Effect of the addition of municipal solid-waste compost on microbial biomass and enzyme activities in soil

Summary Changes in the content of C, N, P, and S in the soil biomass and in phosphatase, urease, protease, deaminase, and arylsulphatase activity, induced by amendment with municipal solid-waste compost, were determined in a clay loam soil during 1 year of incubation at 25° and 35°C.In the unenriched soil (control) decreasing trends in biomass C, biomass N, and biomass S were observed at both temperatures. In the enriched soil, these values increased, reaching a maximum after 1 month. Biomass P, probably due to a slower process of P immobilization, showed different trends. Alkaline phosphomonoesterase, phosphodiesterase, and deaminase activity remained constant after reaching maximum values (3–5 months). Arylsulphatase, urease, and protease activity tended to return to baseline after reaching a maximum (2–3 months).Atrazine, though applied at a dose that was 10 times higher than the recommended field rate, did not modify the chemical and biochemical properties of either the control or the enriched soil.Significant positive and negative correlations between changes in biomass values and changes in enzyme activity were found. The negative correlations are attributed to the delay in the enzymatic response compared with the changes in microbial biomass.     

Effects of municipal solid waste compost amendments on soil enzyme activities and bacterial genetic diversity

Municipal solid waste (MSW) composts have been used to maintain the long-term productivity of agroecosystems and to protect the soil environment from overcropping, changes in climatic conditions and inadequate management; they also have the additional benefit of reducing waste disposal costs. Since MSW may contain heavy metals and other toxic compounds, amendments cannot only influence soil fertility, but may also affect the composition and activity of soil microorganisms. The effects of MSW compost and mineral N amendments in a 6-year field trial on some physical-chemical properties, enzyme activities and bacterial genetic diversity of cropped plots (Beta vulgaris-Triticum turgidum rotation) and uncropped plots were investigated. The compost was added at the recommended and twice the recommended dosage (12, 24 t ha⁻¹). Amendments of cropped plots with MSW compost increased the contents of organic C from 13.3 to 15.0 g kg⁻¹ soil and total N from 1.55 to 1.65 g kg⁻¹ soil. There were significant increases in dehydrogenase (9.6%), β-glucosidase (13.5%), urease (15.4%), nitrate reductase (21.4%) and phosphatase (9.7%) activities. A significant reduction in protease activity (from 3.6 to 2.8 U g⁻¹ soil) was measured when a double dose of compost was added to the cropped plots. No dosage effect was detected for the other enzymes. Changes in the microbial community, as a consequence of MSW amendment, were minimal as determined using denaturing gradient gel electrophoresis, rDNA internal spacer analysis and amplified ribosomal DNA restriction analysis of bacteria, archaea, actinomycetes, and ammonia oxidizers. This indicates that there was no significant variation in the overall bacterial communities nor in selected taxonomic groups deemed to be essential for soil fertility.     

Carbon and nitrogen mineralization of non-composted and composted municipal solid waste in sandy soils

A sterilized, but undecomposed, organic by-product of municipal waste processing was incubated in sandy soils to compare C and N mineralization with mature municipal waste compost. Waste products were added to two soils at rates of 17.9, 35.8, 71.6, and dry weight and incubated at for 90 d. Every 30 d, nitrate and ammonium concentrations were analyzed and C mineralization was measured as total CO₂-C evolved and added total organic C. Carbon mineralization of the undecomposed waste decreased over time, was directly related to application rate and soil nutrient status, and was significantly higher than C mineralization of the compost, in which C evolution was relatively unaffected across time, soils, and application rates. Carbon mineralization, measured as percentage C added by the wastes, also indicated no differences between composted waste treatments. However, mineralization as a percentage of C added in the undecomposed waste treatments was inversely related to application rate in the more productive soil, and no rate differences were observed in the highly degraded soil. Total inorganic N concentrations were much higher in the compost- and un-amended soils than in undecomposed waste treatments. Significant N immobilization occurred in all undecomposed waste treatments. Because C mineralization of the undecomposed waste was dependant on soil nutrient status and led to significant immobilization of N, this material appears to be best suited for highly degraded soils low in organic matter where restoration of vegetation adapted to nutrient poor soils is desired.     

