Land-cover effects on soil organic carbon stocks in a European city

Black carbon in urban soils: land use and climate drive variation at the surface

BACKGROUND

Black carbon (BC) encompasses a range of carbonaceous materials--including soot, char, and charcoal--derived from the incomplete combustion of fossil fuels and biomass. Urban soils can become enriched in BC due to proximity to these combustion sources. We conducted a literature review of BC in urban soils globally and found 26 studies reporting BC and total organic carbon (TOC) content collected to a maximum of 578 cm depth in urban soils across 35 cities and 10 countries. We recorded data on city, climate, and land use/land cover characteristics to examine drivers of BC content and contribution to TOC in soil.

RESULTS

All studies were conducted in the northern hemisphere, with 68% of the data points collected in China and the United States. Surface samples (0-20 cm) accounted for 62% of samples in the dataset. Therefore, we focused our analysis on 0-10 cm and 10-20 cm depths. Urban soil BC content ranged from 0-124 mg/g (median = 3 mg/g) at 0-10 cm and from 0-53 mg/g (median = 2.8 mg/g) at 10-20 cm depth. The median proportional contribution of BC to TOC was 23% and 15% at 0-10 cm and 10-20 cm, respectively. Surface soils sampled in industrial land use and near roads had the highest BC contents and proportions, whereas samples from residential sites had among the lowest. Soil BC content decreased with mean annual soil temperature.

CONCLUSIONS

Our review indicates that BC comprises a major fraction (nearly one quarter) of the TOC in urban surface soils, yet sampling bias towards the surface could hide the potential for BC storage at depth. Land use emerged as an importer driver of soil BC contents and proportions, whereas land cover effects remain uncertain. Warmer and wetter soils were found to have lower soil BC than cooler and drier soils, differences that likely reflect soil BC loss mechanisms. Additional research on urban soil BC at depth and from diverse climates is critical to better understand the role of cities in the global carbon cycle.

Global distribution of surface soil organic carbon in urban greenspaces

Urban greenspaces continue to grow with global urbanization. The global distribution and stock of soil organic carbon (SOC) in urban greenspaces remain largely undescribed and missing in global carbon (C) budgets. Here, we synthesize data of 420 observations from 257 cities in 52 countries to evaluate the global pattern of surface SOC density (0-20 cm depth) in urban greenspaces. Surface SOC density in urban greenspaces increases significantly at higher latitudes and decreases significantly with higher mean annual temperature, stronger temperature and precipitation seasonality, as well as lower urban greenness index. By mapping surface SOC density using a random forest model, we estimate an average SOC density of 55.2 (51.9-58.6) Mg C ha-1 and a SOC stock of 1.46 (1.37-1.54) Pg C in global urban greenspaces. Our findings present a comprehensive assessment of SOC in global urban greenspaces and provide a baseline for future urban soil C assessment under continuing urbanization.

Soil Black Carbon Increases Under Urban Trees with Road Density and Time: Opportunity Hotspots for Carbon Storage in Urban Ecosystems

Black carbon (BC) can comprise a significant fraction of the soil carbon pool in cities. However, vegetation cover and human activity influence the spatial distribution of urban soil BC. We quantified soil total carbon (TC), soil organic carbon (SOC), BC, and total nitrogen (TN) in a medium-sized city in Dallas-Fort Worth, Texas. Soils were sampled to 20 cm depth from underneath 16 paired Quercus stellata (post oak) trees and open lawns. Effects of vegetation cover, road density, and building age (a proxy for time since development) on soil C and N were analyzed. Soil OC concentrations were higher under post oak trees (5.5%) compared to open lawns (3.6%) at 0-10 cm, but not at 10-20 cm depth. In contrast, soil BC and TN did not differ by vegetation cover. There were significant interaction effects between vegetation cover and road density and vegetation cover and building age on soil BC. At 0-10 cm, soil BC concentrations, stock, and BC/SOC ratios increased more with road density under trees than lawns, indicating enhanced atmospheric BC deposition to tree canopies. Black carbon in tree soils also increased with building age as compared to lawn soils, likely due to higher BC retention under trees, enhanced BC losses under lawns, or both. Our findings show that urban tree soils are localized opportunity hotspots for BC storage in areas with elevated emissions and longer time since development. Conserving and planting urban trees above permeable surfaces and soils could contribute to long-term carbon storage in urban ecosystems.

Topsoil Carbon Stocks in Urban Greenspaces of The Hague, the Netherlands

Urbanization influences soil carbon (C) stocks and flows, which, in turn, affect soil-derived ecosystem services. This paper explores soil C storage in urban greenspaces in the Dutch city of The Hague along a transect from the suburban seaside towards the city centre, reflecting a toposequence from dune to peaty inland soils. C storage and C mineralisation potential were evaluated in relation to soil type and greenspace categories. Several soil-quality characteristics were measured, including dissolved organic C, pH, electrical conductivity, nitrogen, phosphorus, sulphur, calcium carbonate, and the water-holding capacity of the soil to evaluate what drives soil C storage in the urban context. The total SOC storage of the upper 30 cm of the greenspaces in The Hague (20.8 km2 with 37% greenspace) was estimated at 78.4 kt, which was significantly higher than assumed given their soil types. Degradability of soil organic matter in laboratory batch tests varied between 0.2 and 3 mg C gSOC−1 day−1. Degradability was highest in the seaside dune soils; however, extrapolated to the topsoil using the bulk density, topsoil C mineralization was higher in the urban forest. Soils beneath shrubs appeared to be hotspots for C storage, accounting for only 13% of the aerial cover but reflecting 24% of the total C storage. Land ownership, land use, greenspaces size, litter management and soil type did not result in significantly different C stocks, suggesting that processes driving urban soil C storage are controlled by different factors, namely land cover and the urbanization extent.

