Methods for estimating types of soil organic carbon and their application to surveys of UK urban areas

Abstract

The occurrence of substantial quantities of black carbon (BC) in urban soil due to local dispersal following incomplete combustion of fossil fuel complicates the determination of labile soil organic carbon (SOC). Estimates of SOC content were made from loss on ignition (LOI) analyses undertaken on samples (0-15 cm depth) from comprehensive soil geochemical surveys of three UK urban areas. We randomly selected 10 samples from each decile of the LOI distribution for each of the surveys of Coventry (n = 808), Stoke-on-Trent (n = 737) and Glasgow (n = 1382) to investigate the proportions of labile SOC and BC. We determined their total organic carbon (TOC) and BC contents, and by difference the labile SOC content, and investigated the linear relationship of the latter with SOC estimates based on LOI analyses. There was no evidence for a difference in the slope of the regression for the three urban areas. We then used a linear regression of labile SOC based on LOI analyses (r 2 = 0.81) to predict labile SOC for all survey samples from the three urban areas. We attribute the significantly higher median BC concentrations in Glasgow (1.77%, compared with 0.46 and 0.59% in Coventry and Stoke-on-Trent) to greater dispersal of coal ash across the former. An analysis of the 30 samples showed that LOI at 450°C accounts for a consistent proportion of BC in each sample (r 2 = 0.97). Differences between TOC (combustion at 1050°C after removal of inorganic carbon) and an LOI estimate of SOC may be a cost-effective method for estimation of BC. Previous approaches to estimation of urban SOC contents based on half the mean SOC content of the equivalent associations under pasture, underestimate the empirical mean value.

Managing soil carbon stocks to enhance the resilience of urban ecosystems

“…Only a few reviews/studies on urban SIC and SOC dynamic and stock have been published (e.g., Lorenz and Lal 2009;Luo et al 2012;Pouyat et al 2006;Rawlins et al 2008;Tong and Dong 2007).…”

Designing a carbon capture function into urban soils

“…and median is 19.63%. The largest proportions of black carbon were all found in the Glasgow samples, where conditioning soil with coal ash was a common practice (Rawlins et al, 2008). The mean and median (16.96%, + 2.15 and 14.39 % respectively) are both reduced when the Glasgow data are removed.…”

“…The area covered by domestic gardens is important, ranging between 11.3 and 24.9% of the land cover of five UK cities (Loram et al, 2007). The soils supporting these different kinds of greenspaces, although often artificial or modified (Rawlins et al, 2008), represent a significant pool of organic carbon in urban areas, but one that in the UK has not attracted much attention and has not been adequately quantified to date. The vegetation, particularly wooded patches and trees, constitutes a conspicuous organic carbon store within urban greenspaces, but globally soils contain three times more organic carbon than vegetation (Lal, 2008).…”

“…It is clear that a history of heavy industry and coal burning can leave a strong and lasting legacy of black carbon in urban soils, but if the Stoke-on-Trent and Coventry samples are typical, this is normally < 20% of the soil organic carbon. However, it is clear that black carbon is occasionally the dominant form of organic carbon as in Glasgow (Rawlins et al, 2008) and parts of Stuttgart, Germany, where it provides 72% of soil organic carbon (Lorenz et al, 2006). Sequestration of organic carbon into urban soils represents an opportunity to reduce net emissions of CO 2 , thus reducing the carbon footprint of a city.…”

“…Black carbon is a highly recalcitrant and altered form of organic carbon, consisting of a range of aromatic hydrocarbon compounds from charred biomass and soot, and which accumulates in soil as it has a very slow rate of decomposition. Urban soils in UK cities have been found to contain black carbon in the range of approximately 4 -61% of their total organic carbon (Rawlins et al, 2008). This range is based on three cities; Glasgow, Stoke-on-Trent and Coventry, where the mean proportion of black carbon is 22.91% (+ 2.52, 1 S.E.)…”