Thomas Rosswall

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.

In situ methane production from acid peat in plant communities with different moisture regimes in a subarctic mire

In situ methane production from acid peat in plant communities with different moisture regimes in a subarctic mire

Seasonal variation of soil microbial biomass—The effects of clearfelling a tropical rainforest and establishment of pasture in the central Amazon

Abstract The effects of clearfelling a tropical rainforest and establishing pasture on soil microbial biomass and nitrogen transformations were assayed monthly over 1 yr in three adjacent systems in the central Amazon region: (1) virgin rainforest; (2) slashed-and-burnt forest; and (3) recently established pasture. The amounts of soil organic matter (SOM) and soil microbial biomass-carbon (biomass-C) were substantial in all systems. Total soil-C ranged between 1.9 and 5.2% depending on management and soil layer, whereas biomass-C ranged between 3.5 and 5.3% of total soil-C. The soil biomass-C decreased upon slashing-and-burning to 64% of its original value (1287 μg g−1) in the forest (0–5 cm soil layer) and increased after establishment of pasture to 1290 μg g−1, but remained unchanged in the deeper 5–20 cm soil layer. No significant seasonal variation was measured in any system or soil layer. Soil respiration responded to management like microbial biomass-C but varied significantly over the season with the smallest respiration found in the driest month (October) and the largest respiration at end of the rains in May. Pools of mineral N varied considerably in all systems and soil layers and displayed identical seasonal variations. The forest topsoil contained the highest amounts (on average 47 μg N g−1) and the pasture soil the smallest amounts (on average 24 μg N g−1). The transition of the forest ecosystem to a pasture resulted in increased NO3− concentrations. Net N-mineralization and net NO3− production monitored during short-term laboratory incubations were used as indices of N mineralization and nitrification. No significant differences in N-mineralization indices were measured between systems, but substantial within season variations were recorded in all systems and soil layers. The variations were synchronized in time with extreme net N-mineralization in September and net N-mineralization in October. Significant nitrification indices were measured in all systems. They were identical in the systems, except for small indices found in topsoil of the slashed and burnt area, where, on the other hand, certain localized areas with extreme nitrification rates were detected.

Microbial biomass as a fraction of potentially mineralizable nitrogen in soils from long-term field experiments

Abstract Aerobic long-term incubations (40-wk) were employed to measure the potentially mineralizable nitrogen (N 0 ) in five 30-yr old cropping systems. The cropping systems consisted of: (1) bare fallow; (2) cropping with no additions; (3) cropping with 80 kg N ha −1 y −1 as Ca(NO 3 ) 2 ; (4) cropping with 80 kg N ha −1 yr −1 as Ca(NO 3 ) 2 plus 1800kg C ha −1 yr −1 as straw; and (5) cropping with 80 kg N ha −1 yr −1 plus 1800 kg C ha −1 yr −1 as farmyard manure. The amounts of N mineralized during the 40-wk incubations were between 93 and 168 μg g −1 (302–543 kg N ha −1 down to 25cm depth) with the lowest value for the fallow and the highest for the farmyard manure treatment. Microbial biomass-C and -N were measured on four occasions during the incubations. The biomass-C showed a rapid decrease to week 4 (to 36% of the initial mass), a slower decrease to week 9 (to 23% of initial mass) and a very slow decline to the final determination at the end of the incubation (to 8% of initial mass). The biomass-N displayed a similar pattern. Two related models were employed to describe the kinetics of N-mineralization during incubation: (1) a two-component first-order; and (2) a simplified special case of the two-component model. In all cases except the straw-amended soil, the simplified two-component model offered the best description of the curves of accumulated mineral-N. The available fraction, N a , of soil organic-N had mineralization rate constants similar to those for mineralization of microbial biomass.

Inhibitory effect of low partial pressures of acetylene on nitrification

Abstract The inhibiting effect of C 2 H 2 on nitrification was investigated in two agricultural soils. Nitrification was totally inhibited at 10 Pa partial pressure of C 2 H 2 which is lower than previously reported for soils. There were no differences in rates of nitrate production between flasks without C 2 H 2 and flasks with C 2 H 2 at 0.01 Pa, while there was an effect on nitrification at 0.1 PaC 2 H 2 . At 0.1 Pa no inhibition was noted during the first 3 days; after this period nitrification was partially inhibited. The inhibiting effect did not cease until 7 days after removal of C 2 H 2 . The sensitivity of nitrification to low concentrations of C 2 H 2 should be noted when denitrification rates are determined by the use of the acetylene inhibition method (usually C 2 H 2 at 10kPa).