Spatio-Temporal Evolution, Prediction and Optimization of LUCC Based on CA-Markov and InVEST Models: A Case Study of Mentougou District, Beijing

With the rapid advancement of urbanization and industrialization, the contradiction between the social economy and resources and the environment has become increasingly prominent. On the basis of limited land resources, the way to promote multi-objective comprehensive development such as economic, social development and ecological and environmental protection through structure and layout regulation, so as to maximize regional comprehensive benefits, is an important task of current land spatial planning. Our aim is to obtain land-use-change data in the study area using remote-sensing data inversion and multiple-model simulation. Based on land suitability evaluation, we predict and optimize the land use structure of the study area in 2030 and evaluate and compare ecosystem services. Based on remote-sensing images and eco-environmental data from 1985 to 2014 in the study area, land use/land cover change (LUCC) and future simulation data were obtained by using supervised classification, landscape metrics and the CA-Markov model. The ecosystem services were evaluated by the InVEST model. The analytic hierarchy process (AHP) method was used to evaluate the land suitability for LUCC. Finally, the LUCC in 2030 under two different scenarios, Scenario_1 (prediction) and Scenario_2 (optimization), were evaluated, and the ecosystem service functions were compared. In the last 30 years, the landscape in the study area has gradually fragmented, and the built-up land has expanded rapidly, increased by one-third, mainly at the cost of cropland, orchards and wasteland. According to the suitability evaluation, giving priority to the land use types with higher environmental requirements will ensure the study area has a higher ecosystem service value. The rapid development of urbanization has a far-reaching impact on regional LUCC. Intensive land resources need reasonable and scientific land use planning, and land use planning should be based on the suitability evaluation of land resources, which can improve the regional ecosystem service function.

Land use and site effects on the distribution of carbon in some humic soil profiles of KwaZulu-Natal, South Africa

Land use effects on the stocks of soil organic carbon (SOC) are generally based on the topsoil. Although subsoil horizons have lower concentrations, they contain a significant amount of SOC which may be more strongly protected than that in the topsoil layers. Analysis of SOC storage must therefore include the whole profile in respect of climate change mitigation. Humic soils in South Africa have high organic C in the A horizon, while the amount of C stocks through the whole profile depth is unknown. This study was conducted at six sites in KwaZulu-Natal Province to determine the effect of land use and site factors on C stocks, texture, pH and extractable aluminium (Al) and iron (Fe) concentrations and their vertical distribution to 100 cm in soils with thick (>45 cm) and thin (<45 cm) humic A horizons. The land use at some sites had been changed from grassland to maize and cultivated pasture and at others from forest to sugarcane farming. Cultivation with field crops reduced the organic C, mainly in the upper 20 cm (from 110 to 22 g C kg⁻¹), with limited effect in deeper layers. The soils with thick humic A horizons and coarser texture stored more C in the deeper layers compared to those with thin humic A horizons and finer texture which had more of the C stocks in the 0–20 cm depth. Although cultivation reduced the soil C stocks in the surface layers, land use did not significantly affect the overall C stock (0–100 cm) at all sites. The high contents of extractable Fe (up to 340 mg kg⁻¹) and Al (up to 3700 mg kg⁻¹) stabilised the soil C and were more important than the effects of either land use or other site factors.

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Humic soils have been converted from native vegetation to cropland and pasture.

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Land use does not affect overall organic carbon stock (0–100 cm) in humic soils.

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Organic carbon distribution in the profile is affected by soil texture.

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Iron and aluminium are more important than land use or site for stabilising carbon.

Spatio-temporal variability in soil CO2 efflux and regulatory physicochemical parameters from the tropical urban natural and anthropogenic land use classes

Urban ecosystems, the heterogeneous and rapidly changing landscape, showed a considerable impact on the global C cycle. However, studies encompassing the spatial differences in urban land uses on soil C dynamics are limited in tropical ecosystems. In this study, seasonal and temporal variability in soil CO₂ efflux (SCE) and its regulatory physicochemical variables under five urban land use classes viz., Bare (BAR), Agriculture (AGR), Plantation (PLT), Grassland (GRA) and Lawns (LAW) were assessed from 2014 to 2016. Bare land use was considered as the reference for observing the variation for different land uses. Seasonal measurements of SCE, soil temperature, moisture content, pH, ammonium-N, nitrate-N and microbial biomass C (MBC) were performed whereas soil organic C (SOC), soil N, and soil physical properties were measured annually. Our results showed a significant (P < 0.01) increase in SCE by 89%, 117%, 132% and 166% for land use types from BAR to AGR, PLT, GRA and LAW, respectively. The results revealed a two-fold increase in SCE from anthropogenically managed urban lawns as compared to bare soil. PLT and LAW land use classes showed higher SOC and N contents. SCE was found positively correlated with temperature, moisture, SOC, soil N and MBC whereas negatively correlated with ammonium-N and nitrate-N (at P < 0.05) for the overall dataset. Soil moisture, temperature, SOC, porosity and pH were identified as the major determinant of urban SCE by explaining 63% of the variability in overall SCE. Further, temperature for BAR and LAW; moisture for PLT; ammonium-N for GRA; and nitrate-N for AGR were identified as the major regulators of SCE for different land use classes. The findings revealed that the interaction of soil temperature and moisture with nutrient availability regulates overall and seasonal variability in SCE in an urban ecosystem. Since these variables are highly affected by climate change, thus, the soil C source-sink relationships in tropical urban ecosystems may further change and induce a positive global warming potential from urban ecosystems.

