The impact of earthworms on the abundance of Collembola: improvement of food resources or of habitat?

I assessed the direct influence of earthworm excretions, and the impact of earthworms through their action on the soil structure (increased macroporosity), on the population dynamics of the collembolan species Heteromurus nitidus. The intestinal content of Collembola arising from cultures on different soil types was observed, and two experimental cultures of H. nitidus were run: (1) a culture performed on an inert substrate supplied either with earthworm casts or with soil as food resource, (2) an experiment using microcosms with cores of two humus forms (moder and calcic mull), in the presence or absence of earthworms. The observation of gut contents revealed that H. nitidus feeds on excrements, the composition of which (ratio organic matter/mineral matter) varies according to the humus form where it lived. Slightly aged (10–15 days) organo-mineral casts of earthworms appeared to be a better food than calcic mull aggregates or organic material from moder. Densities of H. nitidus cultured in cores of calcic mull were higher than in moder, except when cores of moder were inhabited by an anecic earthworm for 2 months. The humus form strongly influenced populations of H. nitidus, firstly because densities of predators were higher in moder than in calcic mull, and probably also because of soil macroporosity. It was concluded that earthworms would affect predation on H. nitidus by creating a network of interconnected macropores in which Collembola can move and find shelter.     

Earthworms and collembola relationships: effects of predatory centipedes and humus forms

Relationships between anecic earthworms (Lumbricus terrestris and Aporrectodea giardi) and the collembolan species Heteromurus nitidus (Templeton, 1835), which is known to be attracted to earthworms, were investigated in an 8-week laboratory experiment. Our aims were (1) to assess whether earthworms influence the population dynamics of H. nitidus, and (2) to study pathways of influence and how earthworm effects are modified by humus forms and predators. Using microcosms with three defaunated humus forms, then provided with earthworms and predators, we intended to demonstrate that, amongst possible favourable effects of earthworms on springtail populations, earthworm activity may provide greater access and more pathways for springtails to explore soil and avoid predation. We expected that the effects of predators (centipedes) on the abundance of H. nitidus would increase from less (calcic mull) to more (moder) compact soil, and we hypothesized that earthworms would reduce predation pressure on H. nitidus by providing escape routes through increased macroporosity. Humus forms and earthworms only affected the population size of H. nitidus under high predation pressure, when collembolan numbers were higher in calcic mull than in moder, and were increased by the presence of earthworms. These results corroborate the hypothesis that earthworms, by increasing soil macroporosity, improve the escape routes for Collembola and thus evade predation. In moder humus earthworms increased the density of H. nitidus whether predators were present or not, so we cannot exclude that earthworms were also directly beneficial to H. nitidus. However, the hypothesis of a functional relationship mediated by soil macroporosity seems relevant since it was supported by differences observed when considering body size. When two size classes were distinguished within populations of H. nitidus (1) the positive effect of earthworms in moder was observed only on larger Collembola (>1 mm), (2) the density of the larger Collembola was decreased by predation only in moder and not in mull, (3) the effects of predators on the smaller individuals were not influenced by the presence of earthworms whatever the humus form, and was not decreased by the presence of earthworms. Nevertheless, factors other than macroporosity may operate as the presence of earthworms in acidic mull led to an unexplained decrease in the abundance of small-sized H. nitidus.     

Movement response of Collembola to the excreta of two earthworm species: Importance of ammonium content and nitrogen forms

