Soil aggregate stabilization by a saprophytic lignin-decomposing basidiomycete fungus I. Microbiological aspects

 We studied the effects of a saprophytic lignin-decomposing basidiomycete isolated from plant litter on soil aggregation and stabilization. The basidiomycete produced large quantities of extracellular materials that bind soil particles into aggregates. These binding agents are water-insoluble and heat-resistant. Water stability of aggregates amended with the fungus and the degrees of biodegradation of the binding agents by native soil microorganisms were determined by the wet-sieving method. The data demonstrated that aggregates supplemented with a source of C (millet or lentil straw) were much more water-stable and resisted microbial decomposition longer than when they were prepared with fungal homogenates alone. Moreover, retrieval of fungal-amended aggregates supplemented with millet during the first 4 weeks of incubation in natural soil exhibited more large aggregate fractions (>2 mm) than the ones supplemented with lentil straw. The possible relationship of the role of basidiomycetes in litter decomposition and soil aggregation is discussed. Received: 27 September 1999     

Influence of soil compaction on carbon and nitrogen mineralization of soil organic matter and crop residues

 We studied the influence of soil compaction in a loamy sand soil on C and N mineralization and nitrification of soil organic matter and added crop residues. Samples of unamended soil, and soil amended with leek residues, at six bulk densities ranging from 1.2 to 1.6 Mg m^–3 and 75% field capacity, were incubated. In the unamended soil, bulk density within the range studied did not influence any measure of microbial activity significantly. A small (but insignificant) decrease in nitrification rate at the highest bulk density was the only evidence for possible effects of compaction on microbial activity. In the amended soil the amounts of mineralized N at the end of the incubation were equal at all bulk densities, but first-order N mineralization rates tended to increase with increasing compaction, although the increase was not significant. Nitrification in the amended soils was more affected by compaction, and NO₃ ^–-N contents after 3 weeks of incubation at bulk densities of 1.5 and 1.6 Mg m^–3 were significantly lower (by about 8% and 16% of total added N, respectively), than those of the less compacted treatments. The C mineralization rate was strongly depressed at a bulk density of 1.6 Mg m^–3, compared with the other treatments. The depression of C mineralization in compacted soils can lead to higher organic matter accumulation. Since N mineralization was not affected by compaction (within the range used here) the accumulated organic matter would have had higher C : N ratios than in the uncompacted soils, and hence would have been of a lower quality. In general, increasing soil compaction in this soil, starting at a bulk density of 1.5 Mg m^–3, will affect some microbially driven processes. Received: 10 June 1999     

Effects of organic and inorganic fertilization on soil bacterial and fungal microbial diversity in the Kabete long-term trial,Kenya

The effects of crop manure and inorganic fertilizers on composition of microbial communities of central high land soils of Kenya are poorly known. For this reason, we have carried out a thirty-two-year-old long-term trial in Kabete, Kenya. These soils were treated with organic (maize stover (MS) at 10 t ha⁻¹, farmyard manure (FYM) at 10 t ha⁻¹) and inorganic fertilizers 120 kg N, 52.8 kg P (N₂P₂), N₂P₂ + MS, N₂P₂ + FYM, a control, and a fallow for over 30 years. We examined 16S rRNA gene and 28S rRNA gene fingerprints of bacterial and fungal diversity by PCR amplification and denaturing gradient gel electrophoresis separation, respectively. The PCR bacterial community structure and diversity were negatively affected by N₂P₂ and were more closely related to the bacterial structure in the soils without any addition (control) than that of soils with a combination of inorganic and organic or inorganic fertilizers alone. The effect on fungal diversity by N₂P₂ was different than the effect on bacterial diversity since the fungal diversity was similar to that of the N₂P₂ + FYM and N₂P₂ + MS-treated. However, soils treated with organic inputs clustered away from soils amended with inorganic inputs. Organic inputs had a positive effect on both bacterial and fungal diversity with or without chemical fertilizers. Results from this study suggested that total diversity of bacterial and fungal communities was closely related to agro-ecosystem management practices and may partially explain the yield differences observed between the different treatments.     

