Inorganic-N release from spent mushroom compost under laboratory and field conditions

PII:

Soil Biol. Biochem. Vol. 30, No. 13, pp. 1689±1699, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0038-0717(97)00264-2 0038-0717/98 $19.00 + 0.00

INORGANIC-N RELEASE FROM SPENT MUSHROOM COMPOST UNDER LABORATORY AND FIELD CONDITIONS D. P. C. STEWART*, K. C. CAMERON and I. S. CORNFORTH Department of Soil Science, P.O. Box 84, Lincoln University, Canterbury, New Zealand (Accepted 3 November 1997) SummaryÐInorganic-N release from soil amended with spent mushroom compost (SMC), a by-product of mushroom production, was measured in three open laboratory incubations (25±308C) and in ®eld lysimeters. Rates of SMC application to the soil were up to 80 t haÿ1 equivalent (0.84% dry weight in the laboratory). SMC contained 1.8% N of which 94% was organic, and had a C-to-N ratio of 17. Small amounts of inorganic-N were leached from SMC in the ®rst incubation (3±18% of that applied). Trends in the data suggested that N in the SMC was initially immobilized in the 20 and 40 t haÿ1 treatments, as shown by modelling using a negative ®rst order exponential term; it was then slowly mineralized according to zero order kinetics. The laboratory optimized model of inorganic-N loss, when modi®ed to account for ®eld soil temperatures, estimated a similar amount of inorganic-N loss as was observed in the ®eld. The CENTURY model overestimated inorganic-N leaching from SMC in the laboratory and underestimated inorganic-N leaching in the ®eld. Fertilizer, containing N, P, K and S, reduced the net amount of inorganic-N recovered from SMC±soil mixtures. The rate of inorganic-N leaching from mushroom compost was considerably slower than from glycine or chicken litter applied at the same N rate. The sterilants applied to mushroom compost during mushroom production and compost sterilization had little e€ect on the rate of inorganic-N leached from the compost; however, hypochlorite and formaldehyde caused a small increase and decrease respectively in the cumulative amount of inorganic-N leached from mushroom-compost-amended soil. The slow rate of release of inorganic-N from SMC-amended soil is predominantly the result of the slow mineralization of recalcitrant organicN in SMC. # 1998 Elsevier Science Ltd. All rights reserved

INTRODUCTION

Spent mushroom compost (SMC) is a by-product of the mushroom industry. It is composed of two layers, a compost layer made from straw, manure and gypsum, and a casing layer made from peat and chalk. These layers are mixed together at the completion of mushroom production. Spent mushroom compost has been used as a soil amendment, particularly for intensive horticultural production. It contains a substantial amount of nitrogen (N) and although Kaddous and Morgans (1986) found that SMC could cause N de®ciency in crops, there is little reported work investigating inorganic-N release from SMC (Maher, 1991, 1994; Maynard, 1993). Maher (1991) investigated inorganic leaching from pots containing a 50:50 mixture of SMC and peat. The pots were leached every 10 d for a total period of 60 d. The SMC, which had been re-composted for an additional 2 months, contained 2.7% N. By the end of the trial 15% of the N added in the SMC had been leached. *Author for correspondence. Present address: AgResearch, c/o Soil, Plant and Ecological Sciences, Lincoln University, Canterbury, New Zealand.

Maynard (1993) investigated nitrate leaching losses from a SMC-amended ®ne sandy loam in the U.S.A. Annual SMC applications containing 365 and 731 kg N haÿ1, made for three consecutive years, had little e€ect on the concentration of NOÿ 3 ±N in the ground water, suggesting that the N mineralization rate from the SMC-amended soil was relatively slow. Maher (1994) reported that applications of up to 400 t SMC haÿ1 to a clay loam soil had little e€ect on the soil NOÿ 3 ±N concentration. During mushroom production a variety of chemicals may be applied to the compost, including: methyl bromide and formaldehyde to sterilize compost following cropping; benomyl to control competing fungi; and hypochlorite in irrigation water to reduce the incidence of disease. There are no reports on the e€ects of these chemicals on N mineralization from SMC. The only information available describes the direct e€ect of soil application of these chemicals on N mineralization. Methyl bromide may inhibit nitrifying bacteria in soils causing an increase in the soil NH+ 4 ±N concentration and a reduction in the concentration of NOÿ 3 ±N, particularly in soils amended with organic-N (Wensley, 1953; Winfree and Cox, 1958; Maw and Kempton,