Several studies reported variable effects of earthworms on microarthropod density and variety. The present study tests the attraction of seven collembolan species belonging to four families, to the excreta of two earthworm species belonging to two families and two ecological categories, Aporrectodea giardi and Hormogaster elisae. Our objectives were (1) to better understand the impact of earthworms on the composition and density of Collembola communities, and (2) to dissect mechanisms involved in the attraction. Experiments were performed in Petri dishes containing two half-disks of filter paper, one with earthworm excreta, i.e. casts or a mix of mucus and urine, and the other with natural soil aggregates or water, respectively. Collembola were introduced half-way between the two half-disks and their number was counted on each half-disk and compared over 140 min. The content of ammonium in casts and mucus-urine of both earthworm species was analyzed to determine whether it altered the responses of Collembola faced with different types of earthworm excreta. The behaviour of Collembola varied strongly among the seven collembolan species, and with type of excreta and earthworm species. Six collembolan species were attracted to the mucus and urine of at least one earthworm species. The mucus-urine mixture of A. giardi, with low ammonium content, was generally more attractive than that of H. elisae, which was even repulsive in some cases, probably because of high levels of ammonium. The attraction to casts of the two earthworm species was less frequent and more variable. Folsomia candida was neither attracted to the casts nor to the mucus and urine of any earthworm species. Therefore, (1) earthworm species with different ecology, and different nitrogen excretion pathway impact differently the behaviour of collembolan species belonging to the same family or arising from the same habitat, and (2) variations in the sensitivity to ammonium among collembolan species partially explain the variable response of Collembola to earthworm excreta.     

Different behavioral patterns of the earthworms Octolasion tyrtaeum and Diplocardia spp. in tallgrass prairie soils: potential influences on plant growth

This study addressed differences between Diplocardia spp. (a native North American earthworm) and Octolasion tyrtaeum (an introduced European species), with respect to behavior, influence on soil microbial biomass, and plant uptake of N in tallgrass prairie soils. We manipulated earthworms in PVC-encased soil cores (20 cm diameter) over a 45-day period under field conditions. Treatments included: (1) control with no earthworms, (2) Diplocardia spp. only, and (3) O. tyrtaeum only. Prior to addition of earthworms, seedlings of Andropogon gerardii (a dominant tallgrass) were established in each core, and a dilute solution of ¹³C-labeled glucose and ¹⁵N-labeled (NH₄)₂SO₄ was added to the soil to facilitate examination of earthworm/microbe/plant interactions. We found that Diplocardia spp. were significantly more active than O. tyrtaeum, and quickly assimilated ¹³C and ¹⁵N from the tracer. Individuals of Diplocardia spp. were present at shallower soil depths than O. tyrtaeum throughout the study. Contrary to expectation, this greater activity of Diplocardia spp. did not result in increased plant productivity. Rather, the activity of Diplocardia spp. was associated with less plant growth and smaller amounts of N acquired by A. gerardii seedlings compared to controls or O. tyrtaeum treatments. We observed few significant influences of earthworm treatments on microbial biomass C or N pool sizes, but the microbial C/N ratio was consistently greater in the presence of Diplocardia spp. relative to O. tyrtaeum. Results of this study indicate that activity of earthworms may enhance competition for N between microbes and plants during the growing season in tallgrass prairie.     

Effects of earthworms on Zn fractionation in soils

Laboratory incubation experiments were conducted to examine the effect of earthworm (Pheretima sp.) activity on soil pH, zinc (Zn) fractionation and N mineralization in three soils. No Zn uptake by earthworms was observed. Zinc addition decreased pH of red soil (soil 1) and hydragric paddy soil (soil 3) by 0.5 and 0.2 unit, respectively, but had no effect on alluvial soil (soil 2). The effect of Zn on soil pH was possibly due to a specific adsorption mechanism between Zn and oxides. Earthworm activity significantly decreased the pH of the red soil, a key factor affecting Zn solubility, but not of the other two soils. Earthworm activity significantly increased DTPA-Zn (DTPA-extractable) and OxFe-Zn (NH₂OH-HCl-extractable) in the red soil, but had little effect on other fractions. In the alluvial soil, earthworm activity significantly increased OxFe-Zn but decreased organic-Zn (organic-associated Zn). In the hydragric paddy soil, earthworm activity significantly increased MgCl₂-Zn (MgCl₂-extractable) and organic-Zn. The level of CaCl₂-extractable Zn in all three soils was not affected by earthworm activity. Nitrogen mineralized as a result of earthworm activity was equivalent to 110, 120 and 30 kg N ha^-1 in soils 1, 2 and 3, respectively. Zinc added at rates less than 400 mg Zn kg^-1 did not seem to affect the activity of N-mineralizing microorganisms. The present results indicated the possibility of increasing the metal bioavailability of relatively low level metal-contaminated soils, with a higher organic matter content, by earthworm inoculation.     

