Urban soils as hot spots of anthropogenic carbon accumulation: Review of stocks, mechanisms and driving factors

Abstract

Urban soils and cultural layers may accumulate C over centuries and consequently large C stocks may be sequestered beneath cities. Processes and mechanisms leading to high C accumulation in urban soils remain unknown. Data on soil organic carbon (SOC), soil inorganic carbon (SIC), black (pyrogenic) carbon (BC), and nitrogen contents and stocks in urban soils were collected from 100 peer‐reviewed papers. The database (770 data points for SOC, SIC, and BC stocks from 116 cities worldwide) was analysed considering the effects of climate and urban‐specific factors (city size, age, and functional zoning) on C stocks. The processes of C accumulation specific for urban soils were analysed, and C sequestration rates were assessed. For the wide range of climatic conditions, total C content in urban soils was 1.5–3 times higher, and C accumulation was much deeper compared with natural soils, resulting in 3–5 times larger total C stocks. Urban SOC stocks increased with latitude, whereas SIC stocks were less affected by climate. City size and age were the main factors explaining intercity differences in C stocks. The intracity variability of C and N stocks was dominated by functional zoning: Large SOC and N stocks in residential areas and large SIC and BC stocks in industrial zones and roadsides were consistent across all climates and for cities of various sizes and ages. Substantial amounts of SOC, SIC, and N are sequestered in the subsoils, cultural layers, and sealed soils, underlining the importance of these hidden stocks for C assessments. Long‐term С input from outside the cities and associated C accumulation coincided with upward soil growth of ~50 cm per century, and continuous accumulation of 15–30 kg C/m2 per century in urban soils and cultural layers. We conclude that, despite the relatively small area of cities, urban soils are hot spots of long‐term soil C sequestration worldwide, and the importance of urban soils will increase in future with global urbanization.

Ecosystem service provision by road verges

“…pollution) and provide further benefits to surrounding landscapes. Each broad landscape type (agricultural, urban and natural areas) demands a different suites of ES, which should be the target of management to enhance ES provision by road verges of the greatest stocks of black carbon (produced from burning fossil fuels; reviewed in Vasenev & Kuzyakov, 2018…”

“…Maps and data were produced by drawing polygons around road verges using satellite imagery from Google Earth and verifying using Google Street View (Google, 2019), then importing to ArcMap 10.5.1 (ESRI, 2017). Road verge widths were calculated by creating centrelines for each road verge, converting them to points at 5 m intervals, measuring the distance from each point to the nearest road verge edge, then multiplying by 2 average value for grasslands (0.054 kg C m −2 year −1 ; reviewed in Conant, Paustian, & Elliott, 2001), which is probably a conservative estimate (Bouchard et al, 2013;Vasenev & Kuzyakov, 2018), then they may sequester 0.015 Gt C/year globally-nearly 1% of the annual carbon sink provided by the world's 4 million km 2 of forests (Pan et al, 2011).…”

Effects of agricultural land use change on organic carbon and its labile fractions in the soil profile in an urban agricultural area

“…In this study, we found that there was no significant difference in the ratios of LOC to TOC caused by the conversion of agricultural land use scenarios, although significantly different contents of TOC and LOC among different agricultural land use scenarios were observed, suggesting that the ratio of LOC to TOC may not be the optimal indicator for assessing the stability and and much more chemical fertilizers were then needed to reinforce the supply of available nutrients for crops, which may further aggravate soil quality degradation and have negative impacts on the sustainability of agricultural land. In addition, the notable changes of subsoil organic carbon and its labile fractions suggest that more attention should be paid to the changes in subsoil carbon caused by agricultural land use changes in urban agricultural areas because agricultural land use is easily affected by rapid urbanization and substantial amounts of SOC are sequestered in the subsoil of suburbs (Vasenev & Kuzyakov, 2018).…”

“…With rapid population growth and economic development, greater demands for agricultural products (i.e., food and bioenergy) have resulted in the expansion of agricultural land areas and enhancement of the degree of agricultural intensification (Alexander et al, 2015;Chou, Dong, Wang, & Fu, 2015;Lai et al, 2016;Piquer-Rodríguez et al, 2018;Vasenev & Kuzyakov, 2018). Compared with agricultural expansion, agricultural intensification plays a major role in meeting the growing demands for agricultural products (Johnson, Runge, Senauer, Foley, & Polasky, 2014).…”

“…In addition, studies on the effects of agricultural practices on SOC dynamics have frequently focussed on the changes in SOC in topsoil (i.e., the top 20 or 30 cm; Hobley, Baldock, Hua, & Wilson, 2017;Wang et al, 2016). Previous studies showed that crop roots can reach a depth of 70 cm in soil and large amounts of SOC are estimated to be stored in the subsurface depths (>20 cm ;Hobley et al, 2017;Vasenev & Kuzyakov, 2018;Zhang et al, 2014). A recent study also revealed the instability of subsoil organic carbon by comparing the isotopic values of carbon between agricultural land and land under native vegetation (Hobley et al, 2017).…”