Soils. Effects of plant-bacterial-amoebal interactions on plant uptake of nitrogen under field conditions. Biology and Fertih'ty. M.

Transcription

1 Biol Fertil Soils (1989) 8: Biology and Fertih'ty Soils Springer-Verlag 1989 Effects of plant-bacterial-amoebal interactions on plant uptake of nitrogen under field conditions M. Clarholm Swedish University of Agricultural Sciences, Department of Microbiology, S Uppsala, Sweden Summary. Bacterial biomass and numbers of bacterivorous naked amoebae were estimated daily in soil associated with barley roots and, to avoid the influence of roots, in soil from a field in its fifth summer under bare fallow. The estimates were associated with two rainfall events and were started just before the first. Increases in bacteria were observed after each rainfall, and bacterial production was about the same size for both treatments. A peak in naked amoebae followed each burst of bacterial production in the root-associated soil, whereas in the fallowed soil protozoan production was low after the first rainfall and undetectable after the second. The bacterial populations in the fallowed soil had yet to decline by the end of the 12-day study, probably because grazing pressure by protozoa was low. Calculations based (1) on shortterm decreases in bacterial biomass in soil close to roots or (2) on the amount of C added to the soil by plants over the growing season indicated that N released via bacterial-protozoan interactions contributed 10% - 17% of the N taken up by the fertilized barley. Key words: Bacteria - Protozoa - Naked amoebae - Soil - Nitrogen mineralization - Ammonium excretion Bacteria and bacterivorous naked amoebae are more numerous in soil surrounding roots than in bulk soil (Darbyshire and Greaves 1967; Bamforth 1976). When protozoa feed on bacteria in water, one-third of the N in the bacterial biomass is converted into protozoan Offprint requests to: Swedish University of Agricultural Sciences, Department of Ecology and Environmental Research, Box 7072, S Uppsala, Sweden biomass, one-third, mainly cell walls and organelles, is excreted as organic N in indigestible parts, and onethird is excreted as ammonium (Fenchel 1982; Goldman et al. 1985). The same relations are most likely valid for bacteria and protozoa in soil since these organisms are only active in the soil water phase. It has been suggested that because bacteria using rootderived C mineralize organic N to support their own growth, naked amoebae feeding on the bacteria should cause a local mineralization of N close to the root (Clarholm 1985b). This mineralization may, in turn, increase the total amount of N available for plant uptake. The series of events outlined above differs in its first step from that described by Elliott et al. (1984a) inasmuch as it implies that bacterial-induced mineralization of the organic N increases as root-derived C becomes available. Elliott et al. (1984a) suggested that bacteria will immobilize N at a higher rate when using root-derived C than when using non-root-derived C sources. In consequence, the flux of inorganic N towards the root should increase as a result of the concentration gradient created. Bacterivorous protozoa will render part of the immobilized N available for root uptake. In the short term, the final result of the two chains of events will be the same, viz. an increase in the availability of N close to the root. In the long term, however, the amount of organic N mineralized in association with the first-mentioned series of events should be greater than that mineralized in association with the event series suggested by Elliott et al. (1984a). That protozoa provide plants with a vital supply of N has been demonstrated experimentally; 6-week-old wheat plants grown in sterilized, unfertilized soil with bacteria and protozoa present contained 62% more N than plants grown in soil with bacteria but without protozoa. The addition of small amounts of glucose twice a week, to mimic an increased release of root-de-

2 374 rived C, increased the difference to 71% (Clarholm 1985 a). The microcosm experiment in the laboratory did not indicate anything about the extent and importance of the interactions in the field, only that the presence of protozoa increased the flow of N from the soil organic matter to the plant. There are few field data to support the hypothesis that bacterial-protozoan interactions contribute substantially to N mineralization. Elliott et al. (1984b) reported that inorganic-n levels were positively correlated with numbers of naked amoebae but negatively correlated with bacterial abundance in monthly estimates in a fallowed dryland wheat plot. Rain has repeatedly been shown to enhance protozoan numbers. Cutler et al. (1922), who made daily counts of bacteria and protozoa in a manured fallow field, reported that rain had a positive effect on protozoa and that there was an inverse relationship between