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TECHNICAL REPORT
Waste Management

Leachate Chemistry of Field-Weathered Spent Mushroom Substrate

Department of Agronomy, The Pennsylvania State Univ., University Park, PA 16802

* Corresponding author (jdc7{at}psu.edu)

Received for publication November 15, 2000.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Passive leaching by rainfall and snowmelt is a popular method to treat piles of spent mushroom substrate (SMS) before its reuse. During this field weathering process, leachate percolates into the underlying soils. A field study was conducted to examine the chemistry of SMS leachate and effects of infiltration. Two SMS piles were deposited (90 and 150 cm in height) over a Typic Hapludult and weathered for 24 mo. Leachate was collected biweekly using passive capillary samplers. The SMS leachate contained high concentrations of dissolved organic carbon (DOC; 0.8–11.0 g L^-1), dissolved organic nitrogen (DON; 0.1–2 g L^-1), and inorganic salts. The pH, electrical conductivity, and acid neutralizing capacity were 6.6 to 9.0, 21 to 66 ds m^-1, and 10 to 75 mmol_c L^-1, respectively. Inorganic chemistry of the leachate was dominated by K⁺, Cl^-, and SO^2-₄. Leachate DOC was predominantly low molecular weight (<1000 Da) organic acids. During 2 yr of weathering, the 90-cm SMS pile released (per cubic meter of SMS) 3.0 kg of DOC, 1.6 kg of dissolved N, and 26.6 kg of inorganic salts. The 150-cm pile released (per cubic meter of SMS) 2.8 kg of DOC, 0.7 kg of dissolved N, and 13.6 kg of inorganic salts. The 150-cm pile retained more water and exhibited lower net nitrification compared with the 90-cm pile. The top 90 cm of soil retained 20 to 89% of the leachate solutes. Weathering of SMS in piles of 90 cm depth or greater may adversely affect ground water quality.

Abbreviations: ANC, acid neutralizing capacity • DOC, dissolved organic carbon • DOM, dissolved organic matter • DON, dissolved organic nitrogen • EC, electrical conductivity • HMW, high molecular weight • IC, ion chromatography • LMW, low molecular weight • LMWOA, low molecular weight organic acid • PL3, leachate collected directly from the 90-cm spent mushroom substrate pile • PL5, leachate collected directly from the 150-cm spent mushroom substrate pile • SL3, soil leachate collected underneath the 90-cm pile at 90 cm soil depth • SL5, soil leachate collected underneath the 150-cm pile at 90 cm soil depth • SC, control soil leachate collected in an adjacent corn field at 90 cm soil depth • SMS, spent mushroom substrate

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
SPENT mushroom substrate (SMS) is an organic waste resulting from commercial mushroom production. In the USA, production of the white button mushroom Agaricus bisporus results in the generation of more than 10⁶ m³ of SMS annually, with half of the material being generated in Pennsylvania alone (Chong and Hamersma, 1997). The major ingredients of SMS are composted straw, hay, peat, horse manure, poultry manure, and gypsum. In addition to these bulk components, SMS may also contain some nutrient additives, such as potassium salts, ammonium nitrate, super phosphate, etc. (Gerrits, 1988). On average, fresh SMS is approximately 60% water (by mass) (Levanon and Danai, 1995), whereas approximately 65% of the dry matter is organic (Gerrits, 1994; Beyer, 1999).

Spent mushroom substrate is regarded favorably as a soil amendment in production agriculture and ecosystem restoration (Chong et al., 1991; Steffen et al., 1994; Stark and Williams, 1994; Wuest et al., 1995). However, a high salt content limits its suitability as a medium for plant growth (Chong and Rinker, 1994; Ciavatta et al., 1993). For this reason, Pennsylvania state law requires that SMS be weathered in the field for 2 yr prior to reuse (Pennsylvania Department of Environmental Protection, 1997).