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1973; MacNish, 1986). Formaldehyde can inhibit the growth of soil fungi for up to 18 months after ®eld application (Martin and Pratt, 1958), and may inhibit nitri®cation in soils (Oslislo and Lewandowski, 1985; Peschke et al., 1991). Benomyl can either inhibit nitri®cation and cause NOÿ 2 ±N to accumulate in the soil or stimulate nitri®cation and ammoni®cation, or have no e€ect on nitri®cation (Mazur and Hughes, 1975; Foster and McQueen, 1977; Smiley and Craven, 1979; Torstenssen and Wessen, 1984). In soils, hypochlorite is rapidly reduced to chloride by organic matter (Gerritse et al., 1992). Chloride salts have been reported to inhibit nitri®cation and may also increase or decrease the rate of ammoni®cation in soil (Agarwal et al., 1971; Broadbent and Nakashima, 1971; McCormick and Wolf, 1980; Golden et al., 1981; Christensen and Brett, 1985). There appears to be little work reported that describes N mineralization from compost. Cheneby et al. (1994) modelled the N mineralization from incubated farm-yard-manure (FYM) compost and soil using a ®rst order equation i.e. Nt ˆ Np …1 ÿ eÿkt t †,

…1†

where Nt is the amount of cumulative net N mineralization over the period of time t, t is the time since the start of the incubation, Np is the potentially mineralizable amount of N, and k1 is the ®rst order exponential rate constant. The N mineralized was assumed to come from one labile fraction (Np), the size of which ranged from 25±34% of the N added in the compost. Murayama et al. (1990) modelled N mineralization from soil amended with rice straw compost also using a ®rst order equation (equation (1)). Although many models have been used to attempt to describe and predict ®eld soil N mineralization, only a few developed under controlled laboratory conditions have been applied to ®eld conditions. Gri€en and Laine (1983) modelled the N mineralization from soils amended with manures during a laboratory incubation at 358C using a ®rst order model (equation (1)). They modi®ed their laboratory model to predict N mineralization in the ®eld. Field k1 rate constants were calculated from

the laboratory k1 rate constants, for each week of the growing season, using a Q10 temperature value of 2 (i.e. the ®eld k1 value is halved for every 108C decrease in temperature below that of the laboratory), and the ratio of ®eld-available moisture to the available-water capacity. The predicted estimates of N mineralization were greater by a mean value of 43% than the amount of N recovered by a corn crop. There was, however, a strong correlation between the plant N uptake and the log of Npk1 for the ®eld temperature and moisture-modi®ed model. The CENTURY model has also been used to model N mineralization (Parton et al., 1987; Metherell et al., 1993). In the CENTURY model added organic material is partitioned into ``metabolic'' or ``structural'' litter pools depending on its lignin-to-N ratio, and soil organic matter is partitioned into three pools (i.e. active, slow and passive). Decomposition rates from all of these pools follow ®rst order kinetics subject to temperature and moisture conditions. Our objectives were to measure and model (using established kinetic models of nutrient release) inorganic-N release from soil amended with SMC, in the laboratory and in the ®eld, and to determine the e€ect of fertilizer on inorganic-N release from SMC. It also aimed to determine if methyl bromide, benomyl, hypochlorite or formaldehyde in¯uence the rate and amount of inorganic-N leached from mushroom-compost-amended soil, and to compare the rate of inorganic-N leached from mushroomcompost-amended soil with that from chicken litter and glycine±glucose-amended soil.

MATERIALS AND METHODS

SMC The SMC used (Table 1) was collected from a mushroom farm and consisted of 85% (by volume) of compost (made from chicken litter, wheat straw and gypsum at an approximate ratio of 13:17:1 respectively by weight) and 15% of a ``casing'' layer (made from Sphagnum peat and chalk at a ratio of 5:1 by volume). The compost was made by conditioning for 6 d in a large heap, followed by windrow composting for 8 d with passive aeration, and

Table 1. Some properties of spent mushroom composta (SMC), mushroom compostb (MC) and chicken litterc (CL)

pH Organic C (%) C:N Total N (%) Nitrate-N (%)d Ammonium-N (%)d Unhydrolysable-N (%)d a

Experiments 1 and 4 (n = 6). Experiments 2 and 3 (n = 4). c Experiment 3 (n = 4). d % of total N present, ND not determined. b

SMCa (SEM)

MCb (SEM)

CLc (SEM)

6.5 (0.1) 31.5 (1.3) 17.2(1.4) 1.83 (0.07) 0.3 (0.1) 6.2 (0.5) 20.3 (2.1)

6.8 (0.0) ND ND 1.81 (0.06) ND 1.5 (0.3) ND

7.1 (0.1) ND ND 4.08 (0.12) ND 13.0 (1.1) 36.2 (5.7)