protozoan abundance and bacterial numbers, as measured by plate counts. Elliott and Coleman (1977), who sampled a native shortgrass prairie soil on two dates, found more protozoa (less than a factor of two) in an irrigated treatment than in an unirrigated control plot, while the protozoan abundance in an irrigation plus fertilization treatment was four times that of the control. When the same soil was brought into the laboratory as undisturbed cores, watered once, and sampled six times in 22 days, the authors observed a threefold increase in protozoa 7 days after the water addition. Daily estimates in the heavily root-influenced humus layer of a pine forest soil shortly after a rain that ended a dry spell showed an initial increase in bacteria followed by an increase in numbers of naked amoebae concomitant with a bacterial decrease (Clarholm 1981). More specifically, on day 2 bacteria were abundant while protozoan numbers were low; by day 3 bacterial numbers had declined sharply while numbers of amoebae had increased; and by day 4 amoebae were very abundant while bacterial numbers were still low. More rain on day 5 led to a second bacterial increase on day 6. Thus, in the space of only 1 day, relations between the gi'oups had totally changed. Studies based on less than daily sampling are therefore of limited value when bacterial-protozoan interactions are to be investigated. Few studies have been made on the effects of plants on bacterial-protozoan interactions. Using weekly samplings, Darbyshire and Greaves (1967) found that populations of naked amoebae and flagellates remained high in soil under white mustard during a 3-week period up to flowering. In contrast, bacterial numbers were high on only one sampling occasion, indicating a high rate of bacterial consumption by the protozoa. The treatment with unplanted soil had low, stable values for all groups throughout the period. Bacterial and amoebal production was four times higher in a soil planted with wheat than in a soil without plants when microbial development was monitored after a dry/wet cycle in the laboratory (Clarholm 1981). Under both treatments, bacteria peaked on day 2 after rewetting, while naked amoebae peaked on day 5. The objectives of the present study were to determine the effects of roots on bacterial biomass and numbers of protozoa in the field during a period of high microbiological activity shortly after a dry spell had been broken. Only 1.5 mm of rain had fallen during the 12 days preceding the start of the study, and initially the cropped soil was drier (-460 kpa, Fig. 1) than the fallow soil (-375 kpa). Observations were also made to estimate the amount of N made available to the plant by protozoan grazing on bacteria. Materials and methods The study was carried out at the Kjettslinge experimental site used by the project "Ecology of Arable Land - The Role of Organisms in Nitrogen Cycling". The field is located 40 km north of Uppsala in central Sweden (60 I0'N, 17 38'E). The soil is classified as a Mollic Gleysol (FAO-UNESCO 1974) and has a loam texture (19 70 clay), an organic-c content of and an N content of o of dry soil, with a ph~ s O~ of 6.3 (Steen et al. 1984). The study used four replicated barley plots, each 560 m 2, which had been sown and fertilized on May 23, 1983, with 12 g N, as Ca(NO3)2, 2.8 g P, 5.2 g K per m 2, and an adjacent, unreplicated, non-fertilized, fallow plot which had been kept bare of vegetation for five summers by repeated harrowing. The few weeds that grew in the fallow plot were removed by hand before the study started, and four subplots were randomly chosen for sampling. In each of the replicated barley plots the first subplot to be sampled was randomly selected. Subsequent samples were taken 1 m away from the preceding sampling site. About 20 cm of a plant row including the above-ground plant part, roots and adhering soil was removed down to the rooting depth (10-15 cm; the rooting depth increased over the sampling period), causing as little disturbance as possible. The barley treatment was always sampled first, and soil from the fallow treatment was sampled to the same depth as the corresponding barley sample. The samples were brought to the laboratory, where the roots plus adhering soil were removed by hand. The number of sampled plants was registered on each occasion. Approximately 2.3 g fresh weight of fine roots with adhering soil (average root : soil ratio 1 : 65) was transferred to a flask with Neff's saline (Page 1967). Part of the diluent was shaken with roots plus adhering soil for 1 h, and then the roots were collected on a sieve, rinsed with the rest of the diluent (total 250 ml), dried with blotting paper, and weighed. The root weight was later subtracted from the root plus soil weight to obtain the amount of soil in each sample. For the fallow treatment, an equivalent amount of soil was taken from each soil sample after mixing. Soil and diluent were mixed in a kitchen blender (Braun, Federal Republic of Germany) at a low speed for 15 s. The protozoa in this suspension were enumerated using a most probable number technique (Darbyshire et al. 1974) with some modifications (Clarholm 1981). After the samples for protozoan estimates had been removed, 1 ml of a concentrated detergent (Calgon, British Drug Houses) so-

