yard waste composting: studies using different mixes of ... · stream for source reduction and...

Compost Science &Utilization, (1993) Vol. 1, No. 3,85-96 # i ? 5 z z v 9 . t g S . d k ’ f d f

Yard Waste Composting: Studies Using Different Mixes of Leaves and Grassin a Laboratory Scale System

Frederick C. Michel Jr., C. Adinarayana Reddy, Larry J. Fomey Department of Microbiology and NSF Center for Microbial Ecology

Michigan State University, East Lansing, Michigan

Composting has become a widely used method of recycling yardwastes such as leaves and grass. However, very little information is available on the chemical changes that occur during the composting of different mixtures of leaves and grass. In this study, three different mixes of leaves and grass were composted at approximately 60% moisture in a temperature controlled laboratory scale system. The mixes, which consisted of all leaves (Mix 1); 2/3 leaves + 1 /3 grass (Mix 2); and 1 /3 leaves + 2/3 grass (Mix 3), had initial C.N ratios of 48,30 and 22, respectively. The compost process was moni- tored by measuring the rate of CO2 evolution, pH, stability, the degree of humification and changes in polysaccharide, carbon, nitrogen and organic matter content. Results showed that the greater the grass content of the mix, the higher the initial pH and the faster the rate of CO2 evolution, organic matter loss and nitrogen loss. After 43 days of composting, Mixes 1,2 and 3, lost, respectively 61 %, 74% and 78% of the cellulose, 57%, 79% and 82% of the hemicellulose and 40%, 49% and 42% of the acid-insoluble organic matter. Humification indices and stability tests indicated that composts produced from the three mixes were well humified and stable.

Introduction Yard wastes (grass clippings, leaves and brush) account for approximately 18% of

U.S. municipal solid wastes and are predicted to surpass 36 million dry tons per year by 1995 (Slivka et al., 1992). Because of the adverse environmental effects of landfilling and incineration, and increasing costs, the U.S. EPA has targeted 25% of the solid waste stream for source reduction and recycling (EPA, 1992). In addition, many eastern states have banned yard waste landfilling and incineration (Kashmanian and Spencer, 1993), while New Jersey has targeted 60% of its solid waste stream for recycling or source reduction (Glenn, 1990).

An attractive alternative to the disposal of yard wastes by landfilling or burning is large scale composting. In this process, leaves or mixtures of leaves, grass and brush are aerobically decomposed giving a topsoil like product, which can be used to en- hance soil fertility (Dick and McCoy, 1993). Yard wastes are particularly good feedstocks for composting since they are source segregated and, therefore, uncontaminat- ed by glass, metal and other non-compostable materials (other than plastic film). Yet, grass alone is an unsuitable compost feedstock due its tendency to become anaerobic and produce strong and noxious odors (Schulz, 1992). At large-scale yard waste composting facilities, leaves are commonly mixed with grass clippings to reduce odor production (Schulz, 1992), balance the C:N ratio, and reduce the moisture content and compaction of fresh grass. The recommended ratio of leaves to grass is at least 2:l (Strom and Finstein, 1989; McNelly, 1990). However, the leaf to grass ratio may vary during the year due to seasonal changes in the available fsedstocks. Furtlwrmore, grass and leaves differ widely in their nitrogen and organic matter content. Very little information is available at this time on the chemical changes that occur during the composting of different mixes of grass and leaves (Poincelot and Day, 1973; Hammouda and Adams, 1986; Strom and Finstein, 1986; Schulz, 1992). Such information will be useful in the design of large scale yard waste composting facilities for the production of marketable compost. We are currently using a temperature controlled laboratory- scale system to study the microbial, chemical and physical changes that occur during yard waste composting under controlled conditions. In this article, we describe the use

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Compost Science & Utilization Summer, 1993 85

F.C. Michel Jr., C. A. Reddy, L. J. Forney

of this laboratory scale system, and examine the effect of different mixes of grass and leaves on the fates of cellulose, hemicellulose and lignin fractions, the level of humification, compost stability, pH, nitrogen loss and the rate of organic matter mineraliza- tion during composting.

Materials And Methods

Feedstock Two feedstocks were used in this study; fresh grass and fresh leaves. These were

obtained from the Granger yard waste composting facility in Lansing, Michigan; from the Michigan State University Sanford woodlot; and from the yard waste composting facility in Canton township, Michigan. The leaves and grass were dried at 37°C to approximately 10% moisture and then passed through a Viking electric hammer mill (Viking Mfg.Co., K346554, Jackson, MI) to give a 3 mm particle size. The three different mixes of leaves and grass used in this study contained on a dry weight basis: all leaves (Mix 1); 1/3 grass + 2/3 leaves (Mix 2); and 2/3 grass + 1/3 leaves (Mix 3). Deionized water was added to the mixes to 60% moisture ( g / g wet wt.), and approximately 500 gm (wet weight) of the mixes were added to each composter.