Inorganic-N release from spent mushroom compost

®nally stage two composting (including pasteurization at 608C for 7 d). Laboratory incubation (experiment 1) An open incubation was done using soil from 0± 250 mm depth of a Templeton ®ne sandy loam (Udic Ustochrept, ®ne loamy, mixed, mesic). Soil samples were taken in the autumn of 1992 from a site (also used for experiment 4) that had been used to grow wheat, followed by 18 months of pasture and then sweetcorn. A factorial design including four rates of SMC (0, 0.21, 0.42 and 0.84% dry SMC which is equivalent to 0, 20, 40 and 80 t moist SMC haÿ1 respectively), two rates of fertilizer (zero or 338 kg N haÿ1, 100 kg haÿ1 of both P and K, and 114 kg S haÿ1) and three replicates were used. The mass of SMC and fertilizer used for each treatment was calculated from the hypothetical surface area of a 250 mm deep column of soil of 25 g mass at the dry bulk density of the ®eld soil (i.e. 1.28 g cmÿ3). The SMC was shredded in a blender and refrigerated until required. Some properties of the SMC are presented in Table 1. The N fertilizer was applied as a split dressing with the equivalent of 150 kg haÿ1 applied initially and 188 kg haÿ1 applied at 8 weeks. The fertilizer (ammonium nitrate, sodium dihydrogen phosphate, potassium chloride and sodium sulphate) was applied in solution. The incubation was done in leaching tubes as described by Ghani et al. (1991). The equivalent of 25 g of oven-dry sieved (4 mm) soil was mixed with the appropriate amount of SMC and 15 g of 3 mm dia glass beads (to maintain aeration) and the appropriate fertilizer solution (250 mL) was then applied to the surface of all the soils. The tubes were incubated at 258C for 16 weeks, and were leached with 100 mL of de-ionised water every 2 weeks. After each leaching the soil moisture in the tubes was equilibrated to ÿ60 kPa by applying a suction to the outlet tube. The leachates were preserved by adding 20 mL Lÿ1 of 3.1% boric acid solÿ ution. Leachates were analyzed for NOÿ 3 and NO2 using anion exchange chromatography (Waters, Millipore, using anion exchange chromatography (Waters, Milford, MA, A-102), and for NH+ 4 using a colorimetric method (Tecator AB, HoÈganaÈs, ASN62-03/84) on a ¯ow injection analyzer. The cumulative net amount of inorganic-N leached (i.e. greater than from the control) was modelled using various simple kinetic models and the CENTURY model (Metherell et al., 1993). SigmaPlot software (Jandel Scienti®c, San Rafael, CA), which employs the Marquardt±Levenberg algorithm and an iterative process, was used for curve ®tting the kinetic models. The ®t of the kinetic models was determined using paired t tests (Kirchner and Lauenroth, 1987), r2 values and the standard errors (SEs) of model parameters.

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After the ®nal leaching, a subsample of soil (5 g moist) was extracted in 50 mL of 2 M KCl to deter+ mine the residual NOÿ 3 ±N and NH4 ±N concentrations. The remaining soil was air dried and sieved (0.7 mm) to remove the glass beads. The soil pH was determined in 0.01 M CaCl2 (using a 1:2.5 ratio of soil to CaCl2 solution). The soil total N concentration was determined using a Kjeldahl digestion followed by NH+ 4 analysis as described previously. E€ect of sterilants (experiment 2) This incubation experiment and experiment 3 were run concurrently using the open incubation technique and other materials and methods as described for experiment 1; however, experiments 2

Fig. 1. Net inorganic-N leached from SMC incubated at 258C ± data and either mixed (®rst/zero) or zero order exponential modelled curves (experiment 1): (a) 20 t SMC haÿ1, (b) 40 t SMC haÿ1 and (c) 80 t SMC haÿ1 (bars are SEDs).

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Table 2. Inorganic-N leaching model parameters (2SE), P values from paired t test and r2 values from the zero and mixed (®rst/zero) order exponential models from a 16 week incubation at 258C (experiment 1) Parameter

SMC rate (t haÿ1) 40

20

80

Mixed (®rst/zero) order model (equation (2)) ÿ3.57 (1.51) ÿ5.69 (2.73) 0.705 (0.135) 1.027 (0.209) 1.000 (1.811) 0.440 (0.394) NS NS 0.904 0.952 a Zero order model (equation (3)) 0.691 (0.091) 0.888 (0.099) ÿ3.393 (0.915) ÿ3.930 (1.000) NS NS 0.907 0.931

NP k0 k1 P r2 k0 c P r2

ND ND ND ND ND 0.860 (0.046) 0.000 (ND) NS 0.924

a

The zero value at t = 0 was omitted from the 20 and 40 t SMC haÿ1 treatments. ND not determined, NS not statistically signi®cant (P r0.05).

and 3 were kept at 308C and the tubes were leached with de-ionised water every week. A factorial design including treatments of plus and minus methyl bromide, benomyl, hypochlorite and formaldehyde was used. This provided eight replicates of the main e€ects and 16 treatments in all. For this experiment SMC could not be used as it had already been treated with a number of chemicals and the e€ect of each chemical could consequently not be determined. Mushroom compost (MC) was therefore obtained after the completion of stage two of the composting process and before spawning with mushroom mycelia. The MC used was treated to try and simulate conditions in SMC as closely as possible, although the MC had not been used for growing mushrooms (Table 1). The compost was put into two wooden boxes and ``casing'' was then applied to the surface of the compost. One box of MC was fumigated with methyl bromide plus chloropicrin (98 and 2% respectively of methyl bromide and chloropicrin, at a rate of 90 g mÿ3) and steam at 408C for 24 h. Each box of compost and casing was vertically partitioned and the remaining treatments were applied in solution to the casing surface of the various compartments at the rate used by the local mushroom producer (Meadow Mushrooms) (i.e. 1.25 g benomyl mÿ2 20 L mÿ2 of 170 mg Cl mLÿ1 hypochlorite solution, and 1 L mÿ2 of 0.8% formalin). A subsample of

