Agricultural factors affecting methane oxidation in arable soil

CH₄ oxidation activity in a sandy soil (Ardoyen) and agricultural practices affecting this oxidation were studied under laboratory conditions. CH₄ oxidation in the soil proved to be a biological process. The instantaneous rate of CH₄ consumption was in the order of 800 mol CH₄ kg^–1 day^–1 (13 mg CH₄ kg^–1 day^–1) provided the soil was treated with ca. 4.0 mmol CH₄ kg^–1 soil. Upon repeated supplies of a higher dose of CH₄, the oxidation was accelerated to a rate of at least 198 mg CH₄ kg^–1 day^–1. Addition of the plant-growth promoting rhizopseudomonad strains Pseudomonas aeruginosa 7NSK2 and Pseudomonas fluorescens ANP15 significantly decreased the CH₄ oxidation by 20 to 30% during a 5-day incubation. However, with further incubation this suppression was no longer detectable. Growing maize plants prevented the suppression of CH₄ oxidation. The numbers of methanotrophic bacteria and fungi increased significantly after the addition of CH₄, but there were no significant shifts in the population of total bacteria and fluorescent pseudomonads. Drying and rewetting of soil for at least 1 day significantly reduced the activity of the indigenous methanotrophs. Upon rewetting, their activity was regained after a lag phase of about 3 days. The herbicide dichlorophenoxy acetic acid (2,4-D) had a strong negative effect on CH₄ oxidation. The application of 5 ppm increased the time for CH₄ removal; at concentrations above 25 ppm 2,4-D CH₄–oxidizing activity was completely hampered. After 3 days of delay, only the treatments with below 25 ppm 2,4-D showed recovery of CH₄–oxidizing activity. This finding suggests that it can be important to include a CH₄–removal bioassay in ecotoxicology studies of the side effects of pesticides. Changes in the native soil pH also affected the CH₄–oxidizing capacity. Permanent inhibition occurred when the soil pH was altered by 2 pH units, and partial inhibition by 1 pH unit change. A rather narrow pH range (5.9–7.7) appeared to allow CH₄ oxidation. Soils pre-incubated with NH ₄ ⁺ had a lower CH₄–removal capacity. Moreover, the nitrification inhibitor 2-chloro-6-trichloromethyl pyridine (nitrapyrin) strongly inhibited CH₄ oxidation. Probably methanotrophs rather than nitrifying microorganisms are mainly responsible for CH₄ removal in the soil studied. It appears that the causal methanotrophs are remarkably sensitive to soil environmental disturbances.     

Influence of oxygen on denitrification and aerobic respiration in soil

Summary The influence of the partial pressure of oxygen on denitrification and aerobic respiration was investigated at defined P02 values in a mull rendzina soil. The highest denitrification and respiration rates obtained in remoistened, glucose- and nitrate-amended soil were 43 1 N₂0 h^–1g^–1 soil and 130 1 O₂ h^–1g^–1 soil, respectively. At -55 kPa matric water potential, corresponding to 40% water saturation, N₂0 was produced only below P0₂ 40 hPa. The K _m, for O₂ was 3.0 x 10^–6 M. Formation of N₂O and consumption of O₂ occurred simultaneously with half maximum rates at P0₂ 6.7–13.3 hPa. Nitrite accumulated in soil below 40 hPa and increased with decreasing pO₂. The upper threshold for N₂0 formation in amended soil was P0₂ 33–40 hPa (39-47 M O₂).     

