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The aim of this study was to determine the extent and timing of mineral N release from a localized source of decomposing residue (hotspot) and its subsequent uptake by a growing plant, in relation to changes in the populations of microbial-feeding nematodes and protozoa. Dried and ground ryegrass labelled with 15N was used to create a hotspot at a depth of 30 cm in a column of clay-loam soil. Wheat plants grown in the columns were harvested after 10, 16, 22, 28 and 34 days and analyzed for total N and 15N content, along with simultaneous measurements of soil mineral N and microfauna. There was a rapid increase in microfauna and mineral N, indicating rapid mineralization activity. A total of 25 per cent of the N initially in the residue was present in the wheat plants after 34 days. The pattern of residue N uptake was sigmoidal, with 62 per cent of residue N uptake occurring between days 16-22. The importance of synchronization between mineralization and plant N uptake is discussed.

Broad-scale approaches seek to integrate information on whole microbial communities. It is widely recognized that culture techniques are too selective and unrepresentative to allow a realistic assessment of the overall structure of microbial communities. Techniques based on fatty acid or metabolic profiles determine the phenotypic composition of the community. Complementary information about the genotypic structure of soil microbial communities necessitates analysis of community DNA. To determine broad-scale differences in soil microbial community structure (i.e., differences at the whole community level, rather than specific differences in species composition), we have applied a community hybridization technique to determine the similarity and relative diversity of two samples by cross hybridization. In previous studies this assay failed with whole-soil community DNA. Usable hybridization signals were obtained using whole-soil DNA, in this study, by digesting the DNA with restriction enzymes before the labeling with a random-primer reaction. The community hybridization technique was tested using a graded series of microbial fractions, increasing in complexity, all isolated from the same soil sample. This demonstrated that single bacterial species and a mixture of cultivable bacteria were less complex and only 5% similar to whole-community DNA or bacteria directly extracted from the soil. Extracted bacterial and whole-community DNA were 75% similar to each other and equally complex. When DNA was extracted from four different agricultural soils, their similarities ranged from 35 to 75%. The potential usefulness of community hybridization applied to soil microbial communities is discussed.

Microbial community DNA was extracted directly from an arable soil experimentally polluted with Cd, Cu, Ni, Pb or Zn, and incubated for 34 months. Broad-scale measurements of changes in microbial community structure were assessed by applying %G+C profiling and community hybridisation techniques to the DNA, and compared with phospholipid fatty acid (PLFA) profiles. %G+C profiles, statistically compared using parameterisation and principal component analysis, differed between treatments. Community hybridisation revealed a range of DNA similarities between treatments. The combination of these two techniques facilitated the interpretation of the data. The differences between the treatments determined by DNA analysis were consistent with the PLFA patterns, and revealed that: (i) microbial communities in metal contaminated soils were different from those in uncontaminated soil, (ii) the microbial community in Cu contaminated soil was different from those in soils contaminated with the other metals, and (iii) the microbial communities in soils contaminated with Pb and Ni were indistinguishable from one another.

Whole‐community DNA was extracted from two pasture soils, the cultivable and directly extractable bacterial subsets of one of the soils, and two aquatic bacterioplankton communities. The percent guanine+cytosine (%G+C) base distribution, derived using thermal denaturation profiles, and parameterised using a generalised logistic equation, varied markedly between the samples. Bacterioplankton communities carried a more even distribution of %G+C than the soil communities, which had higher median %G+C. The directly extracted soil bacterial fraction had the highest median %G+C. There was a distinct trend of increasing complexity, measured using reassociation kinetics, between cultivable, extractable and total soil community DNA. Bacterioplankton communities were of a similar order of complexity to the cultivable soil bacterial fraction.

Numbers of the ciliate protozoan Colpoda steiniiMaupas were increased by the presence of a nematode (Rhabditissp.), in soil and liquid culture. The development of nematode numbers was inhibited by the ciliate. Observations of hatching nematodes, together with counts of total and viable numbers of the bacterium Pseudomonas fluorescens, indicate that Rhabditissp. increased bacterial productivity but that the activity of C. steiniiinhibited feeding by the nematode. When the two bacterial grazers were present in combination C. steiniibenefited from increased bacterial productivity induced by Rhabditissp. Both grazers increased the mineralization of NH4+-N when added separately to soil, and this was further increased when nematodes and ciliates were present in combination. There was no effect of grazing on levels of bicarbonate-soluble phosphate (Pi), while nematodes had a variable effect on levels of soil NO3−-N.

Microbial community DNA was extracted directly from an arable soil experimentally polluted with Cd, Cu, Ni, Pb or Zn, and incubated for 34 months. Broad‐scale measurements of changes in microbial community structure were assessed by applying %G+C profiling and community hybridisation techniques to the DNA, and compared with phospholipid fatty acid (PLFA) profiles. %G+C profiles, statistically compared using parameterisation and principal component analysis, differed between treatments. Community hybridisation revealed a range of DNA similarities between treatments. The combination of these two techniques facilitated the interpretation of the data. The differences between the treatments determined by DNA analysis were consistent with the PLFA patterns, and revealed that: (i) microbial communities in metal contaminated soils were different from those in uncontaminated soil, (ii) the microbial community in Cu contaminated soil was different from those in soils contaminated with the other metals, and (iii) the microbial communities in soils contaminated with Pb and Ni were indistinguishable from one another.

Whole-community DNA was extracted from two pasture soils, the cultivable and directly extractable bacterial subsets of one of the soils, and two aquatic bacterioplankton communities. The percent guanine+cytosine (%G+C) base distribution, derived using thermal denaturation profiles, and parameterised using a generalised logistic equation, varied markedly between the samples. Bacterioplankton communities carried a more even distribution of %G+C than the soil communities, which had higher median %G+C. The directly extracted soil bacterial fraction had the highest median %G+C. There was a distinct trend of increasing complexity, measured using reassociation kinetics, between cultivable, extractable and total soil community DNA. Bacterioplankton communities were of a similar order of complexity to the cultivable soil bacterial fraction.

Adding fresh roots to intact soil cores resulted in marked increases in microbial and microfaunal activity at the resource islands. Microbial activity increased in two phases following root addition. Respiratory activity and concentration of respiratory enzyme (dehydrogenase) in soil adhering to the roots was very high during the first three weeks resulting in anaerobic conditions in the soil. After a period of low respiratory activity and enzyme content, these quantities increased from 6 to 20 weeks, but not enough to maintain anaerobic conditions. Numbers of protozoa peaked earlier than the nematodes. Based on yield coefficients of microbes and bacterivores, the increase in bacterivores was in accordance with root-induced respiration activity. In soil adhering to roots, numbers of bacterial grazers (protozoa and nematodes) were up to 80 and 30 times higher, respectively, than in the surrounding soil. This effect is up to 20 times higher than observed around live root systems, which may suggest that the rhizosphere effect on microbivores could for the major part result from the decomposition of dead segments of the root system.