3 375 lution was added to obtain a final concentration of 0.2% (w : v). The solution was then mixed for 2 min, allowed to rest for 5 min, and then mixed for an additional minute to help disperse the bacteria, which grow in clusters in soil. Slides for direct bacterial counts were prepared on each sampling occasion. At the end of the sampling period all slides were stained with acridine orange (I : aqueous solution), and bacterial counts were made. The slides were observed in a Zeiss epifluorescent microscope at 1000x magnification. At least 200 bacteria were examined in each sample. The bacterial biomass was calculated on the basis of the size-class distribution, assuming nominal values for each size class (Clarholm and Rosswall 1980). For the protozoa two microtiter plates were prepared from each soil sample, and the mean was used as an estimate for the sample. Soil water potentials were calculated from gravimetric estimates of soil water content and a soil water retention curve for the investigated soil. Dry weights of above-ground plant parts and soil samples were estimated after drying them at 85 C. The total N content of the plants was determined after Kjeldahl digestion. 1.6 a b 5 ~ 3 ~ E Results Before the first rain event, the soil planted with barley was drier than the fallow soil (Fig. 1, table at bottom). After the first rain, on day 3, the planted soil dried to -885 kpa by day 5, whereas the fallowed soil remained relatively moist (-159 kpa). After the heavy rain on day 7, which was followed by light showers during subsequent days, the soil water potentials remained low for both treatments. The method used here to count protozoa can be used to enumerate ciliates, flagellates, and naked amoebae if their population densities are higher than 250 individuals g-~ dry soil. Ciliates were spotted in the microtiter plates only occasionally, while flagellate numbers were around 104 g-i dry soil. The temporal dynamics of the flagellate population were similar to those of the bacterial population. Because flagellates are smaller, their grazing impact on bacteria, at the numbers registered, will be much less than that of the amoebae (Elliott and Coleman 1977; Clarholm 1981). Therefore the following discussion is restricted to naked amoebae. Despite the lower moisture deficit in the fallow soil it contained only 17% of the bacterial biomass in the soil under barley before the first rain event. After the first rain, similar responses were observed under the fallow and barley treatments. An increase in bacterial biomass (Fig. i b), which reached its maximum 2 days after the rain, was followed by a peak in naked amoebae (Fig. I a). Although the percentage increase in bacterial biomass was larger in the fallow soil, the peak level was lower compared with that in the root-containing soil, as was the case before the rain. A decrease in bacterial biomass in the latter soil was about equal to the decrease in the fallow soil on days 6-7. However, the increase in the amoebal population associated with the decrease in bacteria was fivefold great- t~ r-t s 27, 25 -I 2O E i, 15 ~'10 i C 1" I Day 1 l ~ '0 1'2 Q,. 10 & veg.soi[ 760 I B 26 k I/+7 Soi[wafer 1 pofenfia[ fal ow , ~0 B9 78 B? 126 [-kpa] Fig. 1. Temporal changes in (a) numbers of naked amoebae (squares), (b) bacterial biomass (circles) and (c) N content per plant (triangles), following a rainfall on a dried-out soil. Filled symbols represent samples taken from soil close to barley roots; open symbols refer to values from fallowed soil. The amount of precipitation and duration of a rain event are indicated by column height and width, respectively. Soil water potentials are given in the table (bottom of figure). Day 1 was June 22, To improve clarity only one part of the SE was drawn; n = 4 er compared with that in the fallow soil. A second, very heavy rainfall on day 7 led to another increase in bacterial biomass under both treatments. Thereafter bacterial biomass declined again in the soil under barley while numbers of naked amoebae increased. In contrast, the bacterial biomass stayed high in the fal-

