Soil Organic Matter Composition in Urban Soils: A Study of Wrocław Agglomeration, SW Poland

Abstract

In urban areas, because of anthropopressure, the transformation of the soil cover can lead to the complete destruction of the natural layout and properties of these soils. The object of this study was to determine the quantity and quality of soil organic matter (SOM) originating in the topsoil horizons of the central part of Wroclaw (SW of Poland). Fractional composition of SOM and humic substances (HS) analysis were performed. Elemental composition and CP MAS ¹³C NMR spectra for the humic acids (HA) were determined, and α (aromaticity) and ω (oxidation) ratios were calculated. Total organic carbon content ranged from 22.39 to 66.1 g kg^–1, while that of total nitrogen ranged from 2.09 to 4.6 g kg¹. In most analysed urban soils, the highest share in SOM was found for residual carbon (CR), while HA of low maturity was the predominant group over FA. CP MAS ¹³C NMR spectroscopy of HA molecules indicated the structure of the samples was dominated by compounds with low aromaticity cores and considerable contents of aliphatic components. In urban soils, efforts should be made to enhance organic matter transformation into more matured and stable forms via, e.g., compost application and chemical treatments, and lawn maintenance should be very strictly controlled and limited.

1. Introduction

The formation of urban soils is the result of the intensive influence of anthropogenic factors on natural soil-forming processes. This phenomenon, observed primarily in urbanised and industrialised areas, promotes considerable transformations of the soil cover [1,2,3]. Moreover, in many cases, it can lead to the complete destruction of the natural layout and properties of these soils. This can result in very specific soils whose properties are continuously influenced by the intensity, direction and time of anthropopressure. Based on many studies [4,5,6], the most important causes have been identified, as well as the direct and indirect effects of human impact on the soil environment in urban–industrial areas. These include construction, redevelopment of urban green areas, industrialisation, regulation or deregulation of water relations, the introduction of waste to the soils, mechanical transformation, and changes in their physical, chemical and biological properties. As a result, the most frequently observed [1,7,8] changes in the soil environment in urban–industrial areas are changes in morphological features, chemical degradation and decreases in their biological activity. Many authors [9,10,11,12] indicate the most recognisable symptoms of anthropopression in urban soils are qualitative and quantitative changes in the composition of soil organic matter. It should be noted, however, that the knowledge of these changes and interactions with mineral fractions is insufficiently recognised, and there are no detailed studies on these issues.

Soil organic matter has a positive effect on several biological, physical, chemical and physicochemical properties of the soil. It improves soil structure, increases water holding capacity, has buffering properties, supplies plants with nutrients and decreases erosion risk [13]. Soil organic carbon filters pollutants, degrades contaminants and is a major sink for atmospheric CO₂, and it could offset fossil fuel emissions by 5 to 15% of global emissions [14]. Among several organic fractions, humic acid and humins are the most capable of bonding to toxic substances. Thus, to increase the capacity to fix organic pollutants in soil, the formation of more humified material in the SOM should be stimulated [9]. Any change in the turnover rate of SOC may alter the atmospheric CO₂ concentration and consequently affect the global climate [15]. Hence, the merge between C sequestration and climate change should not be ignored.

Undoubtedly, the main goals of caring for the quality of organic matter, especially in urban soils, should be its stabilisation, limiting C losses and facilitating the transformation of SOM into more stable substances, which can form complexes with lower turnover rates [16,17]. The transformation of humus compounds in the soil, which depends on the overall ecological conditions, causes changes in most of their properties. Large variations in organic carbon content, especially in the surface layers, are characteristic of urban soils. This is mainly determined by the direction and intensity of the degree of anthropogenic transformation, land use and possible organic fertilisation.

When discussing the properties of organic matter in anthropogenic soils [18,19,20], it is important to remember that they may contain both natural humic substances and organic components of an anthropogenic origin, such as debris, ash, garbage or deposit of airborne particles. It should be emphasised that organic matter in urban soils is characterised by different intensities of susceptibility to transformation, decomposition and humification processes. Thus, the stabilisations of SOM are complex processes that lead to an increase in its turnover time in soil and should not be considered only with their accumulation. Many authors indicate [9,21,22] that it may be caused by an increase in organic compounds with very characteristic fractional compositions. Studies have shown that in anthropopressured areas, the predominant fraction of soil organic matter is non-hydrolysable carbon (residual carbon, CR), remaining in the soil after the extraction of two main fractions of humic substances: humic and fulvic acids. This can be explained by the introduction of specific non-humus organic substances of anthropogenic origin into the surface soil layers of urban areas, e.g., coal dust, soot, petroleum substances and car tire residues, among others. It was also shown that in many cases in the fractional composition of humic substances of the studied urban soils, the humic acids predominated over fulvic acids. In addition, the humic acids studied formed very specific, stable molecules, including complexes and chelated compounds, contributing to the stabilisation of humus.