城市固体废弃物堆肥与化肥对不同土壤植物生长的影响研究

温室盆栽试验研究城市固体废弃物堆肥与化肥对不同土壤黑麦草生长的影响,结果表明,堆肥和化肥可明显增加黑麦草干物质产量,阳春和大安2种土壤处理C50干物质产量分别比对照增加39.53%和109.38%,而NPK处理则分别增产267.44%和406.25%。堆肥与化肥配施处理(NPK C25对阳春和NPK C50对大安)产量最高。堆肥处理明显增加土壤pH、有机碳、土壤有效态磷、钾、铁、锰、锌和铜含量。     

Soil restoration in semiarid Patagonia: Chemical and biological response to different compost quality

Restoration of soils burned by a wildfire using composted amendments of different origin (biosolids and municipal organic wastes) and final particle size (screened and unscreened) was studied after 6 and 12 months of application in a field trial in semiarid NW Patagonia. Composts were applied at 40 Mg ha⁻¹. A fertilized treatment with soluble N (100 kg ha⁻¹) and P (35 kg ha⁻¹), and a non-treated control were also included. As indicators of soil response, chemical (electrical conductivity, pH, organic C, total N, extractable P), biological (potential microbial respiration, potential net N mineralization, N retained in microbial biomass) and physical (temperature and soil moisture) properties were evaluated. Plant soil cover was also estimated. Soil chemical and biological properties showed a high response to organic amendment addition, more evident after the wet season (12 months of application). Soil organic C, total N and extractable P increased significantly with biosolids composts (BC), and soil pH with municipal composts (MC). Potential microbial C respiration and net N mineralization were similar for both MC and BC, and significantly higher than in the control and the inorganic fertilized treatment; when calculated on C or N basis the highest values corresponded to MC. Results imply that in terms of organic C accretion, BC were more effective than MC due to higher amounts of total and recalcitrant C. Screened and unscreened composts did not differ significantly in their effects on soil properties. The increase of organic C with BC did not contribute to increase soil moisture, which was even higher in control plots after the wet season; higher plant cover and water consumption in amended plots could also explain this pattern. Inorganic fertilization enhanced higher plant cover than organic amendments, but did not contribute to soil restoration.     

Effect of composting on extractability and relative availability of heavy metals present in Calcutta municipal solid waste

How composting affects heavy metal content is largely unknown. Accordingly, we investigate the total content of six heavy metals, Cd/Zn ratio and the Zn-equiv, the relative availability and fractionation study to assess the suitability of compost derived out of those for environmental concerns. During composting, total metal content increased but their RA decreased. As a result of composting bioavailable form of metals also decreased. High significant correlations between different forms of heavy metals content and degree of humification were found for all the elements. Composting increases humic acid content than fulvic acid. This transformation is mainly responsible to serve as binding agent for metal thereby moderating the rapid metal mobilization.     

Effect of enriched municipal solid waste compost application on soil available macronutrients in the rice field

Abstract

A study was conducted in the Department of Environmental Sciences, Tamil Nadu Agricultural University, Coimbatore, India, to transform the normal compost into bioactive compost, which has multiple benefits to the crop system. The key players in this transformation process were Azotobacter sp., Pseudomonas sp., Phosphobacteria sp. and the waste materials like poultry litter and spent wash. This enrichment process increases both the quality and nutrient content of the municipal solid waste compost significantly. A study was carried out to evaluate the effect of application of different levels of enriched municipal solid waste compost on the availability of the macronutrient content to the rice field soil. The effect of enriched compost on soil available nutrients was significant. The soil ammonium nitrogen and soil nitrate nitrogen content was found to be high in the plots where the enriched compost was applied along with inorganic fertilizer with the values of 38.87 mg kg^?1 and 32.87 mg kg^?1, respectively. In addition, the availability decreased towards crop growth. The soil available P and K were also increased with enriched compost application to about 22.46 kg ha^?1 and 647 kg ha^?1 compared with control values of 19.44 kg ha^?1 and 518 kg ha^?1, respectively. Both phosphorus and potassium content decreased towards advancement of crop growth.     