An assessment of urban horticultural soil quality in the United Kingdom and its contribution to carbon storage

As participation in urban horticulture grows, understanding the quality of urban horticultural soils is of increasing importance. Until now, case studies of individual cities or gardens have limited the potential of such studies to draw generalised conclusions. Here, we present the first national scale assessment of soil quality in allotments, a dominant form of urban horticulture in the United Kingdom. We sampled soils in 200 allotments in 10 urban areas across Great Britain. We assessed a range of soil quality indicators (carbon and nitrogen concentration, C:N ratio, bulk density, carbon density, pH) comparing them to the quality of soils in rural arable and horticultural land. We present the first estimate of nationwide carbon storage on allotments. We found that allotment gardeners consistently employ management practices conducive to high soil quality. Allotment soil quality differed significantly between soil types but in general soils were of a high quality: low bulk density (0.92 g cm^-3) and high soil organic carbon concentration and density (58.2 mg g^-1 and 58.1 mg cm^-3 respectively). Allotment soil organic carbon concentration was 250% higher than in the surrounding arable and horticultural land. Covering only 0.0006% of Great Britain, allotments contribute a disproportionate 0.05-0.14% of nationwide total organic carbon stocks. This national-scale study provides compelling evidence that small-scale urban horticultural production, unlike conventional horticulture, does not degrade soil quality. Indeed, allotments hold a small but previously unaccounted for carbon stock nationally. Urban horticultural land is a vital part of the urban landscape with effectively functioning soils that should be protected. As public demand for urban horticultural land rises and policy-makers from local to trans-national levels of governance advocate for urban food production, our findings demonstrate that urban horticulture can protect or enhance the ecosystem services provided by soils in cities and towns where the majority of people live.

Diversity patterns and drivers of soil microbial communities in urban and suburban park soils of Shanghai, China

BACKGROUND

The rapid expansion of urbanization leads to significant losses of soil ecological functions. Microbes directly participate in key soil processes and play crucial roles in maintaining soil functions. However, we still have a limited understanding of underlying mechanisms shaping microbial communities and the interactions among microbial taxa in park soils.

METHODS

In this study, the community variations of bacteria and fungi in urban and suburban park soils were investigated in Shanghai, China. Real-time PCR and high-throughput Illumina sequencing were used to examine the microbial abundance and community composition, respectively.

RESULTS

The results showed that soil molecular biomass and fungal abundance in urban park soils were significantly higher than those in suburban park soils, while no significant difference was observed in the bacterial abundance between urban and suburban park soils. The alpha diversity of soil microbes in urban and suburban park soils was similar to each other, except for Chao1 index of fungal communities. The results of similarity analysis (ANOSIM) revealed remarkable differences in the composition of bacterial and fungal communities between urban and suburban park soils. Specifically, park soils in urban areas were enriched with the phyla Methylomirabilota and Verrucomicrobiota, while the relative abundance of Gemmatimonadota was higher in suburban park soils. Moreover, the fungal class Eurotiomycetes was also enriched in urban park soils. Compared with suburban park soils, nodes and average paths of the bacterial and fungal networks were higher in urban park soils, but the number of module hubs and connectors of the bacterial networks and negative interactions among bacterial taxa were lower. Compared with suburban park soils, Acidobacteriota bacterium and Mortierellomycota fungus played more important roles in the ecological networks of urban park soils. Soil available zinc (Zn), available nitrogen (N), pH, and total potassium (K) significantly affected fungal community composition in park soils in Shanghai. Soil available Zn was also the most important factor affecting the bacterial community composition in this study.

CONCLUSION

There were significant differences in the soil molecular biomass, fungal abundance, and the community composition and co-occurrence relations of both soil bacterial and fungal communities between urban and suburban park soils. Soil available Zn played an important part in shaping the structures of both the bacterial and fungal communities in park soils in Shanghai.

The Tea Bag Index—UK: Using Citizen/Community Science to Investigate Organic Matter Decomposition Rates in Domestic Gardens

Gardening has the potential to influence several ecosystem services, including soil carbon dynamics, and shape progression towards the UN Sustainable Development Goals, (e.g., SDG 13). There are very few citizen/community science projects that have been set up to test an explicit hypothesis. However, citizen/community science allows collection of countrywide observations on ecosystem services in domestic gardens to inform us on the effects of gardening on SDGs. The geographical spread of samples that can be collected by citizen/community science would not be possible with a team of professional science researchers alone. Members of the general public across the UK submitted soil samples and buried standardised litter bags (tea bags) as part of the Tea Bag Index—UK citizen/community science project. Participants returned 511 samples from across the UK from areas in their garden where soil organic amendments were and were not applied. The project examined the effects of application of soil amendments on decomposition rates and stabilisation of litter, and in turn, effects on soil carbon and nitrogen concentrations. This was in response to a call for contributions to a global map of decomposition in the Teatime4Science campaign. Results suggested that application of amendments significantly increased decomposition rate and soil carbon, nitrogen, and carbon: nitrogen ratios within each garden. So much so that amendment application had more influence than geographic location. Furthermore, there were no significant interactions between location and amendment application. We therefore conclude that management in gardens has similar effects on soil carbon and decomposition, regardless of the location of the garden in question. Stabilisation factor was influenced more prominently by location than amendment application. Gardening management decisions can influence a number of SDGs and a citizen/community science project can aid in both the monitoring of SDGs, and involvement of the public in delivery of SDGs.

Impacts of urbanization on soil organic carbon stocks in the northeast coastal agricultural areas of China

Dynamic changes in soil organic carbon pools have significant impacts on regional and global carbon balance. Due to rapid development in urbanized areas, the land use changes dramatically, impacting soil organic carbon (SOC) stocks in topsoil. This study aimed to document the impacts of urbanization on SOC stocks in a rapidly urbanized area from northeastern China. A total of 12 auxiliary variables were as SOC predictors including elevation, slope aspect, slope gradient, topographic wetness index, Landsat TM band3, Landsat TM band4, Landsat TM5, and normalized difference vegetation index. Urban-specific variables including population (POP), gross domestic product (GDP), distance to the socio-economic center, and distance to the roads are also considered. A set of 523 (in 1990) and 847 (in 2015) top soil samples with SOC measurement were collected. Two random forest (RF) models, one with all auxiliary variables except urban-specific variable (MA) and the other with all auxiliary variables (MB) were used to map the spatial distribution of SOC stocks in the two periods. Ten-fold cross-validation was conducted to evaluate the performance of RF models. We find that the full auxiliary variables model had a better performance for the both periods. POP and GDP were key auxiliary variables affecting spatial variability of SOC stocks in 2015. Over a 25-year period, SOC stocks decreased from 2.77 ± 1.09 kg m^-2 to 2.16 ± 0.93 kg m^-2, resulting in 3.78 Tg SOC loss in this region. Rapid urbanization led to drastic land- use change, which was the main reason for the decrease of SOC stocks. Additionally, urban-specific variables should be used as the main auxiliary variables when predicting SOC stocks in the areas that experience rapid urbanization. We believe that accurate prediction and mapping of SOC stocks will help manage land use and facilitate soil quality assessment so as to increase soil carbon sequestration in the region.