Toxicity of Pb/Zn mine tailings to the earthworm <Emphasis Type="Italic">Pheretima</Emphasis> and the effects of burrowing on metal availability

Lead and zinc mine tailings from Guangdong, China were found to be toxic to two common species of the earthworm Pheretima, unless the spoil was diluted by at least 20% with an artificial uncontaminated soil. With different soil dilutions, there were significant differences in mortality, growth and Zn uptake between the two species. Zinc and arsenic were principally responsible for the toxicity of the tailings. LC₅₀ values for Zn, Pb, Cu and As for Pheretima were similar to those reported in the literature for Eisenia fetida. Following inoculation with the earthworms, spoil pH, EC and available metals increased; available-Pb and -Zn concentrations increased by up to 48.2% and 24.8% respectively. The findings suggest that earthworms may play an important role in reclamation of mining spoils to achieve a healthy soil supporting sustainable vegetation. Potentially increased toxicity of metals to plants and animals, through increased bioavailability in the presence of earthworms, is discussed in the context of their role in modifying the efficiency of phytoremediation treatments.     

Dynamic study of the burrowing behaviour of Aporrectodea nocturna and Allolobophora chlorotica: interactions between earthworms and spatial avoidance of burrows

The behaviour of earthworms belonging to two different species and ecological types (Aporrectodea nocturna and Allolobophora chlorotica) was studied using two-dimensional (2D) terraria. Two experiments were set up to gain insight into the nature of interactions between these earthworms. Firstly, the evolution of the burrow systems was analysed with the density of the earthworms varying from one to five individuals. Secondly, a burrow system was first established by using one earthworm which was then removed before the introduction of a second earthworm. This second earthworm therefore encountered a burrow system created either by a conspecific earthworm or by an earthworm of the other species. These experiments showed that: (1) intra- and interspecific interactions occur between earthworms, (2) these interactions are dependent on the physical presence of the earthworms, and (3) spatial avoidance can occur (A. chlorotica avoiding burrows created by A. nocturna). The results suggest that earthworm burrow systems are "individual structures", rarely used by other earthworms when inhabited. When abandoned, the burrows may be recolonised depending on the ecological type of the earthworm under consideration.     

Effect of organic manure and the endogeic earthworm Pontoscolex corethrurus (Oligochaeta: Glossoscolecidae) on soil fertility and bean production

A biofertilisation assay was conducted in Maripasoula (French Guiana), testing the effects of three different organic amendments (manioc peels, sawdust and wood charcoal) and the inoculation of the endogeic earthworm Pontoscolex corethrurus on pod production of Vigna unguiculata sesquipedalis and on soil chemical properties (pH, C, N, total and exchangeable P and K). Pod production was highest with manioc peels as available P increased in the soil. Wood charcoal also had a beneficial effect on pod production as it decreased acidity and increased the C:N ratio in the soil. In sawdust-amended soil, pod production did not differ from that in unamended soil. Inoculation of earthworms at a density of 80 sub-adults m^-2 did not significantly affect either pod production or soil nutrient content directly, although it increased the positive effect of manioc peels on pod production. Soil nutrient content, pod production and earthworm density at the end of the experiment were negatively correlated with soil moisture and positively with each other. Despite the strong effect of moisture, this assay demonstrated an interaction between the earthworm P. corethrurus and the legume V. unguiculata sesquipedalis mediated by soil nutrient content and organic matter inputs. We conclude that manioc peels improved soil P availability and were an interesting amendment for legume crops. We discuss also the effect of earthworm inoculation.     

