Rapid urbanization causes the accumulation of large amounts of pollutants, including heavy metals, organic pollutants, and black carbon (BC). BC is the carbonaceous residue generated from the incomplete combustion of fossil fuels and biomass. It plays an important role on the migration of heavy metals and organic pollutants, as well as soil carbon sequestration. BC accumulation due to human activities greatly affects the global carbon budget, helps to drive climate change, and damages human health. To date, few studies have examined how the intensity of urbanization affects the distribution of BC in soils in urban areas. Therefore, the objective of this study is to determine the effects of urbanization intensity on the spatial distribution and content of BC in urban surface soil. We collected samples from 55 sites in South China and used a multi-scale geographical regression model to evaluate the impact of the interference intensity of urbanization on the amount and distribution of BC. Our results showed that the BC content was significantly higher in urban areas (9.74 ± 1.18 g kg−1) than in rural areas (2.94 ± 0.89 g kg−1) and that several urban parks with a higher interference intensity were hotspots of BC accumulation, suggesting that urbanization promoted BC accumulation. Our model revealed that road density was significantly and positively correlated with BC accumulation. Because there are more cars driving in areas with high road density, vehicle emissions may be one of the causes of BC accumulation. Our results also indicated that the impact of urbanization intensity on the BC distribution was sensitive to sampling density.

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Black carbon (BC) is a carbonaceous residue generated by the incomplete combustion of fossil fuels and biomass. It forms an important component of the mangrove carbon pool. Although coastal and marine sediments have long been recognized as important sinks for BC, there are few reports on its distribution in mangrove sediments. This study investigated BC distribution and the associated environmental implications in sediments of various size fractions. Mangrove sediments were collected from 44 sites in the Guangdong, Guangxi, Hainan, and Fujian Provinces of China. Sediments from the Hainan Province were found to have much higher organic carbon (OC) and BC content than those from other provinces. Sediment OC and the BC content showed a significant positive relationship. Sediments from the Guangdong, Guangxi, and Hainan provinces showed significant BC accumulation with an enrichment factor > 1, especially in the size fractions 0.053–0.25 mm and 0.25–2 mm. Sediments from different sampling sites did not show significantly different BC distribution between the various size fractions. A higher mass loading of BC was observed among fine particles in samples from the Fujian Province, and among the larger particles in samples from the Guangdong, Guangxi, and Hainan provinces. Among all the samples, the BC/OC ratio ranged from 0.21 to 0.29, indicating that BC originates from a combination of biomass and mineral combustion. The average δ13C values in the sediments varied between the sampling plots. The highest δ13C values were recorded in the Fujian province, which indicates the proportion of BC from biomass combustion in mangroves in Fujian Province is higher than that in other Provinces.