Feeding a city - Leicester as a case study of the importance of allotments for horticultural production in the UK

The process of urbanization has detached a large proportion of the global population from involvement with food production. However, there has been a resurgence in interest in urban agriculture and there is widespread recognition by policy-makers of its potential contribution to food security. Despite this, there is little data on urban agricultural production by non-commercial small-scale growers. We combine citizen science data for self-provisioning crop yields with field-mapping and GIS-based analysis of allotments in Leicester, UK, to provide an estimate of allotment fruit and vegetable production at a city-scale. In addition, we examine city-scale changes in allotment land provision on potential crop production over the past century. The average area of individual allotment plots used to grow crops was 52%. Per unit area yields for the majority of crops grown in allotments were similar to those of UK commercial horticulture. We estimate city-wide allotment production of >1200 t of fruit and vegetables and 200 t of potatoes per annum, equivalent to feeding >8500 people. If the 13% of plots that are completely uncultivated were used this could increase production to >1400 t per annum, feeding ~10,000 people, however this production may not be located in areas where there is greatest need for increased access to fresh fruits and vegetables. The citywide contribution of allotment cultivation peaked in the 1950s when 475 ha of land was allotments, compared to 97 ha currently. This suggests a decline from >45,000 to <10,000 of people fed per annum. We demonstrate that urban allotments make a small but important contribution to the fruit and vegetable diet of a UK city. However, further urban population expansion will exert increasing development pressure on allotment land. Policy-makers should both protect allotments within cities, and embed urban agricultural land within future developments to improve local food security.

Passive CO2 removal in urban soils: Evidence from brownfield sites

Management of urban brownfield land can contribute to significant removal of atmospheric CO₂ through the development of soil carbonate minerals. However, the potential magnitude and stability of this carbon sink is poorly quantified as previous studies address a limited range of conditions and short durations. Furthermore, the suitability of carbonate-sequestering soils for construction has not been investigated. To address these issues we measured total inorganic carbon, permeability and ground strength in the top 20 cm of soil at 20 brownfield sites in northern England, between 2015 and 2017. Across all sites accumulation occurred at a rate of 1-16 t C ha^-1 yr^-1, as calcite (CaCO₃), corresponding to removal of approximately 4-59 t CO₂ ha^-1 yr^-1, with the highest rate in the first 15 years after demolition. C and O stable isotope analysis of calcite confirms the atmospheric origin of the measured inorganic carbon. Statistical modelling found that pH and the content of fine materials (combined silt and clay content) were the best predictors of the total inorganic carbon content of the samples. Measurement of permeability shows that sites with carbonated soils possess a similar risk of run-off or flooding to sandy soils. Soil strength, measured as in-situ bearing capacity, increased with carbonation. These results demonstrate that the management of urban brownfield land to retain fine material derived from concrete crushing on site following demolition will promote calcite precipitation in soils, and so offers an additional CO₂ removal mechanism, with no detrimental effect on drainage and possible improvements in strength. Given the large area of brownfield land that is available for development, the contribution of this process to CO₂ removal by urban soils needs to be recognised in CO₂ mitigation policies.

Spatial-Temporal Variability of Soil Organic Matter in Urban Fringe over 30 Years: A Case Study in Northeast China

The study on soil organic matter (SOM) is of great importance to regional cultivated land use and protection. Based on data collected via continuous and high-density soil samples (0-20 cm) and socio-economic data collected from household survey and local bureau of statistics, this study employs geostatistics and economic statistical methods to investigate the spatial-temporal variation of SOM contents during 1980-2010 in the urban fringe of Sujiatun district in Shenyang City, China. We find that: (1) as to temporal variation, SOM contents in the study sites decreased from 30.88 g/kg in 1980 to 22.63 g/kg in 2000. It further declined to 20.07 g/kg in 2010; (2) in terms of spatial variation, the closer to city center, the more decline of SOM contents. Contrarily, SOM contents could even rise in outer suburb area; and (3) SOM content variation may be closely related to human factors such as farmers' land use target and behaviour including inputs of chemical and organic fertilizers, types of crops and etc. These findings are conductive to grasp the overall trend of SOM variation and the influence of farmers' land use behaviour on it. Furthermore, they could provide support for policymakers to agricultural planning and land use monitoring, which consequently aids the improvement of soil quality and food production in the urban fringe areas.

Valuing urban green spaces in mitigating climate change: A city-wide estimate of aboveground carbon stored in urban green spaces of China's Capital

Urban green spaces provide manifold environmental benefits and promote human well-being. Unfortunately, these services are largely undervalued, and the potential of urban areas themselves to mitigate future climate change has received little attention. In this study, we quantified and mapped city-wide aboveground carbon storage of urban green spaces in China's capital, Beijing, using field survey data of diameter at breast height (DBH) and tree height from 326 field survey plots, combined with satellite-derived vegetation index at a fine resolution of 6 m. We estimated the total amount of carbon stored in the urban green spaces to be 956.3 Gg (1 Gg = 10⁹ g) in 2014. There existed great spatial heterogeneity in vegetation carbon density varying from 0 to 68.1 Mg C ha^-1 , with an average density of 7.8 Mg C ha^-1 . As expected, carbon density tended to decrease with urban development intensity (UDI). Likely being affected by vegetation cover proportion and configuration of green space patches, large differences were presented between the 95th and 5th quantile carbon density for each UDI bin, showing great potential for carbon sequestration. However, the interquartile range of carbon density narrowed drastically when UDI reached 60%, signifying a threshold for greatly reduced carbon sequestration potentials for higher UDI. These findings suggested that urban green spaces have great potential to make contribution to mitigating against future climate change if we plan and design urban green spaces following the trajectory of high carbon density, but we should be aware that such potential will be very limited when the urban development reaches certain intensity threshold.

