SOIL ORGANIC MATTER QUALITY in a FORMER MINE SITE of NE SPAIN Jaume Bech1, Maria Sokolovska2, Miglena Zhiyanski2, Ekaterina Filc

Silva Balcanica, 13(1)/2012

SOIL ORGANIC MATTER QUALITY IN A FORMER MINE SITE OF NE SPAIN

Jaume Bech1, Maria Sokolovska2, Miglena Zhiyanski2, Ekaterina Filcheva3, Núria Roca1,4

1University of Barcelona, Chair of Soil Science (Plant Biology), Barcelona 2 Forest Research Institute – Sofia, Bulgarian Academy of Sciences 3 Institute of Soil Science – Sofia 4 Facultad de Agronomía, Universidad Nacional del Centro de la Provincia de Buenos Aires – Azul

Abstract

Humus substances are natural ligands in the bonding of elements to complexes. They play an important role in the processes of soil formation as well as in the accumulation and mobility of metal ions. The studied area shows high concentrations of heavy metals, such as (mg kg-1): Cd: 1.03; Cu: 129; Fe: 40120; Mn: 472; Pb: 135 and Zn: 150 (determined by Aqua regia extraction). This site consists of moderately acidic soils (pH ranged 5.2 to 6.2). These studied top soils in the region of the iron mine, exploited in Roman ages near to Poblet monastery (Tar- ragona, Spain), have a low content of soil organic matter. The soil carbon is up to 44% of the total soil organic carbon (SOC) content. The level of humification is medium to high. The type of soil humus is humate – fulvatic in 4 of the studied soils and fulvo – humatic in the other 5. There are no humic acids bonded with alkali ions. The only established humic acids are ‘free’ or bonded with the mobile forms R2O3, which indicates possible processes of soil organic matter migration toward deeper horizons. This is a prerequisite for the contamination of soils and subterranean waters by heavy metals.

Key words: SOM quality, mining site, humic acids, fulvic acids

INTRODUCTION

For the sustainable management of forest ecosystems and improvement of their productivity clarifying of humus formation processes and characteristics of soil organic matter is necessary. The humic and fulvic acids are components of soil organic matter (SOM) and their quantity and chemical characteristics depend on the specific climate conditions (Garcia et al.,1985), altitude, type of vegetation (Singhal et al., 1983; Howard et al., 1998), as well as on the different modes of

30 both humus characteristics – the ratio humic acids/fulvic acids (Cha:Cfa) and the quantity of fulvic acids (FA) bonded with calcium (Ca), could be used for descrip- tion and differentiation of soil horizons. Krupenkov, Ganenko (1970) showed that the distribution of humus acids could be used for differentiation not only of soil types but also of sub-types and some characteristics like type of silviculture. Humus substances are natural ligands in the bonding of elements to complexes. They play an important role in the processes of soil formation as well as in the accumulation and mobility of metal ions. Linear correlation coefficients (0.83; 0.99) were established between organic matter content and extracted with sodium pyrophosphate and DTPA heavy metals (Latterell et al., 1978) or it was pointed a prevail bonded of Cu and Zn (Mirchev, 1978) or Pb, Cd (Shuman, 1998) with soil organic matter. Latterell et al. (1978) underpin sodium pyrophosphate as the only one extragent, which show the high linear correlation between 20 extracted quantities of six elements and soil organic matter. There are investigations on soil organic matter substances and some metals in the polluted areas in Bulgaria (Filcheva et al., 2002, 2004, 2007). These specifics of humus substances are precondition for serious problems in special occasions like long pollution as a result of mining and manufacturing industry. The aim of this study was to investigate the properties and specifics of top soils in the region of ancient iron mine, exploited in Roman ages, near to Monas- tery of Santa Maria de Poblet, region of Tarragona in Spain.