During the weathering process, SMS is leached of soluble constituents by rainwater and snowmelt. Although field studies are lacking, laboratory studies have shown that water extracts of SMS contain high concentrations of organic carbon, K, Ca, Mg, Cl, and SO₄ (Chong and Rinker, 1994). Maher (1994) found that leaching an SMS–peat column with distilled water at 10-d intervals over a 60-d period resulted in leaching of about 94% of total K, 33% of total P, and 15% of total N. The leachate percolates into soil and associated solutes may be transported to ground water. When 170 m³ ha^-1 of SMS were mixed with the top 8 cm of soil, after 210 d of field weathering, 80% of S, 3% of K, 16% of Ca, and 37% of Mg in the applied SMS were recovered in soil leachate collected at 24 cm depth (Stewart et al., 2000). The chemical composition and transport of SMS leachate in field soil environments are not known, despite the fact that they probably affect the quality of receiving waters. In the interest of protecting ground water resources, the chemical properties of SMS leachate, as affected by interaction with soil, warrant further study.

When applied to the field at low rates, SMS appears to be beneficial to soil fertility and it does not pose a threat to water quality. For example, when SMS was spread 2.5 cm thick over a corn field three times per year, ground water quality was not influenced (Kaplan et al., 1995). Wuest et al. (1995) reported that when fresh SMS was applied to a corn field at a thickness of 10 cm, soil cation exchange capacity (CEC), Mg, K, Ca, and P levels were improved, whereas solute loss with runoff and leaching to depth were negligible. If applied to the field at high rates, SMS may pollute ground water. Kaplan et al. (1995) monitored soil water (100 cm depth) and ground water quality under 50-cm piles of weathering SMS over 1 yr. They measured DOC in soil water up to 880 mg L^-1, and concentrations of K⁺, Ca²⁺, Cl^-, and SO^2-₄ were more than 1000 mg L^-1. Ground water concentrations of DOC, K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^-, and SO^2-₄ were 5 to 20 times higher than background values. However, their method of sampling does not permit a quantitative evaluation of solute flux. Also, the conditions employed in that study do not accurately depict the current practice employed by producers, which involves stacking SMS piles to 90 cm height for 2 yr of weathering.

Currently, the Pennsylvania Department of Environmental Protection regulates stacking of SMS piles at or below a height of 90 cm (3 ft) after settling in fields with seasonal ground water tables more than 50 cm from the surface. No information is available about the leachate chemistry of SMS weathered in piles at 90 cm height. The extent to which underlying soils retain leachate solutes produced from such a load remains unclear. Faced with the large quantity of SMS produced each year in Pennsylvania, a recent proposal suggests increasing the allowable height of SMS weathering piles to 150 cm (5 ft). However, little is known about the effect of this practice on soil and ground water quality. Therefore, the objectives of this study were to characterize changes in the chemistry of SMS leachate over the full course of weathering, estimate the amount of associated solute release and subsequent retention by the soil, and evaluate the proposed increase in height of SMS weathering piles.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Field Trial
A field experiment was carried out in Chester County, PA. Two SMS piles were deposited in a fallow agricultural field dominated by grasses. The piles were formed into two rows (20 x 6 m) to be 90 cm (3 ft) and 150 cm (5 ft) in height after settling. The SMS employed in the field experiment was obtained from Elite Mushroom Co., Avondale, PA and was composed mainly of composted horse manure, straw, poultry manure, waste brewer's grains, and gypsum. The experiment site has a mean annual precipitation of 1160 mm, a mean annual temperature of 11.9°C, a slope of 5%, and a very deep, well drained, moderately permeable soil formed in residuum weathered from micaceous schist (fine-loamy, mixed, semiactive, mesic Typic Hapludult). Table 1 contains soil profile descriptions of the study site. Selected chemical properties of the SMS are listed in Table 2.

The field weathering experiment lasted for 2 yr, from July 1997 to July 1999. During weathering, the piles were undisturbed. The SMS and soil leachate samples were collected every 1 or 2 wk, depending on the amount of precipitation. For convenience, leachates from the 90- and 150-cm SMS piles are referred to hereafter as PL3 and PL5, respectively. Soil leachate, collected at 90 cm soil depth underneath the 90- and 150-cm SMS piles and in the corn field, are referred to as SL3, SL5, and SC, respectively.

