Can Organic Matter in Brownfield Soils Reach the Same Level and Quality as in Natural Soils in the Same Area? Study of Soils Long-Term Contaminated With Trace Metals Under Moderate Climate Conditions in Poland

We investigated whether old, self-restored brownfield soils could store similar amounts of soil organic matter (SOM) to natural, reference soils, which would make them valuable for the environment in the fight against global warming. The brownfield soils came from heaps formed after Zn/Pb ore, coal extraction and from the environs of an Fe-smelter. They differed in age (50–400 years); all were covered with grass. We examined: soil pH, texture, enzyme activity, and C, N, P, K, CaCO₃ contents, Zn, Pb and Cd contamination of soils and grasses and biomass, lignin, Ca, K, P, Na contents of grasses. SOM stabilisation was evaluated through the analysis of the humic acids (HAs) composition and optical properties. In all soils, the mean C stocks ranged from 5.8 to 10.3 kg/m² and were similar to these in the reference soils, but the HAs of the brownfield soils had a more aliphatic structure and lower maturity, thus less stabilised SOM than the reference soils. High C accumulation in brownfield soils resulted from the high biomass, especially roots, of the well-adapted plants covering these soils, which were also rich in lignin, accumulated as a protection against stress. A lower SOM stabilisation of brownfield soils than of reference ones was caused by their high TMs contamination, but it differed according to the combined effects of both: the soil age (being more stabilised in older soils) and a disturbance degree (more stabilised in less disturbed industrial soils than mine ones).