Evaluation of rice–legume–rice cropping system on grain yield,nutrient uptake,nitrogen fixation,and chemical,physical, and biological properties of soil

To achieve higher yields and better soil quality under rice–legume–rice (RLR) rotation in a rainfed production system, we formulated integrated nutrient management (INM) comprised of Azospirillum (Azo), Rhizobium (Rh), and phosphate-solubilizing bacteria (PSB) with phosphate rock (PR), compost, and muriate of potash (MOP). Performance of bacterial bioinoculants was evaluated by determining grain yield, nitrogenase activity, uptake and balance of N, P, and Zn, changes in water stability and distribution of soil aggregates, soil organic C and pH, fungal/bacterial biomass C ratio, casting activities of earthworms, and bacterial community composition using denaturing gradient gel electrophoresis (DGGE) fingerprinting. The performance comparison was made against the prevailing farmers’ nutrient management practices [N/P₂O₅/K₂O at 40:20:20 kg ha⁻¹ for rice and 20:30:20 kg ha⁻¹ for legume as urea/single super-phosphate/MOP (urea/SSP/MOP)]. Cumulative grain yields of crops increased by 7–16% per RLR rotation and removal of N and P by six crops of 2 years rotation increased significantly (P < 0.05) in bacterial bioinoculants-based INM plots over that in compost alone or urea/SSP/MOP plots. Apparent loss of soil total N and P at 0–15 cm soil depth was minimum and apparent N gain at 15–30 cm depth was maximum in Azo/Rh plus PSB dual INM plots. Zinc uptake by rice crop and diethylenetriaminepentaacetate-extractable Zn content in soil increased significantly (P < 0.05) in bacterial bioinoculants-based INM plots compared to other nutrient management plots. Total organic C content in soil declined at 0–15 cm depth and increased at 15–30 cm depth in all nutrient management plots after a 2-year crop cycle; however, bacterial bioinoculants-based INM plots showed minimum loss and maximum gain of total organic C content in the corresponding soil depths. Water-stable aggregation and distribution of soil aggregates in 53–250- and 250–2,000 μm classes increased significantly (P < 0.05) in bacterial bioinoculants-based INM plots compared to other nutrient management plots. Fungal/bacterial biomass C ratio seems to be a more reliable indicator of C and N dynamics in acidic soils than total microbial biomass C. Compost alone or Azo/Rh plus PSB dual INM plots showed significantly (P < 0.05) higher numbers of earthworms’ casts compared to urea/SSP/MOP alone and bacterial bioinoculants with urea or SSP-applied plots. Hierarchical cluster analysis based on similarity matrix of DGGE profiles revealed changes in bacterial community composition in soils due to differences in nutrient management, and these changes were seen to occur according to the states of C and N dynamics in acidic soil under RLR rotation.     

Structural and functional response of soil microbiota to addition of plant substrate are moderated by soil Cu levels

In soils, the microbially mediated decomposition of plant residue is a key process with wide ranging effects on ecosystem functioning and stability. Understanding the impact of contamination on this process is of high importance. We investigated the effects of long-term (6 years) copper exposure on the capacity of soil microbiota to decompose newly added resources; dried and ground Medicago truncatula stubble. In addition, the effects on the microbial community structure across the three domains were explored using polymerase chain reaction–denaturing gradient gel electrophoresis rRNA gene profiling. Ecological distances in community structure between treatments was calculated (Kulczynski) and effects tested using PERMANOVA. Clear dose–response relationships were present between microbial respiration (CO₂ evolution) and soil Cu level in soils receiving medic, but not under basal conditions (i.e., no medic added). These show that relatively labile forms of C are needed to drive microbial ecotoxicological responses and that microbial adaptation to the presence of Cu in the soils—after >6 years exposure—was functionally limited. Bacterial, archaeal and fungal communities showed significant (P < 0.05) levels of structural change in soils across the Cu gradient, demonstrating that species replacement had occurred following strong selective pressure. Addition of medic resources to the soils caused significant shifts in the bacterial and archaeal community structure (P < 0.001), which occurred across the entire range of soil Cu levels. For the fungal community, a significant interaction effect was present between Cu and medic addition (P = 0.002). At low Cu levels, medic addition caused large shifts in community structure, but this was negligible under high Cu levels. This was reflected in significant changes in the level of community structural dispersion at low compared with high Cu levels. As such, we show that Cu limits the capacity of soil fungal communities to rapidly respond to new resource capture. Given the primary role of soil fungi in plant material decomposition, this may have wide ranging impacts on wider ecosystem processes including nutrient cycling, trophic interactions, food web stability and energy transfer.     

