Marianne Clarholm

Interactions of bacteria, protozoa and plants leading to mineralization of soil nitrogen

Summary-The capacity of bacteria and protozoa to mineralize soil nitrogen was studied in microcosms with sterilized soil with or without wheat plants. The effect of small additions of glucose or ammonium nitrate or both, twice a week was also tested. Plant dry weight and N-content, number of microorganisms and biomass plus inorganic N were determined after 6 weeks. The introduction of plants profoundly influenced the N tr~sfo~ations. In the presence of root-derived carbon, much more N was mineralized from the organic matter and immobilized mainly in plant biomass. “Total observable change in biomass N plus inorganic N” was negative in the unvegetated soils without additions, while a mmeralization of 1.7 mg N microcosm-’ was observed in microcosms with wheat plants grown with bacteria only. When protozoa were included, the N taken up by plants increased by 75%. Sugar additions resulted in an 18% increase of total N in the shoots when protozoa were present, but had no significant effect in the absence of grazers. Plants with the same root weight were more efficient in their uptake of inorganic N when protozoa were present. Plants grown with protozoa also had a lower R/S ratio, indicating a less stressed N availabiIitv situatron. The lowest ratio was found with N additions m the presence of protozoa. The results indicate that, with energy supplied by plant roots or with external glucose additions, soil bacteria can mineralize N from the soil organic matter to support their own growth. Grazing of the bacteria is necessary to make bacterial biomass N available for plant uptake.

MICROBIAL BIOMASS AND ACTIVITY IN AN AGRICULTURAL SOIL WITH DIFFERENT ORGANIC MATTER CONTENTS

Changes in soil fertility caused by various organic and N-fertilizer amendments were studied in a long-term field trial mostly cropped with cereals. Five treatments were included: (I) fallow, (II) cropping with no C or N addition, (III) cropping with N-fertilization (80 kg ha −1 yr−1), (IV) cropping with straw incorporation (1800kg Cha−1 yr−1) and N-fertilization (80 kg ha−1yr−1), and (V) cropping with addition of farmyard manure (80 kg N + 1800kg Cha−1yr−1). The treatments resulted in soil organic matter contents ranging from 4.3% (I) to 5.8% (V). Microbial biomass and activity were determined by chloroform fumigation, direct counting of fungi (fluorescein diacetate (FDA)-staining and Jones-Mollison agar-film technique) and bacteria (acridine orange staining), most probable number determinations of protozoa, esterase activity (total FDA hydrolysis) and respiration. Both biomass estimates and activity measurements showed a highly significant correlation with soil organic matter. Microbial biomass C ranged from 230 to 600 μg C g−1 dry wt soil, as determined by the fumigation technique, while conversions from direct counts gave a range from 380 to 2260 μg C. Mean hyphal diameters and mean bacterial cell volumes decreased with decreasing soil organic matter content.

Protozoan grazing of bacteria in soil-impact and importance.

Interactions between bacteria and protozoa in soil were studied over 2-week periods in the field and in a pot experiment. Under natural conditions the total biological activity was temporarily synchronized by a large rainfall, and in the laboratory by the addition of water to dried-out soil, with or without plants. In the field, peaks in numbers and biomass of bacteria appeared after the rain, and a peak of naked amoebae quickly followed. Of the three investigated groups—flagellates, ciliates, and amoebae—only populations of the latter were large enough and fluctuated in a way that indicated a role as bacterial regulators. The bacterial increase was transient, and the amoebae alone were calculated to be able to cause 60% of the bacterial decrease. The same development of bacteria and protozoa was observed in the pot experiment: in the presence of roots, amoebic numbers increased 20 times and became 5 times higher than in the unplanted soil. In the planted pots, the amoebic increase was large enough to cause the whole bacterial decrease observed; but in the unplanted soil, consumption by the amoebae caused only one-third of the bacterial decrease.

Biodiversity and species redundancy among litter decomposers

We discuss biodiversity in relation to ecosystem processes, particularly litter decomposition. Three hypotheses concerning the relations between organism groups, diversity and decomposition rates are proposed and tested against data from a two-year straw decomposition experiment. Barley straw mass loss and chemical composition, soil temperature and moisture, and the abundance of bacteria, fungi (total and FDA-active), protozoa, nematodes, microarthropods and enchytraeids were monitored.

Biomass and turnover of bacteria in a forest soil and a peat

Abstract Short-term changes in numbers and biomass of bacteria, determined by direct counts, are descrived for a subarctic mire and for the humus and mineral soil layers of pine forest podsol. For biomass, monthly fluctuations for 15 months are presented. Almost as large fluctuations in bacterial numbers were found during 2 weeks as were found during the whole year. Precipitation resulted in increases in bacterial biomass even when the soil moisture content was non-limiting for bacterial growth, but these increases did not last for more than 1–2 days. Observed rapid declines in bacterial numbers were interpreted as the result of grazing by the microfuana. Changes in cell sizes and shapes after rainfall indicated that even under favourable growth conditions only 15–30% of the bacteria were active. The increases in bacterial biomass were used to calculate a minimum production. For the bacteria in the peat the production over 9 days was 40% of the mean standing crop value per day. The corresponding values for the bacteria of the humus and mineral layer over 13 days were 19 and 15%. The average generation times, estimated from increases in numbers, were 39 h for the peat bacteria and 66 and 55 h for the bacteria of the humus and mineral layers respectively. Based on the number of falls of rain a yearly bacterial production value of 210 g d.w.m−2 was calculated for the forest site. This figure is discussed in relation to the yearly energy input through primary production.

