Functional diagnosis of industrial soils: from a cognitive model to in situ implementation

Industrial activities, such as thermal power plants, induce soil degradation on large areas (e.g. soil sealing, contamination related to fuel, coal and ash deposits, soil compaction). After the cessation of activities, landowners of such sites have a huge land heritage that could be considered to promote rehabilitation projects for new land-uses in the frame of the No Net Land Take by 2050. Therefore, there is a need to develop a robust and easy-to-use approach for landowners that could be implemented by soil techniciens/pratitioners to assess soil functions to measure their potential for future uses.

First a cognitive model linking soil functions to a minimum dataset of indicators was established based on chemical, physical and biological properties of soil as well as vegetation cover. This cognitive model includes 6 soil functions (e.g. plant biomass production) and 17 sub-functions (e.g. phytoavailability of nutrients, nutrients storage) and a minimum set of indicators selected among a large list from research studies and attributed to each sub-function and function.

Then two thermal power plants under closure were selected and a documentary survey was carried out for each site to identify contrasted zones in terms of soil cover, mostly based on the nature of the past activities (e.g. coal, slag or ash deposit, building foundations, fuel storage). Twelve zones considered as homogeneous in terms of vegetation and soil type and distinct from each other were selected on these two sites. In total, 12 soil profiles and 164 soil samples were analysed for various biological (plants, nematodes, microbial communities), chemical and physical parameters.

Our results show contrasting situations. Despite the high vegetation cover of the three different ash deposit zones, their plant diversity indices ranged from very low to medium. The same goes for the area where building foundations were located, but they had very little vegetation cover. Also, the enrichment index (EI) and structure index (SI) of the nematode community showed that ash deposits are degraded, nutrient-poor soils and have a high C/N (>12) while the building foundation areas have a "mature and fertile" soil with optimal C/N.  

Whereas some soils could be considered as natural references as they were not affected by industrial activities, others were Technosols made of 100% artefacts. However, the gradient of anthropisation was surprisingly not correlated to the level of functions that were assessed. As an example, technogenic soils developed from fly ash exhibit high soil functions ratings (e.g. carbon storage).

These initial results suggest that the functioning of these soils must be evaluated according to different scales (e.g. plot scale, surface soils), points of view (biological, chemical and physical) and soil functions (e.g. storage and sequestration of GHG, biodiversity reservoir), to establish their functional profiles and suggest possible future uses.