Increasing amounts of spent mushroom substrate in the Siedlce region and the unsolved problem of its disposal has encouraged researchers to study its properties and, subsequently, the possibility of its utilisation as fertiliser. Samples of (fresh) substrate after 6-week cultivation of the white mushroom Agaricus bisporus in two modern cultivation halls were used as the studied material. The determined parameters included its pH and the content of dry matter, ash, mineral nitrogen as well as total carbon and nitrogen. The main aim of the study was to determine the speciation of organic carbon and metals in spent mushroom substrates. To this end, their sequential extraction was carried out by the method described by HE et al. (1995), recommended for materials with a high organic matter content. The extraction procedure yielded six operational fractions of carbon and metals and separated the humic substances into fractions of fulvic and humic acids and metals bound with them. Sequential application of extraction reagents of increasing ability to extract carbon and metal from compounds (H2O → KNO3 → Na4P2O7 → NaOH → HNO3 → aqua regia) allowed us to assess the potential bioavailability and resistance to biodegradation, as well as current and potential hazards to the environment (phytoavailability, mobility). It has been found that spent mushroom substrate could be recommended as fertiliser because it can enrich soils with organic matter. The largest amounts of carbon (62%) were found in the stable residual fraction, whereas the bioavailable fractions contained about 10% of carbon. Concentrations of metals (Mg, Ca, Fe, Mn, Ba, Sr, Zn, Cu, Ni, Cr, Pb) in the material were variable, but most metals were found in the fraction strongly bound with organic and mineral compounds (30.8-80.6%). Ca, Mg, Mn, Sr, Fe, and Zn were predominantly bound with mobile fulvic acids, whereas Cu, Ni, Ba, and Pb – with more stable humic acids.
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