Phytotron studies to compare nitrogen losses from corn-planted soil by the 15-N balance or direct dinitrogen and nitrous oxide measurements

Summary Containers filled with soil mixed with potassium nitrate highly enriched in ¹⁵N were planted with corn (Zea mays L.) and kept in a phytotron under controlled conditions for 79 days. Soil water content was normally maintained at exactly 60% water-holding capacity (–33 kPa), but it was increased several times to 85% (–5 kPa) for short periods to favour denitrification. The soil headspace was sealed from the phytotron atmosphere and aerated by a continuous stream of air. Nitrous oxide emission was measured by estimating the N₂O concentration differences in the air entering and leaving the containers. Emission of N₂ was estimated by mass spectroscopy from changes in the N₂ composition in the temporarily enclosed soil headspace. Both methods were carefully checked for accuracy by different tests. At specific times during the experiment the distribution of ¹⁵N between plants and soil was determined and a ¹⁵N balance established. Emission of N gases peaked at times of increased water content and reached maxima of 149 and 142 g N pot^–1 day^–1 for N₂O and N₂, respectively. While N losses of 5% ± 2% were indicated by the ¹⁵N balance, only 1.1% ± 0.3% loss from 2.7 g applied N was estimated from the N₂O and N₂ measurements after 79 days. Possible reasons for these differences are discussed.     

Using the spermosphere model technique to describe the dominant nitrogen-fixing microflora associated with wetland rice in two Egyptian soils

Summary This study is an attempt to describe the dominant N₂-fixing microflora associated with the roots of wetland rice. Rice cultivar Giza 171 was grown in a phytotron on two alluvial Egyptian soils for 8 days, a stage when the nitrogenase activity of undisturbed plants reached a level of 245 × 10^–6 mol C₂H₄ h^–1 g^–1 dry weight of leaf. The roots and rhizosphere soils were then used for counting and isolating dominant diazotrophs. Counts and initial enrichment steps were carried out on a selective medium made of an axenic rice plantlet, the spermosphere model, incubated under 1 % acetylene. The counts were very high, exceeding 10⁸ bacteria g^–1 dry weight of rhizosphere soil. Enterobacteriaceae were dominant; most isolates were Enterobacter cloacae belonging to different biotypes in the two soils. Enterobacter agglomerans, Citrobacter freundii and Klebsiella planticola were also present as members of the dominant microflora. Azospirillum brasilense and Azospirillum lipoferum were present as well, but less abundant.     

Acetylene reduction,hydrogen evolution and nodule respiration in Phaseolus vulgaris

Summary In an experiment performed under greenhouse conditions, four cultivars of Phaseolus vulgaris L. (Venezuela-350; Aroana; Moruna; Carioca) were inoculated with three Rhizobium leguminosarum biovar phaseoli strains (C-05; C-40 = CIAT 255; C-89 = CIAT 55) and were fertilized with an N-free mineral nutrient solution. The plants were harvested 25, 40, and 55 days after emergence and the following paramenters were evaluated: Nitrogenase activity of nodulated roots, H₂ evolution by the nodules; relative efficiency of nitrogenase; respiration rates of nodulated roots and detached nodules; dry weight and total N of stems, leaves, pods, roots, and nodules. Generally the bean cultivar, Rhizobium strain, had an effect and there was an interaction effect with both symbiotic partners, on all parameters. On average, nodules represented 23% of total root respiration but the best symbiotic combinations showed lower ratios of C respired to N fixed. The maximum N-assimilation rate (between 40 and 55 days after emergence) of 11.93 mg N plant^–1 day^–1 occurred with the symbiotic combination of Carioca × C-05, while the poorest rate of 0.55 mg N plant^–1 day^–1 was recorded with Venezuela-350 × C-89. The best symbiotic combinations always showed the highest relative nitrogenase efficiency, but the differences in N₂-fixation rates cannot be explained solely in terms of conservation of energy by recycling of H₂. This requires further investigation.     

Effect of high temperature on plasmid curing of Rhizobium spp. in relation to nodulation of pigeonpea [Cajanus cajan (L.) Millsp.]

Summary Fifty-six isolates of Rhizobium and Bradyrhizobium spp. (Cajanus) were studied for their plasmid profile and N₂-fixation efficacy. One to three plasmids were reproducibly detected in all the Rhizobium spp. strains but no plasmid was detected in the Bradyrhizobium spp. strains. Rhizobium sp. strain P-1 was mutagenized by Tn5 and three nod^– and six nod⁺fix^– were screened for symbiotic parameters. Neomycin-sensitive mutants were isolated by elevated temperatrue (40°C) from tranconjugants carrying Tn5 insertions. The high temperature cured these mutants from the single large plasmid present in the parent strain P-1. All these cured mutants were nod^–, indicating that the genes for nodulation were present on this plasmid, which is readily cured at a high temperature (40°C). The high temperature in the semi-arid zones of Haryana could be responsible for the low nodulation of pigeonpea because the plasmid carrying the nodulation genes is cured at 40°–45°C giving rise to non-nodulating mutants.     

