The use of an indigenous microbial consortium, pollutant-acclimated and attached to soil particles (activated soil), was studied as a bioaugmentation method for the aerobic biodegradation of pentachlorophenol (PCP) in a contaminated soil. A 125-1 completely mixed soil slurry (10% soil) bioreactor was used to produce the activated soil biomass. Results showed that the bioreactor was very effective in producing a PCP-acclimated biomass. Within 30 days, PCP-degrading bacteria increased from 105 cfu/g to 108 cfu/g soil. Mineralization of the PCP added to the reactor was demonstrated by chloride accumulation in solution. The soil-attached consortium produced in the reactor was inhibited by PCP concentrations exceeding 250 mg/l. This high level of tolerance was attributed to the beneficial effect of the soil particles. Once produced, the activated soil biomass remained active for 5 weeks at 20 degrees C and for up to 3 months when kept at 4 degrees C. The activated attached soil biomass produced in the completely mixed soil slurry bioreactor, as well as a PCP-acclimated flocculent biomass obtained from an air-lift immobilized-soil bioreactor, were used to stimulate the bioremediation of a PCP-impacted sandy soil, which had no indigenous PCP-degrading microorganisms. Bioaugmentation of this soil by the acclimated biomass resulted in a 99% reduction (from 400 mg/kg to 5 mg/kg in 130 days) in PCP concentration. The PCP degradation rates obtained with the activated soil biomass, produced either as a biomass attached to soil particles or as a flocculent biomass, were similar.

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.

We investigated the changes in microbial biomass C and N, available soil N and neutral sugar composition induced by wheat straw and manure application in four types of central Hokkaido soils. 1. Microbial biomass C and N increased by organic material applications. Microbial biomass N/total N in wheat straw application soils showed a remarkable increase compared with the other two treatment soils. 2. There were significant positive linear correlations among available soil N, microbial biomass N and hot-water extractable C. Available soil N can be estimated by the value of hot-water extractable C. 3. Neutral sugar C increased by organic materials applications, and especially, xylose C increased remarkably by wheat straw application. Neutral sugar compositions were different among three treatments and four soil types. 4. There were significant positive linear correlations between available soil N, microbial biomass N and neutral sugar C. Moreover, microbial biomass N/total N showed a significant positive correlation with xylose C/mannose C. These results explain that the ratio of microbial biomass N in total N is affected by the neutral sugar composition in soil

Clogging of porous media caused by the accumulation of microbial biomass and extracellular polysaccharide (EPS) greatly influences water flow and chemical transport in subsurface environments. A quantitative understanding of the relationship between increase in microbial biomass and reduction in hydraulic conductivity is of critical importance to the simulation of the fate and transport of biologically reactive contaminants in soil and groundwater systems. In this research, soil column experiments were conducted to examine the influence of enhanced biomass production by indigenous soil microbes on the hydraulic conductivity of a river sand. Dextrose-nutrient solution was used to stimulate microbial growth in soil columns. Soil solutions at different locations of the soil columns were sampled to quantify suspended biomass. Destructive sampling was carried out at the end of the percolation runs to determine distributions of attached biomass. About one and one-half orders of magnitude in reduction of hydraulic conductivity was observed 3 weeks after the application of dextrose-nutrient solutions, and different distributions of hydraulic conductivity reduction along the soil columns were obtained from treatments with different dextrose and nitrogen concentrations. A regression equation relating attached microbial biomass and the logarithmic ratio of hydraulic conductivity was obtained. Our experimental results showed no correlation between suspended and attached biomass.

The temporal variation of soil microbial biomass C and N, extractable organic C and N, mineral N and soil-surface CO2 flux in situ in two arabic soils (a sandy loam and a coarse sandy soil) was examined periodically for a full year after field incorporation of 0, 4 or 8 t dry mass ha-1 of oilseed rape straw in late summer. Both unlabelled and 15N-labelled straw were applied. Soil-surface CO2 flux, used as an index of soil respiration, was up to 2-fold higher in the straw-amended treatments than in the unamended treatment at both sites during the first 6-8 wk, but the general temporal pattern was mainly controlled by soil temperature and soil water content. Microbial biomass C and N increased very rapidly after the straw amendments and the 31-49% difference from the unamended treatment persisted throughout the winter. Temporal variations in soil microbial biomass C and N were only within +/- 13-22% of the mean at both sites and in all straw treatments over the 1 y period. Microbial biomass C-to-N ratios were not significantly different between straw treatments and were relatively constant over time. Extractable organic C and N were slightly higher in the straw-amended treatments and were higher in spring and summer than in autumn and winter. More than 90% of the added straw N could be accounted for initially and there was no loss of straw N over the winter period, in spite of a winter rainfall that was twice the 25 y average. Between 52 and 80% of the initial increase in microbial biomass N was derived from the straw N, with up to 27% of the straw N being incorporated into the microbial biomass. Rapid immobilization of soil mineral N occurred simultaneously and the sum of this and the straw-derived microbial biomass N on day 7 exceeded the total increase in microbial biomass N, indicating a very rapid turnover of microbial biomass in the first few days. Significant differences in microbial biomass C and N between the straw treatments were still found after nearly 1 y and the decay constant of straw-derived microbial biomass N was estimated to be ca. 0.26 y-1.