Composting conditions A diagrammatic representation of the composting system used in this study is pre-

sented in Fig. 1. Most of the composting system was contained in a temperature controlled incubator (Lunaire Environmental, Inc. model#10632M-l, Williamsport, PA). The compost chambers consisted of a rubber stoppered 2-liter wide mouth glass jar with two plastic screens (1 cm mesh opening) and one Teflon screen (1 mm mesh opening) which formed a false floor. Aeration was provided through a 5 mm opening in the bottom of each jar. The air supply was stripped of CO, by passing it through a packed bed column filled with 1 1 of 5 N NaOH and the flow rate was measured using a ro- tameter (Aalborg Instruments, Monsey, NY). To avoid moisture losses during composting and the consequent inhibition of the composting process, the air entering the compost chamber was humidified by bubbling into 10 1 of deionized water at the incubation temperature and the air-flow rate was adjusted to provide twice as much oxygen to the compost as indicated by the maximum oxygen uptake rate observed in preliminary experiments. The air flow rate to each composter was set to approximately 100 ml/min during the first 15 days and 50 ml/min thereafter. The exhaust gas from the compost chamber passed through a needle valve used to control the flow rate, and into a column containing 60 ml of anhydrous calcium sulfate (Drierite) absorbent to re- move moisture. This was necessary since water accumulated in the CO, traps. The exhaust gas then flowed into 50 ml of 3 to 5 N NaOH trapping solutions.

The temperature of the incubator was programmed to simulate the temperature during yardwaste windrow composting (Fig. 2; inset). After 7 days the temperature of the incubator was kept constant at 60°C. The temperature within the composts was measured by removing the rubber stopper and placing a thermometer directly into the center of the compost. The temperature within the compost did not deviate more than 4°C from the incubator temperature.

Compost analyses Feedstock Mixes 1,2 and 3 were composted in the laboratory scale system for 52

days. Single samples (20 gm wet weight) were removed on days 1,4,11,18,24,32 and 52 for pH, moisture and ash content determinations, on days 0 and 32 for C and N analysis, on days 0 and 43 for quantitative saccharification, and on days 0 and 52 for

86 Compost Science Z, Utilization Summer, 1993

1 Yard Waste Composting: Studies Using Different Mixes of Leaves and Grass in a Laboratory Scale System

stability and humification measurements. Approximately 10 gm of the sample was dried at 105°C and ashed at 550°C for the

determination of moisture content and organic matter content, respectively. The organic matter (OM) refers to the difference between ash and dry weight. The total

the mean of triplicate measurements of moisture content and organic matter content from preliminary experiments was 4% and

F.C. Michel Jr., C. A. Reddy, L. J . Forney

ing 0,100,200,300,400,500,600 and 800 mg/l phthalic acid as standards. Cellulose, hemicellulose and lignin contents of the compost samples were deter-

mined by the quantitative saccharification method of Saeman et al. (1945). Duplicate samples were dried and ground to a fine powder using a mortar and pestle, diluted 1:lO with conc. H2S04 and incubated with periodic mixing for 1 hour at 30°C. After this initial hydrolysis, the samples were transferred quantitatively to a 1-liter flask and diluted 1:28 with deionized water and then autoclaved for 1 hour. Acid-insolubles were removed by filtering through tared GFC filter paper (Whatman, Maidstone, U.K.). The filter cakes were dried at 105"C, weighed, ashed at 550°C and weighed again. Acid-insoluble organic matter was determined by calculating the difference between the dry weight of the filter cake and the weight of the filter cake ash. Sugar concentrations in the hydrolyzed samples were measured using a Biorad HPX-87P HPLC carbohydrate column equipped with a UV detector (Isco Model 2340, Lincoln, NE). The column temperature was 85°C and the flow rate was 0.5 ml/min. Samples were neutralized with PbCO, to pH>5, centrifuged, and then filtered using Millipore acrodisk syringe filters into autosampler vials. Authentic standards (>98% purity) of glucose, xylose, galactose, mannose and arabinose were also prepared (Sigma Chem- ical, St.Louis MO). All of the glucose obtained from compost hydrolysis was assumed to originate from cellulose. Five carbon sugars and mannose were assumed to originate from hemicellulose. The ash content was determined in samples which were not acid hydrolyzed since the acid hydrolysis solublized some of the inorganic components of the composts.