Biological activity and organic matter mineralization of soils amended with biowaste composts

This study aims to elucidate the significance of compost and soil characteristics for the biological activity of compost‐amended soils. Two agricultural soils (Ap horizon, loamy arable Orthic Luvisol and Ah horizon, sandy meadow Dystric Cambisol) and a humus‐free sandy mineral substrate were amended with two biowaste composts of different maturity in a controlled microcosm system for 18 months at 5 °C and 14 °C, respectively. Compost application increased the organic matter mineralization, the C_mic : C_org ratio, and the metabolic quotients significantly in all treatments. The total amount of C_org mineralized ranged from < 1 % (control plots) to 20 % (compost amended Dystric Cambisol). Incubation at 14 °C resulted in 2.7‐ to 4‐fold higher cumulative C_org mineralization compared to 5 °C. The C_mic : C_org ratios of the compost‐amended plots declined rapidly during the first 6 months and reached a similar range as the control plots at the end of the experiment. This effect may identify the compost‐derived microbial biomass as an easily degradable C source. Decreasing mineralization rates and metabolic quotients indicated a shift from a compost‐derived to a soil‐adapted microbial community. The C_org mineralization of the compost amended soils was mainly regulated by the compost maturity and the soil texture (higher activity in the sandy textured soils). The pattern of biological activity in the compost‐amended mineral substrate did not differ markedly from that of the compost‐amended agricultural soils, showing that the turnover of compost‐derived organic matter dominated the overall decay process in each soil. However, a priming effect occurring for the Dystric Cambisol indicated, that the effect of compost application may be soil specific.     

Assessing the impact of Diplocardia ornata on physical and chemical properties of compacted forest soil in microcosms

 The influence of compaction on Diplocardia ornata (Smith) burrowing and casting activities, soil aggregation, and nutrient changes in a forest soil were investigated using pot microcosms. Treatments included two levels each of compaction, organic matter, and earthworms. Both burrowing and casting activities were more abundant in uncompacted soil than in compacted soil. Bulk density decreased in microcosms of compacted soil containing D. ornata from 1.76 g cm^–3 to 1.49 g cm^–3 over the study period. The overall percent of aggregates in the same size classes in compacted soil was less than the percent of aggregates in uncompacted soil. The mean percent of aggregates in earthworm casts for size classes 0.25–1.00 mm was higher for compacted soil than for uncompacted soil. The reverse was true for aggregates in class sizes 2.00–4.00 mm. Soil compaction also affected soil microbial biomass carbon and soil inorganic N concentrations. These results indicate that the burrowing and casting activities of earthworms in compacted forest soils, as in soils of agricultural and pastured lands, can help ameliorate disturbed soils by improving aggregation, reducing bulk density, and increasing nutrient availability. Received: 1 September 1999     