Effects of municipal solid waste compost and sewage sludge on mineralization of soil organic matter

This work sets out to verify whether the application of municipal solid waste compost (MSWC) or treated urban sewage sludge (USS) organic amendments efficiently promote organic matter (OM) increases in a Haplic Podzol (PZha) and in a Calcic Vertisol (VRcc). For that purpose, carbon (C) mineralization and C kinetic parameters were studied, using a laboratory experimental incubation setup. The results showed that the addition of the amendments to the soils increased their mineralization capacities, and that the highest C mineralization rate was reached at the end of the first 2 d of incubation. The different characteristics of the soils seem to have influenced the C mineralization rates during the 28-d incubation. The USS induced higher C mineralization than the MSWC, and the PZha soil gave rise to higher C mineralization than VRcc. For all treatments, C mineralization adjusted well to an exponential plus linear kinetic model, suggesting that the organic C of the amendments was made up of two organic pools of differing degrees of stability. With the exception of the application of USS 60 t ha⁻¹, all the treatments increased the OM content on both soils, or at least the OM remained constant throughout the incubation.     

Effect of timing of application of municipal solid waste compost on N availability for crops in central Spain

To calculate the correct nitrogen fertilizer rate for crops and the possibility of using municipal solid waste (MSW) compost as an organic amendment, nitrogen mineralization rates were studied by laboratory incubation and field measurements in a soil in central Spain. Nitrogen mineralization rates were studied in a 250-day laboratory soil incubation with two treatments: with and without compost, incubated at 28°C and a moisture content of 70% of field capacity. Three phases are described: (1) no increase in the mineral nitrogen content, (2) a linear increase in the mineral N fraction and, finally, (3) a linear, parallel increase in both mineral N and easily mineralizable organic N fractions. Incubation data were fitted to three different equations. The exponential model proposed by Stanford and Smith (1972) was selected to predict field N mineralization rates. The field experiment was performed using a crop of maize with three treatments: compost applied in February (before sowing), compost applied during sowing and a control (without compost application): sampling was carried out over 14 months. Soil water content was measured periodically. Soil with compost applied in February showed 1.9 and 1.4 times more available nitrogen than soil without compost and compost at sowing, respectively, for the month of maximum accumulation. These results suggest that compost amendments must be applied before sowing. Compost applications were shown to supply the available nitrogen for spring crops. A simulation model showed satisfactory agreement with field data, after correction for soil temperature and water content. Received: 22 July 1996     

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.     

Microbial biomass turnover and enzyme activities following the application of farmyard manure to field soils with and without previous long-term applications

Summary We studied the build-up and turnover of microbial biomass following the addition of farmyard manure to an unmanured soil and to soils from a long-term experiment in which different levels of farmyard manure had been applied for the last 23 years. The application of farmyard manure at 15–90 t ha^-1 to previously unmanured soil increased the microbial biomass during the first 3 months of incubation but a gradual decline occurred with further incubation for up to 12 months. Microbial biomass C was positively correlated with soil organic C and ranged from 1.8% to 2.2% of organic C after 12 months of farmyard manure applications. Biomass turnover increased with the application of farmyard manure, ranging from 0.81 to 0.87 year^-1 with various levels of manure. Amendment of soils from the long-term manure experiment with various levels of farmyard manure led to a build-up and decline in biomass C as seen in the unmanured soils, but biomass C was higher in all treatments compared to the corresponding unmanured soil treatments. Biomass turnover was greater compared to the unmanured soil treatments and it decreased with increasing levels of farmyard manure. The average soil respiratory activity increased with increasing levels of farmyard manure, but respiratory activity per unit of biomass C decreased with increasing levels of manure. Enzyme activities were greater in long-term manured soils compared to unmanured soils amended with various levels of manure. There was a significant correlation between biomass C and enzyme activities.     

Agricultural and ecological aspects of a sandy soil as affected by the application of municipal solid waste composts