Analytical Methods
After collection, the samples were centrifuged at 10200 x g for 15 min and then passed through a 0.5-µm glass fiber filter to remove particulate matter. The filtrates were stored at 4°C prior to analysis. The pH values of leachate samples were measured using a Beckman 310 pH meter (Beckman Coulter, Fullerton, CA) with an Orion ROSS 81-02 combination pH electrode (Thermo Orion, Beverly, MA). Electrical conductivity (EC) was analyzed by an Accumet Model 50 pH/ion/conductivity meter (Fisher Scientific, Pittsburgh, PA) with a YSI 3403 conductance cell (cell constant = 1.0 cm^-1) equipped with an ATC probe to normalize the reading to 25°C. Dissolved organic carbon (DOC) content was determined with a Shimadzu TOC-5000A total organic carbon analyzer and an ASI-5000A auto sampler (Shimadzu, Kyoto, Japan). Acid neutralizing capacity (ANC) was measured using the Gran-titration method (Morel and Hering, 1993). Inorganic anions and low molecular weight organic acids (LMWOAs) were measured by ion chromatography (IC) (Dionex Series 4000I with an AS40 automated sampler, an IonPac AS11 ion exchange column, and a CD20 conductivity detector; Dionex Corp., Sunnyvale, CA). The flow rate was 1 mL min^-1 and run time was 30 min per sample. A gradient mobile phase comprising water and 0.05 M NaOH was employed. Inorganic cations were determined using a Video 22 AA/AE spectrophotometer (Instrumentation Laboratories, Lexington, MA) and Technicon (Franklin, MA) autoanalyzer for NH⁺₄. Total organic N concentrations in leachates were calculated by subtracting NH⁺₄ concentrations from total Kjeldahl N concentrations. Kjeldahl digestion of organic N to NH⁺₄ was performed using standard methods (Clesceri et al., 1998) and NH⁺₄ was measured using a Technicon autoanalyzer.

Allocation of dissolved organic matter (DOM) into low (<1000 Da) and high (>1000 Da) molecular weight fractions was achieved by ultrafiltration and dialysis with an Amicon (Beverly, MA) CH2 ultrafiltration system equipped with an Amicon S1Y1 spiral-wound membrane filter cartridge (regenerated cellulose, 1000-Da molecular weight cutoff). The percent of DOC in each molecular weight fraction was determined. Molar absorptivity was determined using UV spectroscopy (Shimadzu 3101PC UV-Vis-NIR spectrometer) with absorbance at 280 nm normalized by DOC concentration.

Data Analysis
Data are expressed as monthly means of triplicate values weighted by sample volume. Volume weighted mean (VWM) concentrations of all constituents were calculated as follows:

[1]

where C_ai is the concentration of species a in sample i, V_i is the volume of leachate sample i, and n is the number of samples collected in a given month from the set of triplicate samplers.

The mass flux of each solute (i) released from the SMS pile and (ii) leached through the 90-cm soil depth were calculated from the product of volume weighted mean data and associated water fluxes over the 24-mo period. Retention of leachate solutes in the top 90 cm of soil was calculated by the difference between (i) and (ii).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Leachate Flux Pattern
Leachate water fluxes exhibited time dependencies that were similar to those of the precipitation regime (Fig. 1). In the first 4 mo, only a small volume of SMS leachate was collected as most of the precipitation was absorbed by the SMS material. Breakthrough of water from the 150-cm pile proceeded 30 d after it was observed for the 90-cm pile. In the 2-yr field experiment, approximately 51 and 40% of the precipitation passed through the 90- and 150-cm SMS piles, respectively, and percolated to 90 cm depth in the soil.

View larger version (37K): [in this window] [in a new window]   Fig. 1. Rainfall and water flux time series measured under spent mushroom substrate (SMS) piles (PL3 and PL5), at 0.9 m depth in soil under SMS piles (SL3, SL5), and in an adjacent corn field (SC).

View larger version (29K): [in this window] [in a new window]   Fig. 2. pH of the spent mushroom substrate (SMS) leachate (PL3 and PL5) and soil leachate (SL3 and SL5) under SMS piles and in an adjacent corn field (SC). Symbols indicate monthly volume weighted means of triplicates by sampling date.