Influence of microbial populations and residue quality on aggregate stability

Soil structure mediates many biological and physical soil processes and is therefore an important soil property. Physical soil processes, such as aggregation, can be markedly influenced by both residue quality and soil microbial community structure. Three experiments were conducted to examine (i) the temporal dynamics of aggregate formation and the water stability of the obtained aggregates, (ii) the effect of residue quality on aggregation and microbial respiration, and (iii) the effect of fungi and bacteria on aggregation.In the first experiment, 250 μm sieved air-dried soil, mixed with wheat straw, was incubated for 14 days to allow formation of water-stable macroaggregates (>250 μm). Aggregate stability was measured by wet sieving after four different disruptive treatments: (i) soil at field capacity; (ii) soil air-dried and slowly wetted; (iii) soil air-dried and quickly wetted; (iv) 8 mm sieved soil, air-dried and immersed in water (slaking). After 14 days of incubation, maximum aggregation for soil sieved at field capacity was reached; however, these newly formed aggregates were not yet resistant to slaking.During the second experiment, the effect of low-quality residue (C/N: 108) (with or without extra mineral nitrogen) and high-quality residue (C/N: 19.7) (without extra mineral nitrogen) on macroaggregate formation and fungal and bacterial populations was tested. After 14 days, aggregation, microbial respiration, and total microbial biomass were not significantly different between the low-quality (minus mineral nitrogen) and high-quality residue treatment. However, fungal biomass was higher for the low-quality residue treatment compared to the high-quality residue treatment. In contrast, bacterial populations were favored by the high-quality residue treatment. Addition of mineral N in the low-quality residue treatment resulted in reduced macroaggregate formation and fungal biomass, but had no effect on bacterial biomass. These observations are not conclusive for the function of fungal and/or bacterial biomass in relation to macroaggregate formation. In order to directly discern the influence of soil microflora on aggregation, a third experiment was conducted in which a fungicide (captan) or bactericide (oxytetracycline) was applied to selectively suppress fungal or bacterial populations. The direct suppression of fungal growth by addition of fungicide led to reduced macroaggregate formation. However, suppression of bacterial growth by addition of bactericide did not lead to reduced macroaggregate formation. In conclusion, macroaggregate formation was positively influenced by fungal activity but was not significantly influenced by residue quality or bacterial activity.     

Fluorescein diacetate hydrolysis,respiration and microbial biomass in freshly amended soils

The hydrolysis of the fluorescein diacetate (FDA), related to several soil hydrolases, has been utilised to estimate the potential microbial activity of soil freshly amended with a wide range of organic amendments and compared to the size and activity of soil microflora, measured by the microbial biomass C (B _C) and CO₂ evolution, respectively. Three different composting mixtures at different phases of the composting process were added to a semi-arid soil and incubated for 2 months under laboratory conditions. The addition of the organic amendment immediately increased B _C and both measures of microbial activity (FDA and CO₂ evolution). Highly significant correlations were found between FDA hydrolysis and B _C for soil amended with the three composting mixtures (r = 0.81–0.96; P < 0.01), regardless of the origin, composition and degree of stability of the organic amendments. FDA hydrolysis, conversely to CO₂ evolution, was unaffected by the disturbance caused by the soil amendment, indicating that the two parameters probably reflect different aspects of soil microbial activity. FDA hydrolysis could serve as an alternative estimation of the microbial biomass in freshly amended soils, despite the disturbance caused by the exogenous organic matter.     

Spatial changes of soil fungal and bacterial biomass from a sub-alpine coniferous forest to grassland in a humid, sub-tropical region

 Fungal and bacterial biomass were determined across a gradient from a forest to grassland in a sub-alpine region in central Taiwan. The respiration-inhibition and ergosterol methods for the evaluation of the microbial biomass were compared. Soil fungal and bacterial biomass both significantly decreased (P<0.05) with the shift of vegetation from forest to grassland. Fungal and bacterial respiration rates (evolved CO₂) were, respectively, 89.1 μl CO₂ g^–1 soil h^–1 and 55.1 μl CO₂ g^–1 soil h^–1 in the forest and 36.7 μl CO₂ g^–1 soil h^–1 and 35.7 μl CO₂ g^–1 soil h^–1 in the grassland surface soils (0–10 cm). The fungal ergosterol content in the surface soil decreased from the forest zone (108 μg g^–1) to the grassland zone (15.9 μg g^–1). A good correlation (R ²=0.90) was exhibited between the soil fungal ergosterol content and soil fungal CO₂ production (respiration) for all sampling sites. For the forest and grassland soil profiles, microbial biomass (respiration and ergosterol) declined dramatically with depth, ten- to 100-fold from the surface organic horizon to the deepest mineral horizon. With respect to fungal to bacterial ratios for the surface soil (0–10 cm), the forest zone had a significantly (P<0.05) higher ratio (1.65) than the grassland zone (1.05). However, there was no fungal to bacterial ratio trend from the surface horizon to the deeper mineral horizons of the soil profiles. Received: 30 March 2000     

