Environmental and social drivers behind spatial variability of soil carbon in urban green infrastructures of Wageningen

Recent IPCC reports claim carbon neutrality as the key strategy for climate mitigation, therefore compensating greenhouse gases’ emissions by carbon (С) sequestration become the core of climate mitigation measures taken by cities. Developing urban green infrastructures is considered an efficient measure for C sequestration and climate mitigation in cities. However, most of these solutions consider C sequestration in aboveground biomass and ignore the role of urban soil-C stocks. Urban soils’ contribution to C balance in urban ecosystems remains overlooked so far, but gets increasingly important with ongoing climate change. Urban soils are exposed to direct and indirect anthropogenic influences, they are very heterogeneous and dynamic. This variability is driven by both environmental (e.g., vegetation, geomorphology, and parent material) and social (e.g., decisions on maintenance and management) factors. Traditional soil surveys focus on the environmental factor and barely ignore the social drivers, that might be appropriate for natural or agricultural areas, but can hardly be implemented to study soil C stocks in cities.  In the Netherlands, urban areas cover at least 15% of the territory and are projected to expand with more than 1000 km² by 2040, however urban soils remain overlooked and sustainable urban development strategies are not supported by soil data. this study we aimed to explore the effect of natural and social factors on the spatial variability in soil C on Wageningen – a middle-size university town in the Netherlands.

Wageningen is a perfect case study to investigate factors influencing spatial variability of urban soil C. A long history and unique landscape diversity create conditions for high spatial variation in soil-forming factors. Based on the parent materials, the residential blocks outside the center can be subdivided into strata dominated by sandy and clayey soils. Urban expansion and building up new residential blocks, public and private green areas coincided with development and management of urban soils. A random stratified soil survey (n=56) allowed capturing the effect of parent materials, land cover and land-use history. The effect of the social factor was studied by expert interviews with the owners of the green areas (key plots, n=10), where detailed soil survey was done. Expert interviews included information on soil management as well as personal questions. In result, typical ‘portraits’ of landowners/ green-keepers were developed and related to soil C-stocks assessment. It was concluded that land-cover and land-use history/ historical zoning distinguished spatial patterns in soil C at the city level, whereas at the local scale social factors dominated. Moreover, local spatial variability distinguished by differences in maintenance/ management practices (e.g., minimal management in a student house in comparison to an intensive maintenance with irrigation and adding composts in a high-price cottage) was comparable or even higher than total variance at the city level.  This is an important message for urban planners and landscape designers, claiming that the social factors and personal decisions shall not be ignored in climate-resilient strategies and practices to develop and maintain urban green infrastructures.