The heterogeneous and polymorphic nature of the humic substances in soils makes it difficult to develop a universal model of humification processes and a model of humic acid molecules [23,24,25,26,27,28]. Despite there being many studies on the subject [1,9,18,21,29,30,31,32,33,34], the results concerning the transformation of organic matter in soils of urban areas are not conclusive. Many authors [11,12,35,36] explain these very complicated phenomena through the presence of large amounts of inactive forms of non-hydrolysing fractions of organic carbon (residual carbon) from anthropogenic admixtures and direct or indirect human impacts [37,38,39] on the soil environment. Therefore, this study’s purpose was to determine the qualitative and quantitative changes in soil organic matter due to urbanisation and anthropopressure impacts. The obtained results may increase the existing knowledge on the transformation of humus compounds under the specific conditions of the urban environment.

The scientific hypothesis adopted was that in the urban soils, the highest share of organic carbon is incorporated in the residual fractions and the humic substances formed are characterised mostly by aliphatic compounds.

2. Materials and Methods

2.1. Objects of the Studies and Sampling

Wrocław is the capital of Lower Silesia, with a population of approximately 630 thousand, the mean annual temperature is 9 ℃, and the amount of precipitation is 583 mm. The city is located at an altitude of 105 to 156 m above sea level. The landscape of the city is dominated by the Odra River with its branches, canals and backwaters (a total length of 54.5 km within the city limits). The soils of the Old Town at the centre of the Wrocław agglomeration (SW of Poland) were investigated. The study included areas where the intensity of urbanisation impact was limited mainly to tourism, not under the direct influence of industrial plants and not affected by the flood of 1997 [40,41]. In addition, the areas had to meet the following criteria: homogeneous area of at least 400 m², depth of the A soil horizon of at least 15 cm, contents of total organic carbon (TOC) at least 16 g kg^–1 and no supporting organic fertilisation for at least 5 years. Based on the criteria above [42], four study areas—transects with three soil profiles per transect—representing natural soils (control) and anthropogenic soils (in the

Table 1. Localization and general information on investigated soils.

Profile No	GPS Coordinates	WRB Soil Unit	Depth of Debris and Artefacts Layer	Land Use Type
			cm
Control	N 51. 115.11; E 17. 042556	Fluvic Cambisol		walking area. park
1	N 51. 113659; E 17.047577	Mollic Urbic Technosol	40–70	lawn
2	N 51.107843; E 17.043274	Urbic Technosol	25–50	walking area. lawn
3	N 51.114444; E 17.050090	Urbic Technosol	20–65	walking area. lawn

, soil profiles no. 1–3, corresponding to the transect 1–3) were established. For each transect, soil samples were taken from each soil profile (three profiles) at the same depth and thoroughly mixed to obtain one representative sample.

The areas selected for the study were subjected to basic phytosociological analysis. The botanical survey showed a predominance of grasses: 40% Festuca rubra L., 20% Poa pratensis L., 20% Lollium perenne L., in addition to 10% legumes (Trifolium repens L., Trifolium pratense L.) and 10% herbaceous plants (Taraxacum officinale L., Bellis perennis). In each investigated area, an east–west transect was established along which the standard soil profile was performed, and two additional sampling points were equidistant from the profile. The soil unit was defined according to WRB guidelines [42], and samples for analysis were collected from the A horizon in the profile and from the sampling points. Samples were air-dried and ground mechanically to a diameter of 2.0 mm. The coarse fraction was removed, while the fine earths were subjected to laboratory analysis. The mean results from three replications are presented in the tables.

2.2. Basic Physical, Physicochemical and Chemical Properties

To determine the physical, physicochemical and chemical properties of the investigated soils, the following determinations were performed:

• soil texture using an automated dynamometer method to determine soil textural classes according to PSSS and USDA [42,43,44,45];
• pH in 1 mol dm^–3 KCl using potentiometric method (m:v ratio as 1:2.5);
• content of total organic carbon (TOC) and total nitrogen (TN) using a Vario Macro Cube CN analyser (Elementar Analysensysteme GmbH, Germany).