A method of selective inhibition to distinguish between nitrification and denitrification as sources of nitrous oxide in soil

SummaryNitrapyrin and C2H2 were evaluated as nitrification inhibitors in soil to determine the relative contributions of denitrification and nitrification to total N2O production. In laboratory experiments nitrapyrin, or its solvent xylene, stimulated denitrification directly or indirectly and was therefore considered unsuitable. Low partial pressures of C2H2 (2.5–5.0 Pa) inhibited nitrification and had only a small effect on denitrification, which made it possible to estimate the contribution of denitrification. The contribution of nitrification was estimated by subtracting the denitrification value from total N2O production (samples without C2H2). The critical C2H2 concentrations needed to achieve inhibition of nitrification, without affecting the N2O reductase in denitrifiers, must be individually determined for each set of experimental conditions.

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.

Ammonium oxidizer numbers, potential and actual oxidation rates in two swedish arable soils

SummaryThe number of ammonium-oxidizing bacteria was measured with the most probable number (MPN) method while potential ammonium oxidation rates were determined with a chlorate inhibition technique in two arable soils. A new method for measuring actual in situ ammonium oxidation in soil cores is presented.One soil was cropped for 4 years with one of four crop-fertilizer combinations: Unfertilized lucerne ley, unfertilized barley, nitrate-fertilized grass ley, or nitrate-fertilized barley. The highest ammonium oxidizer numbers and potential rates were found in the grass ley. The unfertilized barley had one-third the number and activity of the grass ley. Actual rates were in general 5–25 times lower than potential rates.The other soil was that undergoing a 27-year-old field trial with a fallow and four different cropping treatments: No addition, nitrate, nitrate + straw, or manure. Ammonium oxidizer numbers were highest in the manure and straw treatments. MPN numbers and potential rates were lowest in the fallow treatment. Typical specific potential rates were 30 ng N oxidized cell−1 h−1. Actual rates were in general 40 times lower than potential rates.Actual ammonium oxidation measurements seem to correspond to actual in situ activity at the moment of sampling, whereas the MPN technique and the potential measurements reflect events that occurred weeks to months before the sampling.

Relationships between soil moisture content and nitrous oxide production during nitrification and denitrification

SummaryThe effect of soil water content [60%–100% water-holding capacity (WHC)] on N2O production during autotrophic nitrification and denitrification in a loam soil was studied in a laboratory experiment by selectively inhibiting nitrification with a low C2H2 concentration (2.1 Pa). Nitrifiers usually produced more N2O than denitrifiers. During an initial experimental period of 0–6 days the nitrifiers produced more N2O than the denitrifiers by a factor ranging from 1.4 to 16.5, depending on the water content and length of incubation. The highest N2O production rate by nitrifiers was observed at 90% WHC, when the soil had become partly anaerobic, as indicated by the high denitrification rate. At 100% WHC there were large gaseous losses from denitrification, while nitrification losses were smaller except for the first period of measurement, when there was still some O2 remaining in the soil. The use of 10 kPa C2H2 to inhibit reduction of N2O to N2 stimulated the denitrification process during prolonged incubation over several days; thus the method is unsuitable for long-term studies.

Effect of nitrogen and carbon supply on the development of soil organism populations and pine seedlings - microcosm experiment

Series of pots containing Scots pine seedlings and a humus-sand mixture were watered twice weekly for 398 d with different nutrient solutions (control treatment with complete plant nutrient solution less nitrogen; glucose addition; nitrogen addition; glucose and nitrogen addition). Analyses were made of organic and nitrogen contents in the substrate, plant weights and nitrogen contents, nitrogen fixation and respiration rates, abundance, biomass and in some cases species composition of different soil organism populations. The growth rate of pine seedlings was lowest in series supplied with glucose, which is most easily explained by a deficiency of nitrogen due to microbial immobilization. The fungi and yeast were stimulated by addition of an easily available carbon source whereas the bacteria needed both nitrogen and carbon to maintain high biomass. A positive correlation between fungal feeding soil organisms and amount of fungal mycelium was found while a more complex situation prevailed with regard to bacterial numbers and bacterial feeding nematodes. The systems are evaluated in relation to the different treatments and compared with the field situation. (Less)