We present the results of a plot experiment in which the changes in physical, chemical and physico-chemical properties of a sandy soil were examined after amending the soil with two different composts produced from municipal solid wastes. Triticale (X Triticosecale), cultivated in a 3-y monoculture, was used as a test plant. Both composts differed in their concentrations of heavy metals. Composts were applied non-recurrently in the spring before sowing, at the rates of 18, 36, and 72 t dry matter ha⁻¹. The plots without fertilization, and those fertilized annually with mineral nitrogen (N), phosphorous (P), and potassium (K) were used as controls. Soil samples were collected 1 month after compost application, as well as each year after harvesting. Application of both composts improved soil physical properties, associated with increasing content of organic carbon (OC). Statistically significant increases of total porosity, field water capacity and amounts of plant-available water were found only in the short time after compost application. Despite the fact that soil OC content decreased with time, a C:N ratio clearly increased in the third year after compost application, which was explained by a depletion of N reserve. Both composts caused a large increase of plant-available P, K, and magnesium (Mg), which was observed during the entire period of the experiment. Beneficial changes were also observed in soil humic substances composition. These were confirmed by increased humic acids content and humic/fulvic acid ratios. Soil cation exchange capacity and base saturation increased in all plots amended with composts. This effect was still observed 1 year after compost application, while in the third year it remained significant only at the highest compost rates. Compost originating from industrial areas, even if applied in low amounts, caused a significant increase in total concentration of soil heavy metals. This fact did not result, however, in any substantial changes in soil quality with regard to heavy metals content.     

Organic matter, microbial biomass and enzyme activity of soils under different crop rotations in the tropics

Soil organic matter level, soil microbial biomass C, ninhydrin-N, C mineralization, and dehydrogenase and alkaline phosphatase activity were studied in soils under different crop rotations for 6 years. Inclusion of a green manure crop of Sesbania aculeata in the rotation improved soil organic matter status and led to an increase in soil microbial biomass, soil enzyme activity and soil respiratory activity. Microbial biomass C increased from 192 mg kg^–1 soil in a pearl millet-wheat-fallow rotation to 256 mg kg^–1 soil in a pearl millet-wheat-green manure rotation. Inclusion of an oilseed crop such as sunflower or mustard led to a decrease in soil microbial biomass, C mineralization and soil enzyme activity. There was a good correlation between microbial biomass C, ninhydrin-N and dehydrogenase activity. The alkaline phosphatase activity of the soil under different crop rotations was little affected. The results indicate the green manuring improved the organic matter status of the soil and soil microbial activity vital for the nutrient turnover and long-term productivity of the soil. Received: 7 January 1996     

Lasting microbiological and biochemical effects of the addition of municipal solid waste to an arid soil

 This paper reports the effect of the addition of the organic fraction of municipal solid waste at two different rates on the microbiological and biochemical properties of an arid soil after 8 years. The vegetation that appeared spontaneously just after the amendment was still present 8 years later. The organic matter fractions were higher in the amended soil than in the control soil. Amended soil showed higher values of microbial biomass C, soil basal respiration and dehydrogenase activity than control soil, which reached values near to those of the natural soils in the area. The organic amendment had a positive effect on the activity of enzymes related with C, N, P cycles, particularly when the amendment was at the highest dose. This effect could be also observed on the activity of extracted enzymes. The results indicated that the addition of urban waste could be a suitable technique with which to restore soil quality. Received: 3 July 1998     

Dissolved organic carbon (DOC) as a parameter of compost maturity

Despite numerous investigations of the maturation process of composts, a simple and straightforward parameter which can predict plant response upon compost application has yet to be defined. In light of results accumulated over a decade, we examined simple, chemical parameters of three composts from three types of source materials (municipal solid waste (MSW), separated cow manure (CSM), biosolids (BS)). These materials were composted using different procedures and facilities. The chemical parameters were correlated to the growth response of cucumbers or ryegrass sown in potting media amended with the composts sampled at different stages of the process. The dissolved organic carbon (DOC) concentration of all composts decreased rapidly within the first month, then, towards the end of the process, stabilized at concentration below 4 g kg⁻¹. DOC correlated highly and significantly to the absorbance at 465 nm in all composts, and also to the C/N ratio. Nitrate evolution was similar in all composts, but the final concentrations differed among them. Plant biomass increased with composting time. For CSM and BS compost maximum biomass was reached when the DOC reached levels below 4 g kg⁻¹. DOC concentration is suggested for use as a simple method of determining maturity, with 4 g kg⁻¹ recommended as a threshold level indicating maturity. Absorbance at 465 nm can be used instead of DOC concentration after appropriate calibration.     

