Stability Measurement of Biosolids Compost by Aerobic Respirometry

Oxygen uptake of biosolids compost was measured during both laboratory and full-scale studies. Aerobic respirometry of solid samples of compost provided a precise measure of microbial activity. There was a noticeable decreasing trend in oxygen consumption over 25 days of composting, thereby indicating increasing stability. Moisture content also was found to affect the compost stability. During 48-hour respirometer tests, the compost sample did not dry to the point where respiration was inhibited. Measurement of volatile solids reduction alone during biosolids composting with large quantities of sawdust revealed little about stability.     

The Respiration Rate of Composting Pig Manure

The rate at which oxygen is consumed during composting is a measure of aerobic microbial activity and is linked to the rate of organic material decomposition. The rate of loss in mass is a function of the mass of the degradable organic fraction and is related to oxygen uptake rate by the reaction rate coefficient, k. The decomposition of a pig manure and straw mix was investigated at temperatures between 10°C and 70°C using respirometric techniques. The oxygen concentrations in the reactor were measured continuously for about 4 days and then converted to hourly oxygen uptake rates for each incubation temperature, T. The specific oxygen uptake rate was used to calculate the reaction rate coefficient at T, k_T, for the observed fast and slow stages of decomposition. The effect of the environmental factors was taken into account using a multiplicative approach and a relationship, which expressed k_T for each stage as a function of T, was formulated. The maximum measured rate of activity occurred during the fast stage at 60°C where k_{T fast} = 0.31 day^?1. Activity increased exponentially with the temperature in both stages up to about 60°C. At higher temperatures, the activity slowed but most noticeably in the fast stage. The dependence of k_T on T during each stage was described by a double power expression, which predicted that activity would cease around 73°C. The relationships may be used to improve a compost model that is based on a first order reaction rate kinetics for the decomposition of organic material.     

Biological,Chemical and Physical Properties of Composted Yard Trimmings as Indicators of Maturity and Plant Disease Suppression

A full scale system for composting of fermented, odiferous yardwaste trimmings was developed that produced a stabilized compost, minimized odor generation and prevented leachate formation. Characteristics of the compost highly significantly correlated with composting time included availability of the plant nutrients K, P, Ca and Mg, electrical conductivity, total carbon to nitrogen ratio, cation exchange capacity, stability based on O₂ respirometry, and finally, nitrate-nitrogen concentration. Radish was a good indicator of compost maturity. As the compost matured, suppressiveness to Pythium damping-off increased but it remained conducive to Rhizoctonia damping-off.     

Pilot and Full Scale Evaluations of Leaves as an Amendment in Sewage Sludge Composting

The use of leaf amendment in woodchips/sludge composting was studied in pilot-scale and full-scale operations. Use of leaves at a rate of 20 percent by volume was compatible with present practices and equipment at the Columbus Compost Facility, but higher rates caused materials handling and curing problems and would necessitate system modifications. Only very slight reduction in new woodchip usage was observed for the 20 percent leaf amendment, but output of sieved-finished compost was doubled. Leaf usage increased airflow requirement per unit dry matter during the initial stage of composting but decreased ammonia release throughout the composting process. On the other hand, use of recycled materials, compost and sieved woodchips, increased ammonia release. Adequate moisture and periodic turning in the pilot-scale studies enhanced composting for all treatments. Leaf usage may necessitate forced aeration during curing to keep full size piles aerobic. In addition, leaf usage increased the total materials handling requirement for operating the full-scale system by 16 percent, and it was concluded that careful consideration of materials handling is the major concern for a system's operating efficiency.     

Mass and Nutrient Losses During the Composting Of Dairy Manure Amended with Sawdust or Straw

