This study aims to quantify and establish a mechanistic understanding of the role of soil microbial biomass in soil fertility under intensively cultivated wetland rice. The specific objectives were: a) to quantify the relationships between soil microbial biomass and soil organic carbon content; b) to relate the process of reduction upon flooding to soil organic matter content; and c) to investigate the relationship of soil microbial biomass and soil nitrogen (N) under field conditions. Soil microbial biomass of wetland soils under aerobic incubation was linearly related to a labile carbon pool in the plough layer rather than total soil organic carbon. Labile carbon in the Ap horizon reflects organic inputs and part of soil organic matter. The rate of reduction upon flooding was related to the C:N ratio of the enriched organic fraction but not total soil organic carbon. Soil N status was described on a mass basis relative to N uptake by IR72, where the soil N environment was altered by N fertilizer addition. Soil N was measured as exchangeable, soil solution phase and soil microbial biomass N. Soil microbial biomass increased with increasing soil exchangeable and solution phase ammonium resulting in rapid immobilization of large amounts of added fertilizer N by the soil microbial biomass. These results emphasize the importance of soil microbial biomass in the regulation of soil and fertilizer N supply in flooded soils, highlighting the apparent contrasts in the behavior of the soil microbial biomass under aerobic and anaerobic condition

The effect of roadside metal contamination on the size of the soil microbial biomass was examined in soils from five roadside sites in S.W. London. The soils were predominantly light textured and soil pH ranged from a mean of pH 7.1 at 0.5 m from the road to pH 5.0 at 10 m from the road. Biomass-C, determined by the fumigation-extraction method, and soil ATP were measured at 0.5 and 10 m from the road where toxic metal concentrations in the soil were high and low, respectively. The mean biomass-C at 0.5 and 10 m from the road was 951 micrograms biomass-C g-1 soil and 740 micrograms biomass-C g-1 soil, respectively. There was no evidence that increased soil concentrations of Pb, Cu, Ni, Zn and Cd close to the road had adversely affected biomass size and the larger biomass at 0.5 m was attributed to a positive response to the higher soil pH and carbon levels measured at this distance. A larger CHCl3-extractable soil carbon fraction was measured in 0.5 m soil by Soxhlet analysis and was attributed to hydrocarbon contaminants from vehicle exhausts and road surface materials. The mean ratio of soil ATP to extracted microbial carbon was the same for 0.5 and 10 m soils and the mean ATP content of the biomass was 8.8 +/- 0.55 micromoles ATP g-1 biomass-C. This provided some evidence that differences in hydrocarbon contamination between distances from the road had not affected the fumigation-extraction biomass-C measurements.

The aim of this study was to test whether soil texture and amount of available carbon affect microbial activity and the percentage of the soil microbial biomass that is active. Total and active microbial biomass were determined by the fumigation-incubation method (FI) and the substrate-induced respiration method (SIR), respectively. The ratio biomass-SIR:biomass-FI, indicating the percentage of active microbial biomass, and the ratio C mineralization:biomass-FI, indicating the activity of the microbial biomass, were both higher in soils with a coarse texture than in soils with a fine texture. The ratio C mineralization:biomass-SIR was not affected by soil texture. The ratios biomass-SIR:biomass-FI and C mineralization:biomass-FI were higher in the upper 10-cm soil layer than in the 10-25 cm layer, while the ratio C mineralization:biomass-SIR was not affected by soil depth. During 25 weeks incubation, the amount of biomass-SIR as well as the C mineralization rate decreased significantly. The amount of biomass-FI, however, did not change significantly during incubation. Assuming that the amount of available C decreases both with soil depth and during incubation, the observations indicate that smaller amounts of available carbon coincide with a reduced activity of the biomass and a lower percentage of active biomass. The close similarity between C mineralization rate and amount of biomass-SIR shows that biomass-SIR is a good indicator of microbial activity, whereas the FI method is not suitable for this purpose.

The interaction between microbial growth and S immobilization was investigated in an arable soil amended with oil-seed rape (young leaves) and barley straw (1% w/w). Initially, the rape decomposed more rapidly (40 vs 10% by day 5) and produced a larger microbial biomass (990 microgram C g(-1) soil) than the straw (710 microgram C g(-1) soil). The biomass in both of the amended soils then decreased to amounts 30-50% higher than those in the unamended soil by day 35 and was maintained at levels throughout the 195 day incubation. Most of the rape-S (>80%) and straw-S (>60%) added to the soil was released as SO4(2-)-S or converted to biomass-S in 5 days. By this time, the amount of S assimilated by the biomass in the rape-amended soil was three times that found using straw. Biomass-S in both soils then decreased but remained twice as high in the rape-amended soil over the period of 15-195 days. The biomass in the straw amended soil had a similar C:S (85-120:1) to that of the unamended soil but was narrower (40-50:1) in the rape-amended soil. By day 5, SO4-S in both of the amended soils had increased significantly. The increase in SO4(2-)-S in the rape-amended soil was maintained over the 195 day incubation, suggesting that this S was available for plant uptake. However, by day 15, a net immobilization of soil S by the biomass (25% of soil inorganic S) was found using straw. This immobilized S was retained by the biomass throughout the 195 day incubation and was, therefore, unavailable for plant growth. This suggests that the incorporation of plant residues such as straw which contain low amounts of S may decrease the plant availability of soil S.

