Trash or Treasure? Spent mushroom substrate for trace-metal-contaminated soils: physicochemical effects and leaching.

Spent mushroom substrate (SMS), generated from approximately 44 Mt of global edible mushroom production annually, presents a waste-management challenge with potential value as a soil amendment for contaminated land. Should SMS be used on trace-metal-contaminated soils, and if so, on which soil types and for which metals? This study addresses the question through a three-tier evaluation of unprocessed SMS as an amendment for sandy and loam soils contaminated with Cr, Cu and Zn at 10%, 20% and 30% SMS rates. First, changes in soil pH, water holding capacity, free swelling index and sorption capacity were quantified. Second, gas chromatography-mass spectrometry (GC-MS) screening characterised organic compounds released from SMS under five geochemical scenarios (neutral, acidic, alkaline, weak organic acid and complexing extraction). Third, Cr, Cu and Zn leaching was assessed in column tests on amended soils. SMS amendment increased soil pH from 4.79 to 7.3-7.5 in sandy soil and raised water holding capacity from 98% to 150% at 20-30% rates. SMS released chemically diverse organic fractions including low-molecular-weight organic acids, phenolics, catechols, N-containing compounds and fatty acids. Column tests showed that SMS reduced Cu leaching by 40.5% in sandy soil and 46.0% in loam soil, while Cr leaching decreased by 35.5% and 26.8%, respectively. Zn remained comparatively mobile, especially in loam soil. The findings provide a decision framework for SMS use in remediation: SMS is suitable for sandy soils with Cu and Cr contamination but should be used with caution on Zncontaminated systems and on loam soils where the immobilisation benefit is weaker. SMS use should be preceded by site-specific assessment of soil texture, metal identity and dissolved organic matter dynamics.