MC from the entire depth of each compartment was removed using a core sampler and shredded in a blender. The MC was applied at a rate of 0.84% on a dry weight basis, which is equivalent to 80 t haÿ1 at 66% (w/w) moisture. Comparison with chicken litter and glycine±glucose mixtures (experiment 3) This incubation had three treatments; MC, chicken litter (CL) and glycine±glucose, and three replicates. The MC used was the chemically untreated compost and casing as described for experiment 2 (Table 1). It was also applied at a rate of 0.84% dry weight, giving 168 mg N gÿ1. The CL (Table 1), as used to make SMC at the mushroom farm, was applied at the same rate of N addition as the MC. The glycine was also applied at the same N rate, together with enough glucose to supply a similar amount of C to that applied in the MC treatment (i.e. 2720 mg C gÿ1). The rates of application were equivalent to 0.37, 0.09 and 0.61% (dry weight) respectively of CL, glycine and glucose. The amount of inorganic-N in the leachate from each leaching event was measured, and after the ®nal leaching the amended soils in the tubes were analyzed for pH, and total N, NOÿ 3 ±N and NH+ 4 ±N concentrations as described for experiment 1.

Table 3. E€ect of added fertilizera on the recovery of inorganic-N from SMC-soil mixtures incubated at 258C for 16 weeks (experiment 1, n = 3) Fertilizer

0

20

SMC rate (t haÿ1)

40

80

4.93 110.15

9.81 114.59

98.39 192.02 ÿ9.65 ÿ8.85 83.81 73.02

100.67 184.90 ÿ7.35 ÿ10.75 83.51 59.55

ÿ1

minus plus Leached Soil (net) Net total a

minus plus minus plus minus plus

Added inorganic-N (mg N g ) 0.00 2.47 105.66 107.91 Recovered inorganic-N (mg N gÿ1) 88.56 95.26 172.65 177.22 ÿ9.05 ÿ8.55 ÿ10.95 ÿ8.35 79.51 84.24 56.04 60.96

338, 100, 100 and 114 kg haÿ1, respectively, of N, P, K and S.

Inorganic-N release from spent mushroom compost

1693

Table 4. E€ect of SMC and fertilizera addition on soil chemical properties after 16 weeks of incubation at 258C (experiment 1, n = 3) Initial soil

pH (CaCl2)

5.2

Total-N (%)

0.15 ÿ1

NO3 ±N (mg g ) NH+ 4 ±N

ÿ1

(mg g )

SMC rate (t haÿ1)

Fertilizer

10.6 6.4

minus plus minus plus minus plus minus plus

LSDb (P = 0.05)

0

20

40

80

5.5 5.1 0.13 0.13 4.5 3.3 3.4 2.7

5.2 5.1 0.14 0.15 4.6 4.1 3.8 4.5

5.5 5.1 0.14 0.13 4.0 4.0 3.3 4.1

5.8 5.6 0.15 0.15 5.4 2.8 4.2 3.4

0.5 0.02 2.4 0.9

a

338, 100, 100 and 114 kg haÿ1, respectively, of N, P, K and S. For comparison with the treatment means only, not with the initial soil values.

b

Field incubation (experiment 4) A ®eld incubation was done in lysimeters of fallow soil at a ®eld trial site in paddock D2 at Lincoln University, Canterbury (latitude 43839' south, longitude 172827' east). The soil at the site was described for experiment 1. Undisturbed soil monoliths 176 mm dia and 240 mm deep were taken by inserting P.V.C. lysimeter casings, with an internal cutting ring, over an exposed monolith of soil. The annular gap between the soil and the casing was ®lled with liqui®ed petroleum jelly and allowed to solidify to provide support for the monolith and prevent edge ¯ow during subsequent leaching (Cameron et al., 1990). The smeared lower surface of the soil monolith was removed using an acetone±cellulose acetate paste (Cameron et al., 1990). Depressions in the monolith's base were ®lled with acid washed silica sand, which was secured with ®ne nylon mesh; a P.V.C. drainage collection plates were then glued to the bottom of the monoliths with silicon sealant. The lysimeter shells, leachate collection vessels and monoliths were installed in the ®eld on 23±28 April 1992. Treatments comprised SMC (80 t haÿ1 moist) and a control, with three replicates. The SMC used (Table 1) was applied to the lysimeters on 1 May 1992. The amount of precipitation and irrigation was recorded using an on-site raingauge, and the volume of the leachate was measured. The leachate was preserved with boric acid, and analysed for ÿ ÿ NH+ 4 , NO3 and NO2 as previously described. Field soil temperatures at 100 mm depth were obtained from the Landcare Research Broad®elds meteorolo-