Composting has become an increasingly popular manure management method for dairy farmers. However, the design of composting systems for farmers has been hindered by the limited amount of information on the quantities and volumes of compost produced relative to farm size and manure generated, and the impact of amendments on water, dry matter, volume and nitrogen losses during the composting process. Amendment type can affect the free air space, decomposition rate, temperature, C:N ratio and oxygen levels during composting. Amendments also initially increase the amount of material that must be handled. A better understanding of amendment effects should help farmers optimize, and potentially reduce costs associated with composting. In this study, freestall dairy manure (83% moisture) was amended with either hardwood sawdust or straw and composted for 110-155 days in turned windrows in four replicated trials that began on different dates. Initial C:N ratios of the windrows ranged from 25:1 to 50:1 due to variations in the source and N-content of the manure. Results showed that starting windrow volume for straw amended composts was 2.1 to 2.6 times greater than for sawdust amendment. Straw amended composts had low initial bulk densities with high free air space values of 75-93%. This led to lower temperatures and near ambient interstitial oxygen concentrations during composting. While all sawdust-amended composts self-heated to temperatures >55°C within 10 days, maintained these levels for more than 60 days and met EPA and USDA pathogen reduction guidelines, only two of the four straw amended windrows reached 55°C and none met the guidelines. In addition, sawdust amendment resulted in much lower windrow oxygen concentrations (< 5%) during the first 60 days. Both types of compost were stable after 100 days as indicated by CO₂ evolution rates <0.5 mg CO₂-C/g VS/d. Both types of amendments also led to extensive manure volume and weight reductions even after the weight of the added amendments were considered. However, moisture management proved critical in attaining reductions in manure weight during composting. Straw amendment resulted in greater volume decreases than sawdust amendment due to greater changes in bulk density and free air space. Through composting, farmers can reduce the volume and weights of material to be hauled by 50 to 80% based on equivalent nitrogen values of the stabilized compost as compared to unamended, uncomposted dairy manure. The initial total manure nitrogen lost during composting ranged from 7% to 38%. P and K losses were from 14 to 39% and from 1 to 38%, respectively. There was a significant negative correlation between C:N ratio and nitrogen loss (R₂=0.78) and carbon loss (R₂=0.86) during composting. An initial C:N ratio of greater than 40 is recommended to minimize nitrogen loss during dairy manure composting with sawdust or straw amendments.     

Treatment of High Ammonium–Nitrogen Wastewater from Composting Facilities by Air Stripping and Catalytic Oxidation

Composting municipal wastewater sludge may generate composting wastewater (acid washer water and tunnel wastewater) with high ammonium–nitrogen (NH₄–N) concentration; this kind of wastewater is usually generated in a rather small daily amount. A procedure of air stripping with catalytic oxidation was developed and tested with pilot-scale and full-scale units for synthetic disposal of the high NH₄–N wastewaters from composting facilities. In air stripping, around 90% NH₄–N removal efficiency was reliably achieved with a maximum of 98%. A model to describe the stripping process efficiency was constructed, which can be used for process optimization. After catalytic oxidation, the concentrations in the outlet gas were acceptable for NH₃, NO_X, NO₂, and N₂O, but the NH₃ and N₂O concentrations limited the feasible loading range. The treatment costs were estimated in detail. The results indicate that air stripping with the catalytic oxidation process can be applied for wastewater treatment in composting facilities.     

Effects of Turning Frequency,Leaves to Grass Mix Ratio and Windrow vs. Pile Configuration on the Composting of Yard Trimmings

Because of proposed bans on the landfilling and incineration of leaves, grass and brush, large-scale composting is fast becoming the primary disposal option for yard trimmings in many states. Few systematic studies have been done to compare the effects of turning regime, feedstock mix ratio, or windrow vs. pile configuration on composting and the characteristics of finished compost. In this study, various ratios of leaves, grass and brush were mixed and composted in two series of windrows; and one set of static piles. One windrow series (#1) was turned seven times every four weeks, while the other windrow series (#2), and the piles, were turned once every four weeks. The effects of the different treatments were examined by measuring compost temperature, oxygen concentration, pH, organic matter and moisture content, volatile fatty acid content, bulk density, stability, humification and seed germination indices, total and available nutrient levels, and particle size distribution. Results showed that turning frequency had little impact on oxygen concentrations, VFA content and temperatures during the composting of yard trimmings in windrows, however, in piles temperatures were substantially higher and oxygen concentrations fluctuated greatly. The composts from all the treatments were stable, (oxygen uptake rates < 0.1 mg O₂/g OM/hr) after 60 days of composting regardless of the turning frequency, mix ratio or configuration. The bulk density inereased much more rapidly in frequently turned windrows than in the other treatments and particle sizes were smaller in these windrows. In most respects however, the final composts (day 136) were remarkably similar and none inhibited Cress seed germination or root elongation. The pH of all the composts, and the soluble salts and nitrate levels in composts made with high levels of grass, exceeded guidelines for greenhouse growth media.     