2.3. Comprehensive Studies on Soil Humic Substances (HS)

Humic substances (HS) were extracted from the collected soil materials according to the method that has been found by the International Humic Substances Society (IHSS) to be satisfactory for most soils [32,46,47]. The organic carbon of humic and fulvic acids (CHA and CFA, respectively) and the residual carbon (CR) represented mainly by humin fraction [48] were determined. The obtained humic acid gel was purified with a mixture of 0.1 mol dm⁻³ HCl and 0.3 mol dm⁻³ HF, then dialyzed (7 Spectra/Por Dialysis MWCO: 10,000 membranes) and freeze-dried. In the solid humic acids (HA), the following determinations were performed:

• ash content after ignition at 550 °C;
• elemental composition with 2400 CHN Perkin Elmer (United Kingdom) analyser. Based on the results, the atomic ratios (H/C, N/C, O/C and O/H) and oxidation ratio (ω) were calculated according to the formula [49]:

ω = (2 · O + 3 · N – H) · C⁻¹

(1)

where O, N, H and C are the shares of O, N, H, and C in HA molecules in at. % (atomic percentage):

• ¹³C NMR spectra on the Bruker Advance III 300 MHz spectrometer (Germany) with CP-MAS unit for the range 0–210 ppm. The shares of carbon present in defined organic bonds [9,11,30] were determined: Calkyl (0–45 ppm), CO-alkyl (45–110 ppm), Clig (14–160 ppm), Ccarbox (160–200 ppm), Caliph (0–110ppm), Carom (110–160 ppm). Based on the obtained data, the degree of aromaticity [11,47] was calculated according to the formula:

α = (Carom · Caliph⁻¹) · 100 (%)

(2)

where Carom is the content of carbon pf aromatic bonds (in ppm) and Calif is the content of carbon of aliphatic bonds (in ppm).

2.4. Statistical Analysis

The obtained results were statistically processed with Statistica 13 software. Differences between objects were checked according to Tukey’s test at the significance level of p < 0.05 using ANOVA software, and correlation matrices for selected parameters were performed.

3. Results and Discussion

3.1. Basic Physical and Chemical Properties of the Investigated Soils

The soils characterised varied in their use, genesis and physical and chemical properties (Table 1 and

Table 2. Basic physical and chemical properties of the investigated urban soils.

Profile No.	Depth of A Horizon [cm]	pH (KCl)	Particles > 2.0 mm	Particles < 0.002 mm	USDA Textural Class	TOC	TN	TOC/TN
			%			g kg−1
Control	0–20	6.48	5.5	11.0	loamy sand	22.39 a	2.09 a	10.71 a
1	0–30	6.50	3.3	12.0	sandy loam	66.08 b	4.63 b	14.27 b
2	0–15	6.52	6.1	14.0	sandy loam	53.45 c	2.81 c	19.02 c
3	0–20	6.46	5.8	13.0	sandy loam	58.24 d	3.40 d	17.13 d

Note: ^a,b,c,d—significantly different at p < 0.05.

). The investigated horizons did not differ in texture between the control and the technosols. Analysed samples were characterised by a very low share of coarse fractions in the range of 3.3–6.1% and a low content of clay fraction from 11.0 to 14.0%. Therefore, the impact of anthropopressure in the study areas did not cause changes in soil texture, and the soils did not contain artefacts in the top horizons. Similarly, the activities discussed did not affect the variation in pH among these soils. Although the differences were not significant, values obtained indicated alkalisation processes [9,50,51] caused by the deposition of alkaline dust, mainly from weathered calcareous building mortars, architectural elements and monuments. However, statistical analysis (

Table 3. Correlation coefficients between selected chemical parameters and content of humic substances in the studied soil horizons.

Parameter	Clay	pH KCl	TOC	TN	CHA	CFA	CR
Clay	-
pH KCl	0.841 *	-
TOC	0.576 *	0.306	-
TN	0.128	−0.132	0.876 *	-
CHA	−0.167	−0.395	0.622 *	0.884 *	-
CFA	0.650 *	0.335	0.788 *	0.637 *	0.585 *	-
CR	0.534	0.467	0.654 *	0.409	−0.043	0.150	-

Note: * significant at p < 0.05.

