on the fraction of the silt and a higher content of the carbon in finer fraction isalso a result of high (59%) recovery of the silt and clay (Christensen 1 1992). It shows that not all stabile carbon is bound 0.0177x in the aggregates on the fraction of clay. Therefore, y = 0.3084e r = 0.942 the ratio of the labile and non-labile carbons seems to be a more suitable parameter for assessing the sta- 0 PP bilisation of organic substances in the aggregates. 0 20 40 60 80 100 More productive soils have a higher potential for Figure 4. Correlation between production potential stabilisation of the carbon in the aggregates than less (PP) and the potential of soil potential for productive. The lowest minimum values of L/NL stabilisation of carbon (Ps)

57 Agriculture (Poľnohospodárstvo), 61, 2015 (2): 50−60 changes were the most significant just in the DSA, duction potential of the soil, and the fractions of which again show on a higher importance of the car- dry-sieved aggregates >3 mm play an important role bon content in DSA in the early stages of stabilisa- in the first stages of the carbon stabilisation. tion of the organic substances. The total potential The suitable parameter which reflects the changes of the soil for stabilisation of carbon is the result in carbon stability in the soil is the ratio of labile of the organic substances stabilisation in the all frac- carbon and non-labile carbon (L/NL) in the soil tions of the DSA and WSA, and is a higher in the macro-aggregates, and its lower values indicate a Mollic Fluvisol than in Rendzic Leptosol, which is higher stability of carbon. associated with the production capacity of the soil, Lower values of L/NL were determined at a not only the quality of organic substances. This fact higher pH, at higher content of carbonates and ex- is the best described by the parameter that is the po- changeable alkaline cations, and at higher portions tential of soil for stabilisation of carbon (Ps), which of humic acids free and bound with mobile sesqui- includes portions of the labile and non-labile com- oxides R2O3. ponents in relation to the DSA and WSA according to the equation: Acknowledgements. This project was support- ed by the Scientific Grant Agency of the Ministry of Education of the Slovak Republic and the Slo- vak Academy of Sciences (No. 1/0124/13 Physical stabilisation of organic matter in soils of different ecosystems).

REFERENCES where:

Ps – potential of soil for stabilisation of carbon, ALLISON, L.E. – MOODIE, C.D. 1965. Carbonate.

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