The concentrations of trace elements were determined by plant tissue analysis using plasma emission spectroscopy according to the method of Jones (1977). Duplicate samples were measured and average values are presented. These tests were conduct- ed by the Michigan State University Soil Testing Laboratory.

30 4 0 Days

Mix 3

Mix 2

Mix 1

2

1

O a , P I I 1 : 1 ~ 1 1 1 1 1 1 1 : 1 1 1 , ~ 1 1 1 1 1

0 10 20 30 40 50

Days

Fig. 2. The rate of COP evolution during composting of different mixes of leaves and grass. The composition of the mixes is: all leaves (Mix 1); 2/3 leaves + 1/3 grass (Mix 2); and 1/3 leaves + 2/3 grass (Mix 3). The inset presents the temperature program employed during composting of these mixes.

88 Compost Science Utilization Summer, 1993

Yard Waste Composting: Studies Using Different Mixes of Leaves and Grass in a Laboratory Scale System

TABLE 1. Composition of grass and leaves used to prepare compost feedstock mixesa

Component Grass Leaves

Carbon (%) Nitrogen (%) C:N ratio (g/g) Organic matter (%)

Calcium (%) Magnesium (%) Phospho rus (%) Potassium (%) Aluminum (ppm) Boron (ppm) Copper (ppm) Iron (ppm) Manganese (ppm) Sodium (ppm) Zinc (ppm)

PH

41.6

17 89

2.46

9.3 0.48 0.15 0.26 0.67

31 6 8 6.95

273 27.8

285 58

44.5

48 64

0.93

6.1 1.52 0.27 0.05 0.31

489 65

390

440 252

5.05

74.1

aValues are averages of duplicate measurements except pH and organic matter which are averages of triplicate measur- ments. All units are on a dry weight basis.

Compost stability measurement Compost stability was assessed based on the method described by Iannoti-Frost et

al. (1992). This technique measures the oxygen uptake rate of a compost sample after 24 hours of incubation at 37°C. Relative humidity was measured using a digital hy- grometer (Oakton Co.). The data presented are averages and standard deviations for triplicate samples.

Results And Discussion The characteristics of the grass and leaf feedstocks used to prepare the mixes for

composting are presented in Table 1. The main difference in the grass and leaf feedstocks were the nitrogen content and ash and trace metals content. The grass contained considerably more nitrogen (2.46%) than the fresh leaves (0.93%). The leaves were composed of 36% inorganic matter (ash) and had a greater trace metal content than the grass. The grass was composed of only 13 % inorganic matter, but had a greater potassium, copper and phosphorus content than the leaves.

The composting temperature is known to have a marked effect on microbial diversity (Strom, 1985). In actively composting yard waste, temperatures commonly ex- ceed 60°C, a temperature which favors thermophilic microbes. Therefore, the temperature regime imposed on the compost mixes (Fig.2 inset) was designed to simulate the temperatures found at the center of a yard waste compost windrow (Schulz, 1992) which are more selective for thermophilic microbial species.

The feedstock mixes began to evolve C02 within 24 hours of incubation. The COz evolution rate (mg C02/ g d.w./h) increased steadily for the first eight days and reached a maximum rate between days 8 and 10 (Fig. 2). This initial rise in the rate of C02 evolution paralleled the rise in temperature of the compost incubator (#7 in Fig. 1). The COZ evolution rate gradually decreased until day 22 and then was relatively constant until measurement was stopped on Day 40. The maximum rate of COz evolution observed was 1.1,2.2 and 3.2 (mg C02/ g d.w./h) for Mixes 1,2 and 3 respectively. The calculat-

~



x Y

Y f

50

25

0 0 10 20 30 40 50

Days

Fig. 3. The cumulative CO2-C evolution as a function of the initial carbon content during composting.

ed maximum oxygen uptake rates for Mixes 1,2 and 3, respectively, were 1.3,2.1 and 2.7 (mg 0 2 / g OM/h), assuming a respiratory quotient of 1 (C02:02 ratio). The CO, evolution rate was consistently higher for Mix 3 (2/3 grass + 1/3 leaves) and lowest for Mix 1 (all leaves). This is assumed to be due to the higher nitrogen content of Mix 3. The pat- tern of cumulative CO, evolution (i.e. fraction of initial C mineralized) showed that the percent carbon lost with time reached a relatively constant rate for each of the three mixes after 18 days (Fig. 3). Approximately 36%, 44% and 65%, respectively, of the carbon initially present in Mixes 1,2 and 3, was mineralized to C02.

