Abstract. In view of the role of cities for ensuring a favorable living environment,it is important to study the urban soils since they are formed and developedunder the impact of different by degree and type anthropogenic perturbations.Pedo-chemical studies of urban soils may capture the evolution of different soilcomponents and reveal the different stages of soil matrix transformation. Usingpedogenic and chemical analyses, the present article aims to present the trends ofperturbations of the mineral and organic matrix of urban soils located along thedirection of increasing gradient of urbanization in cursory investigated soil zonesbelonging to the residential and industrial districts of the Sofia city (Bulgaria).The results obtained show that anthropogenic alterations are predominantlyassociated with morphological reorganization of some soils rather than soilcompaction and structure loss. The increase of exchangeable hydrogen contentprovoked by fulvic acid production and leaching can be attributed to the currentnatural perturbations. Anthropogenically induced chemical changes could belinked with increase of the mineral N flux and high ammonium content whichwill influence the existing acid-base status of Sofian soils.
Introduction Globalization processes that started in
the beginning of the 20th centurytransformed cities in a unique assemblage ofnatural, ethnic, aesthetic, production,commercial, social and tourist symbols butalso contributed to the increase of chemicalvulnerability of soils. The enormousgathering of population in the cities
influences all the environmental componentsand substantially changes the soil cover.Usually, urban soils significantly differ inproperties and organization from naturalsoils and should be properly managed(LEHMANN & STAHR, 2007). Specific featuresof urban soils are related to the variation ofsoil acidity and sorption capacity, enrichmentwith organic matter mostly in the form of
Union of Scientists in Bulgaria – PlovdivUniversity of Plovdiv Publishing House
Pedo-chemical perturbations in soils from green ecosystems of the Sofia city (Bulgaria)
non-hydrolysed carbon, low water holdingcapacity, strong compaction and heavymetals contamination (DOICHINOVA, 2006;DOICHINOVA & ZHYANSKI, 2013).
Metrics such as pH and cation exchangecapacity are the most preferable andaccessible indicators for initial assessment ofchemical status of soils (THOMAS, 1996;DINEV, 2011; NIKOVA, 2009). It is a well-known fact that the interpretation of data onphysico-chemical processes running in thesoil adsorbent is a key tool for the sustainablemanagement and protection of soils(HENGLEIN, 1993). Organic matteraccumulation and transformation as primesoil-forming processes in the pedosphere arealso widely discussed (SARDANS et al., 2012;CHENU et al., 2015; BŁONSKA & LASOTA, 2017;PIERSON, 2017; FROUZ & VINDUŠKOVÁ, 2018;ABDELRAHMAN et al., 2018; FILCHEVA, 2018;STUMPF et al., 2018). The issue of globalwarming forces the studies, which increasethe knowledge of domains, resistance andcycling of chemical elements in and out ofecosystems (FINZI et al., 2011; DELGADO-BAQUERIZO et al., 2013; TSOLOVA et al., 2014;PARTON et al., 2015; YUAN & CHEN, 2015; TAN& WANG, 2016; JIAO et al., 2016).
The scarce data on urban soils inBulgaria has aroused the interest in studyingtheir pedochemical characteristics asinformation carriers on modern and relictprocesses of soil formation andtransformation. By studying the cationexchange capacity, base saturation level,content of main exchangeable cations, contentand forms of essential nutrients includingorganic carbon, the present article aimed topresent the trends of transformation ofmineral and organic matrix of urban soilslocated along the direction of increasinggradient of urbanization in cursoryinvestigated soil zones belonging to theresidential and industrial districts of the Sofiacity (Bulgaria).
Materials and MethodsLocation and morphogenesis of studied soilsData on 6 soil types, located at the
Eastern part of the Sofia city are representedin present publication. All soils formecosystems with recreational significanceand some of them have not been previouslystudied. They are distinguished for thefollowing morphogenesis:
Anthropogenically overlappedmoderately leached Smolnitsa, loamic ischaracterized by profile 1 located in“Mladost” residential region (Fig. 1). Thissoil was formed as a result of urbanizationand occupies previously unexplored soilzone. In fact, moderately leached Smolnitsaborders this highly urbanized zoneaccording to the previous studies (ACHKOVet al. 1972). The original soil, Smolnitsa(named after organic clays, smolnitsascomposing soil) is overlapped by layers ofearth calcaric masses, mixed with urbanwaste, pebbles and gravel. Profiledevelopment and morphologicalorganization of new soil includesdifferentiation of organic matter, whichresulted in a bimodal distribution and 3representative horizons for soilmorphogenesis: Ahk (0-15 cm) - C1k (15-65cm) - Аb (65-110 cm).
