Urban Soils as Hotspots for Carbon Storage

Abstract

The services that society extracts from urban ecosystems are becoming increasingly important with increasing urbanization. A potentially crucial ecosystem service is soil carbon (C) storage, as negative soil C balances have the potential to offset some of the anthropogenic greenhouse gas emissions mitigating climate change. This research studied the soil C storage in the urban greenspaces of 2 districts in a typical Dutch city (The Hague districts’ City Centre and Scheveningen, 21 km2 , 37% greenspace) as a case study. The following research question was addressed: ‘What is the carbon storage potential of urban soils in The Hague?’ Soil samples were collected along a transect going from the suburban seaside towards the city centre of The Hague. The transect crossed a toposequence from sandier dune soils to peaty inland soils. Besides soil C densities, several soil-quality characteristics were measured namely dissolved organic C levels, pH, electrical conductivity, nitrogen, phosphorus, and sulphur levels, calcium carbonate, the water-holding capacity of the soil and the degradability of soil organic C.

Although urban soil can be highly disturbed or altered by anthropogenic activities, the high C densities in The Hague suggested that its potential to store C appeared unaffected. Along the transect, a mean C density of 88 t/ha, of which 82 t/ha was considered organic C, was detected, which was higher than the values currently assigned to urban soils in national C inventories. The urban soil C storage was dependent on the type of vegetation, urbanization extent and land ownership. The hypothesized links between land use and soil type were not apparent in this case study, suggesting that processes driving soil C storage are controlled by different factors.

The total soil C storage of the upper 30 cm of the greenspaces in The Hague was estimated at 18.8 kt of C. The use of high spatial resolution GIS data with a scale of 10 x 10 m enabled the inclusion of small patches in the total soil C storage of The Hague, which proved to be significant as the smaller urban greenspaces, which are typical for dense urban centres, contained similar soil C density as the larger urban greenspaces, such as urban forests.

Soil C storage in urban ecosystems is highly variable. How generalizable these results are across other Dutch cities requires further research. Moreover, to translate current soil C stocks to annual C fluxes further research is required. This study found that urban soil C stocks are underestimated, which potentially also is the case for urban soil sequestration rates that are currently applied in C modelling studies.