A substantial loss of the total organic matter (OM) occurred during composting (Fig. 4). After 52 days of composting, the percent organic matter (g organic matter/ g d.w.) of the three mixes decreased from 63% to 48% for Mix 1, from 72% to 50% for Mix 2, and from 80% to 60% for Mix 3. These values represent organic matter losses (g OM lost/g initial OM) of 46%, 59% and 63% respectively. Topsoils typically have an organic matter content of 0.5% to 10% (Paul and Clark, 1989). The moisture content of the compost remained at or slightly above 60% during the composting runs (Fig. 4).

The ”compost stability” was assessed on Day 52 using the method of Iannotti-Frost et al. (1992). This technique measures the oxygen uptake rate of a compost sample after a 24 hour incubation at 37°C. One problem encountered with this method is that only the decrease in oxygen saturation in the system is measured and a quantitative oxygen uptake rate is not calculated. Therefore, it does not allow the comparison of different samples. For this reason, oxygen uptake rates were quantified using the ad- ditional measurements of relative humidity, compost density, organic matter and total flask volume. The results (Table 2) showed that the final composts corresponding to feedstock mixes 1,2, and 3, are quite stable in that they exhibited very low oxygen uptake rates of 0.11,0.09 and 0.09 (mg 0 2 / g OM/h), respectively. By comparison, the initial feedstock mixes had oxygen uptake rates of 1.77,3.49 and 4.85 (mg 0 2 / g OM/h), respectively, for mixes 1,2, and 3.

Since stability criteria for composts have yet to be developed in the U.S., the stability of compost samples was assessed based on the Rottegrad index which is widely used in Germany (Verstraete, W., personal communication). This index relates oxygen uptake rate to the potential for heat generation and compost stability. It divides composts into 5 categories (I to V). Higher numbers correspond to stable composts that have a low potential for self heating. According to this index, each of the three feedstock mixes had an initial Rottegrad index of I while the final (day 52) compost Mixes 1,2 and 3, had Rotte- grad indices of IV-V, V, and V, respectively. According to this criterion, the 52 day old composts produced from feedstock Mixes 1,2 and 3 are quite stable.

90 Compost Science & Utilization Summer, 1993

, Yard Waste Composting: Studies Using Diferent Mixes of Leaves and Grass in a Laboratory Scale System

1

0.8

0.6

0.4

0.2

0

1

0.8

0.6

0.4

0.2

0

1

0.8

0.6

0.4

0.2

0

+ +

Mix 1

+ -I 0 4 1 1 18 24 32 52

Day I

Mix 2 Organic Matter

Ash I

0 4 1 1 18 24 32 52 Day

Mix 3

+ t + -t 0 4 1 1 18 24 32 52

Day

Fig. 4. Changes in organic matter content during composting of the three different feedstock mixes. The absolute amount of ash in the composter at any given time is assumed to be constant. Standard error of the mean based on independent measurements is


TABLE 2. Composition and stability of the initial feedstock mixes and the corresponding final compost

Mix Composition

0, Uptake Ratea (mg 0 2 / g OM/h)

Initial Final

1 All Leaves 1.77 f 0.07 0.11 rf: 0.02 2 1/3 Grass + 2/3 Leaves 3.49 f 0.17 0.09 k 0.03 3 2/3 Grass + 1/3 Leaves 4.85 f 0.26 0.09 k 0.04 aValues are averages of triplicate measurements f standard deviation. Values given are for the initial feedstock mixes and the corresponding final composts after 52 days of composting.

The C:N ratio, nitrogen content, and pH during composting are known to influ- ence volatile N losses (Kissel et al., 1992). In this study, we used feedstock mixes rep- resenting three different initial C:N ratios by varying the ratio of grass to leaves. Feed- stock Mix 1, which had a C:N ratio of 48, represented a nitrogen-limited mix. Feedstock Mix 2 had an initial C:N ratio of 30, a value generally thought to be optimal for organic matter decomposition (Paul and Clark, 1989). Feedstock Mix 3 had a C:N ratio of 22 and represented a nitrogen abundant mix. The C:N ratios of the three feedstock mixes dropped to 24,19 and 13, respectively, after 32 days of composting.