MATERIALS AND METHODS

Studied site It is a former mine situated above ‘Les masies’, little group of houses, near the monastery of Poblet and belongs to Vimbodi village, province of Tarragona (Catalonia, NE Spain). The mine site is located in the low part of a slope of Eastern side of Prades Mountains. Prades Mountains belongs to a Prelitoral Range of Catalonia. They are located in west district of Barcelona and northwest of Tarragona, approximately between 0º55’ to 1º12’ longitude and 41º13’ to 44º24’ of latitude. The general characteristics of studied mining site are presented in Table 1, using data from the nearest meteorological station Riudabella, situated at 608 m a. s. l. The analysis of climatic elements and the diagram of Walter-Gausen showed that the period of water deficiency is between June and August (Fig. 1).

Geology The mining area is over important fault disposed in E-W direction. This fault put in abrupt contact Paleozoic socle of Prades Mountains with Oligocene basin of Conca de Barberá. There is an unevenness of approximately 800 m. The old mine is underlined by Silurian metamorphic shale. In the upper

31 part of mine carboniferous conglomerates recovered by red sandstones of Bund- sandstein are distributed. Silurian metamorphic rocks are dark shale (Fig. 2).

Vegetation The vegetation is dominated by Quercetum ilex galloprovinciale, with mainly: Quercus ilex subsp; Pinus halepensis; Pinus pinea; and the most distributed grass species are Erica arborea; Viburnum tinus, Arbutus unedo; Buplerum fru- ticosum, Rhamnus alaternus; Phillyrea medi; Lonicera implex; Clematis flam- mula, Asparagus acutifolius; Rubia peregrine; Ruscus aculeatus; Smilax aspera; Asplenium adiantum-nigrum, etc. (Bech et al., 2011).

Sampling procedure and methods On the territory of mining site with a total area of 1 ha were chosen nine (9) experimental plots at different distance from the main entrance of the old mine. The first and the second plots (PB1 and PB2) are located out of the mining zone and are considered as control plots for comparison. The experimental plots PB3, PB4 and PB5 were located closer to the control ones. Those from PB6 to PB9 are located in the region of the ancient iron mine at different elongation in a row and the area is often visited by tourists. The sampling procedure was performed in February 2008. The samples were taken from the first 0-30 cm of superficial soil, air dried and sieved through 2 mm according to the requirements of ISO. The analyses of total forms of heavy metals were done after treatment with Aqua regia and determination by AAS Perkin Elmer 310. The soil acidity was determined in distilled water in ratio 1:2.5. The quality and composition of soil organic matter were determined according to the modified method of Turin (120 °С, 45 min, with catalyst Ag2SO4) and method of Kononova, Belchikova (Kononova, 1966; Filcheva, Tsadilas, 2002). Total humic and fulvic acids (Cextractable) were determined after extraction with mixed solution of 0.1 M Na4P2O7 and 0.1 M NaOH; ‘free’ and R2O3 bonded humic and fulvic acids (CNaOH) – after extraction with 0.1 M NaOH and the most dynamic, low molecular fraction of organic matter, so called ‘aggressive’ fulvic acids fraction

(1a6 ) was extracted with 0.05 M H2SO4, in ratio soil:solution = 1:20 for the three extractions. Humic and fulvic acids in both extracts, Cextractable and CNaOH, were separated by acidifying thе solution with sulfuric acid (0.5 M). Optical characteristics (E4/E6) show the degree of condensation and aromatization of humic acids. Humus status criteria were determined by the scheme of Grishina, Orlov (1978) and Grishina (1986).

RESULTS AND DISCUSSIONS:

Heavy metals content and acidity of soil solution In the chemical composition of studied soils located under pine forests

32 some important differences could be outlined. Fe concentrations were extremely high in the top soil layer and the mean value was estimated at 40 120 mg kg-1. In top soil layers of plots from PB1 to PB5 the iron has two times lower concentration and weak variation compared to the other plots, located closer to the mine (Fig. 3). The results obtained show variations of the other studied metal pollutants (Fig. 4). The established variations could be summarized as follows:

for Cu – from 88.96 to 200.16 mg kg-1, mean = 129 mg kg-1; for Zn – from 96.8 to 349.7 1 mg kg-1, mean = 150 mg kg-1; for Pb – from 100.78 to 158.08 mg kg-1, mean 135 mg kg-1; for Mn – from 310.96 to 878.80 mg kg-1, mean = 472 mg kg-1.