WRB classification of soil: UrbicTechnosol (Eutric, Loamic, Humic,Transportic) over Pellic Vertisol (Chernic,Endocalcaric). Profile 2 characterizesTechnogenic soil, moderately deep, loamic,moderately stony (15% coarse surfacefragments’ content in Ah horizon, Fig. 2),classified as Urbic Technosol (Amphyskeletic,Calcaric, Mollic, Transportic). This soil isformed by massive pilling of earth calcaricmaterials onto the moderately leachedSmolnitsa, loamic during the “Mladost”district construction. Profile developmentand morphological organization are resultsof surface accumulation of organic matterand slow weathering of subsoil that isstrongly mixed with urban building artefacts– these processes lead to the formation of athree-layered profile: Ahk (0-21 cm) - C1k (21-52 cm) - C2k (52-85 cm). Parent materials areQuaternary brown alluvial clays andPliocene sands, usually calcaric (YANEV et
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Venera T. Tsolova, Plamen V. Tomov, Ivona P. Nikova, Galina P. Petkova
al., 1992; 1995; BOJINOVA-HAAPANEN, 2014).Profile 3 illustrates the morphogenesis inmoderately leached Cinnamon forest soil,loamic, slightly to moderatelyeroded /Chromic Endocalcic Luvisol (Clayic,Differentic, Humic, Profondic)/ located in theperiphery of the “Mladost” residentialregion (Fig. 3). Profile development is aresult of pedogenetic differentiation(illuviation) of clay content by depth andleaching of base cations. Morphologically,these processes form the following horizons:Аh (0-10 cm) - Вt (10-35 cm) - Вt2 (35-72 cm) -Вtк (72-86 cm) - Сk (86-120 cm). The soil-forming materials according to YANEV et al.,(1992; 1995) and BOJINOVA-HAAPANEN,(2014) are Quaternary diluvial-colluvialmaterials (non-sorted gravel, boulders andclay-sandy deposits) and Pliocene sediments(yellow-rusty clays with layered structure,usually calcaric, clays with sandy matrix andgravel). Profile 4. Alluvial soil, moderatelydeep, slightly stony /Hypereutric Fluvisol(Loamic, Somerimollic)/ distributed in“Drujba” industrial region (fig 4). This soil islocated in an over flooded terrace of theIskar River, in a virgin district, next to the“Sofia Iztok” Thermal-electric Power Plant.Profile development is limited by coarsefragments abundance in subsoil andtherefore the soil formation processesinvolve only the uppermost 15 cm. Theyresulted in morphologically simple profileorganization: Аh (0-15 cm) – C1 (15-40 cm) –C2 (40-80 cm). Parent materials are mostlylarge gravels and boulders with a sandymatrix which lie onto a Pliocene stratumcomposed of sands and grey or greencoloured clays (YANEV et al., 1992; 1995;BOJINOVA-HAAPANEN, 2014). Fig. 5 showsprofile 5 and Alluvial meadow soil,deep /Hypereutric Fluvisol (Epiclayic,Endoloamic, Pachic)/ located in “Drujba”residential region. It is formed within theflooded terrace of the Iskar River by fine-particle alluvial sediments of a Quaternaryand Pliocene origin (ACHKOV et al., 1972).Profile development and morphologicalorganization is also marked by surface
accumulation of organic matter andlithological clay differentiation by depth: Аh
(0-30 cm) – А2 (30-55 cm) – C1 (55-105 cm) –C2 (105-155 cm).Profile 6 is in stronglyleached Smolnitsa, super deep, moderatelyclayey /Pellic Vertisol (Pantochernic,Hypereutric, Relictigleyic)/ distributed in“Mladost” residential region (fig 6). This soiloccupies the higher part of the previouslyunexplored soil zone and neighbours themoderately leached Smolnitsa. This pedonalso consists of organic clays (Smolnitsa),which foster the super deep A-horizondevelopment (reaching up to 165 cm depth).Humus horizon directly lies on parentmaterials - grey-brown Pliocene clayscontaining calcareous nuts (YANEV et al.,1992; 1995; BOJINOVA-HAAPANEN, 2014).