View larger version (28K): [in this window] [in a new window]   Fig. 3. Electrical conductivity (EC) of the spent mushroom substrate (SMS) leachate (PL3 and PL5) and soil leachate (SL3 and SL5) under SMS piles and in an adjacent corn field (SC). Symbols indicate monthly volume weighted means of triplicates by sampling date.

Inorganic Cations in Spent Mushroom Substrate Leachate
The major inorganic cations in SMS leachate were K⁺, Na⁺, Ca²⁺, and Mg²⁺ with concentrations ranging from 72 to 316, 21 to 76, 2 to 116, and 27 to 86 mmol_c L^-1, respectively (Fig. 4). Concentrations of total Fe, Al, and Mn in the leachates were always less than 0.5 mmol L^-1, and Cu, Zn, and Pb were less than 6 µmol L^-1. The time trends in cation concentrations track the EC values (Fig. 3), increasing gradually at early weathering times, reaching a maximum after 6 or 7 mo, and then decreasing slowly. PL5 had higher concentrations of K⁺ and Na⁺ than PL3, but concentrations of Ca²⁺ and Mg²⁺ were similar for PL3 and PL5. Ammonium, which was present in Year 1, later became undetectable (Fig. 4). Since fresh SMS contains little NH⁺₄ (Table 2), leachate NH⁺₄ was a product of either ammonification of amino acids and proteins in SMS, or dissimilatory nitrate reduction. Field weathering of SMS is a passive composting process (Pennsylvania Department of Environmental Protection, 1997) that begins with a fast breakdown of sugars, amino acids, and other easily degradable substances (Finstein and Morris, 1975; Chefetz et al., 2000). The fact that maximum pH and NH⁺₄ coincided for both PL3 and PL5 strongly suggests that the high pH derives from ammonification (Fig. 2 and 4). It is also interesting to note that these maxima arrived 5 mo earlier in the weathering process for PL3 relative to PL5 (Fig. 4a,b). This suggests that the composting process proceeds more rapidly in the 90-cm pile than the 150-cm pile. With increased weathering, NH⁺₄ decreased and eventually was undetectable in SMS leachates. This is probably a result of nitrification.

View larger version (149K): [in this window] [in a new window]   Fig. 4. (a) Cation and anion concentrations of the spent mushroom substrate (SMS) leachate (PL3) and soil leachate (SL3) under SMS piles. Symbols indicate monthly volume weighted means of triplicates by sampling date. (b) Cation and anion concentrations of the spent mushroom substrate (SMS) leachate (PL5) and soil leachate (SL5) under SMS piles and (c) in an adjacent corn field (SC). Symbols indicate monthly volume weighted means of triplicates by sampling date.

Weathering of SMS altered the relative predominance of cations in soil leachate relative to the control; in SC the predominant cation was Ca²⁺ (Fig. 4c) whereas K⁺ was dominant in SL3 and SL5 (Fig. 4a,b). Throughout the weathering process, the major cations in SL3 had relatively stable concentrations (Fig. 4a). But in SL5, after 18 mo, K⁺ concentration increased and finally reached the same level as PL5 (Fig. 4b). The increase in K⁺ is evidently responsible for the concurrent increase in EC (Fig. 3).

Inorganic Anions in Spent Mushroom Substrate Leachate
Chloride, SO^2-₄, and NO^-₃ were dominant inorganic anions in SMS leachate with concentrations ranging from 27 to 196, 79 to 324, and 0.03 to 118 mmol_c L^-1, respectively (Fig. 4). Phosphate concentration was consistently less than 2.5 mmol L^-1. As with the cations, concentrations of all three major inorganic anions peaked at the fifth or sixth month of weathering, and then stabilized after 12 mo. Therefore, in terms of salt release, most of the weathering occurs in the first year.