The manganese content was higher in the superficial soil layers of PB1 and PB2 experimental plots, which are controls. These higher concentrations of Mn are not toxic for plant vegetation, despite of acid pH in the soil layers (рН 5.2- 5.4), because the Fe content was lower for both plots. This fact is underlined in other studies on the influence of site conditions on growing and development of forest plantations over waste banks after iron extraction (Zhelyazkov et al., 1990; 1991; Filcheva et al., 2005). The soils from control plots are rich with Zn – 349.7 mg 1000 g-1. The concentration of Cu is higher for the plots from the region of old mine (from PB6 to PB9). The highest values were detected in soils of PB7 experimental plot (200.16 mg 1000g-1). The content of Cd, which destroys the normal physiological functions and processes in plants, is in very low quantities (from 0.68 to 1.36 mg/kg, mean 1.03 mg/kg). It is well known, that the reaction of soil solution is a result from the mu- tual action of water dissolved substances with inorganic (salts, acids and alka- line substances) and organic origin, colloids, acids with specific and non-specific properties (humic acids, fulvic acids, citric acid, formic acid etc.) as well as of clay minerals (Kowalev, 1986). The root systems through the released substances and different products of microbial bioceanoses in soils, together with decomposition of organic matter have special influence on soil acidity. The studied mining area consists of moderately acidic soils – pH in water ranged 5.2 and 6.2 (Fig. 5). The lower values of pH are typical for the plots outside the old iron mine (from PB1 to PB4). The plot PB5 is an exception and the soil layer there has pH 6.2. According to the data presented in some studies (Sohnitzer, Scinner, 1969; Zhelyazkov et al., 1990), рН of soil solution has important influence on the specifics of interrelation between humus acids and clay minerals. This determines the mineralization and humification rates of organic substances as well as the period of accumulation of humus in technogenic soils. The weathering processes have also impact especially on relatively young soils like technogenic soils, where our studied sites could be referred to.

33 Soil organic matter (SOM or soil humus) The organic carbon (SOC) content (presented in % to dry soil) in the surface soil layers varied between 3.21 % and 10.52 % (Fig. 6). The soils from control plots (PB1 and PB2) have extremely higher soil organic carbon content compared to the other plots. On the territory of the old mine in plots PB6 to PB9 the organic carbon content in soils is lower and did not exceed 6%. The data obtained show that the rate of humus accumulation in regenerated ecosystems from technologi- cal landscapes is not a regular process and confirmed the conclusions of Kowa- lev (1986). This irregularity in humus accumulation could be explained with the established forest ecosystems, which are in conditions of dynamic balance. The possible explanation for the lower soil carbon content is the lower litter input because of the lack of grass coverage and human impact on this area expressed mainly with soil compaction. The content of soil organic matter in studied top soils is medium to high (mean % OC = 6.5) according to Grishina, Orlov (1978). The data obtained show that the organic matter of studied plots (PB1 to PB9) has wider ration between total carbon and total nitrogen (Table 2). C:N is over 20, which determined low degree of transformation of organic materials and complicated humus formation processes. C:N ratio shows, that studied soils have low to very low level of enrichment with total nitrogen and their soil humus is ‘ecologically unbalanced’ and poor of nitrogen (Grishina, 1986). Similar results are presented in other study (Taranov et al., 1979). Under these conditions it is very important the humus substances to be enough sustainable to the oxide- hydrological impacts (Kononova, 1966).

Soil organic carbon (SOC) – extractable and unextractable The higher percent of extractable and unextractable SOC in SOM of soils in PB1 and PB2 and most of the other plots is as a result of the more favourable climatic conditions regarding soil hydrological regime determined by the type of the above ground vegetation – trees with grass cover (Fig. 7). A general specific of organic substances from the superficial soil layers of studied plots on the territory of the old mine (from PB6 to PB9) is the higher percent of carbon in unextractable residuum from the analytical methods. From our data is shown that it varied from 56.7 to 72.49 % and present about 2/3 from the total organic carbon. The organic substances in soils are characterized by lower humification rate. This fact determines the organic matter of the studied soils as easily mobile. The soil carbon extracted by a mixed solution of sodium pyrophosphate and sodium hy- droxide is up to 44 % of the total soil organic carbon content.

