Effects of spent mushroom substrate weathering on the chemistry of underlying soils

Passive weathering of heaped material in the field is a popular method for treating spent mushroom substrate (SMS) before its reuse. During the weathering process, leachate containing high concentrations of dissolved organic matter and inorganic salts is released into the underlying soils, but effects on soil and ground water quality remain uncertain. We conducted a field study to measure the effects of SMS weathering on chemical and morphological properties of underlying soils. Two SMS piles, 20 m long, 6 m wide, and either 90 or 150 cm high, were placed in a fallow agricultural field dominated by grasses and weathered for 24 mo. Soil samples were taken from each genetic horizon under the SMS piles following their removal and analyzed for pH, total organic carbon (TOC), electrical conductivity (EC), water-soluble organic carbon (WSOC), water-soluble inorganic cations and anions, and exchangeable inorganic cations. Compared with an unaffected control, SMS weathering did not raise soil TOC, but did alter soil pH, and significantly increased EC, WSOC, and water-soluble and exchangeable inorganic ions. At 200 cm below the soil surface, the EC, WSOC, and water-soluble Cl^-, NO₃^-, Ca²⁺, Mg²⁺, Na⁺, and K⁺ under SMS piles were 4 to 20 times higher than in unaffected soils. Water-soluble NO₃^- was minimal in the surface soil but peaked in the C horizon (120-180 cm) under the 90-cm SMS pile, indicating that these soils may have little capacity for retaining NO₃^-. Concentration profiles of the different solutes reflect their relative mobilities in the soil environment and indicate the potential for effects on subsurface water supplies.