Dynamics of soil organic carbon and nitrogen associated with physically separated fractions in a grassland-cultivation sequence in the Qinghai-Tibetan plateau

This study is aimed at quantifying organic carbon (C) and total nitrogen (N) dynamics associated with physically separated soil fractions in a grassland-cultivation sequence in the Qinghai-Tibetan plateau. Concentrations of organic C and N of soil, free and occluded particulate organic matter (OM), and aggregate- and mineral-associated OM in different land uses are increased in the following order: 50 years cultivation < 12 years cultivation ≤ native grassland. The prolonged cropping of up to 50 years markedly affected the concentrations of free and occluded particulate OM and mineral-associated OM. After wet-sieving, 43% of native grassland soil mass was found in >1−10 mm water-stable aggregates that stored 40% of bulk soil organic C and N; only 16% and 7% of soil mass containing 16% and 7% of bulk soil organic C and N was >1−10 mm water-stable aggregates of soils cultivated for 12 years and 50 years, respectively. This indicated that losses of soil organic C and N following cultivation of native grassland would be largely related to disruption of >1–10 mm size aggregates and exposure of intra-aggregate OM to microbial attack. Organic C and N concentrations of soil aggregates were similar among aggregate size fractions (>0.05−10 mm) within each land use, suggesting that soil aggregation process of these soils did not follow the hierarchy model. The increase of the C-to-N ratio of free and occluded particulate fractions in the cultivated soils compared to the grassland soil indicated a greater loss of N than C.     

Nutrient Seepage from Operating Turkey Buildings with a Litter System

The effect of selected nutrient amendments and temperature on the biodegradation of pentachlorophenol (PCP) within a soil biopile was studied on a laboratory scale. This was accomplished by monitoring microbial populations, the concentration of PCP and the release of inorganic chloride ions in the contaminated soil. It was found that temperatures of 10, 15 and 20 °C had no significant effect on microbial populations and the percentage of PCP remaining in the soil. However, the nutrient amendments did have a significant effect on the parameters measured. The dairy manure, ammonium nitrate fertilizer and control treatments all experienced some fluctuations in the amount of PCP remaining in the soil over the incubation period and may have been due to the release of initially unextractable bound residues. PCP decreased by 76% in the municipal solid waste compost amended soil, while the concentration of inorganic chloride ions increased. The municipal solid waste compost treatment had significantly higher bacterial and fungal populations. Based on the results of this study municipal solid waste compost may be used as an effective supplemental nutrient amendment for the degradation of PCP in soil biopiles.     

Influence of nitrogen on cellulose and lignin mineralization in blackwater and redwater forested wetland soils

