The effects of Cd and sewage-sludge on microbial biomass and enzyme activities in three soils were studied during two months incubation at 30 +/- 1 degrees C. The sewage-sludge amendment enhanced soil microbial biomass by 8-28%, with the greatest affect in clay-loam and the least in sandy-loam soil. The soil dehydrogenase, alkaline phosphatase and arginine-ammonification activities were enhanced by 18-25, 9-23 and 8-12%, respectively, by sludge. Cadmium addition at 10 micrograms g-1 caused no significant changes in these parameters. However, 50 micrograms Cd g-1 soil detrimentally influenced the soil microbial biomass and enzyme activities; more in sandy-loam than in loam or clay-loam soil. Cadmium adsorption in soils was 31-56%, high in clay-loam and low in sandy-loam soils. A significant negative correlation (r = -0.58* to 0.80*, P less than or equal to 0.05) was observed between DTPA-extractable Cd and microbial biomass. The changes in soil microbial biomass were interpreted as cell death and correlated well with soil dehydrogenase and alkaline phosphatase activities. However, no correlation existed between microbial biomass and arginine-ammonification activity. Sewage-sludge amendment did not mitigate the inhibitory effects of Cd.

Quantitative transformation of soil micromycete biomass was investigated according to the loading of soil by copper doses of 100, 200, 400, 800 and 3,000 mg Cu kg** (-1) of soil. Quantitative determination of soil micromycetes was performed by the direct microscopic method with the aid of membrane filters. The effect of different copper concentrations in soils was investigated by model experiments. Copper doses were specified according to the concentration found in soils from different localities in Slovakia. The results showed a very weak inhibitory effect of copper at concentrations ranging from 100 to 400 mg Cu kg** (-1) of soil on micromycete biomass. Higher copper doses (800 mg Cu) inhibited biomass dynamics more markedly; and a clear toxic effect was found at the dose of 3,000 mg Cu. In this case the toxicity of copper was very distinct, especially in mycelium, the active part of the biomass

The effects of two ergosterol biosynthesis-inhibiting fungicides, epoxiconazole and triadimefon (at 2 and 20 x, and 1 and 10 x field rate, respectively), and of straw amendment on ergosterol and microbial biomass C in a sandy loam soil, were investigated. Initial soil ergosterol and microbial biomass C contents were about 1.6 micrograms g-1 and 330 micrograms g-1 soil, respectively. Both concentrations of the fungicides decreased soil ergosterol content by about 30% after 7 days incubation in unamended soil, following which contents were broadly similar to the control soil. Microbial biomass C remained largely unaffected in the unamended soil, except for a temporary inhibition of about 12% caused by triadimefon after 13 days incubation. After amendment of the soil with wheat straw, the inhibition of ergosterol biosynthesis reappeared, ranging from 4 to 34%, with epoxiconazole having a greater effect than triadimefon, both compared to ergosterol biosynthesis in soils given straw without the biocides. This inhibition was also transient, but was longer-lasting than in the unamended soil. A decrease in soil microbial biomass C, ranging from 5 lo 12%, also occurred, in contrast to the unamended treatments. This indicates that the fungicides were more active against the newly synthesized microbial biomass than against the original population. Soil ergosterol content was a more sensitive measure of pesticide side-effects than microbial biomass C, and could be a useful method in this respect.

The effects of 19 years of cumulative annual field application of five pesticides (benomyl, chlorfenvinphos, aldicarb, triadimefon and glyphosate), applied at, or slightly above, the recommended rates in 25 combinations, on soil microbial biomass and the mineralization of soil organic matter were investigated. Soil samples were taken 1 month after the application of benomyl, chlorfenvinphos and aldicarb in April 1992, and again in October 1992, 1 month after the application of triadimefon and glyphosate. The addition of aldicarb caused a significant increase of 7-16% in soil microbial biomass carbon (biomass C), an effect which appeared to be persistent. This effect of aldicarb was not reflected in the mineralization rate of soil organic C, possibly because the measurements of CO2 evolution showed a greater variation than those of biomass C. Measurement of microbial biomass activity by the substrate-induced respiration method also gave much less precise results than measurements of biomass C by fumigation-extraction. The mineralization of soil organic N to ammonium and then nitrate was mostly unaffected by the pesticide treatments. In the autumn-sampled soil, there was significantly less NH4-N in the aldicarb-treated soil. It is possible that this was due to immobilization by the increased microbial biomass in these treatments, and did not represent a loss to the soil system. The continuous use of these pesticides, either singly or in combination, therefore had no measurable long-term harmful effects on the soil microbial biomass or its activity, as assessed by C or N mineralization.