Chemical studiesThe cation exchange capacityThe cation exchange capacity (Equation
1), the base saturation level and the content ofthe main exchangeable cations in soils weredetermined under the GANEV & ARSOVA(1980) method . This method determines thecontribution of both the permanent,preferential charges (on basal surfaces, TCA)and variation charges of soil colloids (basicallypH dependent exchange including the lateralsurfaces, TA) to the cation exchange capacityby titration of soil extracts (obtained by mixedsolution of 1.0 n sodium acetate and 0.2 npotassium maleate having pH 8.25) with 0.04 nsodium hydroxide solution in the presence ofphenolphthalein to determine TA andsubsequent titration of the above eluate with0.04 n complexon III (after dilution up to 200cm3 with deionized water and addition of 10cm3 of triethanolamine and 2 cm3 of 5.0 npotassium hydroxide solution, non-carbonateto achieve pH 12-13) in the presence ofchromium-blue to determine ТCA (Equation 1):
T8.2=TCA+ TA (cmol/kg) (1)
Exchangeable Al: in 1.0 n calciumchloride filtrate obtained as a soil:extractantratio 1:25 by titration with 0.04 n sodium
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Pedo-chemical perturbations in soils from green ecosystems of the Sofia city (Bulgaria)
hydroxide in the presence ofphenolphthalein. When soil pH <4.0,exchangeable hydrogen ions, HA, should bedetermined first - 1 drop of methyl orange isadded to the calcium chloride filtrate andtitrated with 0,04 n sodium hydroxide andthen this solution is treated to determine theexchange aluminium.
Total acidity (exchange H8.2) iscalculated by the Equation 2:
exch.Al + HA = exch.H8.2 (cmol/kg) (2)
Exchangeable calcium: 50 cm3 of themixed sodium-acetate and potassium-maleatesolution is diluted with deionized water to 100cm3. Then 5 cm3 of triethanolamine (1: 1), 1 cm3
5.0 n potassium hydroxide solution and achromium blue (calcon) are added to achieveintensive purple-red colouring. The solution istitrated slowly with 0.01 n complexon III to adeep blue colour.
Sum of exchangeable calcium andmagnesium: A new 50 cm3 of the filtrate isfilled up with deionized water to about 100cm3. Five cm3 of triethanolamine (1: 1) and asolid mixture of eriochrome black are addedto reach pH of 9.5-10.0 and titrated slowlywith 0.01 n complexon III to a deep bluecolour. Exchange magnesium is determinedby the difference between the sum of twoalkaline earth cations and exchange calcium.
The base saturation level (V) iscalculated in percentages as the differencebetween the magnitude of total cationexchange capacity (T8.2) and total acidity(exchange H8.2) relative to the magnitude oftotal cation exchange capacity.
Humic substances content and compositionThe content of extractable humus fractions
was determined using the Kononova-Belchikova method (FILCHEVA & TSADILAS,2002) in four extracts at soil: solution ratio 1:20.
Total organic carbon was determined bythe modified dichromate oxidation method (theoxidation of the soil sample with 0.4 N K2Cr2O7
and concentrated H2SO4 in a ratio 1:1 at 120 0Сfor 45 min. in the presence of Ag2SO4 followedby a titration with 0.2 N Mohr’s salt). Humuscontent is calculated by multiplication oforganic carbon content with the coefficient1.724.
Content of humic (HA) and fulvic (FA)acids – in a mixed solution of 0,1 M Na4P2O7
and 0,1 M NaOH, and separation of FA by 0,5M H2SO4 as an acidifying agent.
Content of free or linked to sesquioxideshumic and fulvic acids representing thepotentially mobile HA and FA – extracted with0,1 M NaOH.
Content of the low molecular (aggressive)fraction of fulvic acids – in extracts with 0,05 MH2SO4.
Optical hallmarks (E4/E6) are determinedin HA-fraction as a ratio of the optical densitiesat 465 and 665 nm.
Elemental and speciation assaysContent of ferromagnesian trace elements
was determined after sample mineralizationwith aqua regia (ISO 11466:1995) via AAC (ISO11047:1998) on a Perkin-Elmer 2100.