 Microcosms were used to determine the influence of N additions on active bacterial and active fungal biomass, cellulose degradation and lignin degradation at 5, 10 and 15 weeks in soils from blackwater and redwater wetlands in the northern Florida panhandle. Blackwater streams contain a high dissolved organic C concentration which imparts a dark color to the water and contain low concentrations of nutrients. Redwater streams contain high concentrations of suspended clays and inorganic nutrients, such as N and P, compared to blackwater streams. Active bacterial and fungal biomass was determined by direct microscopy; cellulose and lignin degradation were measured radiometrically. The experimental design was a randomized block. Treatments were: soil type (blackwater or redwater forested wetlands) and N additions (soils amended with the equivalent of 0, 200 or 400 kg N ha^–1 as NH₄NO₃). Redwater soils contained higher concentrations of C, total N, P, K, Ca, Mn, Fe, B and Zn than blackwater soils. After N addition and 15 weeks of incubation, the active bacterial biomass in redwater soils was lower than in blackwater soils; the active bacterial biomass in blackwater soils was lower when 400 kg N ha^–1, but not when 200 kg N ha^–1, was added. The active fungal biomass in blackwater soils was higher when 400 kg N ha^–1, but not when 200 kg N ha^–1, was added. The active fungal biomass in redwater wetland soils was lower when 200 kg N ha^–1, but not when 400 kg N ha^–1, was added. Cellulose and lignin degradation was higher in redwater than in blackwater soils. After 10 and 15 weeks of incubation, the addition of 200 or 400 kg N as NH₄NO₃ ha^–1 decreased cellulose and lignin degradation in both wetland soils to similar levels. This study indicated that the addition of N may slow organic matter degradation and nutrient mineralization, thereby creating deficiencies of other plant-essential nutrients in wetland forest soils. Received: 7 April 1999     

Influence of nitrogen on atrazine and 2, 4 dichlorophenoxyacetic acid mineralization in blackwater and redwater forested wetland soils

 Microcosms were used to determine the influence of N additions on active bacterial and fungal biomass, atrazine and dichlorophenoxyacetic acid (2,4-D) mineralization at 5, 10 and 15 weeks in soils from blackwater and redwater wetland forest ecosystems in the northern Florida Panhandle. Active bacterial and fungal biomass was determined by staining techniques combined with direct microscopy. Atrazine and 2,4-D mineralization were measured radiometrically. Treatments were: soil type, (blackwater or redwater forested wetland soils) and N additions (soils amended with the equivalent of 0, 200 or 400 kg N ha^–1 as NH₄NO₃). Redwater soils contained higher concentrations of C, total N, P, K, Ca, Mn, Fe, B and Zn than blackwater soils. After N addition and 15 weeks of incubation, active bacterial biomass in redwater soils was lower when N was added. Active bacterial biomass in blackwater soils was lower when 400 kg N ha^–1, but not when 200 kg N ha^–1, was added. Active fungal biomass in blackwater soils was higher when 400 kg N ha^–1, but not when 200 kg N ha^–1, was added. Active fungal biomass in redwater soils was lower when 200 kg N ha^–1, but not when 400 kg N ha^–1, was added. After 15 weeks of incubation 2,4-D degradation was higher in redwater wetland soils than in blackwater soils. After 10 and 15 weeks of incubation the addition of 200 or 400 kg N ha^–1 decreased both atrazine and 2,4-D degradation in redwater soils. The addition of 400 kg N ha^–1 decreased 2,4-D degradation but not atrazine degradation in blackwater soils after 10 and 15 weeks of incubation. High concentrations of N in surface runoff and groundwater resulting from agricultural operations may have resulted in the accumulation of N in many wetland soils. Large amounts of N accumulating in wetlands may decrease mineralization of toxic agricultural pesticides. Received: 26 June 1998     

Role of amendments on N cycling in Mediterranean abandoned semiarid soils

Soils found in semiarid areas of the Mediterranean Basin are particularly prone to degradation due to adverse climatic conditions with annual rainfall <300 mm and high temperatures being responsible for the scant vegetal growth and the consequent lack of organic matter. A three-year field experiment was conducted to test the potential of two organic amendments (sludge and compost) to improve soil quality and plant growth in a semiarid degraded Mediterranean ecosystem. Since little is known about N dynamics in such assisted ecosystems, we investigated the effects of this practice on key processes of the global N cycle. Besides soil chemical and biological parameters and vegetation cover, we measured absolute and specific potential nitrification and denitrification rates and quantified the size of the ammonia oxidising and denitrifying bacterial populations via quantitative PCR (amoA and nirS genes). At the end of the experiment soil fertility, microbial activity and plant growth had improved in treated plots. Amendments increased the amount of ammonia oxidisers and denitrifiers in soil, but the relative proportion of these groups varied in relation to the total microbial community, being higher in the case of ammonia oxidisers but not in the case of denitrifiers. As a consequence, significantly higher potential nitrification and denitrification rates were measured on a global basis in amended soils. Yet specific activities (potential rate/gene copy numbers) were lower for ammonia oxidisers in amended soils and for denitrifiers in sludge treated soils than those observed in control plots. Organic amendments influenced resource availability, the size and the activity patterns of microbial populations involved in long-term N dynamics. Therefore N cycling processes may play a key role to assist sustainable restoration practices in semiarid degraded areas.     

