Solute release from weathering of spent mushroom substrate under controlled conditions

Weathering of piled material in the field is a popular method to treat spent mushroom substrate (SMS) before reuse. During the weathering process, rainfall and snowmelt pass through SMS piles and a large amount of solutes is released in the leachate. To investigate solute release patterns, the field weathering process was simulated under controlled conditions in the laboratory. Fresh SMS was packed in an acrylic column (20 cm i.d.) to 150 cm height and leached intermittently with a cumulative total of 230 cm of deionized water over 180 days. Leachate was collected and analyzed for dissolved organic carbon (DOC), dissolved organic nitrogen (DON), electrical conductivity (EC), and inorganic salts. Solute release patterns were described using first order models, and total released solutes were calculated. The SMS leachate had DOC, DON and EC values ranging from 450 to 15, 500 mg L⁻¹, 50 to 1, 700 mg L⁻¹, and 3 to 50 dS m⁻¹, respectively. The major inorganic cations were K⁺, Na⁺, Ca²⁺, Mg²⁺ and NH₄⁺, and anions were Cl⁻ and SO₄²⁻. Release of DOC, DON, and bivalent cations Ca²⁺ and Mg²⁺ were described by a first order Exponential Rise to Maximum model, while releases of monovalent ions Cl⁻, K⁺, Na⁺ and NH₄⁺ were described as a first order Sigmoidal Logistic process, and SO₄²⁻ release was best modeled by a Sigmoidal Chapman equation. Following six months and 230 cm applied water, 3.1 kg of DOC, 0.58 kg of dissolved N, and 8.6 kg of inorganic salts were leached per cubic meter of bulk SMS (220 kg oven dry mass). Weathering of SMS involves a significant removal of nutrients from the composted material, which can contribute to pollution of soil and groundwater.