Effect of land use and soil properties in the feasibility of two sequential extraction procedures for metals fractionation

Several sequential extraction procedures are widely applied for metals chemical fractionation in the literature. However, their limitations to be used in different soils and metals have not been discussed in detail. This study compares two of the most commonly used extraction methods for metals chemical fractionation: Tessier and BCR. The objectives were to i) assess the differences between concentrations of metals extracted in each fraction by both Tessier and BCR procedures; ii) elucidate if soil properties affected the extraction ability of each fraction from both procedures; and iii) evaluate how land use contributes to different chemical metal distribution. Results indicated that both methods provide similar results when were applied to the same soil, since non-significant differences were found in metal concentrations between both methods at each fraction. Conversely, when we compared among land uses, significant differences were found in the metal concentration between both methods, especially between agricultural/urban/industrial against forest soil. Redundancy analysis showed that in carbonate-rich soils, BCR extraction method could cover up the real concentration of exchangeable metals with those bound to the carbonate phase, being the Tessier method the most suitable one for this kind of soils. Therefore, although sequential extraction is a useful tool to understand the distribution of metals in soil, the method used must be selected according to the land use and specific soil characteristics, taking into account at least, soil carbonate content.

Effect of Large-Scale Cultivated Land Expansion on the Balance of Soil Carbon and Nitrogen in the Tarim Basin

Land reclamation influences the soil carbon and nitrogen cycling, but its scale and time effects on the balance of soil carbon and nitrogen are still uncertain. Taking the Tarim Basin as the study area, the impact of land reclamation on the soil organic carbon (SOC), total nitrogen (TN), and carbon to nitrogen (C:N) ratio was explored by the multiple temporal changes of land use and soil samples. Remote sensing detected that cropland nearly doubled in area from 1978 to 2015. Spatial analysis techniques were used to identify land changes, including the prior land uses and cultivation ages. Using land reclamation history information, a specially designed soil sampling was conducted in 2015 and compared to soil properties in ca. 1978. Results found a decoupling characteristic between the C:N ratio and SOC or TN, indicating that changes in SOC and TN do not correspond directly to changes in the C:N ratio. The land reclamation history coupled with the baseline effect has opposite impacts on the temporal rates of change in SOC, TN and C:N ratios. SOC and TN decreased during the initial stage of conversion to cropland and subsequently recovered with increasing cultivation time. By contrast, the C:N ratio for soils derived from grassland increased at the initial stage but the increase declined when cultivated longer, and the C:N ratio decreased for soils derived from forest and fluctuated with the cultivation time. Lower C:N ratios than the global average and its decreasing trend with increasing reclamation age were found in newly reclaimed croplands from grasslands. Sustainable agricultural management practices are suggested to enhance the accumulation of soil carbon and nitrogen, as well as to increase the C:N ratio to match the nitrogen deposition to a larger carbon sequestration.

Soil Organic Carbon Increases in Semi-Arid Regions while it Decreases in Humid Regions Due to Woody-Plant Encroachment of Grasslands in South Africa

Grasslands and savannas are experiencing intensive land-cover change due to woody plant encroachment. This change in land cover is thought to alter soil carbon (C) and nitrogen (N) storage in these ecosystems. Some studies have reported a negative correlation between soil C and N and mean annual precipitation while others have indicated that there is no relationship with mean annual precipitation. We quantified the changes in C and N pools and δ¹³C and δ¹⁵N values to a depth of 1 m in pairs of encroached and adjacent open grassland sites along a precipitation gradient from 300 mm to 1500 mm per annum in South Africa. Our study showed a negative correlation between changes in soil organic C stocks in the 0–100 cm soil layer and mean annual precipitation (MAP). The most humid site (1500 mm MAP) had less C in shrub-encroached sites while the drier sites (300–350 mm MAP) had more C than their paired open grasslands. This study generally showed soil organic C gains in low precipitation areas, with a threshold value between 750 mm and 900 mm. Our threshold value was higher than that found in North America, suggesting that one cannot extrapolate across continents.

Changes of Ecosystem Services and Landscape Patterns in Mountainous Areas: A Case Study in the Mentougou District in Beijing

Land use types have been strongly modified across mountainous areas. This has substantially altered the patterns and processes of ecosystems and the components of ecosystem services (ESs), and could in turn impact the sustainable development. In the mountainous Mentougou district of Beijing, we explored the changes in land use type (cropland, orchard, forested land, scrubland, grassland, bare land, water bodies, wasteland and built-up land), landscape patterns and ESs as well as their interactions during the past 30 years (1985–2014). The ESs included water yield (WY), carbon stocks (CS) and soil retention rate (SR). The results showed that 23.65% of the land use changed and the wasteland decreased by 80.87%. As for ESs, WY decreased by 47.32% since the year 2000, probably due to the increases in temperature and evapotranspiration. Although the decrease of forested land led to the decrease of CS, the increase of vegetation coverage improved SR. CS decreased by 0.99%from 1990 to 2014, and SR increased by 1.38% from 1985 to 2014. Landscape patterns became fragmented and dispersed, and MPS and CS, SHDI and SR were significantly negatively correlated. IJI and CS was positively correlated. This indicated that landscape patterns were highly correlated with ESs. In order to maintain the sustainable development of ESs, we should not only plan land use types, but also consider the rationality of landscape patterns.