) showed that despite the increased pH values, this parameter did not significantly affect most of the chemical properties analysed. Significant differences in chemical properties of the studied surface levels were found for TOC and TN contents (Table 2). The organic carbon contents in the surface horizons of the investigated soils were high and ranged from 22.39 to 66.08 g kg⁻¹. Enrichment of the surface layers of anthropogenic urban soils in organic matter can be explained using organic fertilisers and peat at the stage of preparation of the studied area for tourism. This phenomenon has been previously observed in the historical parts of cities [9,12,50,51,52]. Moreover, the authors indicated that the dry and wet deposition of coal dust and soot from point sources of emissions might play an important role in the TOC contents in these areas.

Additionally, TN contents in anthropogenic soils were significantly higher than in natural soil (Table 3) and associated with TOC content [11,21,51,53]. It should also be noted that pet excrement might be an additional source of both TOC and TN in the studied surface horizons. The consequences of quantitative changes of TOC and TN in the investigated soils were changes in the TOC/TN ratios (Table 3). The obtained results showed that the surface horizons of the studied anthropogenic soils (profiles 1–3) were characterised by a generally wider C/N ratio range (14.27–19.02) in comparison with natural soils (10.71) from the same area. Similar observations were found by other authors [9,11,51,54], and the increase in TOC/TN ratio values was explained by the presence of anthropogenic additives containing high contents of inactive forms of TOC and small amounts of nitrogen in the surface horizons of urban soils. Furthermore, it should be noted that all the chemical properties investigated except pH were significantly influenced by the increased TOC content.

3.2. Fractional Analysis of Soil Organic Matter

In the studied soil horizons, the processes of SOM transformations, modified by anthropogenic factors, caused qualitative and quantitative changes in the fractional composition of HS (Figure 1,

Table 4. Fractional composition of humic substances from top soil horizons of the investigated urban soils.

Profile No.	CHA	CFA	CR	CHA/CFA
	g kg−1
Control	6.94 a	3.72 a	8.82 a	1.90 a
1	32.51 b	8.99 b	21.61 b	3.60 b
2	9.63 c	9.25 b	21.11 b	1.04 c
3	8.62 d	5.82 c	42.40 c	1.50 d

Note: ^a,b,c,d—significantly different at p < 0.05.

). Regardless of the genesis of the investigated soils, the highest content and share (% of TOC) for the CR fraction was found in soils from transects 2 and 3. These compounds are considered non-hydrolysing carbon source.

The results obtained from all the studied anthropogenic soils were significantly higher than the control soil, reaching values from 21.11 to 42.40 g kg^–1 in profiles 2 and 3, which represented 39.5 and 72.8% of TOC, respectively. Although the occurrence of substantial amounts of CR is characteristic of urban soils [9,11,33], the importance of these compounds in the urban soil ecosystem has not yet been clearly established.

In transect 1, investigated soils were not subjected to such anthropogenic pressure as, unlike transects 2 and 3, it was not a walking area; therefore, the highest content and share (in % of TOC) among humic substances was found for the humic acids fraction (Table 4, Figure 1).

Chemical analyses of HS enabled the qualitative and quantitative characterisation of the direction and intensity of humification based on the changes in CHA and CFA as well as in the CHA/CFA index (Figure 1, Table 4). The results showed that humic acids prevailed over fulvic acids (quantity and share in TOC) in all investigated urban soils. The amounts of CHA in the horizons from anthropogenic soils were significantly higher than those that were from natural soils: 6.94 k kg^–1, reaching the value of 32.51 g kg^–1 in soils from transect 1. The amount of CFA obtained from anthropogenic soils was also significantly higher than that obtained from control soils (Table 4). However, compared to the values for CHA, it should be noted that the environmental conditions of the study area limit the formation of FA. The obtained results indicate the stimulation of humification processes in the studied area, resulting in the humus formation with mainly humic acids (Table 2). Many authors [9,11,21,33] claim that this phenomenon can be explained by the presence of carbonates of anthropogenic origin in the soil, which optimises pH and modifies the biological activity of soil microflora and microfauna, limits mineralisation and stimulates the humification process. The most common parameter used to determine the direction and intensity of SOM transformations is the CHA/CFA ratio [9,11,18,55,56]. Values of CHA/CFA ≥ 1 indicate the presence of more stable humus formations. Due to the observed enhanced HA synthesis and limited FA formation, this parameter reached values above 1 in all soils studied (Table 4). Furthermore, the values obtained for the anthropogenic soils were significantly different from those obtained for the control soil. However, a significant increase compared to the control (1.9) was observed only for soils from transect 1 (3.6), a lawn without a walking area, while profiles 2 and 3 showed significantly lower values of CHA/CFA ratio (1.04 and 1.50, respectively). These differences were caused by the CHA content in profile 1 (32.51 g kg^–1), which was several times higher compared to the studied natural and other anthropogenic soils.