Soil microbial biomass, dehydrogenase activity, bacterial community structure in response to long-term fertilizer management

This study describes the effects of balanced versus nutrient-deficiency fertilization on soil microbial biomass, activity, and bacterial community structure in a long-term (16 years) field experiment. Long-term fertilization greatly increased soil microbial biomass C and dehydrogenase activity, except that the P-deficiency fertilization had no significant effect. Organic manure had a significantly greater (P<0.05) impact on the biomass C and the activity, compared with mineral fertilizers. Microbial metabolic activity (dehydrogenase activity per microbial biomass C) was significantly higher (P<0.05) under balanced fertilization than under nutrient-deficiency fertilization. General bacterial community structure was analyzed by PCR-denaturing gradient gel electrophoresis (DGGE) targeting eubacterial 16S rRNA gene. Mineral fertilization did not affect the DGGE banding pattern, while specific DGGE band was observed in organic manure-fertilized soils. Phylogenetic analysis showed that the change of bacterial community in organic manure-fertilized soil might not be because of the direct influence of the bacteria in the compost, but because of the promoting effect of the compost on the growth of an indigenous Bacillus sp. in the soil. We emphasize the importance of balanced-fertilization, as well as the role of P, in maintaining soil organic matter, and promoting the biomass and activity of microorganisms.     

Soil microbial biomass and nitrogen dynamics in a turfgrass chronosequence: A short-term response to turfgrass clipping addition

A mechanistic understanding of soil microbial biomass and N dynamics following turfgrass clipping addition is central to understanding turfgrass ecology. New leaves represent a strong sink for soil and fertilizer N, and when mowed, a significant addition to soil organic N. Understanding the mineralization dynamics of clipping N should help in developing strategies to minimize N losses via leaching and denitrification. We characterized soil microbial biomass and N mineralization and immobilization turnover in response to clipping addition in a turfgrass chronosequence (i.e. 3, 8, 25, and 97 yr old) and the adjacent native pines. Our objectives were (1) to evaluate the impacts of indigenous soil and microbial attributes associated with turf age and land use on the early phase decomposition of turfgrass clippings and (2) to estimate mineralization dynamics of turfgrass clippings and subsequent effects on N mineralization of indigenous soils. We conducted a 28-d laboratory incubation to determine short-term dynamics of soil microbial biomass, C decomposition, N mineralization and nitrification after soil incorporation of turfgrass clippings. Gross rates of N mineralization and immobilization were estimated with ¹⁵N using a numerical model, FLAUZ. Turfgrass clippings decomposed rapidly; decomposition and mineralization equivalent to 20-30% of clipping C and N, respectively, occurred during the incubation. Turfgrass age had little effect on decomposition and net N mineralization. However, the response of potential nitrification to clipping addition was age dependent. In young turfgrass systems having low rates, potential nitrification increased significantly with clipping addition. In contrast, old turfgrass systems having high initial rates of potential nitrification were unaffected by clipping addition. Isotope ¹⁵N modeling showed that gross N mineralization following clipping addition was not affected by turf age but differed between turfgrass and the adjacent native pines. The flush of mineralized N following clipping addition was derived predominantly from the clippings rather than soil organic N. Our data indicate that the response of soil microbial biomass and N mineralization and immobilization to clipping addition was essentially independent of indigenous soil and microbial attributes. Further, increases in microbial biomass and activity following clipping addition did not stimulate the mineralization of indigenous soil organic N.     

Effects of conventional tillage on biochemical properties of soils

Modification of soil environment by different farming practices can significantly affect crop growth. Tillage causes soil disturbance, altering the vertical distribution of soil organic matter and plant nutrient supplies in the soil surface, and it may affect the enzyme activity and microbial biomass which are responsible for transformation and cycling of organic matter and plant nutrients. In this study, the influence of three conventional tillage systems (shallow plowing, deep plowing and scarification) at different depths on the distribution and activity of enzymes, microbial biomass and nucleic acids in a cropped soil was investigated. Analysis of variance for depth and tillage showed the influence of the different tillage practices on the activity of some enzymes and on the nucleic acids. Glucosidase, galactosidase, nitrate reductase and dehydrogenase activity were significantly affected by the three tillage modalities. Activity in the upper layer (0–20 cm) was higher in the plots tilled by shallow plowing and scarification than in those tilled by deep plowing. Positive relationships were observed between the soil enzymes themselves, with the exception of urease and pyrophosphatase activity. Moreover, significant correlations were found between DNA and β-galactosidase, and between RNA and β-glucosidase, β-galactosidase, alkaline phosphatase and phosphodiesterase. α-Glucosidase, β-galactosidase, alkaline phosphatase and phosphodiesterase were highly correlated with biomass C determined by the fumigation-extraction method. Received: 27 June 1996     