Infectivity and acetylene reduction of diazotrophic rhizosphere bacteria in wheat (Triticum aestivum) seedlings under gnotobiotic conditions

Summary Wheat seedlings were inoculated with rhizosphere nitrogen-fixing bacteria and grown gnotobiotically for 15 days. The growth medium consisted of semisolid agar with or without plant nutrients. The bacteria, isolated from roots of field-grown wheat, were three unidentified Gram-negative rods (A1, A2, E1), one Enterobacter agglomerans (C1) and two Bacillus polymyxa (B1, B2). A strain of Azospirillum brasilense (USA 10) was included for comparison.Nitrogenase activity (acetylene reduction activity, ARA) was tested on intact plants after 8 and 15 days of growth. In semisolid agar without plant nutrients, five isolates showed ARA of 0.01–0.9 nmol C₂H₄ plant^–1 h^–1, while the two strains of B. polymyxa had higher ARA of 3.3–10.6 nmol C₂H₄ plant^–1 h^–1.Plant development was not affected by inoculation with bacteria, except that inoculation with B. polymyxa resulted in shorter shoots and lower root weight.Transmission electronmicroscopy of roots revealed different degrees of infection. A. brasilense, A1 and A2, occurred mainly in the mucilage on the root surface and between outer epidermal cells (low infectivity). B. polymyxa strains and E1 were found in and between epidermal cells (intermediate infectivity) while E. agglomerans invaded the cortex and was occasionally found within the stele (high infectivity).     

Physiological and symbiotic characteristics of Rhizobium fredii isolated from subtropical-tropical soils

Summary Physiological and symbiotic characteristics were identified in Rhizobium fredii isolated from subtropical-tropical soils. The generation times of R. fredii Taiwan isolated-SB 357 and -SB 682 were 1.7 and 2.5 h, respectively. These strains were associated with acid production in yeast-extract mannitol medium. They were able to use hexoses, pentose, sucrose, trehalose and raffinose. Strain SB 357 can resist a high concentration of kanamycin (100 g ml^–1 and penicillin (400 g ml^–1). It can tolerate up to 2.34% NaCl and 1031.3 mosmol kg^–1 (23.4 bars). The growth rate of R. fredii SB 357 under the concentration of approximately 450 mosmol kg^–1 (10.2 bars) was not affected by salinity, but responded to osmotic pressure. Both strains (SB 357 and SB 682) isolated from subtropical-tropical soils were able to form an effective N₂-fixing symbiosis with the US soybean cv Clark lanceolate leaflet.     

Fluxes of methane and carbon dioxide from soil under forest, grazing land, irrigated rice and rainfed field crops in a watershed of Nepal

Field evolution of CH₄ and CO₂ from soils under four dominant land uses in the Mardi watershed, western Nepal, were monitored at 15-day intervals for 1 year using closed chamber techniques. The CH₄ oxidation rate (mean±SE, g CH₄ m^–2 h^–1) in the forest (22.8±6) was significantly higher than under grazing land (14±2) and an upland rainfed maize and millet system (Bari) (2.6±0.9). Irrigated rice fields (Khet) showed an oxidation rate of 6±0.8 g CH₄ m^–2 h^–1 in the dry season (December–May) but emitted a mean rate of 131 g CH₄ m^–2 h^–1 in the rainy season and autumn (June–October). The evolution of CO₂ ranged from 10 mg CO₂ m^–2 h^–1 in the Bari in January to 1,610 mg CO₂ m^–2 h^–1 in the forest in July. Higher evolution of CO₂ (mean±SE, mg CO₂ m^–2 h^–1) was observed in the Bari (399±39) and forest (357±36) compared to Khet (246±25) and grazing (206±20) lands. The annual emission of CO₂ evolution varied from 86.6 to 1,836 g CO₂ m^–2 year^–1. The activation energy for CH₄ and CO₂ varied between 16–283 and 80–117 kJ mol^–1, respectively. The estimated temperature coefficient for CO₂ emission varied from 2.5 to 5.0. Temperature explained 46–51% of the variation in CO₂ evolution, whereas it explained only 4–36% of the variation in CH₄ evolution.     