Chemical Fractionation of Soil Humus The content of fulvic acids (FA) is highest in experimental plots PB1 and PB2 and predominates over the humic acids (HA) (Fig. 8). The increase of fulvic

34 acids in soils from controls could be explained with the higher content of coarse roots, participating in the processes of humus formation. Similar results are typical for forest process of soil formation under mountainous climatic conditions.

The aggressive fraction of fulvic acids – 1a (extracted by 0.1 N H2SO4) is the most dynamic (mobile) low molecular fraction of soil organic matter. The synthesis of humus starts from this fraction and the decomposition of humus ends with it, so this fraction is the bonding section in the ‘humification-mineralization’ balanced systems (Kononova, 1966). It is in very low quantities (absolute and relative percents) for all studied soil samples. The humic acids (ha) are natural ligands, which bond the elements in complexes characterized with different stability, more stable than complexes with fulvic acids (fa) (Filcheva, 1976, 2004). For the studied experimental plots the humic acids predominate in soils more polluted with heavy metals (PB4, PB6, PB8 and PB9). Comparison of data obtained from the chemical fractionation of soil humus in studied soils allow us to underline that under conditions of technogenic landscapes the type of humus accumulation is dependent mainly on hydrother- mal conditions influencing the processes of destruction and humification of litter and energetic balance of the system (Volobuev, 1974).

Characteristic of humus acids The degree of humification in studied soils, e.g. the percentile participation of humic acids in the soil organic matter composition, is low (10-20) and moder- ate for the PB6 (determined by the scheme of Grishina, Orlov, 1978; Orlov, 1985) (Table 2). The type of soil humus is humic-fulvic (Cha/Cfa – 0.5-1) in four of the studied plots (PB1, PB2, PB3, PB7) and fulvic-humic (Cha/Cfa – 1-1.5) – for the other five plots (PB4, PB5, PB6, PB8, PB9). This is a precondition for stable complexes formation between humic acids and heavy metals. The stability of formed complexes depends on the type of metal ion and on the maturity of humus substances (Filcheva, 1976, 2007). It could be noted that despite of the content of humic and fulvic acids and unextractable residuum in soil organic matter of technogenic soils the ratio Cha/Cfa varies in similar degrees like in the natural soils of the region – Cambisols (IUSS, 2006). There are no humic acids bonded with alkali-earth ions. The only established humic acids are ‘free’ or bonded with the mobile forms of R2O3, e.g. the simplest in their structure and most mobile, which indicates possible processes of soil organic matter migration towards deeper horizons. This is a prerequisite for the contamination of soils and subterranean waters with heavy metals. The type of organic substances fall through the litter input on the soil surface and the acid parent materials (the sediments dominate – dark shale, marls and sandstones) could explain the results obtained.

35 The optical density could be used as a index for humus quality of studied soils and presents the ratio Е4/Е6, characterizing the nature of humic acids (Table 2). This ratio varies in very limited degrees and does not exceed 11. The lowest values show indirect relation with the condensation of aromatic nuclei of humic acids. Humic acids in the surface horizons are characterized by low degree of condensation and aromatisation. They are much closer to the optical characteristic of fulvic acids, which create possibilities for leaching of mobile humic compounds.

CONCLUSIONS

Higher content of soil organic carbon is determined in soils from the control plots РВ1 and РВ2 compared to the experimental plots within the zone of ancient mine. In control plots the fulvic acids predominate in the composition of soil organic matter. Humic acids content predominates over the fulvic acids in the soils from the zone of ancient mine. It could be supposed that more stable complexes with metal ions could be formed in these plots. The low content of ‘aggressive’ fraction of fulvic acids in all studied soils indicates the formation of stable forms of organic matter and stable organic-mineral complexes. The level of humification is medium to high. The type of soil humus is humate – fulvatic in four of the studied soils and fulvo – humatic the other five plots. There are no humic acids bonded with alkali ions. The only established humic acids are ‘free’ or bonded with the mobile forms R2O3, which indicates possible processes of soil organic matter migration toward deeper horizons. This is a prerequisite for the contamination of soils and subterranean waters by heavy metals.

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