Response of an agricultural soil to pentachlorophenol (PCP) contamination and the addition of compost or dissolved organic matter

The response of a fresh, agricultural soil when contaminated with pentachlorophenol (PCP) and supplemented with compost (C) or dissolved organic matter (DOM) was studied in the laboratory. The concentration of PCP and the changes in various functionally related properties (i.e. microbial biomass, basal respiration, soil hydrolase and oxidoreductase activity) were measured over 150 d. Variations in the main physical and chemical properties of the soils were also monitored. Two different doses of compost (C1 = 0.27% and C2 = 0.83%, corresponding to 10 and 30 t ha⁻¹, respectively) or DOM (D1 = 0.07% and D2 = 0.2%) equivalent to the carbon content of the two compost doses C1 and C2 were used and the following five systems were investigated: soil (S), soil–compost (S-C1 and S-C2) and soil–DOM (S-D1 and S-D2). PCP concentrations declined progressively and significantly with time. This effect was most pronounced for the soils amended with the lower compost dose C1 (S-C1) and with the two DOM (S-D1 and S-D2) amounts. Significantly reduced amounts of PCP were extracted after its 500-d residence in the various systems. Higher amounts of the residual PCP were extracted from the humic acids (HA), fulvic acids (FA) and humin–mineral (HU) fractions of the 500 d aged samples than from the same unfractionated samples, indicating that the residual PCP preferentially accumulated in the organic fractions of soil. The soil showed an endogenous microbial activity as indicated by basal respiration, microbial biomass and all the enzymatic activities tested (dehydrogenase, glucosidase, phosphatase, arylsulphatase and urease). Addition of the PCP severely depressed some of the tested biochemical properties suggesting an inhibitory effect on microbial activity. Conversely, higher basal respiration, and similar β-glucosidase and phosphatase activities were measured in comparison with the controls. No significant effects were observed following the addition of two doses of the compost or the DOM. Fungal colonies belonging to the taxonomic group of Ascomycetes and identified as Byssochlamys fulva developed with time in all the PCP-contaminated samples. Growth of B. fulva in vitro in the presence of PCP showed that the isolate was tolerant to 12.5 and 25 mg l⁻¹ PCP and degraded 20% of its initial concentration in 8 d. Overall, the results indicate that many complex processes occurred in the contaminated soil and combinations of these determined the response to PCP contamination. The sorption of PCP to the soil matrix (which increased with time) and its degradation/transformation by indigenous soil microbial activity were likely involved. Both the processes appeared to be favoured by the presence of dissolved organic matter.     

Microbial response to the addition of glucose in low-fertility soils

Addition of soluble organic substrates to soil has been shown to either increase or restrict the rate of microbial CO₂–C evolution. This has been attributed to a priming effect resulting from accelerated or decreased turnover of the soil organic matter including the soil microflora. We investigated microbial responses to small glucose-C additions (10–50 μg C g¹ soil) in arable soils either amended or not with cellulose. An immediate CO₂–C release between 0 and 69 h (equivalent to 59% of glucose-C applied) was measured. However, only half of the CO₂–C respired could be attributed to the utilisation of glucose-C substrate, based on the percentage of ¹⁴C–CO₂ evolved after the addition of a ¹⁴C-labelled glucose tracer. Thus, although no evidence of an immediate release of ‘extra’ C above the rate applied as glucose-C was observed, the pattern of decomposition for ¹⁴C-glucose suggested utilisation of an alternate C source. Based on this, a positive priming effect (1.5 to 4.3 times the amount CO₂–C evolved that was attributed to glucose-C decomposition) was observed for at least 170 h in non-cellulose-amended soil and 612 h in cellulose-amended soil. Two further phases of microbial activity in cellulose-amended soils were attributed to either activation of different microbial populations or end-product inhibition of cellulase activity after glucose addition. During these subsequent phases, a negative priming effect of between 0.1 and 1.5 times was observed. Findings indicate that the response of the microbial community to small additions of soluble organic C substrate is not consistent and support the premise that microbial response varies in a yet to be predicted manner between soil type and ecosystems. We hypothesise that this is due to differences in the microbial community structure activated by the addition of organic C and the timing of soluble organic substrate addition with respect to the current dissolved organic C status of the soil.