Root exudate effects on the bacterial communities, CO2 evolution, nitrogen transformations and ATP content of rhizosphere and bulk soils

The ATP content, soil respiration, bacterial community composition, and gross N mineralization and immobilization rates were monitored under laboratory condition at 25 °C for 28 d in a model system where low molecular weight root exudates (glucose and oxalic acid) were released by a filter placed on the surface of a forest soil also treated with ¹⁵N, so as to simulate rhizosphere conditions. Periodically, the soil was sampled from two layers, 0-2 and 6-14 mm below the filter's surface, which were indicated as rhizosphere and bulk soils, respectively. The isotope dilution technique was used to determine the effect of these low molecular weight organic compounds (LMWOCs) on gross N mineralization and immobilization rates. From 0 to 3 d both glucose and oxalic acid amended soils showed a rapid evolution of CO₂, more pronunced in the latter treatment together with a decrease in the amount of mineral N of the rhizosphere soil, probably due to N immobilization. Nevertheless, these changes were accompanied by a very small increase in the net ATP content probably because the low C application rate stimulated microbial activity but microbial growth only slightly. A positive ‘priming effect’ probably developed in the oxalic acid amended soil but not in the glucose amended soil. Gross N mineralization and immobilization rates were only observed in the rhizosphere soil, probably due to the greater C and N concentrations and microbial activity, and were a little higher in both amended soils than in the control soil, only between 1 and 7 d. Both glucose and oxalic acid influenced the bacterial communities of the rhizosphere soil, as new bands in the DGGE profiles appeared at 3 and 7 d. Glucose induced lower changes in the bacterial community than oxalic acid, presumably because the former stimulated a larger proportion of soil microorganisms whereas the latter was decomposed by specialized microorganisms. Peaks of net daily soil respiration and net ATP content and the appearence of new dominant bacterial populations were shifted in time, probably because there was less ATP synthesis and DGGE patterns changed after complete substrate mineralization.     

Changes in enzyme activities and microbial biomass after “in situ” remediation of a heavy metal-contaminated soil

Microbial properties such as microbial biomass carbon (MBC), arylsulfatase, β-glucosidase and dehydrogenase activities, and microbial heterotrophic potential, together with several chemical properties such as pH, CaCl₂ soluble heavy metal concentrations, total organic carbon and hydrosoluble carbon were measured to evaluate changes in soil quality, after “in situ” remediation of a heavy metal-contaminated soil from the Aznalcóllar mine accident (Southern Spain, 1998). The experiment was carried out using containers, filled with soil from the affected area. Four organic amendments (a municipal waste compost, a biosolid compost, a leonardite and a litter) and an inorganic amendment (sugarbeet lime) were mixed with the top soil at the rate of 100 Mg ha⁻¹. Unamended soil was used as control. Agrostis stolonifera L. was sown in the containers. The soil was sampled twice: one month and six months after amendment application. In general, these amendments improved the soil chemical properties: soil pH, total organic carbon and hydrosoluble carbon increased in the amended soils, while soluble heavy metal concentrations diminished. At the same time, higher MBC, enzyme activities and maximum rate of glucose mineralization values were found in the organically amended soils. Plant cover was also important in restoring the soil chemical and microbial properties in all the soils, but mainly in those that were not amended organically. As a rule, remediation measures improved soil quality in the contaminated soils.     

