Guo 225-234

Pennsylvania generates over 50,000 m of spent mushroom substrate (SMS) annually due to production of the white button mushroom Agaricus bisporus (Chong and Hamersma 1997). Improper disposal of this volume of organic waste may cause environmental problems such as air, water and soil pollution. Considering that its major ingredients include incompletely composted straw, hay, manure, gypsum and potash, SMS may be reused as potting soil, topsoil or soil amendments in agriculture, horticulture and ecosystem restoration (Stark and Williams 1994; Wuest et al. 1995). However, a high salt content limits its suitability as a plant growth medium without dilution (Ciavatta et al. 1993; Chong and Rinker 1994), and fresh SMS is usually stacked in the field for two years of weathering or “passive composting” to remove soluble salts prior to land application. During the weathering process, rainwater and snowmelt percolate through SMS piles and a large amount of solutes is leached out. Field studies with lysimeter sampling showed that SMS leachate from 90 cm and 150 cm high piles contained high concentrations of dissolved organic carbon (DOC) and inorganic salts. The DOC concentrations ranged from 80011,100 mg L, and K, Ca, Cl and SO 4 2concentrations ranged from 72-316, 2-116, 27-196 and 79-324 mmol c L, respectively. In two years, 90 cm piles released (per cubic meter of SMS) 3.0 kg of DOC and 26.6 kg of inorganic salts. Piles of 150 cm height released (per cubic meter of SMS) 2.8 kg of DOC and 13.6 kg of inorganic salts (Guo et al. 2001a). Solute release from organic composting including SMS weathering determines the pretreatment efficiency and related environmental impacts. As labile components in organic wastes are decomposed and leached out, the composting product becomes mature. Meanwhile, released solutes may enter into surface water with runoff or migrate into groundwater by percolation. For example, Holcomb et al. (2000) found that leaching each pot of SMS with three volumes of irrigation water for 15 minutes removed excessive salts and the leached SMS was then an effective growth medium for most ornamental plants. The solutes released during SMS field weathering of 90-cm and 150cm piles migrated to at least 200 cm depth in soil profiles in two years, posing a threat to groundwater with high water tables (Guo et al. 2001b). Kaplan et al. (1995) observed that groundwater concentrations of DOC, Compost Science & Utilization, (2004), Vol. 12, No. 3, 225-234