Effects of moisture on soil microorganisms and nematodes: a field experiment

The effects of soil moisture changes on bacteria, fungi, protozoa, and nematodes and changes in oxygen consumption were studied in a field experiment. In one plot the soil was drip-irrigated daily for 10 days, while an adjacent plot experienced one rainfall and was then allowed to dry out. Oxygen consumption was the parameter measured which responded most rapidly to changes in soil moisture content. Lengths of fluorescein diacetate-active hyphae paralleled oxygen consumption in both plots. Total hyphal length was not affected by one rainfall but increased from 700 mg−1 dry weight soil to more than 1,600 m in less than 10 days in the irrigated plot. In the rain plot, bacterial numbers doubled within 3 days and declined during the following period of drought. In the irrigated plot, numbers increased by 50% and then remained constant over the duration of the study. Only small changes in protozoan numbers were observed, with the exception of the last sampling date in the irrigated plot when large numbers of naked amoebae were recorded 2 days after a large natural rainfall. Nematode numbers, especially obligate root feeders, increased in both treatments. The increases were caused by decoiling rather than growth. The results indicate that fungal respiration was dominating, while bacteria, lacking a suitable source of energy, were less active, except for the first days.

Microbial biomass P, labile P, and acid phosphatase activity in the humus layer of a spruce forest, after repeated additions of fertilizers

A 20-year-old forest fertilization trial was used to investigate the effects of repeated P additions on P availability in the humus layer of a Norway spruce forest soil. N was supplied annually, and P, K, and micronutrients were supplied every 4th year. The last P application was made 2 years before the investigation started. Microbial P concentrations in the P+NK+micro-amended plots were about half as high as those in the control and the N-only treatment. In plots without P amendments, around 50% of the total P in the humus layer was found in microorganisms, whereas in P-amended plots the figure was around 25%. The block supporting more rapid tree growth, situated on the middle of a slope, showed a significantly higher microbial biomass P concentration than the less productive block at the bottom of the slope. Labile P concentrations did not vary between treatments and thus could not have directly contributed to the treatment-related differences in total microbial biomass P. Acid phosphatase activities were around three times lower in the sites treated with P+NK+micro-nutrients. Two sources are suggested for the acid phosphatases, active excretion by living roots and fungi and passive release from ruptured cells. For all eight plots investigated, there was a positive correlation (R=0.83) between acid phosphatase activity and the microbial P concentration. The P concentration in current-year needles was the lowest in the N-only treatment at 1,13 mg g-1 dry weight, and the highest in the P+NK + micronutrients + lime treatment at 1.92 mg g-1 dry weight. The P:N ratio in needles varied from 0.115 in the P+NK + micronutrients + lime plots to 0.068 in the N-only plots. The latter value is at the level where P is considered to be the growth rate-determining nutrient.

Effects of plant-bacterial-amoebal interactions on plant uptake of nitrogen under field conditions

SummaryBacterial 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 short-term 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.

Microbial nitrogen transformations in the root environment of barley

Abstract To determine the influence of barley roots on microorganisms and N-transfonning processes in soil, numbers of nitrifiers and potential nitrification and denitrification rates were measured every week for 5 wks. The barley plants were grown in growth chambers in which the root-containing soil layer (A) was separated from three outer soil layers (B, C, D). The numbers and biomass of bacteria, numbers of flagellates and amoebae, total and FDA-active hyphal lengths, microbial biomass carbon and respiration were also determined. The numbers of ammonium oxidizers were positively correlated with root biomass but did not differ significantly between soil layers. Potential ammonium oxidation was stimulated in the root-layer, while potential nitrite oxidation was stimulated in the B- and C-layers. The denitrification activity (measured anaerobically in the presence of excess No − 3 ) was positively correlated with root biomass in the A-layer. Denitrification activity in the B-layer was positively correlated with the water content of the soil. When roots grew near the nets separating the root layer from the other layers, denitrification activity was stimulated in the next layer (B). We propose that nitrite oxidation in the root zone partly depends on the reduction of nitrate. This would explain why nitrite-oxidizer numbers were usually several orders of magnitude higher than ammonium-oxidizer numbers. Bacterial numbers decreased between wks 1 and 5. Increases in bacteria, naked amoebae and flagellates in all layers between wks 2 and 3 indicated that bacteria were produced until wk 3. There were no signs of bacterial production after wk 3. The total length of hyphae and the length of FDA-active hyphae were not significantly different between layers. However, both of these parameters, as well as total microbial biomass carbon and respiration, were consistently highest in the A-layer.

Fungi, bacteria and protozoa in soil from four arable cropping systems

SummaryThe effects of four cropping systems on soil microorganisms were investigated during 3 years. The cropping systems were B0, barley without nitrogen fertilizers; B120, barley with 120 kg N ha−1 year−1; GL, grass ley receiving 200 kg N ha−1 year−1; and LL, lucerne ley without nitrogen fertilizer additions. At samplings in September during three consecutive years no differences were found between treatments. Total fungal lengths ranged between 0.7 and 2.0 × 103 m and bacterial numbers between 3.5 and 7.2 × 109 g−1 dry wt. soil.Twenty samplings over 3 years in B120 and in GL indicated higher numbers of bacteria and protozoa during the growing season, except for periods with moisture stress. No clear seasonal trends were found for the fungi. When comparing mean values for the 20 samplings, the grass ley contained significantly (P < 0.05) higher numbers of amoebae. Means of the bacterial numbers and biomass, total and FDA-active hyphal lengths were also higher or equal (FDA-active hyphae) but not significantly so.Seventy-nine per cent of the bacterial biomass and 73% of the total fungal lengths were found in the top soil, where also 85% of the oxygen was consumed.