Short-term measurements of uptake of nitrogen fixed in the rhizospheres of sorghum (Sorghum bicolor) and millet (Pennisetum americanum)

Summary In a series of short-term experiments root systems of young sorghum and millet plants inoculated with N₂-fixing bacteria were exposed to ¹⁵N₂-enriched atmospheres for 72 h. The plants were grown in a normal atmosphere for up to 22 days after the end of the exposure to allow them to take up the fixed N₂. Environmental conditions and genotypes of sorghum and millet were selected to maximise N₂-fixation in the rhizosphere. Detectable amounts of fixed N (> 16 g/plant) were rapidly incorporated into sorghum plants grown in a sand/farmyard manure medium, but measurable fixation was found on only one occasion in plants grown in soil. N₂ fixation was detectable in some experiments with soil-grown millet plants but the amounts were small (2–4 g/plant) and represented less than 1 % of plant N accumulated over the same period. In many cases there was no detectable ¹⁵N₂ incorporation despite measurable increases in ethylene concentration found during an acetylene reduction assay.Published as ICRISAT Journal Article No. JA 740     

Effect of encapsulated calcium carbide on dinitrogen,nitrous oxide,methane, and carbon dioxide emissions from flooded rice

Summary The efficiency of N use in flooded rice is usually low, chiefly due to gaseous losses. Emission of CH₄, a gas implicated in global warming, can also be substantial in flooded rice. In a greenhouse study, the nitrification inhibitor encapsulated calcium carbide (a slow-release source of acetylene) was added with 75, 150, and 225 mg of 75 atom % ¹⁵N urea-N to flooded pots containing 18-day-old rice (Oryza sativa L.) plants. Urea treatments without calcium carbide were included as controls. After the application of encapsulated calcium carbide, 3.6 g N₂, 12.4 g N₂O-N, and 3.6 mg CH₄ were emitted per pot in 30 days. Without calcium carbide, 3.0 mg N₂, 22.8 g N₂O-N, and 39.0 mg CH₄ per pot were emitted during the same period. The rate of N added had a positive effect on N₂ and N₂O emissions, but the effect on CH₄ emissions varied with time. Carbon dioxide emissions were lower with encapsulated calcium carbide than without. The use of encapsulated calcium carbide appears effective in eliminating N₂ losses, and in minimizing emissions of the greenhouse gases N₂O and CH₄ in flooded rice.     

Quantification and potential availability of non-symbiotically fixed 15N in soil

Summary Non-symbiotic N₂ fixation was studied under laboratory conditions in two soils from Pakistan (Hafizabad silt loam and Khurrarianwala silt loam) and one from Illinois, USA (Drummer silty clay loam) incubated in a ¹⁵N-enriched atmosphere. N₂ fixation was greatest with the Drummer soil (18–122 g g^–1 soil, depending upon the soil treatment) and lowest with the Khurrarianwala soil (4–81 g g^–1 soil). Fixation was increased by the addition of glucose, a close correlation being observed between the amount of glucose added and the amount of N₂ fixed in the three soils (r = 0.96). Efficiency of N₂ fixation varied with soil type and treatment and was greatest in the presence of added inorganic P. Application of Mo apparently had a negative effect on the amount and efficiency of N₂ fixation in all the soils. The percentage of non-symbiotically fixed ¹⁵N in potentially mineralizable form (NH ₄ ⁺ -N released in soil after a 15-day incubation period under anaerobic conditions) was low (2%–18%, depending upon the soil treatment), although most of the fixed N (up to 90%) was recovered as forms hydrolysable with 6N HCl. Recovery in hydrolysable forms was much greater for the fixed N than for the native soil N, indicating that the former was more available for uptake by plants.     