Stability Evaluation of Mixed Food Waste Composts

As interest in food waste composting grows, so does the need for proven composting methods. Stability testing has been proposed as a compost quality assurance tool. We conducted this study to: (i) to evaluate the efficacy of simple outdoor composting methods in producing a compost with a low, stable decomposition rate, and (ii) to determine the reliability of simple, 4-h compost stability evaluation methods. Composting was conducted outdoors in winter and spring in Eugene, Oregon without moisture addition. Mixed food waste was combined with screened dairy solids and ground yard trimmings. Sawdust was used to cover windrows for the first 27 d of composting. Compost windrow temperatures remained above 55°C for 30+ d. Carbon dioxide evolved with several 4-h test methods was strongly correlated (r² > 0.7) with CO₂ evolved using a 48-h test. A limited-turn windrow (LTW) composting system produced compost with slightly greater stability than a passively aerated windrow (PAW) composting system. Food waste compost samples had a low CO₂ evolution rate after 71 to 99 d using either composting system. Compost CO₂ evolution rate at 25°C decreased with composting time, reaching approximately 1 to 4 mg CO₂-C g compost C^?1 d^?1 for the PAW method and 0.5 to 2 mg CO₂-C g compost C^?1 d^?1 for the LTW method. Putrescible organic matter in food waste was effectively decomposed in outdoor windrows using composting methods that did not employ forced aeration, self-propelled windrow turners, or manufactured composting vessels. Several 4-h stability tests showed promise for implementation as quality assurance tools.     

Effect of Treated Wood on Biosolids Composting

The possibility of using construction and demolition (C&;D) waste wood as a bulking material in biosolids composting was investigated. Potential contaminants in C&;D waste wood include arsenic (As), chromium (Cr), and copper (Cu) from treated wood, and lead (Pb) from paints. Untreated and treated woodchips from C&;D wood were mixed with biosolids, composted using an aerated static pile process, and cured. There were no significant differences between untreated and treated woodchips with respect to composting process, time to stability, or product quality. Composting parameters monitored included moisture content, pH, electrical conductivity, organic matter degradation, fecal coliform levels, and stability by respirometry. Finished compost quality was evaluated in terms of potential toxic elements (PTE) levels. PTE values in treated woodchips (26 ± 35, 29 ± 41, 56 ± 46, and 5 ± 5 µg·g^?1 for As, Cr, Cu, and Pb, respectively) were higher and more variable than those in the untreated woodchips (3 ± 3, 17 ± 8, 13 ± 2, and 0.5 ± 0.0 µg·g^?1). However, both untreated and treated wood compost products met Canadian Council of Ministers of the Environment Category B values for PTE. In addition, and only molybdenum (Mo) and Cu exceeded Category A thresholds. Biosolids were the most significant contributor of Mo, while Cu contributions came from both biosolids and wood chips; some samples of pressure-treated wood showed concentrations of Cu in the range of 765 to 8,455 µg·g^?1. The results of this study suggest that treated wood from C&;D recycling facilities will not significantly degrade the quality of biosolids compost products.     

Relating Compost Measures of Stability And Maturity to Plant Growth

Assessment of compost maturity is important for successful use of composts in agricultural and horticultural production. We assessed the “maturity” of four different sawdust-based composts. We composted sawdust with either cannery waste (CW), duck manure (DM), dairy (heifer) manure (HM) or potato culls (PC) for approximately one year. Windrows were turned weekly for the first 60 days of composting, covered for four winter months and then turned monthly for six more months. We measured compost microbial respiration (CO₂ loss), total C and N, C:N ratio, water soluble NO₃-N and NH₄-N, dissolved organic carbon (DOC), pH and electrical conductivity at selected dates over 370 days. Compost effects on ryegrass biomass and N uptake were evaluated in a greenhouse study. We related compost variables to ryegrass growth and N uptake using regression analysis. All composts maintained high respiration rates during the first 60 days of composting. Ammonium-N concentrations declined within the first 60 days of composting, while NO₃-N concentrations did not increase until 200+ days. After 250+ days, DM and PC composts produced significantly more ryegrass biomass than either CW or HM composts. Total C, microbial respiration and water-extractable NO₃-N were good predictors of compost stability/maturity, or compost resistance to change, while dissolved organic carbon, C:N ratio and EC were not. The compost NO₃-N/CO₂-C ratio was calculated as a parameter reflecting the increase in net N mineralization and the decrease in respiration rate. At ratio values >8 mg NO₃-N/mg CO₂-C/day, ryegrass growth and N uptake were at their maximum for three of the four composts, suggesting the ratio has potential as a useful index of compost maturity.     