Relationships between total metals, CaCl2-extractable metals and soil microbial biomass were investigated in a sandy loam soil (Cuckney series) at Gleadthorpe Experimental Husbandry Farm, U.K. The metals occurred because sewage sludges, enriched either with different rates of the single metals Zn, Cu or Ni, or with combinations of the metals (Zn and Cu or Zn and Ni) at different rates, were applied in 1982 and again, in some cases, in 1986. The observed increases in total soil metal concentrations were generally in good agreement with the intended soil metal additions. However, the proportional amounts of metals extracted by CaCl2 differed between metals. Calcium chloride extracted a maximum of about 42% of total Zn, 9% of total Cu and 26% of total Ni in the sludged soil, but very much less (3% of total Zn, 1% of total Cu and 2% of total Ni) in control soils (i.e. the soils that never received sewage sludge). Neither Zn, Cu or Ni present singly in soils at below current EC permitted total soil metal concentrations decreased the amounts of soil microbial biomass. However, Cu at about 4.9 times and Zn at about 2.3 times permitted limits decreased the amounts of soil microbial biomass by 51 and 36%, respectively, when present separately, compared to the control soil. The soils which contained either Cu or Zn separately at about 1.4 times permitted limits contained about 12% less biomass C than the control soil. In contrast, Cu and Zn in combination at about 1.4 and 1.2 times permitted limits, respectively, decreased the biomass by about 29%, and soils containing Cu and Zn in combination at 1.8 and 1.4 times the limits contained 53% less biomass than the control soil. Thus a combination of Zn and Cu decreased the amount of biomass at lower soil metal concentrations than were required when either metal was present singly, suggesting the effects were additive. Biomass C as a percentage of total soil organic C in soils contaminated singly with higher rates of Zn or Cu or with both metals in combination was less than half that in the soil which received no sludge, uncontaminated sludge or sludge contaminated with lower rates of metals. Thus, this statistic provides a sensitive indicator of the effects of heavy metals on microbial biomass.

It is established that the dependence between biomass of soil microorganisms, determined by method of substrate-induced respiration, and the constant of pesticides decomposition in soils is directly proportional. Assessment of soil ability to self-purification of pesticides is given using the amount of biomass of soil microorganisms. The value of microorganisms biomass is inversely proportional to complete decomposition of pesticides and may be considered as a qualitative criterion for determining the soil ability to self-purification of pesticides

Streptomycin and cycloheximide were applied singly or in combination to an arable or a forest soil to determine their effects on N mineralization, nitrification, CO2 evolution, and microbial biomass C and N. Only slight differences were observed in concentrations of both inorganic N forms, CO2 evolution and amounts of microbial biomass C and N between the streptomycin-treated arable soil and the respective control. Cycloheximide singly or in combination with streptomycin stimulated N mineralization, depressed NO3- production as well as the increase in microbial biomass C and N observed in the untreated arable soil. The C:N ratio of soil microbial biomass fluctuated around 4 with cycloheximide singly or in combination with streptomycin during the 0-10 day incubation; the ratio was generally higher than 4.5 but never exceeded 5 in the streptomycin-treated arable soil and in the respective control. The inhibition of nitrification in the arable soil by the fungicide prompted the question concerning the relative importance of autotrophic and heterotrophic nitrification rates; the inhibition of the NO3- formation by nitrapyrin and the absence of stimulation by peptone indicated that the potential for autotrophic nitrification in this soil is greater than that for heterotrophic nitrification. The addition of antibiotics to the forest soil increased respiration and NH4(+)-N concentration with the highest effect with both antibiotics. Both biocides singly or in combination depressed the decrease of NO3(-)-N concentration observed in the control; they markedly depressed microbial biomass C and N during the early incubation period; then both microbial biomass values increased probably because surviving microorganisms utilized organic compounds released from antibiotic-killed cells.

Total above ground biomass production was highly significant for the different fallow periods. The five year old fallow fields had the highest net above ground biomass production. Nutrient concentration in different plant component was not significantly affected by the fallow period. The nutrient accumulation in the above ground biomass (kg/ha) was significantly affected by the fallow period. Duncan Multiple Range Test revealed that, the nitrogen, phosphorus, potassium, and calcium accumulation in the above ground total biomass was not significantly different for the first three years of the fallow period. The ratio of the nutrient content in the leaf biomass to the total above ground biomass decreased with the progress of the fallow period. The soil chemical properties was affected by the fallow period though analysis of variance of many of these soil chemical property changes did not show any significant differences among the fallow periods. Soil pH decreased with the progress of the fallow period while the organic matter content increased in the first three years of the fallow period. The available phosphorus was significantly affected by the fallow period and it increased with the progress of the fallow period up to three years. Other soil properties that was affected by the duration of the fallow period was the water holding capacity, soil moisture and particle density, though the effect was not significantly different. Soil texture was not affected by the fallow period.

The quantity of microbial biomass was measured in a range of soils by the fumigation-incubation (FI) and fumigation extraction (FE) techniques. The air-dried soils were rewetted and incubated before the measurements were made. The FE method for biomass C was calibrated from the relationship between the flush of extractable organic C (Ec) and microbial-C, as estimated by the FI technique in the case of N, the size of the flush of total extractable N (EN) was compared to the flush of mineralization N (FN) by FI. Two different soils were chosen for further calibration by labelling (14C) microorganisms in situ (kEC factor). Biomass C (FE) accounted for 1% of soil C in the finer soils and only 0.4% of soil C in the coarser soils. Biomass content was related to soil texture, presumably because texture influences the ability of soils to preserve microbial biomass. The FI method appeared to underestimate biomass C (relative values) in the coarser soils. The FE technique gave higher biomass C estimates in these soils. The relationship between the flush (EC) and soil biomass C (FI) forced through zero was: biomass C = (2.42 +/- 0.164) Ec (r = 0.90). Somewhat different results were obtained when the FE method was calibrated by in situ labelling. A weaker relationship was obtained for the flush (EN) and the flush (FN) by FI (r = 0.80).