gical station at Lincoln, which is approximately 3 km from the ®eld trial site. Experiment 4 ran over the winter and spring of 1992, after which time the soil in the lysimeters was subsampled, dried, and sieved (2 mm) prior to analysis (except for the soil samples extracted for ÿ NOÿ 3 and NH4 which were kept refrigerated in a ®eld-moist state). The soil was analysed for ÿ total N, NOÿ 3 and NH4 , and pH (in water) as previously described, and for organic C using a modi®ed Walkey±Black method (Blakemore et al., 1987). RESULTS

First incubation (experiment 1) Although the data were variable, there were small (non-statistically signi®cant) net amounts of inorganic-N leached from the SMC over the duration of the trial (Fig. 1). The trends in the 20 and 40 t SMC haÿ1 data suggest that N was immobilized immediately after the SMC was applied. Almost all the N detected in the leachate was NOÿ 3 ±N, with very little organic-N. The inorganic-N leached from experiment 1 was modelled using a mixed (®rst/zero) order model (Fig. 1, Table 2). i.e.: Nt ˆ Np …1 ÿ eÿkt t † ‡ k0 t,

…2†

where k0 is a zero order rate constant. The ®rst order term modelled the gross cumulative N immobilization (i.e. negative) and the zero order term modelled the gross cumulative N mineralization (i.e. positive). The magnitude of Np indicates

Table 5. Main e€ects of selected chemicals on the cumulative inorganic-N leached (mg gÿ1) from mushroom compost plus soila after 6 weeks incubation at 308C (experiment 2, n = 16) Treatment

Minus

Plus

LSD (P = 0.05)

Methyl bromide Benomyl Hypochlorite Formaldehyde

75.13 73.94 73.01 77.70

75.75 76.94 77.87 73.18

3.88 3.88 3.88 3.88

a

0.84% mushroom compost and 99.16% soil by dry weight.

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E€ect of sterilants (experiment 2) Although the methyl bromide treatment had no signi®cant e€ect on the cumulative amount of inorganic-N leached over the duration of the trial (Table 5), it did increase the initial rate of leaching slightly (data not shown). Benomyl had no signi®cant e€ect on the amount of inorganic N leached (Table 5). More inorganic-N was leached from the hypochlorite-treated soil-MC mixture than from the untreated soil-MC (Table 5), although initially the rate of leaching was slightly faster from the untreated soil-MC mixture (data not shown). In contrast, formaldehyde slightly reduced the amount of inorganic-N leached (Table 5). The treatments had little e€ect on the soil pH, + and total N, NOÿ 3 ±N and NH4 ±N concentrations at the end of the incubation (data not shown).

Fig. 2. Comparison of cumulative inorganic-N leached from mushroom compost plus soil with chicken litter or glycine±glucose plus soil applied at the same N rate (168 mg N gÿ1 soil, experiment 3, bars are SEDs).

Comparison with chicken litter and glycine±glucose mixtures (experiment 3)

the potential amount of N immobilization. Alteratively, all the SMC treatments could be modelled using a zero order model, i.e. Nt ˆ k0 t ‡ c,

The rate of inorganic-N leaching from MCamended soil was slower than from CL or glycine± glucose-amended soil over the duration of the experiment (Fig. 2). The rate of inorganic-N leaching was initially most rapid from the CL-amended soil. However, after the ®rst week the most rapid rate of inorganic-N leached was from the glycine±glucoseamended soil (Fig. 2). The cumulative net inorganic-N recovered in the leachate by the end of the experiment was equivalent to approximately 2, 40 and 56% of the N applied in MC, CL and glycine respectively. By the end of the incubation the MC-amended soil had a higher pH and total N concentration than any other soil treatment (Table 6). The MCtreated soil also had a higher concentration of NOÿ 3 ±N than the glycine±glucose-amended soil and a higher concentration of soil NH+ 4 ±N than the CL-amended soil (Table 6).

…3†

where c is a constant. For the zero-order model to ®t, the data had to be simpli®ed by omitting the ®rst data point (zero at t = 0) from the 20 and 40 t SMC haÿ1 treatments. These zero-order models all included an intercept term, except for the 80 t haÿ1 treatment which went through the origin (Fig. 1(c), Table 2). The models' ®tted curves were not statistically di€erent from the data and had high r2 values (Table 2). The CENTURY model substantially overestimated the amount of inorganic-N leached from SMC (i.e. CENTURY simulations were 11, 28 and 65 mg gÿ1 greater, respectively, for 20, 40 and 80 t SMC haÿ1 treatments than the cumulative net data at 16 weeks i.e. Figure 1). Although more inorganic-N was leached from fertilized treatments, this increase was less than the amount of fertilizer N applied (Table 3). N was mineralized from all treatments as more inorganicN was recovered than was applied (Table 3). Less N was mineralized from fertilized than unfertilized treatments (Table 3). By the end of the ®rst incubation the highest soil pH and total-N concentrations were in the 80 t SMC haÿ1 treatments; the treatments had less e€ect on soil inorganic-N concentrations (Table 4).