Total nitrogen content was quantified bythe modified Kjeldahl method (BDS ISO11261:2002) and the main mineral nitrogenforms – by procedure of BREMNER & KEENEY(1965).
Carbonate content was measuredfollowing ISO 10693:1995 protocol whichreproduces the Scheibler method.
Sample pre-treatment and pH determinationSoil samples were pre-treated according to
BDS ISO method (11464:2012) and pH wasmeasured in 2.5:1 water soil suspension (1 partsoil and 2.5 parts deionized water) according tothe protocol given by GANEV & ARSOVA (1980).
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Venera T. Tsolova, Plamen V. Tomov, Ivona P. Nikova, Galina P. Petkova
Fig. 1. View and location of profile 1 in overlapped moderately leached Smolnitsa.
Fig. 2. View and location of profile 2 in Technogenic soil.
Fig. 3. View and location of profile 3 in moderately leached Cinnamon forest soil.
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Pedo-chemical perturbations in soils from green ecosystems of the Sofia city (Bulgaria)
Fig. 4. View and location of profile 4 in Alluvial soil.
Fig. 5. Location and view of profile 5 in Alluvial meadow soil.
Fig. 6. Location and view of profile 6 in strongly leached Smolnitsa.
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Venera T. Tsolova, Plamen V. Tomov, Ivona P. Nikova, Galina P. Petkova
Results and DiscussionSoils created as a result of urbanization
(profiles 1 and 2) are characterized with veryslightly alkaline reaction (Table 1) andmiddle content of carbonates (in the interval3-5%). They have middle colloidal activity(Т8,2 from 30 to 45 cmol/kg), according to theclassification given by GANEV (1990) andsorption interactions, transforming theslightly acidic positions of mineral colloidsinto the hydrogen-acidic complex. Due to thepresence of carbonates, this acidic-hydrogenform of TA, i.e., exchangeable retention ofhydrogen cations upon the slightly acidicpositions of mineral colloids can beconsidered physico-chemical characteristicof soils, which originated from thehydromorphic stage in their genesis andwater self-ionization catalytic effect. Signs ofhydromorphism are evident in many soilcharacteristics (clay mineralogy, organiccarbon state and transformation) and leave amark on their current genesis. Concerningclay mineralogy, the predominance ofmixed-layered smectite-vermiculitestructures was found in Pliocene claysdistributed in the “Mladost” residentialdistrict (BOJINOVA-HAAPANEN, 2014) whichconfirm our data and suggestions. Thismineral might be indirectly identifiedthrough the typically high total magnesiumcontent (or “pseudo” total contentdetermined in the aqua regia extract, Table1). The sorption capacity which is not veryhigh also suggests a lack of pure smectitesand together with transformation of biotite(that is present in Smolnitsas) intovermiculite could explain CEC values.Magnesium, despite of its high content, isnot the main exchangeable cation and doesnot cause magnesium salinization.
Hydromorphism and furthertransformation of the mineral matrix turnhydrogen in the second abundant exchangeion after calcium. As a result of highhydrolytic acidity (exch. Н8,2) the basesaturation level is under 93%, which isconsidered a critical minimum for lack ofdeleterious acidity in soils (PALAVEEV &
TOTEV, 1985; TRENDAFILOV, 1992). Thedeleterious acidity in soils is notaccompanied by toxic aluminiumavailability (exch. Al) and possibility for Aldesorption in the soil solution – it is limitedonly to destabilizing role and chemicalactivity of exchangeable hydrogen.
The content of hydrolytic acidity inTechnogenic soils (profile 2) sharplydecreases under 21 cm depth (over 2 times)and that positively influences subsoil basesaturation status (V). Although the sorptionpotential in surface horizon of Technosol isgenerally lower, its hydrolytic aciditycontent is close to that in topsoil of theoverlapped Smolnitsa. Obviously, thehydrolytic acidity in topsoil of these newly-formed soils originated from biogenicprocesses associated with well-developedmeadow vegetation (TSOLOVA & TOMOV,2018) rather than clay mineralogy, becausethis is the only profile wherein thevermiculite is not occurred (as wementioned above the higher content ofmagnesium than calcium is indicative forvermiculite presence).