Nitrous oxide emissions from the wheat-growing season in eighteen Chinese paddy soils: an outdoor pot experiment

To identify the key soil parameters influencing N₂O emission from the wheat-growing season, an outdoor pot experiment with a total of 18 fertilized Chinese soils planted with wheat was conducted in Nanjing, China during the 2000/2001 wheat-growing season. Average seasonal N₂O-N emission for all 18 soils was 610 mg m^-2, ranging from 193 to 1,204 mg m^-2, approximately a 6.2-fold difference between the maximum and the minimum. Correlation analysis indicated that the seasonal N₂O emission was negatively correlated with soil organic C (r²=0.5567, P<0.001), soil total N (r²=0.4684, P<0.01) and the C:N ratio (r²=0.4530, P<0.01), respectively. A positive dependence of N₂O emission on the soil pH (r²=0.3525, P<0.01) was also observed. No clear relationships existed between N₂O emission and soil texture, soil trace elements of Fe, Cu and Mg, and above-ground biomass of the wheat crop at harvest. A further investigation suggested that the seasonal N₂O-N emission (E, mg m^-2) can be quantitatively explained by E=1005-34.2SOC+4.1S_a (R²=0.7703, n=18, P=0.0000). SOC and S_a represent the soil organic C (g kg^-1) and available S (mg kg^-1), respectively.     

Soil carbon and nitrogen dynamics in Lumbricus terrestris. L. middens in four arable, a pasture, and a forest ecosystems

Lumbricus terrestris' middens contain large concentrations of organic material and have been characterized as microenvironments distinct from the surrounding soil. The direct and indirect consequences of midden formation on nutrient cycling dynamics and organic matter pools in various ecosystem types have not received much consideration. Therefore, we focused on the differences in C and N dynamics between midden and bulk soil samples in four corn (Zea mays L.) agroecosystems, a rotational pasture and a deciduous forest, in June, July and August of 1996, in Ohio, USA. Paired earthworm midden and bulk soil samples were analyzed for mineral N (NH₄⁺-N and NO₃^--N), dissolved organic N, microbial biomass N (MBN) and carbohydrate C (CarbC). Additionally, coarse litter, fine litter, particulate organic matter, and soil organic matter fractions were separated and analyzed for total C, total N and C:N ratios. Mineral and dissolved N levels were higher in the midden soil relative to those in the bulk soil for all ecosystem types, except for only NO₃^--N levels in two highly fertilized agroecosystems and in the pasture. MBN, CarbC, and total C and N levels for all organic fractions were significantly greater in the earthworm midden samples relative to these in the bulk samples across all ecosystem types. The plan defined by principal component analysis clearly separated two main groups: (1) includes the forest, the pasture and the less fertilized cornfields and the midden effect is to increase slightly the organic matter content and strongly the inorganic N content, and (2) includes the heavily fertilized agroecosystems and the midden effect is also to increase the organic matter content but to decrease the inorganic N content. We concluded that L. terrestris' middens significantly raised overall soil C and N levels relative to the bulk soil, in a variety of ecosystem types, and, given the abundance of earthworm middens, these macrosites should receive important attention when evaluating nutrient cycling processes at the systems level.     

Ecotoxicity of mercury to Eisenia fetida, Enchytraeus albidus and Folsomia candida

Despite growing concerns about the potential adverse effects of elevated mercury concentrations in the environment, only a few toxicity data are available for soil invertebrates. The chronic toxicity of mercury (II) was therefore assessed for Eisenia fetida, Enchytraeus albidus and Folsomia candida using standard test protocols. The 21-day EC₅₀ (the concentration causing 50% effect) for the cocoon production of E. fetida was 9.16 mg Hg kg^-1 dry wt. Based on the reproduction, a 42-day EC₅₀ of 22.0 mg Hg kg^-1 dry wt was observed for E. albidus while for F. candida the 28-day EC₅₀ was 3.26 (2.45-4.05) mg Hg kg^-1 dry wt. Although these data can be considered as a step forward in the assessment of the potential risks of mercury in terrestrial environments, further research is needed to evaluate the influence of soil parameters on the toxicity of mercury and to quantify the effect of ageing on the bioavailability.     