Field trial (experiment 4) The soil moisture and temperature ¯ux during the ®eld trial are presented in Fig. 3. The winter and spring of 1992 were exceptionally wet (592 mm precipitation) (Fig. 3(a)) compared with the longterm mean precipitation for the district (386 mm). The soil temperature decreased during the ®rst 5± 10 weeks of the trial and increased thereafter (Fig. 3(b)).

Table 6. E€ect of mushroom compost (MC), chicken litter (CL) or glycine±glucose applied at the same N rate (168 mg N gÿ1 soil) on the soil chemical properties after six weeks incubation at 308C (experiment 3, n = 3) Property

MC

CL

Glycine

LSD (P = 0.05)

pH Total N (%) NO3 ±N (mg gÿ1) ÿ1 NH+ 4 ±N (mg g )

5.8 0.20 6.1 4.6

5.1 0.16 1.6 2.2

5.1 0.16 1.1 3.8

0.2 0.02 4.8 1.5

Inorganic-N release from spent mushroom compost

1695

Fig. 3. Soil moisture and temperature ¯ux during experiment 4 (1 May to 25 November 1992) (a) cumulative drainage, daily and cumulative precipitation and irrigation; (b) soil temperature at 100 mm depth.

A ®eld-optimized model was developed by ®tting the laboratory model (equation (3), converted to kg haÿ1 units) to the ®eld data (experiment 4) and allowing the laboratory model parameters to optimize to this data. The laboratory-optimized model was also modi®ed to allow for di€erences in temperature or drainage conditions between the laboratory and the ®eld. The model was modi®ed for temperature using the hypothetical relationship between the soil microbial decomposition activity and the soil temperature as used in the CENTURY model (Parton et al., 1987; Metherell et al., 1993) i.e.   45 ÿ stemp 0:2 Ma ˆ 10     45 ÿ stemp  exp 0:076 1 ÿ …4† 10 where Ma is the soil microbial activity of decomposition, and stemp is the soil temperature (8C). It was assumed that di€erences in the soil temperature between the laboratory and the ®eld had no in¯uence on the loss of inorganic-N added in the SMC. Model rate constants were converted to ®eld rate constants for each increment between the collection of leachate using:

"

kfield

#   Maftemp N0 Ni ‡ klab
ˆ klab

N0 ‡ Ni Malabtemp N0 ‡ Ni …5†

where k®eld is the rate constant for one leaching increment in the ®eld, klab is the rate constant from the laboratory model, N0 is the organic fraction of N, Ni is the inorganic fraction of N, Maftemp is the relative microbial activity for the particular ®eld

Fig. 4. Field net inorganic-N loss from 80 t SMC haÿ1 ± data, ®eld-optimized model curve, and temperature-modi®ed laboratory-optimized model curve (experiment 4).

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Table 7. E€ect of 80 t SMC haÿ1 on soil chemical conditions in the (®ne sandy loam) lysimeters after 30 weeks (experiment 4, n = 3) Property pH ÿ1

NO3 ±N (mg g ) NH+ 4 ±N

ÿ1

(mg g )

Total N (%) C-to-N ratio

Depth (mm)

Control

SMC

LSD (P = 0.05)

0±100 100±200 0±100 100±200 0±100 100±200 0±100 100±200 0-100 100-200

5.2 5.2 17.2 11.1 4.1 4.0 0.18 0.19 11.7 11.5

6.0 5.6 14.8 10.7 5.0 5.8 0.22 0.18 11.5 11.8

0.1

soil temperature during the leaching interval, and Malab‡emp is the relative microbial activity at the temperature used for the laboratory incubation (258C). The laboratory-optimized models were also modi®ed to allow for the lower relative amount of drainage in the ®eld, i.e. the parameters for each leaching interval were also expressed on a fractional pore volume basis. After an initial 3 week lag, inorganic-N was rapidly leached from SMC for approximately 2 weeks (Fig. 4). After this period the rate of inorganic-N leaching declined to a relatively constant rate (Fig. 4). The net amount of inorganic-N leached from the SMC-treated lysimeters was less than the amount of inorganic-N applied (viz. 26 cf. 31 kg N haÿ1 respectively), which indicates that net N immobilization could have occurred. The (gross) amount of inorganic-N leached from the soil only lysimeters was 35 kg N haÿ1. The (unmodi®ed) laboratory-optimized model overestimated the cumulative net loss of inorganic-N from SMC by approximately 56 kg haÿ1. Modifying this model for di€erences in either ®eld temperature (Fig. 4) or drainage conditions improved the model's ®t; however, it then underestimated the loss of inorganic-N by 11 and 22 kg haÿ1, respectively. The CENTURY model also underestimated the loss of inorganic-N by 18 kg haÿ1. Although the measured data were not statistically di€erent from the ®eld-optimized zero-order model prediction (P < 0.05, r2=0.872), there were consistent trends of a more rapid inorganic-N loss at 5±10 weeks and a slower loss at 20± 30 weeks than the model predicted (Fig. 4). By the end of the trial SMC had increased the soil total N concentration (0±100 mm depth) and pH (0±200 mm depth), but had no signi®cant e€ect on soil inorganic-N concentrations or C-to-N ratios (Table 7). DISCUSSION