The carbonates in the Ah horizon ofmoderately leached Cinnamon forest soils(profile 3) take part in neutralization of acidproducts generated by biodegradableprocesses. This gradually leads to theirdepletion and acidification of soilenvironment to рН 5.9. The strong linearcorrelations shown on fig 7 reveal theprevalence of exchange hydrogen cationsonto slightly acidic positions and pHdependence on the exchange hydrogencontent.
The exchangeable acidity (exch. Al)also occurs in topsoil in a concentration thatcould be toxic for many pasture species(CORANGAMITE REGION "BROWN BOOK"). Itusually appears as a result of aciddestruction of clay minerals, which can beseen in the ratios: Т8,2 in Ah/ Т8,2 in Ck < 1and Т8,2 in Вtк/Т8,2 in Ck > 1 (GANEV, 1990).The colloid degradation in Ah is moderateaccording to the classification given byGANEV (1990) and shows that this process is
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Pedo-chemical perturbations in soils from green ecosystems of the Sofia city (Bulgaria)
still running slowly. The moderatelyleached Cinnamon forest soil also hasmoderately high sorption capacity butsmaller buffer potential which, as it wasmentioned above, decreases as a result ofincreasing acidity in surface horizons.
Alluvial soils from the Iskar river valley(profiles 4 and 5) are formed of sedimentswith different coarse fragments contents.The stonier soils (profile 4) are moderatelycolloidal (Т8,2 from 20 to 30 cmol/kg) withhigh neutralizing potential (V over 80 cmol/kg). Their acidic systems also saturate thevariation charges on colloidal surfaces withhydrogen and evoke slightly acidic reaction(according to ATANASOV et al., 2009classification) - 6.1-6.9. Studied parametersdecrease downwards the profile depth,resembling the distribution in somenormally developed, genetically old soilsand do not follow the lithological differencesbetween separate horizons.
Alluvial-meadow soils (profile 5) aremoderately colloidal (Т8,2 from 30 to 45cmol/kg) and mostly moderately acidic (рН5,1-6,0). They have the highest hydrolyticacidity among studied soils and respectivelythe lowest base saturation level within thewhole depth (Table 1). The base saturationlevel in the interval 77-86% defines a middlerange of deleterious acidification of soilsaccording to the classification scheme set byBulgarian legislation (ORDINANCE № 4).
The features of strongly leachedSmolnitsa (profile 6) reveals the evolution ofsoils distributed in the peripheral part of thispreviously unexplored soil zone. They areneutral, highly colloidal soils with highneutralizing potential which is slightly lowerin A’ and A’’ as a result of the listedhypergenic processes. These soils aredistinguished with small amount of slightlyacidic charges (TА) in the humus horizonwhich is presumably due to the slow in situtransformation of biotite into vermiculite,which suggests a lack of defects in thecrystal structures (due to the lack oftransportation) and a small formation oflateral surfaces yet. These processes can be
more clearly observed in the last sub-horizonwhere the content of slightly acidic positionsis the smallest and the content ofexchangeable magnesium – the highest(Table 1). The content of “pseudo-total“magnesium (from 565 to 607,5 mg/kg) ishigher than the content of calcium (420-560mg/kg) within the whole profile depthwhich evidences for the strong leaching ofcarbonates probably in the form of ironcarbonates due to the low content of“pseudo-total“ iron too (from 1,06 to 1,30%).These data support the opinion of STRANSKI(1936) that a hidden process of podsolizationtakes place in the black Sofian soils, since itcan't be diagnosed by usual morphologicalfeatures. This phenomenon is also observedby NIKOVA & TSOLOVA (2018) in arableSmolnitsas from the Sofia valley.
Organic matrix hallmarksSurface horizon of the Overlapped
(Buried) Smolnitsa (profile 1, Table 1) isvery rich in organic matter (5.52% humus),despite of soil recent creation (about 45years ago). The humus is of Mull type,abundant in humic acids (CHA/CFA > 2,0)which dominated along the entire depth.Humic acids are strongly condensed (E4/E6
= 3.87), very hydrophobic and slightlymobile polymers. They are strongly boundto the mineral matrix having in mind thedominance of Ca-humates (100%). The lowmolecular (aggressive) fraction of fulvicacids is also present in descendingconcentrations (from 0,8 in topsoil to 0,4 g/kg in buried horizon). The ratios of C:N(14.04-10.40) in this epipedon indicatemiddle to high enrichment of hydrocarbonswith N (Fig. 8). These N-dressedcompounds are active source ofammonium-N and may provoke the soiltoxicity (BRITTO & KRONZUCKER, 2002).Values obtained for main mineral forms ofN illustrate this trend (fig 8) considering aprincipally low content of nitrate-N in soils.