Denitrification and nitrogen immobilization as affected by organic matter and different forms of nitrogen added to an anaerobic water-sediment system

The effects of wheat straw and different forms of N on denitrification and N immobilization were studied in an anaerobic water-sediment system. The water-sediment system was supplemented with various combinations of wheat straw and ¹⁵N-labelled and unlabelled (NH₄)₂SO₄ or KNO₃, and incubated anaerobically at 30°C for 10 days. ¹⁵N-labelled and unlabelled NO _inf3 ^sup- , NO _inf2 ^sup- , NH _inf4 ^sup+ , and organic N were determined in the water-sediment system. The gases evolved (N₂, CO₂, N₂O, and CH₄) were analyzed by gas chromatography at regular intervals. Larger quantities of ¹⁵N₂–N and organic ¹⁵N were formed in wheat straw-amended systems than in non-amended systems. Trends in CO₂ production were similar to those of N₂–N evolution. The evolution of N₂O and CH₄ was negligible. Denitrification processes accounted for about 22 and 71% of the added ¹⁵NO _inf3 ^sup- –N in the absence and presence of wheat straw, respectively. The corresponding denitrification rates were 3.4 and 12.4 g ¹⁵Ng^-1 dry sediment day^-1. In systems amended with ¹⁵NO _inf3 ^sup- –N and ¹⁵NO _inf3 ^sup- +NH _inf4 ^sup+ –N without wheat straw, 1.82 and 1.58%, respectively, of the added ¹⁵NH _inf3 ^sup- –N was immobilized. The corresponding figures for the same systems supplemented with wheat straw were 5.08 and 4.10%, respectively. Immobilization of ¹⁵NO _inf4 ^sup+ –N was higher than that of ¹⁵NO _inf3 ^sup- –N. The presence of NO _inf3 ^sup- –N did not stimulate NH _inf4 ^sup+ –N immobilization.     

Asymbiotic dinitrogen fixation by tundra in the Imnavait Creek drainage,Alaska, USA

Summary N₂ fixation by free-living microorganisms was investigated at an intensively studied low Arctic site near Toolik Lake in the northern foothills of the Brooks Range, Alaska, during July 1987. Four characteristic vegetation associations along an elevational gradient were assayed using minimally disruptive in situ acetylene reduction assay methods. The acetylene reduction rates did not differ significantly among vegetation associations. The mean rate for the site was 9.60 mol m^–2 h^–1 or 90 g N m^–2 day^–1, which is within the range of values given for other Arctic and alpine tundra studies. The complex microtopography and resulting patchy distribution of free-living and phycobiont diazotrophs is the most likely cause of the high spatial variability in acetylene reduction activity. Rates were most variable among samples from the lowest position, a riparian site. The potential contribution of heterotrophic diazotrophs was examined through a laboratory enrichment study. Soils from the two lower slope positions showed dramatic responses to added C, suggesting that heterotrophs may contribute fixed N₂ to this system.     

GREENHOUSE GAS EMISSIONS FROM AN ALKALINE SALINE SOIL CULTIVATED WITH MAIZE (ZEA MAYS L.) AND AMENDED WITH ANAEROBICALLY DIGESTED COW MANURE: A GREENHOUSE EXPERIMENT

Sludge derived from cow manure anaerobically digested to produce biogas (methane; CH₄) was applied to maize (Zea mays L.) cultivated in a nutrient-low, alkaline, saline soil with electrolytic conductivity 9.4 dS m^?1 and pH 9.3. Carbon dioxide (CO₂) emission increased 3.1 times when sludge was applied to soil, 1.6 times when cultivated with maize and 3.5 times in sludge-amended maize cultivated soil compared to the unamended uncultivated soil (1.51 mg C kg^?1 soil day^?1). Nitrous oxide (N₂O) emission from unamended soil was -0.0004 μg nitrogen (N) kg^?1 soil day^?1 and similar from soil cultivated with maize (0.27 μg N kg^?1 soil day^?1). Application of sludge increased the N₂O emission to 4.59 μg N kg^?1 soil day^?1, but cultivating this soil reduced it to 2.42 μg N kg^?1 soil day^?1. It was found that application of anaerobic digested cow manure stimulated maize development in an alkaline saline soil and increased emissions of CO₂ and N₂O.     