First incubation The apparent immobilization of N during the ®rst 2±4 weeks in the 20 and 40 t SMC haÿ1 treatments (Fig. 1) is probably because the amount of inorganic-N added was not sucient to supply the initial N demand of the soil microorganisms, the

3.6 3.7 0.03 2.9

amount of inorganic-N added in the 80 t SMC haÿ1 treatment appeared to be sucient to meet this demand. Immobilization of N following SMC applications in the ®eld, particularly at a rate of 20 t haÿ1, has been recorded (Stewart et al., 1998) After any initial immobilization, N was mineralized in a zero order pattern, which is consistent with a single pool of organic-N that is partially resistant to soil micro-organisms and mineralization. The amount of N mineralized in this experiment was small and of similar magnitude to that reported from SMC by Maher (1991), and from other composts (Mattingly, 1956; Tester and Parr, 1983; Herbert et al., 1991; Cheneby et al., 1994). Other trials have also recorded N immobilization following compost applications to soil (Mattingly, 1956; Tester et al., 1977; Tester and Parr, 1983; Sims, 1990; Herbert et al., 1991; Sims et al., 1992; Aoyama and Nozawa, 1993; Zaccheo et al., 1993; Paul and Beauchamp, 1994). Neither of the kinetic models used in this study (i.e. mixed (®rst/zero) or zero order models) had been used before to model N mineralization from soils amended with compost. However, Bonde et al. (1988) used a mixed (®rst/zero) order model to describe N mineralization from FYM plus N fertilizer, and Addiscott (1983) used a zero order model to describe N mineralization from FYM. Applying fertilizer decreased the amount of N mineralization that occurred from SMC-soil mixtures (Table 3). Adding inorganic-N to the soil may reduce the rate of C or N mineralization from soil or residue organic matter (Stotzky and Mortensen, 1957; Fog, 1988; Clay and Clapp, 1990; Liljeroth et al., 1990; Liljeroth et al., 1994), and this was con®rmed by our results. This may result from the added N disturbing the balance of competition between speci®c microorganisms, blocking the production of ligninase enzymes, increasing the breakdown of easily available cellulose and increasing the accumulation of recalcitrant ligno-cellulose, increasing the formation of toxic browning precursors as amino compounds condense with polyphenols, and being absorbed by soil micro-organisms via ``luxury uptake'' and hence delaying N mineralization (Fog, 1988).

Inorganic-N release from spent mushroom compost

The CENTURY model could not accurately simulate the inorganic-N leached from SMC in this experiment. The CENTURY model partitions organic nitrogen added to soil, based on its lignin-to-N ratio, into either a ``structural'' pool with a slow turnover or a ``metabolic'' pool with a more rapid turnover (Metherell et al., 1993). SMC has a lignin content of approximately 17% (Gerrits, 1969), and most of its organic-N was partitioned into the ``metabolic'' pool of CENTURY. As SMC is partially resistant to microbial degradation, CENTURY's ®rst order kinetic simulation of N mineralization from this pool (and to a lesser extent from the ``structural'' pool), subject to temperature and moisture conditions, overestimated inorganic-N leaching from SMC. E€ect of sterilants The initial increase in the inorganic-N leaching rate from the methyl bromide treatment is probably the result of sterilization and heat-induced mineralization of microbial biomass. However this e€ect did not appear to in¯uence the more recalcitrant organic-N in MC, as methyl bromide had no signi®cant e€ect on the total amount of inorganic-N leached (Table 5) or on the total N concentration in the soil at the end of the experiment. The increase in inorganic-N leached of 6.7% following hypochlorite treatment (Table 5) is probably the result of increased ammoni®cation. During the ®rst week of the incubation the rate of inorganic-N leaching from the hypochlorite treated compost was slower and could have been the result of increased being largely ammoni®cation, with the NH+ 4 retained on the soil exchange complex. Alternatively, initial mineralization may have been inhibited by the hypochlorite. After this lag period, the NH+ 4 was rapidly nitri®ed and NOÿ 3 ±N was leached (Table 5). Agarwal et al. (1971) and McCormick and Wolf (1980) have reported that the addition of chloride salts to the soil may increase the ammoni®cation rate. Although some of these sterilants did in¯uence the amount of inorganic-N leached from MC amended soil, the magnitude of these e€ects was small (i.e. <7%). Obviously the chemical treatment of the MC was considerably diluted by the soil in this experiment, and hence the soil provided a buffer to any chemical e€ects on soil micro-organisms and N mineralization. Comparison with chicken litter and glycine±glucose mixtures The rate of inorganic-N leaching from MC was slow and is attributed, not to any chemical inhibition of N mineralization as this MC received none of the chemicals used in experiment 2, but to the slow mineralization rate of its organic-N. The organic-N in SMC is incorporated into large lignin±cellulose molecules during the composting pro-