4 376 low soil, and there was no sign of any new increase in the numbers of naked amoebae. The uptake of N by above-ground plant parts was mainly confined to two periods (Fig. lc), an initial one between days 5 and 6, and a second one between days 9 and 12. The total N uptake by above-ground plant parts during the experimental period was 13.6 mg N per plant, which is equivalent to 28 kg N ha -I. The increase in plant dry weight was 64.1 mg per plant, and the growth curve followed that of N uptake. Discussion The growth dynamics of the bacterial and amoebal populations following rain on a dry, cropped soil followed the expected pattern based on previous observations in the laboratory (Clarholm 1981) and the field (Clarholm 1981; Schnarer et al. 1986a). Although the finding that bacterial numbers decreased before the numbers of naked amoebae began to increase seems anomalous, it can be explained by the observation that soil amoebae are adapted to exploit short periods of high food availability. When bacteria are produced, the amoebae feed first and divide later (Fenchel 1987). At the very least, they can double their size before division. Because the indirect method used for their enumeration does not take size into account, it is not possible to detect protozoan growth until the division has occurred. The high levels of bacterial production in the fallow plot were unexpected. The almost daily estimates showed that bacteria gradually built up in the fallow plots after the second rainfall, with standing-crop values approaching those of the root-containing soil on the last sampling date. Values from other occasional samplings of the same fallow treatment (Schnarer et al. 1986b) indicated that the "protected" biomass (Van Veen et al. 1984) for that soil was around mg g-1 dry weight of soil. This, in turn, indicated that the heavy rain stimulated the production of bacteria in the fallow plots. After day 7, the numbers of naked amoebae were low in the fallow plots although ample food appeared to be available, judged by bacterial abundance. Selective bacterial feeding has been reported for protozoa (Singh 1941 a; Dive 1973), and bacterial species differ in their nutritional value (Singh 1941b; Burnbanck 1942). Energy/C availability was quite different under the two treatments; no new carbonaceous material had been added to the fallow for five summers whereas the barley plots had been given yearly and daily additions. Other species of bacteria less palatable to amoebae might have proliferated to a greater extent in the fallow than in the cropped soil, which could explain the large bacterial biomass observed there. In a microcosm study with nutrient-rich oceanic water from an upwelling area (Lucas et al. 1987), photosynthetic green algae and bacteria peaked simultaneously on day 4, followed by a peak in flagellated protozoa on day 7. This peak was followed by a second increase in bacteria between days 9 and 25, and another slow increase in the numbers of flagellates, which continued until the end of the experiment (day 42). The C source for the first bacterial peak most likely consisted of products released by the living green algae. The bacterial genera contributing most to this peak were Vibrio spp. and Pseudomonas spp., which were readily eaten by the flagellates. This series of population fluctuations is comparable in many respects to those occurring around living roots. The bacteria in the second peak were primarily of the genera Acinetobacter, Flavobacterium, and Cytophaga. The second bacterial population, although similar in size to the first, resulted in a much lower level of protozoan production, i.e., about one-third as high as the first protozoan peak. Thus, the bacteria forming the second peak were apparently less palatable to the protozoa. This last sequence could be seen as equivalent to the situation in the fallow plots. Sampling for a few more days would most likely have provided additional information about the development in the fallow plots. The results from the protozoan estimates were, however, not available until 14 days after the microtiter plates had been inoculated. Moreover, it was not possible to complete the bacterial counts until after all the field work had been carried out. At the time, therefore, there were no indications that further samplings were needed. Nevertheless, on the basis of the results of Lucas et al. (1987) and the "protected biomass" concept, it seems reasonable to assume that the bacteria decreased some days thereafter, concomitant with a small increase in amoebae. On June 23 (day 2), 10.7 g N m -2 was found in the inorganic form, of which 95% occurred as NO3 (Bergstr6m 1986) and was most likely available for plant uptake. Despite the apparently high N availability, the two periods of rapid N uptake by above-ground plant parts (Fig. lc) were concomitant with the two rapid decreases in bacterial biomass (Fig. 1 b). Days 7-9 were characterized by favourable moisture but an increasing bacterial biomass. No appreciable plant uptake of N was observed during that period. It is possible that both plant uptake and bacterial production were initiated by the rainfall. However, the fact that N uptake by the plants coincided with the decrease in bacteria could also be interpreted as a sign that a local mineralization of NH~- around the root had contributed to the N uptake, even though N was present as NO~-. Cox and Reisenauer (1973) reported that wheat plants supplied with 200~tg NO~- and 10 ~tgnh~