Composting Pulp and Paper Mill Sludge—Effect of Temperature and Nutrient Addition Method

Pulp and paper mill sludges (PMS) are a significant byproduct of the paper making industry world-wide, and composting with mineral nutrients in Tasmania is viewed as the most environmentally suitable method to convert this material into a horticultural product, thus eliminating the need for landfilling. The major control variables for composting PMS with a high C:N ratio are nutrient and temperature management. Addition of the nutrient requirement prior to composting can result in significant nutrient loss by leaching and may lead to ground water pollution. Alternatively, the nutrient requirement may be added incrementally during composting, thereby decreasing the risk of nutrient loss. Control of temperature is also important as this affects the metabolic activity of microorganisms and may determine the rate at which a cured compost can be produced. This study therefore examined the relationship between the method of nutrient addition and temperature on composting of PMS, using small-scale reactors designed to simulate conditions in a large-scale mechanically turned windrow. The rate of PMS decomposition as determined by the rate of CO₂ production and O₂, consumption was higher at 55°C than at 35°C. The time to produce a cured compost could be shortened by 30-50 days if composting was undertaken at the higher temperature. The method of nutrient addition had no effect on the respiratory activities of compost microbiota or rates of decomposition, but had a major influence on pH which determined the intensity and period of ammonia volatilization. If pH was controlled, then incremental nutrient addition could be advantageous from the perspective of nutrient conservation.     

EFFECTS OF GYPSUM ENHANCED COMPOSTS ON YIELDS AND MINERAL COMPOSITIONS OF BROCCOLI AND TALL FESCUE

Gypsum (CaSO₄·2H₂O) addition during composting of manure or biosolids can reduce ammonia nitrogen losses and represents a new method for controlling odors. Additional work is needed, however, to test the ability of the gypsum-containing composts to support plant growth and affect uptake of nutrients and heavy metals. A field study using broccoli (Brassica oleracea L. var. italica) and a growth chamber study using tall fescue (Festuca arundinacea) were conducted by application of composts at 10 Mg ha^?1 for broccoli and 10 and 25 Mg ha^?1 for tall fescue. Compared to composts without gypsum, at 10 Mg ha^?1, gypsum composts significantly increased or had a strong trend to increase yields of broccoli and tall fescue. Gypsum composts affected concentrations of nutrient elements but did not increase concentrations of environmental concern elements in broccoli flowers and tall fescue tissue. Thus gypsum composts can be safely applied to soils to enhance crop growth.     

Maturity and Stability Evaluation of Composted Yard Trimmings

The objective of this research was to evaluate a variety of stability and maturity indices for yard trimmings compost produced in the Puget Sound region of western Washington State. Compost samples were collected periodically during a 133-d composting cycle at a commercial composting facility, showing that indices of compost respiration rate were sensitive indicators of compost quality. All respiration rate indices identified a period of high respiration rates during active composting (first 27 d), and a period of relatively stable respiration rates during the latter part of curing (70 to 133 d). Chemical tests of compost solids showed less promise as maturity indicators, but provided valuable information on final compost quality. Mature yard trimmings compost had a C:N of 12, an NH₄-N to NO₃-N ratio of less than 4, a cation exchange capacity (CEC) of 400 cmol per kg of compost-C, and a pH between 6.5 and Seed germination tests and sensory tests (color and odor) were of limited value in assessing compost maturity. Fully-cured compost produced with forced aeration had a Solvita CO₂ test value of 6 to 7 and a respiration rate via the alkaline trap method of 2 mg CO₂-C g compost-C^?1 d^?1. It reheated less than 2°C in an insulated Dewar flask in a 7 d incubation. Further evaluation and calibration of respiration test protocols for compost quality assurance testing programs are recommended.     