An experiment was conducted to determine if spatial nutrient heterogeneity affects mean plant size or size hierarchies in experimental populations of the weedy annual Abutilon theophrasti Medic. (Malvaceae). Heterogeneity was imposed by alternating 8 X 8 X 10 cm blocks of low and high nutrient soil in a checkerboard design, while a homogeneous soil treatment consisted of a spatially uniform mixture of the two soil types (mixed soil). Populations were planted at three densities. The effect of soil type on the growth of individuals was determined through a bioassay experiment using potted plants. The high nutrient, low nutrient, and mixed soil differed in their ability to support plant growth as indicated by differences in growth rates and final aboveground biomass. Concentrations of N, K, P, and Mg, measured at the end of the growing season in the experimental plots, also differed among all three soil types. Nevertheless, nutrient heterogeneity had little effect at the population level. Mean maximum leaf width measured at midseason was greater for populations on heterogeneous soil, but soil treatment did not affect midseason measurements of plant height, total number of leaves per plant, or canopy width. Population density affected all these parameters except plant height. When aboveground biomass was harvested at the end of the growing season, soil treatment was found to have no main effect on mean plant biomass, total population biomass, the coefficient of variation in plant biomass, or the combined biomass of the five largest plants in the population, but mean plant biomass was greater for populations on heterogeneous soils at the intermediate planting density. Mean plant biomass, total population biomass, and the coefficient of variation in plant biomass all varied with planting density. Mortality was low overall but significantly higher on homogeneous soil across all three densities. Soil heterogeneity had its strongest effect on individuals. In heterogeneous treatments plant size depended on the location of the plant stem with respect to high and low nutrient patches. Thus, soil nutrient heterogeneity influenced whether particular individuals were destined to be dominant or subordinate within the population but had little effect on overall population structure.

We developed a simple method that can be used to estimate the biomass and growth rates of microbial functional groups in soil. The method is derived from basic principles and can be used to estimate the biomass of organisms that can mineralize specific substrates added to soil. We adapted the substrate-induced growth-response (SIGR) model that was originally used to analyze curves of substrate disappearance or cumulative CO2 production. The present model utilizes data describing the rate of CO2 production from substrates added to soil. We used two unique systems to demonstrate the applicability of this method. In one test of the model we added glucose to alpine tundra soil to estimate the biomass that could respond to a labile carbon source. We also derived biomass estimates from a widely used substrate-induced respiration (SIR) model for the same soil. Overall the SIGR method yielded conservative biomass estimates (mean = 194 micrograms C g-1 soil) when compared to the SIR estimates (mean = 436 micrograms C g-1 soil). In the second test we used a soil to which a known biomass of a specific functional group (i.e. pentachlorophenol-mineralizers) was added. In this case the SIGR method also gave a conservative estimate of 0.05 micrograms C g-1 compared to a death-rate adjusted value of 0.11 micrograms C g-1 for the actual inoculum added to the soil. The SIGR model also estimated maximum specific growth rates (0.11-0.12 h-1) similar to those measured in independent experiments (0.09 h-1) for the Sphingomonas sp. that was added to the soil. Using our model we were able to obtain biomass estimates and growth rates for microbial functional groups without using calibrations needed for other physiological methods. Overall the SIGR approach gives conservative estimates of the active biomass that can mineralize specific carbon substrates added to soil.

The soil microbial biomass S fraction of total organic S in soil is considered to be relatively labile and the most active S pool for S turnover in soil. Its significance has been demonstrated in studies of S deficiency in agronomic situations and in those of S pollution from high atmospheric inputs. The utility of the CHCl3 fumigation-extraction technique for the measurement of microbial S has been proved for a range of soils and conditions. The various methodologies currently available are discussed, including the need for determination of the conversion (Ks) factor. Microbial S values, summarized from the available literature, ranged from 3 to 300 microgram S g-1 dry weight soil. They were generally greater in grassland than in arable systems, though the greatest values were obtained in the few examples from forest and peatland soil systems. Microbial S values showed direct relationships with both microbial C and with total soil organic S. Again, there were significant differences between arable and grassland systems. The effect of factors such as organic and inorganic inputs as well as soil physical conditions on microbial S are described. Microbial S turnover rates were estimated from seasonal, 35S-labelling and modelling studies. These rates varied between an approximately annual turnover rate in undisturbed soils up to 80 year-1 following the addition of readily available substrates. Prospective future research areas are also outlined.