The initial pHs of Mixes 1,2, and 3 were 6.1,7.4, and 9.0, respectively. During composting, the pH of Mixes 2 and 3, which contained grass, reached a pH of 8.1 by day 10 and the values remained fairly constant at that level through day 52 (Fig. 5). The pH of the all leaves mixture (Mix l), on the other hand, remained lower than the other mixes through most of the composting run in that it rose to pH 7 by day 4, remained at that level through day 32, and then rose to pH 7.8 by day 52. The lower pH may be due to the higher C:N ratio of the all leaves mix.

As indicated above, during the composting of Mixes 2 and 3, the pH reached 8.1 at which nitrogen losses due to ammonia volatilization are likely to occur. Consistent with this idea, Mixes 2 and 3 lost 23% and 27% of their initial nitrogen, respectively, after 32 days of composting while the all leaves mix (Mix 1) lost only 6% of its initial nitrogen during this period. The absolute amount of nitrogen lost from Mixes 1,2 and 3 was 0.55,3.31 and 5.27 (mgN/g initial d.w.). Thus in absolute terms, Mix 3 (2/3 grass + 1 /3 leaves) lost almost an order of magnitude more nitrogen than Mix 1 (all leaves). This is significant considering the fact that composts containing grass are often asso- ciated with odor problems and that nitrogen compounds are one of the primary components of composting odors (Kissell et al., 1992). In spite of nitrogen loss by volatilization, however, the overall nitrogen content of Mixes 1,2 and 3 rose during composting since carbon losses outweighed nitrogen losses (Table 3).

TABLE 3. Carbon and nitrogen content, C:N ratio, and nitrogen loss during composting

Carbon contentb Nitrogen contentC N-loss !%> P%) C:N ratio (%I

Mixa DayO Day32 DayO Day32 DayO Day32

1 44.6 27.0 0.93 1.11 48 24 6 2 43.2 32.8 1.44 1.76 30 19 23 3 42.9 33.0 1.95 2.49 22 13 27

aThe composition of the mixes is described in Table 2. h h e values presented are averages for duplicate measurements. 'The values represent single measurements but the standard error of the mean between samples in independent measurements was less than 2%.

92 Compost Science Utilization Summer, 1993


3 10

0 10 20 30 40 SO Days

Fig. 5. pH profiles during composting. Values represent single measurements.

Carbohydrates (cellulose and hemicellulose) accounted for 24%, 28% and 32% of feedstock Mixes 1 , 2 and 3, respectively. After 43 days of composting carbohydrates accounted for only 13%, 8% and 13% of the composts corresponding to mixes 1,2 and 3, respectively (Fig. 6). Approximately 61%, 74% and 78% of the cellulose and 57%, 79% and 82%, of the hemicellulose, respectively, in Mixes 1,2 and 3, was lost after 43 days of composting. In a study of leaf decomposition during windrow composting, Poincelot and Day (1973) reported initial cellulose (24%) and nitrogen (1.28%) contents for fresh leaves that were almost identical to those of the leaves (Mix 1) used in this study. However, the rate of cellulose loss in windrows (31 % loss in 200 days) was much slower than the rates observed in our laboratory scale composting system (61% loss in 43 days). The faster rate of cellulose loss observed in the laboratory scale compost system described here, is probably due to improved aeration, moisture control and the reduced particle size of the milled leaves.

The acid-insoluble organic matter (primarily lignin) decreased by 40%, 49% and 42% for Mixes 1,2 and 3, respectively, during composting (Fig. 6). It should be noted, however, that this so called "lignin" fraction also contains acid-insoluble proteins and humic acids in addition to lignin. Inoko et al. (1979) have shown that between 8% and 22% of the acid-insoluble organic matter obtained from city refuse composts were nitrogen containing compounds. However, the greater than 40% reduction in the acid- insoluble organic matter indicates that a substantial portion of the lignin was mineralized during the composting of the leaf and grass mixtures.

The "other" fraction obtained during the quantitative saccharification procedure represents acid soluble organic matter such as fulvic acids, fats, oils, lipids, phenols, tannins and proteins. This fraction remained relatively constant during composting of the leaves mix (Mix l), but was reduced substantially during the composting of the mixes containing grass (Mixes 2 and 3).