Despite the higher mass of N in overlying SMS, NO^-₃ concentrations were lower in PL5 than in PL3 (Fig. 4a,b). Leachate NO^-₃ is a product of ammonium oxidation. Prior work has shown that ammonification is predominant in early phases of composting and the resulting high pH inhibits nitrification (Myrold, 1998). As a result, the SMS leachates contained little NO^-₃ at early weathering times (Fig. 4a,b). Accumulation of NH⁺₄ under oxic conditions stimulates the growth of NH⁺₄ oxidizing bacteria and increases leachate NO^-₃ concentrations. This was observed for PL3, where oxic conditions appeared prevalent enough to generate high NO^-₃ concentrations and NH⁺₄ concentrations were subsequently diminished (Fig. 4a). The low concentration of NO^-₃ in PL5 is suggestive of the predominance of anoxia in the deeper pile (i.e., nitrification was inhibited by a lack of O₂ or denitrification occurred in the lower wet portion of the pile). The prevalence of anoxic conditions in the 150-cm pile was also evident from structural data on organic matter diagenesis (Chefetz et al., 2000) and the fact that upon removal of the SMS piles, the bottom of the 150-cm pile was relatively wet and enriched in noxious sulfide odors. This suggests that increasing SMS pile height from 90 to 150 cm slows the SMS weathering process by decreasing the extent of aerobic composting. Reduced rates of microbial activity with increased pile depth are consistent with the observation that PL5 showed a maximum NH⁺₄ concentration 5 mo later than PL3 (Fig. 4a,b).

The top 90 cm of soil retained a large quantity of SMS-derived inorganic anions (Guo et al., 2001) and concentrations of the three major anions were significantly reduced in soil leachate. Anion retention at pH > 7 (Table 1) is counterintuitive because the density of positive charged sites on soil particles is expected to be small. However, the high ionic strength of SMS leachate would increase the charge density on Al or Fe oxides and hydroxides that are below their point of zero charge at ambient pH (Chan et al., 1980). Soil retention of anions may also occur through adsorption of ion pairs or via inner-sphere complexation reactions on soil (hydr)oxides (Thomas and Swoboda, 1970; Sposito, 1989). Indeed, retardation of anion transport relative to bulk water has been observed in variable-charge soils at circumneutral pH (Kinjo and Pratt, 1971; Chan et al., 1980).

Despite reductions in the soil, NO^-₃ concentrations in SL3 and SL5 ranged from 7 to 38 mmol L^-1 and 1 to 24 mmol L^-1, respectively. These concentrations greatly exceed the USEPA drinking water standard of 0.73 mmol L^-1 NO^-₃. After the SMS piles were removed, continuous monitoring of NO^-₃ in SL3 and SL5 over the following year revealed concentrations ranging from 2 to 32 mmol L^-1 (data not shown), implying that both 90- and 150-cm SMS piles have the potential to pollute ground water with nitrate.

In addition to increased concentrations, weathering of SMS altered the anion composition of soil leachate relative to the corn field control. In SC, the predominant anion was NO^-₃, but in SL3 and SL5, Cl^- was dominant. After 18 mo of weathering, the concentrations of Cl^- in SL5 exceeded that in PL5, indicating breakthrough of retarded anions. This corresponds to the rise of EC and K⁺ concentration in SL5 at the same time (Fig. 3 and 4b).

Dissolved Organic Carbon and Nitrogen
The DOC concentrations of PL3 and PL5 ranged from 800 to 7900 and 1500 to 11100 mg L^-1, respectively (Fig. 5a). These SMS leachates contain much higher DOC concentrations than natural waters. Leachate from organic horizons of forest soils, which generate some of the highest DOC concentrations in natural ecosystems, contain 10 to 100 mg L^-1 of DOC (Currie et al., 1996). For both SMS piles, DOC increased gradually at the beginning, reached a maximum after 5 or 6 mo of weathering, and then decreased slowly. This trend is consistent with patterns for EC, and major ions (Fig. 3 and 4), and supports the contention that weathering of SMS mainly occurs in the first year. Reduced concentrations of DOC in soil leachate indicates that a large quantity of DOC was removed in the top 90 cm of soil, either via adsorption or biodegradation (Fig. 5a). However, compared with the control (ca. 8.5 mg L^-1), soil leachate under the SMS piles was still 40 to 200 times higher in DOC.

View larger version (29K): [in this window] [in a new window]   Fig. 5. (a) Dissolved organic carbon (DOC) and (b) organic nitrogen (DON) concentrations of the spent mushroom substrate (SMS) leachate (PL3 and PL5) and soil leachate (SL3 and SL5) under SMS piles and in an adjacent corn field (SC). Symbols indicate monthly volume weighted means of triplicates by sampling date.