5 377 produced more dry mass and ended up with a 38 7o higher N content compared with wheat plants given NO~- only. These results indicated that small additions of NH4 ~ in the all-nitrate situation had a strong growth-promoting effect. Increased growth was observed with additions in the gg range, with an interpolated maximum around 30 gg NH4 ~ (15o70) of inorganic N. The contribution from the bacterial biomass to the total N taken up by the plant was calculated on a day-to-day basis from bacterial decreases, based on the following data and assumptions: (1) Bacteria are composed of 10% N (Fenchel and Blackburn 1979), of which one-third is mineralized (Fenchel 1982) and taken up by the plants; (2) the field contained 226 plants m -2 (Petterson 1989) with a total root mass of 100 g dry weight m -2 (Hansson et al. 1987); and (3) the root density in the samples of rootassociated soil was, on average, 50 times higher than the average root density in the field. Using these figures, the first bacterial decrease was estimated to have accounted for 11 7o of the N uptake while the second decrease accounted for These values should be considered as minimum estimates since bacteria will also have been consumed while their numbers were increasing. Calculations for the same field based on the estimated input of root-derived C over the whole growing season (Clarholm 1985b) indicated that the N contribution from the bacterial biomass to the plants was 10% - 17O7o. Hunt et al. (1987) estimated that naked amoebae accounted for 14% of the N taken up by plants, when modelling N in a detrital food web in a shortgrass prairie. For that system, 27O/o of the N was mineralized through consumption of bacteria, nematodes being the other large group of bacterial consumers. In a phytotron study in which maize was grown in soil containing high levels of added 15NO~- (Haider et al. 1987), between 14 /0 and 20O7o of the N used by the plants came from non-tagged organic matter. The authors postulated a tight coupling between N mineralization from soil organic matter and root uptake, since the 15NO3 pool was not diluted. By the end of the experiment 500 mg untagged N had been mineralized in the treatment with maize plants, while the corresponding figure for the soil in the unplanted pots was only 150 mg. The 350 mg N additionally mineralized in the maize treatment all ended up in the plants. Root-initiated mineralization of organic N should be most important in non-fertilized soils or in fertilized soils after the added N has been taken up. The latter case applies to a grass ley, where the period of active growth and N uptake is much longer than in an annual crop like barley. According to budget calculations for the Kjettslinge experimental site, 8.6 g m -2 more N was mineralized annually from the organic matter in a grass-ley soil than from the organic matter in a soil cropped with barley (Paustian et al. 1989). The higher rate of mineralization in the grass-ley treatment did not lead to a higher rate of N loss compared with barley (Paustian et al. 1989); instead, this N was incorporated into the plants. Indirect evidence suggested a close coupling between N mineralization and plant uptake in the grass-ley, i.e., the immobilization of inorganic N by straw incubated in the grass-ley was lower compared with that of straw incubated under the barley treatments (Andr6n 1987). The observations and calculations presented here indicate that the presence of plants had a strong influence on the numbers of naked amoebae whereas the numerical effects on bacteria were small. In retrospect, it is clear that the use of the five-season-fallow plots as the control was a mistake; besides the lack of plants, too many other variables differed as well. Thus, no field study has yet been carried out with a resolution high enough to adequately determine the effects of plants on bacterial-protozoan interactions. My observations suggest that plants benefit from local N mineralization as a result of the associated increase in bacterial-protozoan interactions around the roots. Root-initiated N mineralization seems to be tightly coupled to root uptake and appears to be higher in perennial systems than in annual ones. Acknowledgment. I thank K. Paustian and T. Lindberg (Swedish University of Agricultural Sciences, Uppsala, Sweden), J.M. Tiedje (Michigan State University, Michigan, USA) and E.R. Ingham (Oregon State University, Oregon, USA) for helpful discussions and constructive criticism. The work was carried out within the "Ecology of Arable Land" project and was supported by the Swedish National Science Research Council, Stockholm, Sweden. References Andr6n O (1987) Decomposition of shoot and root litter of barley, lucerne and meadow fescue under field conditions. Swed J Agric Res 17: Bamforth SS (1976) Rhizosphere-soil microbial comparisons in subtropical forests of southeastern Louisiana. Trans Am Microsc Soc 95: Bergstr6m L (1986) Distribution and temporal changes of mineral nitrogen in soils supporting annual and perennial crops. Swed J Agric Res 16: Burnbanck WD (1942) Physiology of the ciliate Colpidium colpoda: I. The effect of various bacteria as food on the division rate of Colpidium colpoda. Physiol Zool 15: Clarholm M (1981) Protozoan grazing of bacteria in soil - impact and importance. Microb Ecol 7: Clarholm M (1985 a) Interactions of bacteria, protozoa and plants leading to mineralization of soil nitrogen. Soil Biol Biochem 17: Clarholm M (1985b) Possible roles for roots, bacteria, protozoa and fungi in supplying nitrogen to plants. In: Fitter AH, Atkinson D, Read DJ, Usher MB (eds) Ecological interactions in the soil. Spec Publ No 4, Br Ecol Soc, Blackwell, Oxford, pp

6 378 Clarholm M, Rosswall T (1980) Biomass and turnover of bacteria in a forest soil and a peat. Soil Biol Biochem 12:49-57 Cox WJ, Reisenauer HM (1973) Growth and ion uptake by wheat supplied nitrogen as nitrate or ammonium or both. Plant and Soil 38: Cutler DW, Crump LM, Sandon H (1922) A quantitative investigation of the bacterial and protozoan population of the soil, with an account of the protozoan fauna. Phil Trans R Soc B 211: Darbyshire JF, Greaves MP (1967) Bacteria and protozoa in the rhizosphere of Sinapis alba L., Trifolium repens L. and Lolium perenne L. Can J Microbiol 13: Darbyshire JF, Wheatley RF, Greaves MP, Inkson RHE (1974) A rapid micromethod for estimating bacterial and protozoan populations in soil. Rev Ecol Biol Soil 11: Dive D (1973) La nutrition holozoique des protozoaires cili6s: Ses cons6quences dans l'6puration naturelle et artificielle. Ann Biol 12: Elliott ET, Coleman DC (1977) Soil protozoan dynamics in a shortgrass prairie. Soil Biol Biochem 9: Elliott ET, Coleman DC, Ingham RE, Trofymow JA (1984a) Carbon and energy flow through microflora and microfauna in the soil subsystem of terrestrial ecosystems. In: Klug M J, Reddy CA (eds) Current perspectives in microbial ecology. Am Soc Microbiol, Washington DC, pp Elliott ET, Horton K, Moore JC, Coleman DC, Cole CV (1984b) Mineralization dynamics in fallow dryland wheat plots, Colorado. Plant and Soil 76: FAO-UNESCO (1974) Soil map of the world, vol 1. Geneva, FAO- UNESCO Fenchel T (1982) Ecology of heterotrophic microflagellates: II. Bioenergetics and growth. Mar Ecol Prog Ser 8: Fenchel T (1987) Ecology of protozoa. Brock/Springer Series in contemporary bioscience, Berlin Heidelberg New York Fenchel T, Blackburn TH (1979) Bacteria and mineral cycling. Academic Press, New York London Goldman JC, Caron DA, Andersen DK, Dennett MR (1985) Nutrient cycling in microflagellate food chains: I. Nitrogen dynamics. Mar Ecol Prog Ser 24: Haider K, Mosier A, Heinemeyer O (1987) The effect of growing plants on denitrification at high nitrate concentrations. Soil Sci Soc Am J 51: Hansson A-C, Pettersson R, Paustian K (1987) Shoot and root production and nitrogen uptake in barley with and without nitrogen fertilization. Z Acker/Pflanzenbau 158: Hunt HW, Coleman DC, Ingham ER, Ingham RE, Elliott ET, Moore JC, Rose SL, Reid CPP, Morley CR (1987) The detrital food web in a shortgrass prairie. Biol Fertil Soils 3:57-68 Lucas MI, Probyn TA, Painting SJ (1987) An experimental study of microflagellate bacterivory: Further evidence for the importance and complexity of microplanctonic interactions. S Aft J Mar Sci 5: Page FC (1967) Taxonomic criteria for limax amoebae with description of 3 new species of Hartmannella and 3 of Valkampfia. J Protozool 14: Paustian K, Andr6n O, BostrOm U, Clarholm M, Hansson A-C, Johansson G, Lindberg T, Pettersson R, Sohlenius B (1989) Carbon and nitrogen budgets for four agroecosystems with annual and perennial crops with and without fertilization. J Appl Ecol (in press) Pettersson R (1989) Above-ground growth dynamics and net production of spring barley in relation to nitrogen fertilization. Swed J Agric Res (in press) Schn~irer J, Clarholm M, BostrOm S, Rosswall T (1986a) Effects of moisture on soil organisms and nematodes: A field experiment. Microb Ecol 12: Schnarer J, Clarholm M, Rosswall T (1986b) Fungi, bacteria and protozoa in soil from four arable cropping systems. Biol Fertil Soils 2: Singh BN (1941 a) Selectivity in bacterial food by soil amoebae in pure and mixed cultures and in sterilized soil. Ann Appl Biol 28:52-64 Singh BN (1941 b) The influence of different bacterial food supplies on the rate of reproduction of Colpoda stein#, and the factors influencing encystation. Ann Appl Biol 28:65-73 Steen E, Jansson PE, Persson J (1984) Experimental site of the 'Ecology of Arable Land' project. Acta Agric Scand 34: Van Veen JA, Ladd JN, Frissel MJ (1984) Modelling C and N turnover through the microbial biomass in soil. Plant and Soil 76: Received February 27, i989