Dehydrogenase activity, redox potential, and emissions of carbon dioxide and nitrous oxide from Cambisols under flooding conditions

Samples from topsoils (0-10 cm) of 16 Polish arable Cambisols developed from different parent materials (sand, silt, sandy gravel, loess, loam and clay), were incubated under flooded conditions with NO₃^-. Dehydrogenase activity, redox potential (Eh), and emissions of CO₂ and N₂O were measured. According to dehydrogenase activity, the soils were divided into two groups: those of low activity (I), where the final dehydrogenase activity was <0.03 nmol triphenylformazan (TPF) g^-1 min^-1, and those with high final dehydrogenase activity (II), >0.03 nmol TPF g^-1 min^-1. Generation of CO₂ and of N₂O under flooded conditions was shown to be significantly related to dehydrogenase activity. Soil dehydrogenase activity increased curvilinearly with organic matter content, showed a maximum at pH 7.1, and decreased curvilinearly with Eh. The final cumulative CO₂ production increased linearly with soil organic matter content and curvilinearly with dehydrogenase activity and decreased linearly with Eh. The most significant relationship was found with dehydrogenase activity (R²=0.74, P<0.001). The final cumulative N₂O production decreased linearly with Eh and increased curvilinearly with pH and dehydrogenase activity but linearly with organic matter content; the most significant relation being found with dehydrogenase activity (R²=0.69, P<0.001). The CO₂:N₂O ratio in the gases evolved increased curvilinearly with Eh and decreased with dehydrogenase activity and N₂O and CO₂ production.     

A spontaneous chlorate-resistant mutant of Azospirillum brasilense Sp245 displays defects in nitrate reduction and plant root colonization

Azospirillum, a soil bacterium capable of colonizing plant roots, can reduce NO₃^-. In this work, a spontaneous chlorate-resistant mutant of Azospirillum brasilense Sp245, named Sp245chl1, was phenotypically characterized. The mutant is defective in both assimilatory and periplasmic dissimilatory nitrate reductase activity. Using the gusA reporter gene methodology, Sp245chl1 was found to be significantly affected in its ability to colonize roots of wheat and rice seedlings.     

Effects of endogeic earthworms on soil processes and plant growth in coniferous forest soil

Summary The effects of the endogeic earthworm, Aporrectodea caliginosa tuberculata (Eisen) on decomposition processes in moist coniferous forest soil were studied in the laboratory. The pH preference of this species and its effects on microbial activity, N and P mineralization, and the growth of birch seedlings were determined in separate pot experiments. Homogenized humus from a spruce stand was shown to be too acid for A. c. tuberculata. After liming, the earthworms thrived in the humus and their biomass increased (at pH above 4.8). In later experiments in which the humus was limed, the earthworms positively influenced the biological activity in humus and also increased the rate of N mineralization. A. c. tuberculata increased the growth of birch seedlings, with increases observed in stems, leaves, and roots. Neither NH ₄ ⁺ -N fertilizer nor mechanical mixing with artificial worms affected seedling growth. No plant-growth-affecting compounds (e.g., hormone-like compounds) due to the earthworms were present in the humus. The shoot: root ratio in the birch seedlings was not affected by either the earthworms or the fertilizer. The experiments revealed the impact of earthworm activity on soil processes and plant growth.     