Acid Neutralizing Capacity of Spent Mushroom Substrate Leachate
Acid neutralizing capacity (ANC) is the equivalent sum of all the bases that can be titrated with a strong acid to an equivalence point. It is a useful parameter for measuring the environmental effect of SMS leachate and its potential utility in remediation of acid soils and other acid-affected sites (Stark and Williams, 1994; Buswell, 1994). It indicates the net deficiency of protons in an aqueous solution (Stumm and Morgan, 1996) and the capacity of the solution to buffer additional proton inputs against changes in pH. In these leachate-affected solutions, ANC is likely to be dominated by weakly acidic organic anions.

Leachates from the 90- and 150-cm SMS piles had ANC values ranging from 10 to 45 and 25 to 74 mmol_c L^-1, respectively (Fig. 6). These values are much higher than those measured for the SC because the leachates and SMS-affected soil solutions contain high concentrations of DOM. In general, ANC values tracked DOC concentrations quite closely (Fig. 6), indicating the importance of organic anions to the buffering of proton inputs in these systems. For example, the sudden increase in ANC of SL5 after 18 mo is a result of the increase in DOC discussed above. Subtle differences between ANC and DOC indicate that the ANC of DOM is not constant with time, but changes over the course of organic matter diagenesis that accompanies SMS weathering. In particular, changes in the relative predominance and total concentrations of carboxylic and phenolic functional groups will affect the C-normalized values of ANC (Sposito, 1989).

View larger version (31K): [in this window] [in a new window]   Fig. 6. Acid neutralizing capacity (ANC) of the spent mushroom substrate (SMS) leachate (PL3 and PL5) and soil leachate (SL3 and SL5) under SMS piles and in an adjacent corn field. Symbols indicate monthly volume weighted means of triplicates by sampling.

View larger version (29K): [in this window] [in a new window]   Fig. 7. Molar absorptivity at 280 nm of dissolved organic matter in the spent mushroom substrate (SMS) leachate (PL3 and PL5) and soil leachate (SL3 and SL5). Symbols indicate monthly volume weighted means of triplicates by sampling date.

Molecular Size Distribution of Leachate Dissolved Organic Matter
Leachate and soil solution DOM were separated into low molecular weight (LMW) and high molecular weight (HMW) fractions by ultrafiltration through a membrane with a nominal molecular weight cutoff of 1000 Da. This cutoff is derived from calibration with globular proteins, so it may not represent the true molecular mass of DOM molecules. However, ultrafiltration provides a means for comparing DOM molecular size across treatments and through time. The percentage of DOM that is HMW is shown in Fig. 8. Evidently, leachate DOM was composed mainly of LMW constituents. High molecular weight DOM constituted 4 to 26% and 7 to 50% of the total DOC in PL3 and PL5, respectively. Investigations of DOM from other composting environments indicate similar results. Harmsen (1983) showed that 33% of leachate DOC from a methanogenic landfill was HMW, whereas Russo et al. (1986) found that more than 56% of landfill leachate DOC passed through a 500-Da filter. In general, LMW constituents are dominant components of soluble OM in a wide range of environments, while macromolecules are less prevalent (Peuravuori and Pihlaja, 1997).

View larger version (27K): [in this window] [in a new window]   Fig. 8. Percentage of total leachate dissolved organic carbon (DOC) that did not pass through a 1000-Da molecular weight cutoff ultrafiltration membrane. The terms PL3 and PL5 refer to pile leachate solutions. The terms SL3 and SL5 refer to soil leachate solutions. Symbols indicate monthly volume weighted means of triplicates by sampling date.