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cess (Maggioni, 1981; Schisler, 1982; Gerrits, 1988), making it relatively unavailable. In contrast the small and relatively available glycine molecules were rapidly mineralized, particularly as there was a readily available source of C for the soil microbial community. CL contained a considerable amount of NH+ 4 ±N which was nitri®ed and leached, and there was also mineralization of the more labile organic-N in the CL. Field trial An approximately similar proportion of SMC-N was leached from this trial compared with the pot trial of Maher (1991) (viz. 8% cf. 15% of applied N). The initial ¯ush of inorganic-N leached was probably from the inorganic-N applied in the SMC. As expected, the rate of inorganic-N leaching was slower in the ®eld and hence the (unmodi®ed) laboratory-optimized model overestimated the cumulative loss. However, modi®cation of the laboratory model for either ®eld soil temperatures or drainage conditions gave an underestimate of the inorganicN loss. As the organic-N in SMC is partially resistant to microbial degradation, the lower temperature in the ®eld would have had less e€ect on N mineralization than if the organic-N in SMC was readily mineralizable. Also, although there was less drainage in the ®eld, the drainage water was probably more ecient at leaching as a result of ¯ow through soil macro-pores. Macro-pores were destroyed by the preparation of laboratory soil columns but were preserved in the ®eld lysimeters. This was shown by the greatest peak of inorganic-N in the leachate occurring before one pore volume of drainage had occurred in the ®eld. Hence modifying the model for either ®eld soil temperatures or drainage conditions overestimated the e€ect on the change in N mineralization rate from SMC. There may have been some gaseous losses from denitri®cation in this and the other experiments. Although no measurements of gaseous N losses were made in these experiments, we have found that there is little potential for large denitri®cation losses from SMC-amended plots (i.e. <6 g N haÿ1 dÿ1 on 80 t SMC haÿ1 plots on a warm day following a 44 mm, irrigation event). Macro-pore ¯ow in the ®eld lysimeters probably also caused the di€erence in shape of the inorganicN leaching curves between the laboratory and the ®eld. Hence the zero order model from the laboratory data did not ®t the ®eld data very well. Field conditions, including freezing-thawing and wettingdrying cycles stimulating mineralization, and soil macro-pores allowing the rapid loss of NOÿ 3 ±N, could explain the underestimate of inorganic-N leaching from SMC in the ®eld by the CENTURY model. No attempt was made to quantify the e€ects of di€erences in the soil structure between the labora-

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D. P. C. Stewart et al.

tory and the ®eld when inorganic-N leaching was modelled. In order to produce better models of ®eld mineralization from organic soil amendments, future research should attempt to combine the e€ects of nutrient mineralization, soil temperature and the amount of drainage, with soil structural e€ects on N mineralization and movement. In¯uence of soil The nature of the organic-N in SMC is likely to have the greatest in¯uence on its rate of mineralization. Also, because of the growth requirements of mushrooms, SMCs are likely to have similar properties. Only one soil was used in this work and its favourable conditions for microbial activity resulted in N mineralization from soil only treatments with no appreciable lag period. However, soils with lower pHs, greater clay contents, nutrient de®ciencies, or high C-to-N ratios could have considerably slower rates of N mineralization from them and also cause slower N mineralization from SMC if they are mixed with it. There has been no work reported comparing the rate of inorganic-N release from SMC when mixed with di€erent soils. CONCLUSIONS

The slow release of inorganic-N from SMC is predominantly the result of recalcitrant organic-N, but may also be attributable to formaldehyde inhibition of mineralization. Immobilization of N occurred in the ®eld and appeared to occur brie¯y in the laboratory also. Mixed (®rst/zero) and zero order kinetic models accurately described inorganicN leaching from SMC in the laboratory. After modi®cation for ®eld soil temperatures, the zero order model estimated a similar amount of inorganic-N loss as was observed in the ®eld. Adding fertilizer reduced the amount of mineralization of organic-N that occurred from SMC in the laboratory. SMC is suitable as a slow-release source of N for crops but applications, particularly initially and at low rates, may cause N immobilization. AcknowledgementsÐWe wish to thank Bruce Main for helping with modelling and his comments on the manuscript, Maureen McCloy and Roger Cresswell for chemical analysis, Neil Smith for helping to install the lysimeters, David Yinil for helping set up two of the laboratory incubations, and Meadow Mushrooms Limited for funding this study. REFERENCES

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