Comparison of a gas detection tubes test with the traditional alkaline trap method to evaluate compost stability

Compost stability is an important parameter of compost quality. Among tests proposed to evaluate compost stability, microbial respiration is one of the better accepted tests. Variations in rates of CO₂ evolution during composting were studied in two pilot pruning waste piles using a windrow composting system. To measure the CO₂ production rate, two methods were compared: the alkaline trap test and gas detection tubes. Both respiration tests indicated increasing compost stability with processing time, but CO₂ evolution rates from the alkaline trap method were higher than values from the gas detection tube method. A first-order kinetic equation was used to describe CO₂ evolution over time. A linear relationship (r=0.81, p<0.01) was found between the two methods. Although both methods could distinguish unstable compost from stable compost, CO₂ detection tubes were easier to use and gave results in a shorter period of time.     

SE–Structures and Environment: Physical Properties of Composting Material: Gas Permeability,Oxygen Diffusion Coefficient and Thermal Conductivity

Composting is one of the oldest bio-technological processes used by human beings. It can be defined as the partial decomposition of heterogeneous organic matter by a mixed microbial population in a moist, warm and aerobic environment. In the organic matter, a dense population of various micro-organisms is found. The micro-organisms use organic matter, minerals, water and oxygen for their growth and metabolic activity. Each microbial species has an optimal temperature at which growth and multiplication rates are maximal. The oxygen concentration plays a dominant role in these processes. To optimize the composting process, the above-mentioned aspects must be known in detail. A simulation model offers an appropriate tool to reach this goal. Such a model must especially describe distributions of temperature and oxygen concentrations because these are considered as the most important process parameters. Reliable results can only be obtained if the physical properties of the composting material are well known under various conditions. In this paper, measurements of gas permeability, oxygen diffusivity, and thermal conductivity of the composting material are presented.Generally it is found that the gas permeability decreases as the gas velocity increases. For raw material, the gas permeability decreases with the wetness, whereas for older material there is no clear relationship. For composting material which has been turned once, the gas permeability is larger than for raw material. The oxygen diffusion coefficient is proportional to the gas-filled volume fraction to the power 1·5. There is no clear relationship between the oxygen diffusion coefficient and the age of the material. It is found that at a given temperature and for volume fractions of solid phase of 0·33 or less, the thermal conductivity increases linearly with the volume fraction of the liquid phase. The thermal conductivity is not influenced by the age of the composting material. The thermal conductivity increases with temperature.     

Variability of Atmospheric Ammonia In High-Rise,Caged Layer Composting

High concentrations of atmospheric ammonia (NH₃) can impact poultry and human health. During composting inside high-rise, caged layer facilities, high concentrations of NH₃ are produced due to low carbon to nitrogen ratios of composting materials and the confined building environment. This study characterized the spatial and temporal variability of NH₃ during in-house composting as a preliminary step to identifying control measures. Boric acid solutions and gas sensors were used to measure NH₃ in 2 m × 7.5 m grid patterns for three high-rise laying hen structures during composting. Spatial variability was evident in all buildings, with areas of higher NH₃ concentration near the center of buildings away from ventilation fans. Ammonia concentrations in the composting area frequently exceeded human health standards for 8-hour and 10-minute exposure periods of 25 and 35 μL L^?1, respectively. Ammonia concentrations were lower in cage areas of high-rise structures due to the negative pressure ventilation system venting gas directly from the composting area to the outside of buildings. Over a 6-week composting cycle, NH₃ generally increased as compost accumulated in the structure. Over 1-day periods of time, NH₃ concentrations varied with fluctuations in outdoor air temperatures and fan operation. During turning of compost, atmospheric NH₃ reached a high of nearly 50 μL L^?1 for over 30 minutes. Monitoring NH₃ and altering the ventilation of poultry houses could reduce NH₃ concentrations below critical levels at peak times such as during turning. However, ventilation as a solution to high NH₃ levels may not be environmentally sustainable. Other alternatives such as chemical and process controls, structural changes, or biofiltration should be explored to prevent NH₃ volatilization or remove NH₃ from air vented during in-house composting.     