Current and historical land use influence soil-based ecosystem services in an urban landscape

Urban landscapes are increasingly recognized as providing important ecosystem services (ES) to their occupants. Yet, urban ES assessments often ignore the complex spatial heterogeneity and land-use history of cities. Soil-based services may be particularly susceptible to land-use legacy effects. We studied indicators of three soil-based ES, carbon storage, water quality regulation, and runoff regulation, in a historically agricultural urban landscape and asked (1) How do ES indicators vary with contemporary land cover and time since development? (2) Do ES indicators vary primarily among land-cover classes, within land-cover classes, or within sites? (3) What is the relative contribution of urban land-cover classes to potential citywide ES provision? We measured biophysical indicators (soil carbon [C], available phosphorus [P], and saturated hydraulic conductivity [K_s ]) in 100 sites across five land-cover classes, spanning an ~125-year gradient of time since development within each land-cover class. Potential for ES provision was substantial in urban green spaces, including developed land. Runoff regulation services (high K_s ) were highest in forests; water quality regulation (low P) was highest in open spaces and grasslands; and open spaces and developed land (e.g., residential yards) had the highest C storage. In developed land covers, both C and P increased with time since development, indicating effects of historical land-use on contemporary ES and trade-offs between two important ES. Among-site differences accounted for a high proportion of variance in soil properties in forests, grasslands, and open space, while residential areas had high within-site variability, underscoring the leverage city residents have to improve urban ES provision. Developed land covers contributed most ES supply at the citywide scale, even after accounting for potential impacts of impervious surfaces. Considering the full mosaic of urban green space and its history is needed to estimate the kinds and magnitude of ES provided in cities, and to augment regional ES assessments that often ignore or underestimate urban ES supply.

Urban parks provide ecosystem services by retaining metals and nutrients in soils

Urban greenspaces provide ecosystem services like more natural ecosystems do. For instance, vegetation modifies soil properties, including pH and soil organic matter content, yet little is known about its effect on metals. We investigated whether the accumulation and mobility of heavy metals, nutrients and carbon is affected by plant functional types (evergreen or deciduous trees, lawns) in urban parks of varying ages in southern Finland. Plant types modified soil physico-chemical parameters differently, resulting in diverging accumulation and mobility of metals and other elements in park soils. However, the effects of plant functional type depended on park age: lawns in parks of ca. 50 y old had the highest contents of Cr, Cu, Fe, Mn, Ni, and Zn, and in these, and older parks (>100 y old), contents of most metals were lowest under evergreen trees. The mobility of metals and other elements was influenced by the amount of water leached through the soils, highlighting the importance of vegetation on hydrology. Soils under evergreen trees in young parks and lawns in intermediately-aged parks were most permeable to water, and thus had high loads of Ca, Cr, Cu, Fe, Ni, tot-P and tot-N. The loads/concentrations of elements in the leachates was not clearly reflected by their content/concentration in the soil, alluding to the storage capacity of these elements in urban park soils. Our results suggest that in urban systems with a high proportion of impermeable surfaces, park soil has the potential to store nutrients and metals and provide an important ecosystem service particularly in polluted cities.

Soil organic carbon distribution in roadside soils of Singapore

Soil is the largest pool of organic carbon in terrestrial systems and plays a key role in carbon cycle. Global population living in urban areas are increasing substantially; however, the effects of urbanization on soil carbon storage and distribution are largely unknown. Here, we characterized the soil organic carbon (SOC) in roadside soils across the city-state of Singapore. We tested three hypotheses that SOC contents (concentration and density) in Singapore would be positively related to aboveground tree biomass, soil microbial biomass and land-use patterns. Overall mean SOC concentrations and densities (0-100 cm) of Singapore's roadside soils were 29 g kg^-1 (4-106 g kg^-1) and 11 kg m^-2 (1.1-42.5 kg m^-2) with median values of 26 g kg^-1 and 10 kg m^-2, respectively. There was significantly higher concentration of organic carbon (10.3 g kg^-1) in the top 0-30 cm soil depth compared to the deeper (30-50 cm, and 50-100 cm) soil depths. Singapore's roadside soils represent 4% of Singapore's land, but store 2.9 million Mg C (estimated range of 0.3-11 million Mg C). This amount of SOC is equivalent to 25% of annual anthropogenic C emissions in Singapore. Soil organic C contents in Singapore's soils were not related to aboveground vegetation or soil microbial biomass, whereas land-use patterns to best explain variance in SOC in Singapore's roadside soils. We found SOC in Singapore's roadside soils to be inversely related to urbanization. We conclude that high SOC in Singapore roadside soils are probably due to management, such as specifications of high quality top-soil, high use of irrigation and fertilization and also due to an optimal climate promoting rapid growth and biological activity.

Soil surface temperatures reveal moderation of the urban heat island effect by trees and shrubs

Urban areas are major contributors to air pollution and climate change, causing impacts on human health that are amplified by the microclimatological effects of buildings and grey infrastructure through the urban heat island (UHI) effect. Urban greenspaces may be important in reducing surface temperature extremes, but their effects have not been investigated at a city-wide scale. Across a mid-sized UK city we buried temperature loggers at the surface of greenspace soils at 100 sites, stratified by proximity to city centre, vegetation cover and land-use. Mean daily soil surface temperature over 11 months increased by 0.6 °C over the 5 km from the city outskirts to the centre. Trees and shrubs in non-domestic greenspace reduced mean maximum daily soil surface temperatures in the summer by 5.7 °C compared to herbaceous vegetation, but tended to maintain slightly higher temperatures in winter. Trees in domestic gardens, which tend to be smaller, were less effective at reducing summer soil surface temperatures. Our findings reveal that the UHI effects soil temperatures at a city-wide scale, and that in their moderating urban soil surface temperature extremes, trees and shrubs may help to reduce the adverse impacts of urbanization on microclimate, soil processes and human health.