Short-term patterns of carbon and nitrogen mineralisation in a fallow field amended with green manures from agroforestry trees

The mineralisation of green manure from agroforestry trees was monitored with the objective to compare the temporal dynamics of mineralisation of litter from different species. Green manures from five agroforestry tree species were used on a fallow field during the long rainy season of 1997 (March-August) and from two species in the following short rainy season (September-January) in western Kenya. Different methods, i.e. measurements of isotopic ratios of C in respired CO₂ and of soil organic matter (SOM) fractions, soil inorganic N and mass loss from litterbags, were used in the field to study decomposition and C and N mineralisation. Soil respiration, with the separation of added C from old soil C by using the isotopic ratio of ¹³C/¹²C in the respired CO₂, correlated well with extractable NH₄⁺ in the soil. Mineralisation was high and very rapid from residues of Sesbania sesban of high quality [e.g. low ratio of (polyphenol+lignin)/N] and low and slow from low quality residues of Grevillea robusta. Ten days after application, 37% and 8% of the added C had been respired from Sesbania and Grevillea, respectively. Apparently, as much as 70-90% of the added C was respired in 40 days from high quality green manure. Weight losses of around 80%, from high quality residues in litterbags, also indicate substantial C losses and that a build-up of SOM is unlikely. For immediate effects on soil fertility, application of high quality green manure may, however, be a viable management option. To achieve synchrony with crop demand, caution is needed in management as large amounts of N are mineralised within a few days after application.     

Cultivable heterotrophic N<Subscript>2</Subscript>-fixing bacterial diversity in rice fields in the Yangtze River Plain

Heterotrophic N₂-fixing bacteria are a potentially important source of N₂ fixation in rice fields due to the moist soil conditions. This study was conducted at eight sites along a geographic gradient of the Yangtze River Plain in central China. A nitrogen-free solid malate-sucrose medium was used to isolate heterotrophic N₂-fixing bacteria. Numbers of the culturable N₂-fixing bacteria expressed as CFU (colony forming units) ranged between 1.41ǂ.42᎒⁶ and 1.24ǂ.23᎒⁸ in the sampled paddy field sites along the plain. Thirty strains with high ARA (acetylene reduction activity) were isolated and purified; ARA of the strains varied from 0.9 to 537.8 nmol C₂H₄ culture^-1 h^-1, and amounts of ¹⁵N fixed ranged between 0.008 and 0.4866 mg·culture^-1·day^-1. According to morphological and biochemical characteristics, 14 strains were identified as the genus Bacillus, 2 as Burkholderia, 1 as Agrobacterium, 4 as Pseudomonas, 2 as Derxia, 1 as Alcaligenes, 1 as Aeromonas, 2 as Citrobacter, and 3 strains belonged to the corynebacter-form group.     

Naidid worms (Oligochaeta, Naididae) in paddy soils as affected by the application of legume mulch and/or tillage practice

Reproduction, intrinsic rate of natural increase and population density of naidid worms were investigated in submerged paddy fields and the laboratory. No tillage plus legume-mulching increased the population density of naidid worms. Soil treatments with neither tillage nor legume mulch, and tillage practice alone, did not increase the number of worms. Dero dorsalis Ferronnière was dominant in soil of the no-tillage treatment. In laboratory experiments, legume-mulching with the proper amount of dissolved O₂ accelerated asexual reproduction of D. dorsalis through zooid budding. For the legume and aeration treatment, (N_i+1-N_i) N_i^-1 values (where N_i and N_i+1 are the populations at times t=i and t=i+1) were plotted against N_i+1. Utilizing this linear relation, this data fitted the logistic curve (r²=0.885, P<0.05). Based on the linear relation, the intrinsic rate of natural increase (r), carrying capacity (K), and doubling time (T) were calculated as 0.2125 day^-1, 12,666 m^-2, and 3.26 days, respectively. The amounts of legumes applied were highly correlated with the population of D. dorsalis, indicating that the weight of legume is a limiting factor with respect to carrying capacity. A literature review indicated a significant correlation (P<0.01) between intrinsic rate of natural increase and maximum body length of naidids with temperature conversion of the growth rate. Sexually mature worms were rarely found in submerged paddy fields. Sexual reproduction seems to be adopted in response to soil desiccation after paddy field drainage.     