The basic features of organic matter(OM) in surface horizon of Technogenic soils(profile 2) are: morphologically
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Venera T. Tsolova, Plamen V. Tomov, Ivona P. Nikova, Galina P. Petkova
homogeneous humus system of wellhumificated organic matter (CHA: Ctotal x 100,in % = 17.3%) formed by soil-biomesinteractions and medium humus content(2.59%); Rhizomull type of humus whereinthe very strongly condensed and stablehumic acids (C4/E6 = 3,50) are absolutelyprevalent (CHA/CFA = 3,25). The content oforganic carbon (OC) sharply drops (up to 1.8g/kg) in subsoil, where potentially mobileOM is only composed of FA (up to 33% oftotal C). The degree of OC enrichment withN is very low especially in topsoil (C:N21.74) and could be primary attached to thefeatures of newly formed organic matteroriginated from cereal plant species whichare dominant in this ecosystem (TSOLOVA &TOMOV, 2018), soil biota activity and lowatmospheric inputs of N.
The humus-accumulative horizon (Ah)of Cinnamon forest soil (profile 3, Table 1) isaltered by erosion and this affected carbonstocks - it is moderately rich in organicmatter (2.86% humus) likewise Technogenicand Alluvial soils. The prevalence of humicacids is slightly pronounced there (CHA/CFA
= 1.17) and the degree of condensation oftheir aromatic nuclei is lower (E4/E6 = 4.08),although this does not change HAhydrophobicity and structure. FA contentsharply drops beneath 35 cm and positivelyinfluenced the OM humification rate. Theincrease content of humus acids fractionsevokes the naturally occurring leachingprocess and acidification of pH (5.9). Theinteraction between pH and potentiallymobile fractions of humus acids, respectivelyfulvic acids is evidenced by the statisticallysignificant correlation between them (R² =0.73 for both fractions and R² = 0.85 for fulvicacids).
C:N ratios (10.44-12.29) in this epipedonindicates high degree of organic matterenrichment with nitrogen and respectivelysimilar rate of release of NH4-N.
The status of organic matter in the nextthree profiles (№№ 4, 5 and 6) differentiatesfrom described above. All of them arecharacterized with almost equal amount of
HA and FA or lack of HA (like in Alluvialsoils - profiles 4 and 5). Fulvic acids aremainly mobile and partially aggressivehaving in mind the aggressive fractionscontents (up to 25% of the total FA fraction).HA are stable, strongly condensed polymersmostly bound to Ca. Organic matter ishighly abundant in nitrogen but mineral Ncontent is lower than in profiles 1, 2 and 3(C:N values fluctuate in the interval 8.6-20.2with average value 11.54 mg/kg, Fig. 8). Thelow variation of C:N values by depth revealsthe ancient age of humic substances andtheir stability in diverse soil environments.On the other hand, the older organic colloidshave low reactivity (MCBRIDE et al., 1997;BRADL, 2004; COUTRIS et al., 2012) and highresistance to biodegradation and thereforeplay a minor role in CEC.
The results obtained confirm thefundamental finding that transformationprocesses of biogenic products in soils aremuch more intense than those of the mineralcomponents. The presented study showsthat biogenic transformation in the modernurban environment is a multifactorialprocess, dependent on all environmentalcomponents.