3.3. Elemental Composition of Humic Acids

The analysis of the elemental composition of humic acids (

Table 5. Elemental composition, atomic ratio and degree of internal oxidation of HA from A horizons of urban soils.

Profile No	C	H	O	N	H/C	N/C	O/C	O/H	ω
	Atomic %
Control	32.28 a	42.95 a	20.05 a	2.81 a	1.33 a	0.09 a	0.62 a	0.47 a	0.17 a
1	36.85 b	42.10 b	19.40 b	1.98 b	1.16 b	0.05 b	0.53 b	0.46 a,d	0.07 b
2	36.15 c	42.58 c	19.45 b	2.96 a	1.20 c	0.08 a	0.53 b	0.44 b,c	0.12 c
3	36.25 c	43.04 a	19.31 b	2.91 a	1.24 d	0.08 a	0.56 c	0.45 c,d	0.13 a

Note: ^a,b,c,d means followed by the same letter are not significantly different at p < 0.05.

) from the studied horizons showed the predominance of hydrogen, which ranged from 42.1 (soils from transect 1) to 43.04 atomic % (soils from transect 3), and the values obtained for profiles 1 and 2 were significantly lower compared to the control. The share of carbon ranged from 32.28 (control) to 36.85 (soils from transect 1) atomic % and was significantly higher in HA from anthropogenic soils compared to natural soil. The content of N in HA molecules varied from 1.98 (soils in transect 1—lawn) to 2.96 (soils in transect 2—walking area) and only HA from soil 1 were characterised by significantly lower nitrogen content compared to the control and other investigated urban soils (Table 5). The analysis of the O share (Table 5) in HA from anthropogenic soils (profiles 1–3) showed a slight variation from 19.31 to 19.45 atomic % in profiles 3 and 2, respectively. Furthermore, the values obtained were significantly lower compared to the control soil (20.05 atomic %). The obtained results reveal that humic acids from the surface horizons of the studied soils are characterised by a low degree of humification and low level of maturity [36,54,57].

Differences in the elemental composition of the studied HA resulted in changes in the atomic ratios of individual elements (Table 5). The obtained results allowed the determination of the structure of the investigated molecules through the evaluation of the condensation of aromatic compounds and their maturity. The calculated H/C ratios showed low variability in the studied anthropogenic soils, ranging from 1.16 to 1.24 in profiles 1 and 3, respectively, and these ratios were significantly lower compared to the control fluvic cambisol (1.33). The values of O/C ratios in the HA of the studied technosols ranged between 0.53 and 0.56 and were significantly lower than in the control soil (0.62). Moreover, the results obtained for the O/H ratio showed that the samples from profiles 2 and 3, 0.44 and 0.45, respectively, differed significantly from the HA of the control soils (0.47). Taking into consideration the H/C, O/H and O/C results, it can be assumed that all investigated HA have a complex structure containing lignin–protein components with a marked carbohydrate component [11,55,56,58]. However, the values of these parameters obtained for HA from natural soil indicate higher degrees of oxidation and higher levels of maturity compared to the investigated technosols. Humic acids extracted from the surface horizons of the studied soils showed a low diversity of N/C indices, (Table 5) and a significantly lower value of this parameter was found for profile 1. Based on these results, N is the element that differentiates the studied HA the least. The investigated HA were characterised by different levels of the oxidation ratio “ω” (Table 5), ranging from 0.07 to 0.17 (profile 1 and control, respectively). Significantly lower values were observed for the technosols samples, while the highest value was observed for the fluvic cambisol. The obtained values indicate that the humification processes occurring in the control profile led to the formation of HA molecules with a higher humification degree in comparison to the samples from anthropogenic soils, which suggests less stable and less suitable biotransformation processes in the studied technosols in comparison to the fluvic cambisols.

3.4. ¹³C NMR Spectra of the Investigated HA

The ¹³C NMR spectra allow the structure of humic acids to be assessed based on qualitative and quantitative analysis of the C, H, O and N bonds [11,49,59]. Moreover, it is possible to assess the stability of the organic structures formed, excluding organic–mineral interactions [57,58].