The humification index (Sequi et al., 1986) is a ratio of non-humic extractables to total humified extractables (humic + fulvic acid). The Humification index is used to in- dicate the relative amount of humified material (ie. humic + fulvic acid) in various samples and have been used as a partial indicator of compost maturity (Inbar et al., 1990; Ciavatta et al., 1990). This index tends to decrease during composting due to the humification of organic matter. Humified materials such as soils have a humification index less than 0.5 while non-humified materials generally have a humification index

present in day 0 and day 52 samples using the method of Ciavatta et al. (1990). The results showed an increase in the total amount of extractable carbon and extractable humic + fulvic acids after 52 days of composting of the three mixes (Table 4). The non- humified extractable material decreased substantially during the same period. The

greater than 1.0 (Ciavatta et al. 1990). We measured the amount of humified material . --



TABLE 4. Initial and final humification measures for the three feedstock mixes

and their corresponding final composts

Humic + Fulvic Acids Nonhumic fraction (HA+FA) (NHF) Humification Index

mgC/gOM mgC/gOM [NHF/(HA+FA)I

Mixa Initial Final Initial Final Initial Final

1 37f 3 60f 1 3 3 f 2 2 1 f 4 0.99 f 0.16 0.34 2 0.08 2 3 5 f 3 60f 1 3 6 f 2 2 2 f 4 1.00 f 0.20 0.37 k 0.07 3 3 3 f 3 5 2 f 2 3 9 f 2 2 1 f 1 1.17 f 0.15 0.40 f 0.05

aThe composition of the mixes is described in Table 2. The length of composting was 52 days. Values are triplicate measurements + one standard deviation.

100

75

50

25

0 Day 0 Day 43

100

75

50

25 0 Day 0 Day 43

100

75

50

25

0

Cellulose

Hemicellulose

Acid-insoluble organic matter

Day 0 Day 43

Fig. 6. Amounts of cellulose, hemicellulose, acid-insoluble organic matter, ash and "other" (unaccounted for C) components of feedstock Mixes 1,2 and 3, before and after composting for 43 days. Acid-insoluble organic matter includes lignin, humates and some proteins. Values represent averages of duplicate samples.

94 Compost Science & Utilization Summer, 1993


humification index of the three mixes decreased from 0.99 to 0.34 for Mix 1, from 1.0 to 0.37 for Mix 2, and from 1.17 to 0.40 for Mix 3. The humification index values obtained in this study are consistent with values reported by other investigators (Sequi et al., 1986; Inbar et al., 1990) and show that the final composts produced from our three feedstocks were mature and well humified.

Conclusions Composting is the accelerated aerobic biotransformation of plant polysaccharides,

and other organic compounds to CO, and humic substances, by microorganisms. The laboratory scale system used in this study was designed to provide an optimal envi- ronment for composting. In the laboratory system, aeration was continuously sup- plied, the moisture content was kept at 60%, and the temperature was controlled at 60°C. These composting conditions led to the production of a well humified and stable compost product at a rate much faster than that generally observed during windrow composting.

The organic matter composition of the finished composts produced from the three feedstock mixes (Mixes 1,2 and 3) used in this study were very similar. By dry weight, the "finished" composts were composed primarily of inorganic matter or ash (38-49%) and acid-insoluble organic matter (lignin, humic acids, proteins) (27-33%). Cellulose and hemicellulose comprised less than 13% while unidentified components accounted for from 11 to 16% of the final compost mixes. The humification indices calculated for the composts produced from the three mixes were not significantly different.

The primary difference in the composts produced from the three mixes was the nitrogen content. Nitrogen is an important component of compost due to its value as a plant nutrient. During composting, nitrogen compounds are immobilized by microorganisms but are also lost as volatiles. When grass and leaf mixtures are composted nitrogen losses by volatilization increase as more grass is added to the mix. For instance, Mix 3 (2/3 grass + 1/3 leaves) lost almost an order of magnitude more nitrogen than Mix l (all leaves). Grass contained higher levels of nitrogen than leaves and could be used to increase the nitrogen content of composting leaves to both speed the rate of decomposition and give compost products with a higher nitrogen content. However, grass addition also leads to higher rates of oxygen depletion from the compost, potentially leading to anaerobic conditions and the development of odors due to incomplete organic matter oxidation (eg. volatile fatty acids).

Assessment of the oxygen stability and humification of the final compost products produced in the laboratory scale system showed that each of the three mixes produced composts that were well humified and stable.

Acknowledgements We thank Dr. Elis Owens for reviewing this manuscript, Dr. H.E.'Grethlein for the use of his laboratory facilities, and Sharon Debar for helping with the Kjehldahl nitrogen measurements. This research was supported in part by Michigan Agriculture Experi- ment Station and by Research Excellence funds from the State of Michigan to the NSF- Center for Microbial Ecology and the Center for Crop Bioprocessing.

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