The elevated NH4 and NO3 contents inprofiles 1, 2 and 3 can be related to humaninduced urea saturation of soils (they are alsoused for strolling pets) which may entail ahigher rate of ammonification and amplifythe nitrogen cycling. In areas where organicmatter is more abundant in nitrogen (profile4, 5 and 6) the main additional source of N isgreenhouse gas emissions (or theirprecursors - NOx, CO and NMVOCs) whichmay also affect the cycle of nitrogentransformations. All profiles are located inclose proximity to bustling traffic arteriesand simultaneously in the direction ofprevailing winds (from the north andnortheast) which distribute thecontamination from the industrial zoneknown with its strong negative impact onthe environment (UZUNOV et al., 1996;FAITONDJIEV et al., 2000; DIMITROVA et al.,2010). The higher temperature of topsoils of
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Pedo-chemical perturbations in soils from green ecosystems of the Sofia city (Bulgaria)
profiles 4, 5 and 6 (up to 3-4 oC) supports theassumption for differentiation of mineralnitrogen fluxes during the anthropogenicimpact, although the significant correlationbetween organic nitrogen and carbon (fig 9)shows that humus is the major source ofnitrogen. This is also among reasons foralkaline pH values in profiles 1, 2 and 3regardless of photochemical smog andnitrous oxide (dinitrogen monoxide, N2O)acidifying effect.
In urban environments carbon andnitrogen cycles are still coupled (Fig. 9),although the plant diversity in studiedecosystems does not imply substantialnitrogen revenues.
Some more important statistic dataregarding the organic carbon abundance canbe noted: the established average content oforganic carbon in the surface layer of studied
urban soils, 19.3 g/kg is almost 2-fold lowerthan the content of organic carbon in virginleached Smolnitsas of Bulgaria - 35 g/kg(FILCHEVA, 2007). This content is equivalentto the average content (19.1 g/kg) in thesurface horizon of grasslands in Bulgaria(TÓTH et al., 2013) but higher than thecontent in topsoil of grasslands in the Sofiavalley - 12.5 g/kg (LUCAS 2015).
Comparing the results for the meanvalue of CEC in topsoil of Bulgariangrasslands, extracted for LUCAS (2015) - 36,7cmol/kg shows a close average value onlyfor moderately leached Cinnamon forestsoils (profile 3) and Alluvial-meadow soils(profile 5). The lower content of clay fractionand smectite-vermiculite as well, can explainthese lowest CEC values in normallysupplied with organic carbon Alluvial soils(profile 4).
Fig. 7. Correlations between hydrolytic acidity and variation charges TA (left), andhydrolytic acidity and pH (right) in moderately leached Cinnamon forest soil.
Fig. 8. Mineral nitrogen content and C:N ratios in urban soils.
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Venera T. Tsolova, Plamen V. Tomov, Ivona P. Nikova, Galina P. Petkova
Fig. 9. Correlations between total N and C content in root layers of urban soils, and mineralN and pH (average values).
Table 1. Chemical data on studied urban soils in the city of Sofia.
characteristics of studied urban soils areresult of positive and negativetransformation of their matrix. Positivechanges are mostly related with organicmatrix and intensive processes of humus-formation and accumulation. Main factorsthat favour accumulation of organic carbonin studied soils are high content of silt andclay fractions which are typically humusfractions since they contain high amount ofhumus and humino-mineral complexes;organic clays, smolnitsas, comprising in theparent materials of some soils (because theyare rich in organic carbon), as well as thestability of humus acids and their lowmobility. Stability of humic acids is relatedwith their dense heterocyclic structure andhigh nitrogen enrichment. Recently formed
humus is also well humificated and rich inhighly condensed humic acids. For thisreason, organic colloids are predominantlymature, persistent and slightly active.Simultaneously, in acidic soil horizons evena slight increase of FA content enhances thepH dependence on their content.
Positive changes of the mineralmatrix are derived from mineral colloids andslow transformation of biotite intovermiculite – this process may reduce thesoil hydrolytic acidity. Mineral colloidspredominantly determine the sorptioncapacity and acidic complexes in studiedsoils. Negative changes of the mineral matrixare provoked from:
Acid destruction of clay mineralsoccurring as a consequence of the naturallyoccurring soil-forming and weatheringprocesses of low intensity and associated
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Venera T. Tsolova, Plamen V. Tomov, Ivona P. Nikova, Galina P. Petkova
processes of slight dispersion anddisintegration of the mineral matrix;
Increase of exchangeable hydrogencontent above the exchangeable magnesiumlevels up to the second abundant exchangeion after calcium although the H-destabilizing role itself is difficult todistinguish.
Anthrogenically induced changes, wherethey can be identified, can increase mineral Ncontent and fluxes and may influence theexisting acid-base status of soils due to theinput of neutral, alkaline (urea, ammonia) oracidifying agents (water soluble compoundsof CO2, NO2) present in the groundtroposphere of the Sofia city.
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