Analysis of the ¹³C NMR spectra of humic acids isolated from investigated surface horizons indicated differences in both absorption intensity and carbon contents of particular structures (

Table 6. Selected carbon bonds and structural parameters of the HA molecules from the investigated urban soils.

Profile No	Calkyl	CO-Alkyl	Caliph	Carom	Clig	Ccarbox	α
	[%]
Control	27.62	27.99	55.61	22.89	9.13	21.10	29.16
1	28.51	29.84	58.35	21.04	9.72	20.66	26.50
2	24.24	30.03	54.27	22.52	7.76	23.21	29.33
3	26.57	27.84	54.41	22.48	8.24	23.11	29.24

, Figure 2). The results showed the predominance of C_aliph structures, the share of which in the molecules of all humic acids studied was over 50% and differed between the control soils and the technosols (Table 6). An intense signal was observed for 30 ppm in the control soil and in profiles 1 and 3. It is associated with methylene groups in alkyl chains and polypeptides and CH₃ acetyl groups, bound to aliphatic structures, whose natural source is mainly plant material [11,60,61,62,63]. The highest intensity of 57 ppm, related to the presence of OCH₃ groups in the polypeptides, was observed both in the fluvic cambisol and in the investigated technosol (Figure 2, Table 6). The decrease in the range of 73 ppm for profiles 1–3 may indicate a low share of cellulosic and hemicellulosic organic carbon in HA molecules from the studied anthropogenic soils.

Despite the observed differences in absorption, the share of CO-alkyl in HA from the technosols studied was higher in comparison with the control. Furthermore, CO-alkyl shares below 30% were observed [9,21,54] in alkaline urban soils with specific bioactivity, where O-alkyl components are decomposed and incorporated into other HA structures. Two carbon absorption maxima of the aromatic structures were found in the analysed samples: 130 and 150 ppm (Figure 2). Absorption at 130 ppm occurred in all samples, while the peak at 150 ppm, associated with the presence of lignin–carbon structures, was the most intense for profile 1. Similar results in soils of urban parks, gardens and lawns were observed by Lorenz et al. [11]. A clear peak at 160–200 ppm was observed in all HA studies. This signal is due to the carboxyl groups present mainly in carboxylic acids and esters [9,19,61,62] and the share of these structures ranged from 20.66 to 23.21% (Table 6). The authors indicated that an increase in the content of carboxyl groups may be connected to the biotransformation of lignins and some hydrophobic [63,64,65] compounds, introduced into alkaline urban soils with organic fertilizers. A similar phenomenon was observed in the studied soils, and higher shares of these structures were found in the technosol (profiles 2 and 3), containing a lower share of lignins (Table 6). The degree of aromaticity (α), determining the share of aliphatic and aromatic structures in the molecules of HA investigated, was in the range of 26.50–29.33 (Table 6). A lower value of this parameter in comparison with the control was found in profile 1 (26.50 and 29.11 respectively). Therefore, the factor differentiating the HA from the soils studied is likely the total share of aliphatic components. Moreover, it should be noted that all HA investigated were dominated by aliphatic structures with a substantial amounts of carboxylic groups.

4. Conclusions

The studied area of Wroclaw included both natural (fluvic cambisol) and human-transformed (technosols) soils, which were influenced by anthropopressure of various degrees. It mainly caused an increase in the TOC content of the studied surface horizons, from various sources, and a decrease in organic matter decomposition confirmed by a wider C/N ratio. The SOM of the urban soils subjected to the anthropogenic pressure was dominated by the non-hydrolysing, residual carbon fraction, especially humins, indicating the presence of compounds resistant to biotransformation. In the studied technosols, environmental conditions stimulated humification processes, leading to the formation mainly of HA, although of lower maturity and stability in comparison to fluvic cambisol. Elemental composition and ¹³C NMR spectroscopy indicated that the structure of the studied HA was dominated by the compounds with a low aromatic core and a considerable content of aliphatic components. Moreover, the key factor differentiating the studied HA molecules was the share of aliphatic structures.

In urban soils, efforts [1,2,66] should be made to enhance organic matter transformation into more matured and stable forms. The use of bulky organic amendments, e.g., composts, may be the best method of providing organic matter in these soils over prolonged periods. Chemical treatments as well as lawn maintenance, especially herbicide application and litter removal, should be very strictly controlled, limited and in some cases, avoided. Thus, rational management of urban soils is the most reasonable way to improve the qualitative and quantitative parameters of SOM.