Ecological role of vertebrate scavengers in urban ecosystems in the UK

Recent research has demonstrated how scavenging, the act of consuming dead animals, plays a key role in ecosystem structure, functioning, and stability. A growing number of studies suggest that vertebrate scavengers also provide key ecosystem services, the benefits humans gain from the natural world, particularly in the removal of carcasses from the environment. An increasing proportion of the human population is now residing in cities and towns, many of which, despite being highly altered environments, contain significant wildlife populations, and so animal carcasses. Indeed, non-predation fatalities may be higher within urban than natural environments. Despite this, the fate of carcasses in urban environments and the role vertebrate scavengers play in their removal have not been determined. In this study, we quantify the role of vertebrate scavengers in urban environments in three towns in the UK. Using experimentally deployed rat carcasses and rapid fire motion-triggered cameras, we determined which species were scavenging and how removal of carcass biomass was partitioned between them. Of the 63 experimental carcasses deployed, vertebrate scavenger activity was detected at 67%. There was a significantly greater depletion in carcass biomass in the presence (mean loss of 194 g) than absence (mean loss of 14 g) of scavengers. Scavenger activity was restricted to three species, Carrion crows Corvus corone, Eurasian magpies Pica pica, and European red foxes Vulpes vulpes. From behavioral analysis, we estimated that a maximum of 73% of the carcass biomass was removed by vertebrate scavengers. Despite having low species richness, the urban scavenger community in our urban study system removed a similar proportion of carcasses to those reported in more pristine environments. Vertebrate scavengers are providing a key urban ecosystem service in terms of carcass removal. This service is, however, often overlooked, and the species that provide it are among some of the most disliked and persecuted.

Carbon sequestration through urban ecosystem services: A case study from Finland

Plants and soil are natural regulators of atmospheric CO2. Whereas plants sequester atmospheric carbon, soils deposit it for decades. As cities become increasingly more densely built, the available land area for such ecosystem services may decrease. We studied seven different housing areas in the Finnish city of Espoo to ascertain the extent to which site efficiency affects to the ecosystem services if the full life-cycle GHG emissions of these areas are taken into account. The results show that the impact of CO2 uptake through carbon sinks in growing plants and the uptake of soil organic carbon vary greatly. Its share of all emissions varied from a marginal value of 1.2% to a more considerable value of 11.9%. The highest potential was calculated for a detached house located on a large site, while the weakest was calculated for compact apartment blocks. The study revealed that in order to quantify this potential more accurately, several knowledge gaps must first be addressed. These include impartial growth algorithms for Nordic wood species, missing accumulation factors for soil organic carbon in cold climates and statistical maintenance scenarios for gardens.

A review on the role of organic inputs in maintaining the soil carbon pool of the terrestrial ecosystem

Among the numerous sources of greenhouse gases, emissions of CO2 are considerably affected by changes in the extent and type of land use, e.g., intensive agriculture, deforestation, urbanization, soil erosion, or wetland drainage. As a feasible option to control emissions from the terrestrial ecosystems, the scientific community has explored the possibility of enhancing soil carbon (C) storage capacity. Thus, restoration of damaged lands through conservation tillage, crop rotation, cover cropping, reforestation, sub-soiling of compacted lands, sustainable water management practices, and organic manuring are the major antidotes against attenuation of soil organic C (SOC) stocks. In this research, we focused on the effect of various man-made activities on soil biotic organics (e.g., green-, farm-yard manure, and composts) to understand how C fluxes from various sources contribute to the establishment of a new equilibrium in the terrestrial ecosystems. Although such inputs substitute a portion of chemical fertilizers, they all undergo activities that augment the rate and extent of decay to deplete the SOC bank. Here, we provide perspectives on the balancing factors that control the mineralization rate of organic matter. Our arguments are placed in the background of different land use types and their impacts on forests, agriculture, urbanization, soil erosion, and wetland destruction.

Influence of population density on the concentration and speciation of metals in the soil and street dust from urban areas

Street dust and soil from high, medium and low populated cities and natural area were analysed for selected physical-chemical properties, total and chemical speciation of Zn, Pb, Cu, Cr, Cd, Co, Ni to understand the influence of human activities on metal accumulation and mobility in the environment. The pH, salinity, carbonates and organic carbon contents were similar between soil and dust from the same city. Population density increases dust/soil salinity but has no influence on metals concentrations in soils. Increases in metal concentrations with population density were observed in dusts. Cu, Zn, Pb, Cr can be mobilized more easily from dust compared to the soil. In addition, population density increase the percentage of Pb and Zn associated to reducible and carbonate phase in the dust. The behaviour of metals except Cd in soil is mainly affected by physico-chemical properties, while total metal influenced the speciation except Cr and Ni in dusts.

Black Carbon Contribution to Organic Carbon Stocks in Urban Soil

Soil holds 75% of the total organic carbon (TOC) stock in terrestrial ecosystems. This comprises ecosystem-derived organic carbon (OC) and black carbon (BC), a recalcitrant product of the incomplete combustion of fossil fuels and biomass. Urban topsoils are often enriched in BC from historical emissions of soot and have high TOC concentrations, but the contribution of BC to TOC throughout the urban soil profile, at a regional scale is unknown. We sampled 55 urban soil profiles across the North East of England, a region with a history of coal burning and heavy industry. Through combined elemental and thermogravimetic analyses, we found very large total soil OC stocks (31-65 kg m(-2) to 1 m), exceeding typical values reported for UK woodland soils. BC contributed 28-39% of the TOC stocks, up to 23 kg C m(-2) to 1 m, and was affected by soil texture. The proportional contribution of the BC-rich fraction to TOC increased with soil depth, and was enriched in topsoil under trees when compared to grassland. Our findings establish the importance of urban ecosystems in storing large amounts of OC in soils and that these soils also capture a large proportion of BC particulates emitted within urban areas.

Modelling short-rotation coppice and tree planting for urban carbon management - a citywide analysis

The capacity of urban areas to deliver provisioning ecosystem services is commonly overlooked and underutilized. Urban populations have globally increased fivefold since 1950, and they disproportionately consume ecosystem services and contribute to carbon emissions, highlighting the need to increase urban sustainability and reduce environmental impacts of urban dwellers. Here, we investigated the potential for increasing carbon sequestration, and biomass fuel production, by planting trees and short‐rotation coppice (SRC), respectively, in a mid‐sized UK city as a contribution to meeting national commitments to reduce CO₂ emissions.

Iterative GIS models were developed using high‐resolution spatial data. The models were applied to patches of public and privately owned urban greenspace suitable for planting trees and SRC, across the 73 km² area of the city of Leicester. We modelled tree planting with a species mix based on the existing tree populations, and SRC with willow and poplar to calculate biomass production in new trees, and carbon sequestration into harvested biomass over 25 years.