Suppression of methane oxidation in aerobic soil by nitrogen fertilizers,nitrification inhibitors,and urease inhibitors

Concentrations of CH₄, a potent greenhouse gas, have been increasing in the atmosphere at the rate of 1% per year. The objective of these laboratory studies was to measure the effect of different forms of inorganic N and various N-transformation inhibitors on CH₄ oxidation in soil. NH ₄ ⁺ oxidation was also measured in the presence of the inhibitors to determine whether they had differential activity with respect to CH₄ and NH ₄ ⁺ oxidation. The addition of NH₄Cl at 25 g N g^-1 soil strongly inhibited (78–89%) CH₄ oxidation in the surface layer (0–15 cm) of a fine sandy loam and a sandy clay loam (native shortgrass prairie soils). The nitrification inhibitor nitrapyrin (5 g g^-1 soil) inhibited CH₄ oxidation as effectively as did NH₄Cl in the fine sandy loam (82–89%), but less effectively in the sandy clay loam (52–66%). Acetylene (5 mol mol^-1 in soil headspace) had a strong (76–100%) inhibitory effect on CH₄ consumption in both soils. The phosphoroamide (urease inhibitor) N-(n-butyl) thiophosphoric triamide (NBPT) showed strong inhibition of CH₄ consumption at 25 g g^-1 soil in the fine sandy loam (83%) in the sandy clay loam (60%), but NH ₄ ⁺ oxidation inhibition was weak in both soils (13–17%). The discovery that the urease inhibitor NBPT inhibits CH₄ oxidation was unexpected, and the mechanism involved is unknown.     

Vertical distribution of nematodes in arable soil under grass (Festuca pratensis) and barley (Hordeum distichum)

Summary The connection between faunal composition and soil factors is discussed in this study on vertical distribution of soil nematodes under grass and barley. The investigation was undertaken on the field site of a Swedish integrated research project Ecology of Arable Land. The Role of Organisms in Nitrogen Cycling. Higher nematode number (7.6 × 10⁶ m^–2) and biomass (340 mg dry wt. m^–2) were found under a 4-year-old grass ley than under barley (5.0 × 10⁶ m^–2; biomass, 136 mg dry wt. m^–2). Plant feeders dominated under the grass ley (3.2 × 10⁶ m^–2 whereas under barley the bacterial feeders (2.4 × 10⁶ m^–2) were the most abundant feeding group. Number, biomass, mean individual size and various community parameters indicated a much better nutritive situation for the nematodes under grass than under barley. The vertical changes in the various parameters, including proportion of egg-carrying females, indicated an increasing food shortage for the nematode populations towards greater depths. In the top soil, predation could be an important factor in regulating nematode number.Dedicated to the late Prof. Dr. M.S. Ghilarov     

Effects of various nitrogen fertilizers on emission of nitrous oxide from soils

Summary Field studies of the effects of different N fertilizers on emission of nitrous oxide (N20) from three Iowa soils showed that the N₂O emissions induced by application of 180 kg ha^–1 fertilizer N as anhydrous ammonia greatly exceeded those induced by application of the same amount of fertilizer N as aqueous ammonia or urea. On average, the emission of N₂O-N induced by anhydrous ammonia was more than 13 times that induced by aqueous ammonia or urea and represented 1.2% of the anhydrous ammonia N applied. Experiments with one soil showed that the N₂O emission induced by anhydrous ammonia was more than 17 times that induced by the same amount of N as calcium nitrate. These findings confirm indications from previous work that anhydrous ammonia has a much greater effect on emission of N₂O from soils than do other commonly used N fertilizers and merits special attention in research relating to the potential adverse climatic effect of N fertilization of soils.Laboratory studies of the effect of different amounts of NH₄OH on emission of N₂O from Webster soil showed that the emission of N₂O-N induced by addition of 100 g NH₄OH-N g^–1 soil represented only 0.18% of the N applied, whereas the emissions induced by additions of 500 and 1 000 g NH4OH-N g^–1 soil represented 1.15% and 1.19%, respectively, of the N applied. This suggests that the exceptionally large emissions of N₂O induced by anhydrous ammonia fertilization are due, at least in part, to the fact that the customary method of applying this fertilizer by injection into soil produces highly alkaline soil zones of high ammonium-N concentration that do not occur when urea or aqueous ammonia fertilizers are broadcast and incorporated into soil.     