Evaluation of the selective respiratory inhibition method for measuring the ratio of fungal:bacterial activity in acid agricultural soils

A procedure for the measurement of the fungal and bacterial contribution to substrate-induced respiration was tested in three arable soils. Glucose and different amounts of cycloheximide (eukaryote inhibitor) and streptomycin sulfate (prokaryote inhibitor) were added to soil suspensions, and respiration (CO₂ evolution) was measured. Streptomycin sulfate concentrations from 10 to 120 mg ml^–1 soil solution caused a stable inhibition of respiration. Amounts of cycloheximide ranging from 5 to 35 mg ml^–1 showed an increasing inhibition. In a test with separate and combined addition of the antibiotics at maximum inhibitory concentrations, inhibition by streptomycin was completely overlapped by cycloheximide. This indicated non-target inhibition which may lead to overestimation of fungal respiration. Experiments with sterilized soils inoculated with either fungi or bacteria confirmed that streptomycin selectively inhibited bacteria. Cycloheximide, however, did not only inhibit fungal respiration already at 2 mg ml^–1, but also increasingly inhibited bacterial respiration at increasing concentrations. Only at less than 5 mg cycloheximide ml^–1 was the condition of selective fungal inhibition fulfilled. When 2 mg cycloheximide and 10 mg streptomycin sulfate ml^–1 were applied, the sum of the separate inhibitions almost equalled the combined inhibition by the mix of both inhibitors in field samples. This method yielded fungal:bacterial respiration ratios of 0.50 to 0.60, and confirmed the dominance of bacteria in Dutch arable soils. The ratios obtained by the selective inhibitors were not correlated with, and were higher than, ratios of fungal:bacterial biovolume (0.19 to 0.46) as determined by microscopy and image analysis. Similar measurements in a forest soil (A-horizon) raised doubts on the reliability of the fungal inhibition by cycloheximide in this soil. It is concluded that the separate:combined inhibition ratio should always be checked, and comparison with other approaches is recommended. Received: 17 September 1996     

The formation of water-stable coprolite aggregates in soddy-podzolic soils and the participation of fungi in this process

The water-stability of soil and coprolite aggregates in soddy-podzolic soils and the participation of fungi in the formation of water-stable aggregates from earthworm (Aporrectodea caliginosa) coprolites were assessed. The water stability of the soil and coprolite aggregates in the soils increased in the following sequence: potato field—mown meadow—mixed forest. The fungal mycelium reserves increased in the same sequence. The water stability of the coprolite aggregates of Aporrectodea caliginosa inhabiting these soils is 2–2.5 times higher than that of the soil aggregates of the same size (3–5 mm). The inhibition of the growth of fungi by cycloheximide decreased the water stability of the coprolite aggregates, on the average, by 15–20%.     

Effect of plant materials on microbial transformation of amino sugars in three soil microcosms

Amino sugars, being predominantly of microbial origin, can help elucidate the role of microbes in carbon and nitrogen cycling in soils. However, little is known about the microbial degradation and synthesis of soil amino sugars as affected by plant-derived organic materials. We conducted a 30-week microcosm study using three soils amended with soybean leaf or maize stalk to investigate changes in the amounts and patterns of amino sugars over time. The total soil amino sugar content initially increased during the incubation, but later decreased. Amino sugar content of soil amended with maize stalk peaked at an earlier time than it did for soybean leaf, suggesting nutrient quantity and substrate composition influence microbial transformation. Temporal dynamics of the proportion of total soil amino sugar to organic matter after plant material addition conformed to parabolic models (r > 0.8; p < 0.01), which tended to converge over time. The models predicted that the proportions would ultimately approach the initial values as determined before amendment. These findings suggest that soil organic matter has the ability to maintain a baseline steady-state level of amino sugars, and support the interpretation of soil amino sugar reservoir as two components: the Stable Pool (SP) and the Transition Pool (TP).