Influence of soil parameters on the effect of 3,4-dimethylpyrazole-phosphate as a nitrification inhibitor

Nitrification inhibitors specifically retard the oxidation of NH₄⁺ to NO₂^- during the nitrification process in soil. In this study, the influence of soil properties on the nitrification-inhibiting effect of 3,4-dimethylpyrazole-phosphate (DMPP), a newly developed nitrification inhibitor, has been investigated. Based on short-term incubation experiments, where the degradation of DMPP could be largely disregarded, the oxidation of the applied NH₄⁺ was more inhibited in sandy soils compared with loamy soils. The influence of soil parameters on the relative NO₂^- formation could be described by a multiple regression model including the sand fraction, soil H⁺ concentration and soil catalase activity (R²=0.62). Adsorption studies showed that the binding behaviour of DMPP was influenced markedly by soil textural properties, viz. the clay fraction (r²=0.61). The adsorption of DMPP was found to be an important factor for the inhibitory effect on NH₄⁺ oxidation in a short-term incubation (r²=0.57). It is concluded that the evaluated soil properties can be used to predict the short-term inhibitory effect of DMPP in different soils. The significance of these results for long-term experiments under laboratory and field conditions needs further investigation.     

Cultivar-specific dinitrogen fixation in Vicia faba studied with the nitrogen-15 natural abundance method

N fixation by different faba bean (Vicia faba) cultivars was studied using the natural abundance method. The delta ¹⁵N ('¹⁵N) values of the faba beans and the reference plants differed by 4.6-7.0‰. The non-nodulating V. faba cv. F48 seems to be the best reference plant for nodulated and N₂-fixing V. faba. Significant differences occurred in the quantity of N₂ fixation of six V. faba cultivars. The average fraction of N derived from air (FNdfa) estimated from leaf material ranged between 69 and 80%. Shoot-based estimates of N fixation varied between 200 and 360 kg N ha^-1. N fixation was affected more by differences in FNdfa than by differences in total N accumulation. Fixation data calculated with the non-nodulated reference plant V. faba cv. F48 were lower than those calculated with cabbage (Brassica oleracea) and ryegrass (Lolium perenne) as reference plants. Of all reference plants, non-N₂-fixing V. faba cv. F48 has a root system and temporal pattern of N assimilation that is the one most similar to that of N₂-fixing V. faba plants. Cv. F48 showed senescence as did the other V. faba cultivars after pod-fill was complete, whereas cabbage, ryegrass and camomile had a later senescence period. N fixation during pod-filling appears more important for a good yield than N₂ fixation abilities in the earlier growth period. The best V. faba cultivars left about 100 kg N ha^-1 in residual material on the field as fertilization for the following crops.     

Microfungal community structure in anthropogenic birch stands in central Finland

We describe the soil microfungal communities in 30-year-old birch (Betula pendula Roth) stands planted either on former spruce forest soil (BS) or on former arable soil (BF) and compare these with the soil microfungal communities in spruce forests (S), arable fields (F) and old deciduous forests (D). Fungi were isolated from 0- to 3-cm and 3- to 6-cm samples collected in September 1997 and May 1998. Principal components analysis differentiated fungal communities in the S and BS sites from those in the other site types. The Morisita-Horn index of similarity indicated that fungal communities in the F and BF sites were less similar to those in the other site types. Fungal communities of the BS and S sites were more similar in the 3- to 6-cm layer than in the 0- to 3-cm layer, suggesting that it takes >30 years to replace the spruce litter and associated fungal community. Similarity between fungal communities in the F and BF sites was low, indicating significant changes in the fungal community composition following afforestation of field soil by birch. Fungal communities in the BS and BF sites were more similar to each other than those of their original site types. Fungal communities of the BS, BF and S sites were more similar to those of the D sites than those of the F sites were to the D sites. Differences between the fungal communities of the different site types were attributed primarily to litter quality, earthworm community, and secondarily to organic matter, pH, KCl-extractable NH₄⁺, and PO₄^3-.