Gut load and food-retention time in the earthworms Lumbricus festivus and L. castaneus: A field study

Summary Lumbricus festivus and L. castaneus consume dung. In the field, below cow pats, their gut loads were about 0.15 and 0.14 g dry weight g^-1 ash-free dry weight of worm, respectively, but in free soil the loads were higher, about 0.21 and 0.19 g g^-1 ash-free dry weight of worm. The gut contents of dung were lighter than the total ingested material, at about 0.10 and 0.07 g dry weight g^-1 ash-free dry weight of worm, respectively. Field experiments showed that the retention time of dung ranged from >9 to 15 h for L. festivus, and from >3 to 6 h for L. castaneus. The experiments also indicated that L. festivus exploited 20- and 36-day-old dung in different ways, since the gut load was lower in those worms consuming 20-day-old dung than in those consuming 36- to 40-day-old dung. On the basis of these results the calculated consumption rate for L. festivus is 0.08 g dung day^-1 g live weight of worm^-1, and for L. castaneus 0.15 g dung day^-1, with retention times assumed to be at maximum, 15 h, for L. festivus and 6 h for L. castaneus. These calculations indicate that our field population of worms (75 g m^-2) consumes 10–15 t dung ha^-1 180 days^-1, corresponding to the amount of dung produced by 2–3 dairy cows.     

Contribution of denitrification and autotrophic and heterotrophic nitrification to nitrous oxide production in andosols

To quantify the contribution of denitrification and autotrophic and heterotrophic nitrification to N₂O production in Andosols with a relatively high organic matter content, we first examined the effect of C₂H₂ concentrations on N₂O production and on changes in mineral N contents. The optimum C₂H₂ concentration for inhibiting autotrophic nitrification was 10 Pa. Secondly, and Andosol taken from an arable field was incubated for 32 days at 30°C at 60, 80, and 100% water-holding capacity with or without the addition of NH ₄ ⁺ or NO _inf3 ^sup- (200 mg N kg^-1), and subsamples collected every 4–8 days were further incubated for 24 h with or without C₂H₂ (10 Pa). At 60 and 80% water-holding capacity with NH ₄ ⁺ added, 87–92% of N₂O produced (200–250 g N₂O–N kg^-1) was derived from autotrophic nitrification. In contrast, at 100% water-holding capacity with or without added NO _inf3 ^sup- , enormous amounts of N₂O (29–90 mg N₂O–N kg^-1) were produced rapidly, mostly by denitrification (96–98% of total production). Thirdly, to examine N₂O production by heterotrophic nitrification, the Andosol was amended with peptone or NH ₄ ⁺ (both 1000 mg N kg^-1)+citric acid (20 g C kg^-1) and with or without dicyandiamide (200 mg N kg^-1). Treatment with citric acid alone or with citric acid+dicyandiamide suppressed N₂O production. In contrast, peptone increased N₂O production (5.66 mg N₂O–N kg^-1) mainly by denitrification (80% of total production). However, dicyandiamide reduced N₂O production to 1.1 mg N₂O–N kg^-1. These results indicate that autotrophic nitrification was the main process for N₂O production except at 100% water-holding capacity where denitrification became dominant and that heterotrophic nitrification had a lesser importance in the soils examine.Dedicated to Professor J. C. G. Ottow on the occasion of his 60th birthday