Urban soil organic carbon and its spatial heterogeneity in comparison with natural and agricultural areas in the Moscow region
Highlights
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Urban soils are heterogeneous and contribute strongly to the regional spatial variability of SOC.
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Urban SOC contents (average 3.3%) significantly exceeded non-urban one (average 2.7%).
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“Cultural layer” is the key factor influencing urban carbon stocks.
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Soil depth, soil type and land-use factors determined more than 30% of total variance.
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Urban SOC should not be ignored in regional and global carbon assessments.
Introduction
Terrestrial ecosystems are a major player in the global carbon cycle, acting as carbon stocks and carbon sources (Ouimet et al., 2007). Soil organic carbon (SOC) is the largest carbon stock in terrestrial ecosystems accounting for about 2000 PG C (Janzen, 2004). Carbon sequestration is a widely accepted soil ecosystem function (Blum, 2005, Kudeyarov et al., 2007, MEA — Millennium Ecosystem Assessment, 2005). Regional analysis of SOC stocks is important and receives increasing attention in e.g., land-use planning (Gruniberg et al., 2010, Krogh et al., 2003, Phachomphon et al., 2010). Although quite a few studies focus on analyzing and mapping SOC for natural and agricultural areas (Guo and Gifford, 2002, Stoorvogel et al., 2009, Zhou et al., 2007), urban areas are often excluded from these regional carbon assessments. General literature indicates various factors to influence SOC variation in a region: soil type (Dobrovolsky and Urussevskaya, 2004), land-use (Lal, 2002, Zhou et al., 2007), and the level of urbanization (Lorenz and Lal, 2009, Poyat et al., 2006). So far, very little is known about SOC in urban environments. However, urbanization is now one of the predominant pathways of land-use change (Saier, 2007, Seto et al., 2011). In contrast with the global average of 2%, regionally urban lands can occupy up to 10% and their total extent is expanding (Denisov et al., 2008, Pickett et al., 2011). Therefore, it is necessary to understand the contribution of urban soils to the regional SOC stocks.
The urban environment can be characterized by a number of specific features and processes that influence soil formation and functioning (Imhoff et al., 2004, Vrscaj et al., 2008). Soil movement and transformation during construction and greenery work, functional zoning, soil sealing, and settlement history determine the SOC stocks in urban soils with extremely high spatial variability (Prokofieva and Stroganova, 2004). In contrast to the often gradual changes in natural areas, urban soils may exhibit abrupt changes due to anthropogenic influence. The spatial heterogeneity is further complicated by a specific profile distribution. Profiles of typical urban soils may display two SOC maximums (Gerasimova et al., 2003, Lorenz and Lal, 2009). The first one is connected with humus-accumulative horizon and the second one corresponds to the so-called “cultural layer”.
The concept of the cultural layer originates in archeological research, where it was used to define the age of artifacts and describe the settlement history (Alexandrovskiy et al., 1998, Avdusin, 1980). Afterwards the cultural layers of several ancient Russian towns were studied as part of soil morphological research (Kaidanova, 1992, Sycheva, 1994). The cultural layers and soils buried under them were shown to be a single complex, developing in time (Sycheva, 1994). A number of specific soil features, such as a high level of heavy metal accumulation and soil microbiological communities, non-typical for topsoil were described for cultural layers (Evdokimova, 1986, Marfenina et al., 2008, Sycheva, 2006). From a carbon stock perspective, the cultural layers include wooden remains, coal and buried non-urban horizons (Prokofieva and Stroganova, 2004) (Fig. 1). Organic carbon contents in the cultural layer may be as high as 3–5% or even more (Dolgikh and Aleksandrovskii, 2010, He and Zhang, 2009). Their depth depends on the age of the settlement and varies from 10 cm to several meters (Alexandrovskaya and Alexandrovskiy, 2000). So far, most of carbon assessments focus mainly on topsoil (Food and Agriculture Organization of the United Nations (FAO), 1995, Nilsson et al., 2000). However, ignoring this cultural layer may lead to a considerable underestimation of the SOC stocks.
This paper aims to provide insight into the importance of urban SOC compared with agricultural and natural areas. In addition it tries to identify and analyze the principal factors influencing the spatial variability of SOC contents. The study was implemented in Moscow region that due to its high level of urbanization provides an excellent test case.
Section snippets
The study area
The Moscow region is located in the European part of Russia (55° northern latitude and 37° eastern longitude) covering 46.700 km2. Four main zonal soil types are found in the region: podzols in the north, sod-podzolic soils in the center, and gray forest soils and chernozems in the south. In addition intrazonal alluvial soils are located in the flood-plains of the Moscow and Oka rivers (Shishov et al., 2004). SOC contents in the topsoil of natural areas vary from 1–2% in podzols up to 6–7% in
Importance of urban SOC
The average SOC contents (0–150 cm) in urban areas (3.3 ± 1.9%) turned out to be significantly higher (p < 0.05) than in non-urban areas (2.7 ± 1.6%). Although SOC in urban areas is generally excluded from the regional assessment (e.g., Burghardt, 2002, Schaldach and Alcamo, 2007, Schulp and Verburg, 2009), this study shows that cities can have considerable carbon stocks. This conclusion is confirmed if we look at individual soil types. On chernozems and sod-podzolic soils, SOC contents in urban areas
Conclusions
Urban soil organic carbon pools remain as one of the least known carbon pools. Studies on urban ecosystems have been traditionally ignored by ecologists and soil scientists (Byrne, 2007, Grimm et al., 2000). This study demonstrated the importance of the urban SOC. In the Moscow region the urban SOC contents were found to be comparable or higher than ones of natural and agricultural areas. This contradicts the popular assumption that urban SOC can be ignored. The main source of carbon
Acknowledgments
The project was partly supported by Governmental Grant # 11.G34.31.0079, the Russian Foundation Basic Research grants # 11-04-01376 and # 11-04-02089. The authors thank Julia Pereverzeva for assistance with soil chemical analysis as well as Rik Leemans, Oleg Makarov, Tatyana Prokofieva and Nadezhda Ananyeva for valuable suggestions and useful comments.
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