In an attempt to elucidate the composition of the LMW DOM, SMS and soil leachate solutions were analyzed for six common aliphatic low molecular weight organic acids (LMWOAs: formic, acetic, malonic, oxalic, phthalic, and citric) using ion chromatography (IC). These acids were present in DOM solution at very low concentrations, mostly 0.02 to 0.4 mmol L^-1, and never in excess of 3 mmol L^-1 (data not shown). Formic acid was the most prevalent of those measured and it was always present in SMS leachate. At the onset of weathering, SMS leachate contained relatively higher concentrations of these identifiable acids but, as the weathering progressed, their concentrations were reduced. After Month 12, only formic acid was detectable. Evidently, these acids were consumed during transport through the soil and they were rarely detected in soil leachate samples after the first few months. None of the LMWOAs was detected in the SC samples.

Release of Leachate Solutes from Spent Mushroom Substrate Weathering and Their Retention in Soil
The flux of solutes from SMS piles to soils was calculated from water flux and solute concentration data. Release of leachate solutes from SMS piles and their retention in the top 90 cm of soil are listed in Table 3. Over the 2-yr weathering duration, solute efflux from the 90- and 150-cm piles included 27.3 (PL3) and 43.4 (PL5) Mg ha^-1 of DOC, 3.0 and 4.3 Mg ha^-1 of DON, and 242 (PL3) and 206 (PL5) Mg ha^-1 of inorganic salts. Total effluxes of Cl^-, SO^2-₄, and NH⁺₄ were higher for the 150-cm pile (32.2 vs. 28.1, 71.6 vs. 66.1, 5.2 vs. 1.0 Mg ha^-1, respectively, for PL5 vs. PL3) but releases of Na⁺, K⁺, Ca²⁺, Mg²⁺, and NO^-₃ were lower (9.9 vs. 10.6, 63.8 vs. 68.2, 7.9 vs. 13.0, 5.3 vs. 6.73, 10.7 vs. 47.2 Mg ha^-1, respectively), especially when normalized to SMS volume as in Table 3. Despite higher leachate solute concentrations, the flux of water from the 150-cm pile was only 80% of that released from the 90-cm pile (Table 3). In other words, the 150-cm pile retained much more water, as was reflected in the greater predominance of anoxic conditions. Greater anoxia in the 150-cm pile probably contributed to the fact that it released only 23% of the NO^-₃ that was generated during weathering of the 90-cm pile. Given that NH⁺₄ and DON efflux was higher for the 150-cm pile, we infer that nitrification was inhibited and greater denitrification occurred there, although gas efflux was not measured directly. Given the concern regarding nitrate contamination of ground water, SMS release of this mobile contaminant is critical to assessing the risk of SMS weathering (USEPA, 1996).

View this table: [in this window] [in a new window]   Table 3. Release of leachate solutes and their retention in the top 90 cm of soil during the 2 yr of field weathering of spent mushroom substrate (SMS).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Leachate from SMS field weathering contains high concentrations of DOM and inorganic salts. The major cations in the leachate are K⁺, Na⁺, Ca²⁺, and Mg²⁺, and major anions are DOM, Cl^-, SO^2-₄, and NO^-₃. Potassium and Cl^- are the predominant inorganic ions. Leachate DOM is mainly composed of low molecular weight (<1000 Da) organic acids and it has molar absorptivities of 200 to 600 L mol^-1 C cm^-1.

In 2 yr of weathering the 90- and 150-cm SMS piles generated 100 and 80 cm of leachate, respectively. In the 90-cm pile, a cubic meter of SMS released about 3.0 kg of DOC and 26.6 kg of inorganic salts. In the 150-cm pile, 2.8 and 13.6 kg m^-3 of DOC and inorganic salts were released. The top 90 cm of soil retained 20 to 89% of the leachate solutes against leaching during the 2-yr weathering cycle. Although the larger pile released a smaller flux of nitrate into ground waters, the total fluxes (in Mg ha^-1) of DON and NH⁺₄ were higher. Mineralization of DON in the underlying soil and subsequent nitrification is likely. For both pile heights, high flux rates of dissolved N, DOM, and inorganic solutes certainly alter the quality of ground water with high seasonal water tables (ca. 1 m depth) and probably affect deeper systems as well.

	ACKNOWLEDGMENTS

 
This research was supported by the Mushroom Industry Farmer Based Research (MIFBAR) Program in the State of Pennsylvania. We thank Mr. Vincent Santucci for providing access to an ideal site for this research project. The comments of three anonymous reviewers improved the manuscript.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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