An area of 11 km² comprising 15% of the city met criteria for tree planting and had the potential over 25 years to sequester 4200 tonnes of carbon above‐ground. Of this area, 5·8 km² also met criteria for SRC planting and over the same period this could yield 71 800 tonnes of carbon in harvested biomass.

The harvested biomass could supply energy to over 1566 domestic homes or 30 municipal buildings, resulting in avoided carbon emissions of 29 236 tonnes of carbon over 25 years when compared to heating by natural gas. Together with the net carbon sequestration into trees, a total reduction of 33 419 tonnes of carbon in the atmosphere could be achieved in 25 years by combined SRC and tree planting across the city.

Synthesis and applications. We demonstrate that urban greenspaces in a typical UK city are underutilized for provisioning ecosystem services by trees and especially SRC, which has high biomass production potential. For urban greenspace management, we recommend that planting SRC in urban areas can contribute to reducing food–fuel conflicts on agricultural land and produce renewable energy sources close to centres of population and demand.

We demonstrate that urban greenspaces in a typical UK city are underutilized for provisioning ecosystem services by trees and especially SRC, which has high biomass production potential. For urban greenspace management, we recommend that planting SRC in urban areas can contribute to reducing food–fuel conflicts on agricultural land and produce renewable energy sources close to centres of population and demand.

Global drivers and tradeoffs of three urban vegetation ecosystem services

Our world is increasingly urbanizing which is highlighting that sustainable cities are essential for maintaining human well-being. This research is one of the first attempts to globally synthesize the effects of urbanization on ecosystem services and how these relate to governance, social development and climate. Three urban vegetation ecosystem services (carbon storage, recreation potential and habitat potential) were quantified for a selection of a hundred cities. Estimates of ecosystem services were obtained from the analysis of satellite imagery and the use of well-known carbon and structural habitat models. We found relationships between ecosystem services, social development, climate and governance, however these varied according to the service studied. Recreation potential was positively related to democracy and negatively related to population. Carbon storage was weakly related to temperature and democracy, while habitat potential was negatively related to democracy. We found that cities under 1 million inhabitants tended to have higher levels of recreation potential than larger cities and that democratic countries have higher recreation potential, especially if located in a continental climate. Carbon storage was higher in full democracies, especially in a continental climate, while habitat potential tended to be higher in authoritarian and hybrid regimes. Similar to other regional or city studies we found that the combination of environment conditions, socioeconomics, demographics and politics determines the provision of ecosystem services. Results from this study showed the existence of environmental injustice in the developing world.

Urban tree effects on soil organic carbon

Urban trees sequester carbon into biomass and provide many ecosystem service benefits aboveground leading to worldwide tree planting schemes. Since soils hold ∼75% of ecosystem organic carbon, understanding the effect of urban trees on soil organic carbon (SOC) and soil properties that underpin belowground ecosystem services is vital. We use an observational study to investigate effects of three important tree genera and mixed-species woodlands on soil properties (to 1 m depth) compared to adjacent urban grasslands. Aboveground biomass and belowground ecosystem service provision by urban trees are found not to be directly coupled. Indeed, SOC enhancement relative to urban grasslands is genus-specific being highest under Fraxinus excelsior and Acer spp., but similar to grasslands under Quercus robur and mixed woodland. Tree cover type does not influence soil bulk density or C∶N ratio, properties which indicate the ability of soils to provide regulating ecosystem services such as nutrient cycling and flood mitigation. The trends observed in this study suggest that genus selection is important to maximise long-term SOC storage under urban trees, but emerging threats from genus-specific pathogens must also be considered.

Urban cultivation in allotments maintains soil qualities adversely affected by conventional agriculture

Modern agriculture, in seeking to maximize yields to meet growing global food demand, has caused loss of soil organic carbon (SOC) and compaction, impairing critical regulating and supporting ecosystem services upon which humans also depend. Own‐growing makes an important contribution to food security in urban areas globally, but its effects on soil qualities that underpin ecosystem service provision are currently unknown.

We compared the main indicators of soil quality; SOC storage, total nitrogen (TN), C : N ratio and bulk density (BD) in urban allotments to soils from the surrounding agricultural region, and between the allotments and other urban greenspaces in a typical UK city. A questionnaire was used to investigate allotment management practices that influence soil properties.

Allotment soils had 32% higher SOC concentrations and 36% higher C : N ratios than pastures and arable fields and 25% higher TN and 10% lower BD than arable soils.

There was no significant difference between SOC concentration in allotments and urban non‐domestic greenspaces, but it was higher in domestic gardens beneath woody vegetation. Allotment soil C : N ratio exceeded that in non‐domestic greenspaces, but was lower than that in garden soil.

Three‐quarters of surveyed allotment plot holders added manure, 95% composted biomass on‐site, and many added organic‐based fertilizers and commercial composts. This may explain the maintenance of SOC, C : N ratios, TN and low BD, which are positively associated with soil functioning.

Synthesis and applications. Maintenance and protection of the quality of our soil resource is essential for sustainable food production and for regulating and supporting ecosystem services upon which we depend. Our study establishes, for the first time, that small‐scale urban food production can occur without the penalty of soil degradation seen in conventional agriculture, and maintains the high soil quality seen in urban greenspaces. Given the involvement of over 800 million people in urban agriculture globally, and its important contribution to food security, our findings suggest that to better protect soil functions, local, national and international urban planning and policy making should promote more urban own‐growing in preference to further intensification of conventional agriculture to meet increasing food demand.

Maintenance and protection of the quality of our soil resource is essential for sustainable food production and for regulating and supporting ecosystem services upon which we depend. Our study establishes, for the first time, that small‐scale urban food production can occur without the penalty of soil degradation seen in conventional agriculture, and maintains the high soil quality seen in urban greenspaces. Given the involvement of over 800 million people in urban agriculture globally, and its important contribution to food security, our findings suggest that to better protect soil functions, local, national and international urban planning and policy making should promote more urban own‐growing in preference to further intensification of conventional agriculture to meet increasing food demand.