The immobilization and mineralization of N following plant residue incorporation were studied in a sandy loam soil using 15N-labelled field pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.) straw. Both crop residues caused a net immobilization of soil-derived inorganic N during the complete incubation period of 84 days. The maximum rate of N immobilization was found to 12 and 18 mg soil-derived N g-1 added C after incorporation of pea and barley residues respectively. After 7 days of incubation 21% of the pea and 17% of the barley residue N were assimilated by the soil microbial biomass. A comparison of the 15N enrichments of the soil organic N and the newly formed biomass N pools indicated that either residue N may have been assimilated directly by the microbial biomass without entering the soil inorganic N pool or the biomass had a higher preference for mineralized ammonium than for soil-derived nitrate already present in the soil. In the barley residue treatment, the microbial biomass N was apparently stabilized to a higher degree than the biomass N in the pea residue treatment, which declined during the incubation period. This was probably due to N-deficiency delaying the decomposition of the barley residue. The net mineralization of residue-derived N was 2% in the barley and 22% in the pea residue treatment after 84 days of incubation. The results demonstrated that even if crop residues have a relative low C/N ratio (15), transient immobilization of soil N in the microbial biomass may contribute to improved conservation of soil N sources.

Winter cover crops may affect the short- and long-term N availability in soil depending on the quantity, quality, and degradation rate of biomass returned to the soil. We examined the effects of several cover crops on soil inorganic and organic N levels in a winter cover crop-silage corn (Zea mays L.) double-cropping system that was initiated in 1987. High biomass N concentrations (BMN) in the above- and belowground biomass of the leguminous cover crops corresponded to high levels of inorganic N and water-soluble N, but low levels of water-soluble C and carbohydrate compared with the nonleguminous cover crops. The BMN above which there was net N mineralization 4 wk after residue incorporation was 17.9 g N kg⁻¹. The organic N from the aboveground biomass degraded rapidly. The first-order rate constants for the degradation of organic N and C in the cover crops were significantly correlated. This, coupled with a significant correlation between the soil organic N (SON) levels and cumulative biomass C added, indicated the importance of biomass C inputs in organic N retention in the soil. The cover crops had variable short- and long-term effects on soil N availability. Whereas rye (Secale cereale L. cv. Tetra Petkus) and annual ryegrass (Lolium multiflorum Lam. cv. Billion) were ineffective in increasing soil inorganic N levels, they were more effective than hairy vetch (Vicia villosa Roth subsp. villosa), Austrian winter pea (Lathryrus hirsutus L.), and canola (Brassica napus L. cv. Santana) in increasing SON accumulation because of a higher biomass potential and a larger input of biomass C. Scientific Paper no. 9604-09, Dep. of Agronomy and Soils, College of Agric. and Home Economics Research Center, Washington State Univ., Pullman, WA 99164.

The effect of increasing soil CO2 concentration was studied in six different soils. The soils were incubated in ambient air (0.05 vol.% CO2) or in air enriched with CO2 (up to 5.0 vol.% CO2). Carbon dioxide evolution, microbial biomass, growth or death rate quotients and glucose decay rate were measured at 6, 12 and 24 h of CO2 exposure. The decrease in soil respiration ranged from 7% to 78% and was followed by a decrease in microbial biomass by 10-60% in most cases. High CO2 treatments did not affect glucose decay rate but the portion of Cgluc mineralized to CO2 was lowered and a larger portion of Cgluc remained in soils. This carbon was not utilized by soil microorganisms.

Two methods to estimate soil microbial biomass, viz. substrate-induced respiration (SIR) and fumigation-extraction (FE), were compared using 40 beech (Fagus sylvatica L.) forest soils. Soil chemical properties, microbial biomass and activity indices differed over a wide range but generally fitted a normal distribution. Microbial biomass C estimates by SIR and FE were significantly correlated with r = 0.94. Microbial biomass C was significantly correlated with soil pH, soil organic C, K2SO4-extractable C and especially with the basal respiration rate, irrespective of the method used. As expected from the basic principles of an extraction and a physiological method, FE was more affected than was SIR by soil organic matter, with SIR being more affected by soil pH and basal respiration. The development of different microbial community structures at different pH values affects the SIR and FE methods in reverse directions, increasing the differences between the results of the two methods. Estimates of microbial biomass C-to-soil organic C ratio by SIR and FE were significantly interrelated, with r = 0.89 (P < 0.0001) and were closely connected with soil pH. Estimates of the metabolic quotient qCO2 by SIR and FE were also significantly correlated, but with r = only 0.54 (P < 0.001).

Glucose-C was repeatedly added to organic Kalmia humus from two sites of contrasting spruce productivity (i.e. rich and poor) and basal respiration, microbial biomass and metabolic quotient (qCO2) were measured over a 438 day incubation and compared to post-incubation soil mineral-N pools, anaerobic N mineralization rates, the nutritional deficiency index (NDI) of soil microbial communities and final soil weights. Repeated measures analysis of variance revealed a strong overall soil effect on basal respiration and microbial biomass and a strong overall glucose effect on microbial biomass and qCO2, whereas the analysis of within-subjects effects revealed strong interactions of the time factor with both soil and glucose on all three measurements. In the poor soil, glucose supported high microbial biomass and therefore low qCO2, until the end of the incubation, whereas in the rich soil, glucose also supported high microbial biomass and low qCO2, but these converged towards control (i.e. no glucose) values before the end of the incubation. Mineral-N pools were high in the rich control treatment only, whereas the NDI was high only in the rich + glucose treatment. Anaerobic N mineralization rates did not differ statistically among treatments. Glucose significantly decreased mass loss in the rich soil but not in the poor soil. The data support the conclusion that glucose addition to the rich soil inhibits microbial utilization of nutrient-containing soil OM for maintenance energy thus exacerbating nutritional deficiencies, whereas glucose addition to the poor soil does not affect soil N cycling. Based on results of analytical pyrolysis performed on soil subsamples prior to the incubation, we hypothesize that higher amounts of tannins measured in the poor humus may have chemically immobilized and ultimately controlled availability of N.