Recycling of Two-Phase Olive-Mill Cake “Alperujo” by Co-composting with Animal Manures

The use of poultry manure or goat/sheep manure in the co-composting of the two-phase olive-mill cake “alperujo” (ALP) with olive leaf (OL) is compared by studying organic-matter mineralization and humification processes during composting and the characteristics of the end products. For this, two different piles (P1 and P2) were prepared using ALP with OL mixed with poultry manure (PM) and goat/sheep manure (GSM), respectively, and composted by the turned windrow composting system. Throughout the composting process, a number of parameters were monitored, such as temperature, pH, electrical conductivity (EC), organic matter (OM), OM losses, total organic carbon (C_org), total nitrogen (N_t), C_org/N_t ratio, and the germination index (GI). In both piles, the temperature exceeded 55 °C for more than 2 weeks, which ensured maximum pathogen reduction. Organic-matter losses followed a first-order kinetic equation in both piles. The final composts presented a stabilized OM and absence of phytotoxins, as observed in the evolution and final values of the C_org/N_t ratio (C_org/N_t < 20) and the germination index (GI > 50 percent). Therefore, composting can be considered as an efficient treatment to recycle this type of waste, obtaining composts with suitable properties that can be safely used in agriculture.     

Composting Hog Manure/Sawdust Mixtures Using Intermittent and Continuous Aeration: Ammonia Emissions

Odorous emissions from manures have become a significant problem. Preliminary work on composting hog manure with sawdust had indicated that intermittent aeration could reduce ammonia emissions during this process. This paper presents results from four additional runs with a total of 22 pilot-scale vessels that have confirmed that ammonia emissions are affected by aeration. The pilot-scale vessels consisted of insulated, stainless steel, 205 L drums that either received continuous (high/low rate, thermostatically controlled blowers) or intermittent (5 min on high rate, 55 min off) aeration. Ammonia emissions, air flow rates, carbon dioxide production, oxygen utilization, and temperatures at four locations in each vessel were monitored. Ammonia emissions under intermittent aeration were roughly 50% less than those from the continuously aerated vessels. However, this appeared to result more from total air flow than from the aeration technique used. A linear regression of emissions versus total air flow data for all vessels yielded a fit of y = 0.1309x + 29.385 (y being total ammonia emitted [in g of N] and x being total air flow [in kg]) with an R² = 0.6808. Since air flow termination was relatively arbitrary, this only means basically, that ammonia emissions were doubled for a quadrupling of air flow. Under intermittent aeration, the minimum oxygen level in the exhaust air occasionally dropped to as low as 1%. So the aeration pattern used probably represents the lowest one suitable for maintaining aerobic conditions. Within this constraint, however, lower air flow appears to be suitable for reducing odorous ammonia emissions.     

Towards an agronomic assessment of N2O emissions: a case study for arable crops

Agricultural soils are the main anthropogenic source of nitrous oxide (N₂O), largely because of nitrogen (N) fertilizer use. Commonly, N₂O emissions are expressed as a function of N application rate. This suggests that smaller fertilizer applications always lead to smaller N₂O emissions. Here we argue that, because of global demand for agricultural products, agronomic conditions should be included when assessing N₂O emissions. Expressing N₂O emissions in relation to crop productivity (expressed as above‐ground N uptake: ‘yield‐scaled N₂O emissions') can express the N₂O efficiency of a cropping system. We show how conventional relationships between N application rate, N uptake and N₂O emissions can result in minimal yield‐scaled N₂O emissions at intermediate fertilizer‐N rates. Key findings of a meta‐analysis on yield‐scaled N₂O emissions by non‐leguminous annual crops (19 independent studies and 147 data points) revealed that yield‐scaled N₂O emissions were smallest (8.4 g N₂O‐N kg⁻¹N uptake) at application rates of approximately 180–190 kg N ha⁻¹ and increased sharply after that (26.8 g N₂O‐N kg⁻¹ N uptake at 301 kg N ha⁻¹). If the above‐ground N surplus was equal to or smaller than zero, yield‐scaled N₂O emissions remained stable and relatively small. At an N surplus of 90 kg N ha⁻¹ yield‐scaled emissions increased threefold. Furthermore, a negative relation between N use efficiency and yield‐scaled N₂O emissions was found. Therefore, we argue that agricultural management practices to reduce N₂O emissions should focus on optimizing fertilizer‐N use efficiency under median rates of N input, rather than on minimizing N application rates.     

Effect of lignite humic acid treatment on the rate of decomposition of wheat straw

Comparisons were made between the rates of microbial respiration during the incubation of milled wheat straw in the presence or absence of a dispersal of lignite humic acids. Lignite treatment significantly reduced both the rate of O₂ consumption and CO₂ evolution from the straw substrate over a 4-week period. In view of this observed inhibitory effect, additions of exogenous humic acids from lignite during composting might have a practical application as a means of increasing the microbial stability of horticultural composts derived from plant waste materials.